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Coastal ecosystem functioning often hinges on habitat‐forming foundation species that engage in positive interactions (e.g. facilitation and mutualism) to reduce environmental stress. Seagrasses are important foundation species in coastal zones but are rapidly declining with losses typically linked to intensifying global change‐related environmental stress. There is growing evidence that loss or disruption of positive interactions can amplify coastal ecosystem degradation as it compromises its stress mitigating capacity. Multiple recent studies highlight that seagrass can engage in a facultative mutualistic relationship with lucinid bivalves that alleviate sulphide toxicity. So far, however, the generality of this mutualism, and how its strength and relative importance depend on environmental conditions, remains to be investigated. Here we study the importance of the seagrass‐lucinid mutualistic interaction on a continental‐scale using a field survey across Europe. We found that the lucinid bivalve Loripes orbiculatus is associated with the seagrasses Zostera noltii and Zostera marina across a large latitudinal range. At locations where the average minimum temperature was above 1 °C, L. orbiculatus was present in 79% of the Zostera meadows; whereas, it was absent below this temperature. At locations above this minimum temperature threshold, mud content was the second most important determinant explaining the presence or absence of L. orbiculatus. Further analyses suggest that the presence of the lucinids have a positive effect on seagrass biomass by mitigating sulphide stress. Finally, results of a structural equation model (SEM) support the existence of a mutualistic feedback between L. orbiculatus and Z. noltii. We argue that this seagrass‐lucinid mutualism should be more solidly integrated into management practices to improve seagrass ecosystem resilience to global change as well as the success of restoration efforts. [ABSTRACT FROM AUTHOR]

We explored the sulfur dynamics and the relationships between sediment sulfur and nutrient pools, seagrass structural and physiological variables and sulfide intrusion in native ( Posidonia oceanica , Cymodocea nodosa ) and exotic ( Halophila stipulacea ) Mediterranean seagrasses at six sites affected by cumulative anthropogenic pressures to understand the factors controlling sulfide intrusion in seagrass. Sensitive indicators of seagrass stress (leaf TN, δ 15 N, TS, F sulfide ) were increased at several sites, implying that seagrasses are under pressure. Sulfide intrusion was not related to sediment TOC but it was negatively related to shoot size and below-ground biomass. Sulfide intrusion in seagrass tissue was high in P. oceanica (12–17%) and considerably higher in C. nodosa (27–35%). Intrusion was particularly high in H. stipulacea (30–50%), suggesting that its possible biogeographical expansion due to warming of the Mediterranean may result in accumulation of sulfides in the sediments and hypoxia/anoxia with further implications in ecosystem function. [ABSTRACT FROM AUTHOR]

Tourist development is rapid on the South-West coast of the Black Sea, increasing the pressure on the coastal ecosystem. Zostera marina and Zostera noltii are abundant seagrasses in the Black Sea, but little is known about their sensitivity towards coastal developments. In this study plant nutrient content (N and P), sulphide intrusion, sediment organic matter, pore water pools of nutrients and sulphides, sulphate reduction rates as well as water column nutrients and chlorophyll- a concentrations were studied in a mixed seagrass meadow ( Z. marina L. and Z. noltii Hornem.) located in an area receiving untreated sewage waters from a point source in Sozopol Bay, South-Western Black Sea. In addition, studies included population level metrics like density, biomass and shoot morphology. The point source impacted water quality in the area leading to local nutrient enrichment, development of filamentous macroalgae and reduced water transparency. Although sulphide intrusion was detected in Z. marina and thus indicating existence of sulphide pressure on the plants, there were no significant effects on the seagrass distribution except for Z. noltii being absent from the meadows close to the point source. The nutrient levels were low in the plants and in the sediments compared to global Z. marina and Z. noltii meadows, but the observed blooms of filamentous macroalgae, increase in sulphide intrusion and higher N contents of the plants during the peak tourist season are signs of eutrophication in Sozopol Bay most likely as a result of increased recreational activity on the coast. [ABSTRACT FROM AUTHOR]

Gaseous sulfide intrusion into seagrasses growing in sulfidic sediments causes little or no harm to the plant, indicating the presence of an unknown sulfide tolerance or detoxification mechanism. We assessed such mechanism in the seagrass Zostera marina in the laboratory and in the field with scanning electron microscopy coupled to energy dispersive X-ray spectroscopy, chromatographic and spectrophotometric methods, and stable isotope tracing coupled with a mass balance of sulfur compounds. We found that Z. marina detoxified gaseous sediment-derived sulfide through incorporation and that most of the detoxification occurred in underground tissues, where sulfide intrusion was greatest. Elemental sulfur was a major detoxification compound, precipitating on the inner wall of the aerenchyma of underground tissues. Sulfide was metabolized into thiols and entered the plant sulfur metabolism as well as being stored as sulfate throughout the plant. We conclude that avoidance of sulfide exposure by reoxidation of sulfide in the rhizosphere or aerenchyma and tolerance of sulfide intrusion by incorporation of sulfur in the plant are likely major survival strategies of seagrasses in sulfidic sediments. [ABSTRACT FROM AUTHOR]

The build-up of sulphide concentrations in sediments, resulting from high inputs of organic matter and the mineralization through sulphate reduction, can be lethal to the benthos. Sulphate reduction is temperature dependent, thus global warming may contribute to even higher sulphide concentrations and benthos mortality. The seagrass Posidonia oceanica is very sensitive to sulphide stress. Hence, if concentrations build up with global warming, this key Mediterranean species could be seriously endangered. An 8-year monitoring of daily seawater temperature, the sulphur isotopic signatures of water ( δ 34 S water), sediment (δ 34 S CRS) and P. oceanica leaf tissue (δ 34 S leaves), along with total sulphur in leaves ( TS leaves) and annual net population growth along the coast of the Balearic archipelago (Western Mediterranean) allowed us to determine if warming triggers P. oceanica sulphide stress and constrains seagrass survival. From the isotopic S signatures, we estimated sulphide intrusion into the leaves ( F sulphide) and sulphur incorporation into the leaves from sedimentary sulphides ( SS leaves). We observed lower δ 34 S leaves, higher F sulphide and SS leaves coinciding with a 6-year period when two heat waves were recorded. Warming triggered sulphide stress as evidenced by the negative temperature dependence of δ 34 S leaves and the positive one of F sulphide, TS leaves and SS leaves. Lower P. oceanica net population growth rates were directly related to higher contents of TS leaves. At equivalent annual maximum sea surface water temperature ( SST max), deep meadows were less affected by sulphide intrusion than shallow ones. Thus, water depth acts as a protecting mechanism against sulphide intrusion. However, water depth would be insufficient to buffer seagrass sulphide stress triggered by Mediterranean seawater summer temperatures projected for the end of the 21st century even under scenarios of moderate greenhouse gas emissions, A1B. Mediterranean warming, therefore, is expected to enhance P. oceanica sulphide stress, and thus compromise the survival of this key habitat along its entire depth distribution range. [ABSTRACT FROM AUTHOR]

Abstract: The study aimed at examining the relationship between epiphyte dynamics and carbon metabolism in seagrass ecosystems under nutrient enrichment. Temporal variability of epiphytes and factors controlling their dynamics (i.e. environmental conditions, substratum availability, substratum stability and herbivore pressure) were assessed in a fish farm impacted and an unaffected Mediterranean seagrass (Posidonia oceanica) meadow in the Aegean Sea (Greece). The factors controlling epiphyte dynamics responded differently to nutrient enrichment and partly interacted, rendering their cumulative effect on epiphyte load difficult to elucidate. Yet epiphytes accumulated on seagrass leaves near to the fish farm throughout the year, contributing 2 times more in above-ground biomass at cages than the control station. Reduction in substratum availability (i.e. decrease in leaf biomass) and increase in herbivore pressure affected epiphyte load, albeit their effects were not strong enough to counterbalance the effect of nutrient input from fish farm effluents. Moderate yet continuous nutrient input possibly stimulated epiphyte growth in excess of herbivory, shifting the control of epiphytes from top-down to bottom-up. Epiphyte accumulation affected carbon metabolism in the seagrass ecosystem by contributing to enhanced dissolved organic carbon (DOC) release, but seagrass loss was so acute that increased epiphyte cover could not counterbalance the decrease in community carbon production which was mainly driven by seagrass decline. [Copyright &y& Elsevier]

Seagrasses are threatened by multiple anthropogenic stressors, such as accumulating drift algae and increasing temperatures (associated with eutrophication and global warming, respectively). However, few seagrass experiments have examined whether exposure to multiple stressors causes antagonistic, additive, or synergistic effects, and this has limited our ability to predict the future health status of seagrass beds. We conducted a laboratory experiment to test whether abundance of Gracilaria comosa (3 levels; 0, 1.2, and 3.4 kg WW m), an algae that is resistant to wide environmental fluctuations (e.g. light, temperature, salinity, and oxygen levels), has negative effects on the small ephemeral seagrass, Halophila ovalis and whether the effects are exacerbated by high temperature (3 levels; 20, 25, and 30°C). We found an additive negative effect of the two stressors when tested simultaneously on 14 seagrass performance measures, with most data variability explained by the drift algae. For the individual plant performance measures (above- and below-ground growth and mortality, leaf area, internode distance, and root length and root volume), we found 5 additive effects, 4 synergistic effects, and 5 effects that were significant only for drift algae. We also documented a significant additive effect of drift algae and temperature on dissolved porewater sulphide (DS). A follow-up correlation analysis between DS and the 14 plant performance measures revealed significant or near-significant linear correlations on 9 of these responses (above- and below-ground growth, leaf area and weight, leaf mortality, and internode distance). In summary, we showed (a) that a stress-resistant drift algae can have strong negative effects on a small ephemeral seagrass, (b) this negative effect can increase both additively and synergistically with increasing temperature depending on performance measure, and (c) the negative effects may be mediated by a build-up of porewater DS. An implication of our findings is that resource managers aiming to preserve healthy seagrass beds in an almost certain future warmer world should increase efforts to keep drift algae populations low. [ABSTRACT FROM AUTHOR]

Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP, DIP) flux across the sediment–water interface was assessed in fish farm impacted and pristine seagrass ( Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of P. oceanica community health. [ABSTRACT FROM AUTHOR]

Abstract: The potential threat to seagrasses of the invasive algae, Gracilaria vermiculophylla was assessed through metabolic indicators under experimental conditions. Net leaf photosynthesis (LNP) and dark respiration (LDR) were measured from leaf segments of Zostera marina shoots under different loads of G. vermiculophylla (control, low 2.2kg FW m−2 and high 4kg FW m−2) in mesocosm experiments separated in tanks at four temperatures (19, 23.5, 26 and 30°C). LNP decreased in the presence of the high density G. vermiculophylla mat (25% on average), being the most severe reductions at 30°C (35% less in high). LDR did not respond significantly to differences in algal biomass, whereas a progressive increase was found with increasing temperatures (3.4 times higher at 30°C than at 19°C). Sulphide in porewater was measured weekly in order clarify the role of sediment conditions on seagrass metabolism, and increased both with algal biomass (29% in high) and temperature (from 0.5mM at 26°C to 2.6mM at 30°C), but changes in LNP and LDR were not correlated with sulphide concentrations. Seagrass survival rates showed decreasing trend with algal biomass at all the temperatures (from 74% to 21% survival). G. vermiculophylla showed harmful effects on Z. marina metabolism and survival with synergistic effects of temperature suggesting greater impact of invasive species under future higher water temperatures. [Copyright &y& Elsevier]

Community metabolism and dissolved organic and inorganic nutrient fluxes were assessed in impacted from fish farm discharges and reference vegetated ( Posidonia oceanica) and adjacent unvegetated communities in the Aegean Sea, Greece. Both metabolism and nutrient fluxes significantly differed between impacted and reference communities, but the effect depended on community type and time of year. Net community production (NCP) in the impacted vegetated community decreased by 60%, respiration (R) by 34%, and gross primary production (GPP) by 44%. The GPP:R ratio declined more (35%) in the impacted unvegetated than in the corresponding vegetated community (15%), implying that proximity to the fish farm has a severe impact on the unvegetated community, leading to imbalanced metabolism (GPP < R) and heterotrophic (GPP:R = 0.9) conditions. Higher release of dissolved organic and inorganic carbon, nitrogen, and phosphorous was observed in the impacted vegetated community compared to the corresponding unvegetated one, implying intensification of mineralization in the seagrass community. On an annual scale, the impacted vegetated community supported increased DOC efflux by 204%, DON by 1639%, NH4 by 122%, and NO3 by 26%, whereas it supported release of DOP and PO4 compared to the reference community, which removed these dissolved nutrients from the water column. The impacted unvegetated community supported an annual increase of DOC efflux by 208% and PO4 by 42% and it released DON, NH4, NO3, and DOP, whereas the reference community took up these nutrients. Proximity to the fish farm altered the ecosystem state by lowering the productivity and by enhancing the nutrient release. [ABSTRACT FROM AUTHOR]

Abstract: Sulfur and oxygen dynamics in the seagrasses Thalassia testudinum and Syringodium filiforme and their sediments were studied in the US Virgin Islands (USVI) in order to explore sulfide intrusion into tropical seagrasses. Four study sites were selected based on the iron concentration in sediments and on proximity to anthropogenic nutrient sources. Meadow characteristics (shoot density, above- and below-ground biomass, nutrient content) were sampled along with sediment biogeochemistry. Sulfide intrusion was high in T. testudinum, as up to 96% of total sulfur in the plant was derived from sediment-derived sulfides. The sulfide intrusion was negatively correlated to the turnover of sulfides in the sediments regulated by both plant parameters and sediment sulfur pools. Sediment iron content played an indirect role by affecting sulfide turnover rates. Leaf production was negatively correlated with sulfide intrusion suggesting that active growth reduced sulfide intrusion. Sulfide intrusion was lower in S. filiforme (up to 44%) compared to T. testudinum consistent with a higher internal nighttime oxygen concentrations found for S. filiforme. When S. filiforme can take advantage of its ability to maintain high internal oxygen concentrations, as was the case on the USVI, it could increase its success in colonizing unvegetated disturbed sediments with potentially high sulfide concentrations. [Copyright &y& Elsevier]

Abstract: Morphology, total sulfur content and stable sulfur isotopic composition of Zostera marina were examined in the Baltic Sea–Skagerrak transition zone through surveys. The seagrass meadows were denser and less productive at the low salinities in the Baltic Sea (salinity 6–7psu), and total sulfur accumulations in plants were lower and δ34 S values were higher compared to the west coast of Sweden (salinity 21–29psu). The δ34 S values of the three plant compartments (leaves, rhizomes, roots) indicated lower sulfide invasion at low salinities, which was mainly due to environmental conditions (e.g. low epiphytic biomass, low sediment organic matter and low sulfate concentration) and plant characteristics (productivity, shoot morphology). Between 13% and 63% of the sulfur in the plants was derived from sediment sulfides with highest percentages in the roots (27–63%) and lower in rhizomes (13–50%) and leaves (14–51%). The high sulfide invasion on the west coast of Sweden was coincident with high sediment organic matter, probably increasing sulfide pressure on the plants, and high epiphytic biomass, probably constraining the oxygen dynamics in the plants and enhancing sulfide invasion. Regional and spatial variability in the δ34 S were extensive, emphasizing the need for detailed analysis of local sources when applying stable sulfur isotopes in food web analyses. The observed invasion of sulfides suggests sulfide as a contributing factor to reported declines of Z. marina in the Skagerrak region. [Copyright &y& Elsevier]

Abstract: Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina. In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.]. [Copyright &y& Elsevier]

Abstract: The effect of increased sediment sulfide concentrations on the sulfur isotopic composition (δ34S), total sulfur (TS) and elemental sulfur (S0) concentrations in plant tissues was studied for the two seagrasses Zostera marina (3 weeks in laboratory) and Posidonia oceanica (4 months in situ). Porewater sulfide concentrations were experimentally regulated and plants exposed to high sediment sulfide concentrations had δ34S signals closer to the δ34S of sulfide, whereas plants exposed to no / low sulfide concentrations had δ34S signals closer to the δ34S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to high sulfide concentrations. The δ34S varied between the plant tissues in both species with the leaves having more positive δ34S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS and S0 concentrations were higher in plants exposed to sulfide in both experiments suggesting that sulfur derived from sediment sulfide accumulated in the plants. The δ34S signal in S0 was similar to sediment sulfide verifying that S0 found in the seagrasses originated from sediment sulfide. Direct comparisons of δ34S in the two different seagrasses and across the treatments were not possible due to large differences in δ34S of the sulfur sources. Fsulfide adjusted for these differences and may be a useful alternative, when δ34S of the sulfur sources varies between study sites. There were no significant effects of sulfide exposure on plant growth and mortality in Z. marina and P. oceanica after 3 weeks and 8 weeks exposure, respectively, but P. oceanica showed indications of reduced growth and higher mortality after 16 weeks of sulfide exposure probably due to sulfide invasion/toxicity. [Copyright &y& Elsevier]

Abstract: The impact of milkfish fish pens on the distribution of the alpheid shrimp Alpheus macellarius was studied in the Bolinao area, Philippines. In addition, the impact of the alpheid shrimp on sediment biogeochemistry, including organic matter mineralization, nutrient and sulfur cycling was compared in sediments affected and unaffected by fish farming. The fish farming activity had negative impact on the distribution of shrimps by reducing the abundance up to 60% in the vicinity of the net pens, and to almost absence inside the net pens. At a farm site abandoned seven month prior to sampling the abundance of shrimps was still low and sediment pools of reduced sulfur high, indicating a slow benthic recovery. The shrimp had large physical impact on the sediments, and the mineralization rates were stimulated by the presence of the shrimp in pristine sediments, whereas the fish farm impacted sites showed high mineralization rates both in sediments with and without the presence of the shrimp suggesting that the organic matter enrichment stimulated the mineralization in these low-organic sediments, whereas the shrimp had less impact. Sulfate reduction rates and in particular pools of sulfides increased in the farm impacted sediments, and sulfate reduction was an important process for organic matter mineralization (>67%) in all sediments. Although the shrimp had minor effect on the organic matter mineralization and nutrient fluxes in the farm impacted sediments, they appeared to increase the oxidation of sulfides by increasing the zone of oxidized surface sediment and thereby improving the sediment conditions for recolonization after fish farming has ceased. [Copyright &y& Elsevier]

Abstract: The effect of filamentous algae invasion into Zostera marina meadows on water quality, sediment sulfur pools and sulfide invasion into plant tissues was studied experimentally. Sulfide invasion was assessed through analysis of sulfur isotopic composition (δ 34S) and total sulfur (TS) concentrations in plant tissues. The algal mats (5 and 10 cm thickness) depleted oxygen in the mats and increased the pools of sulfides in the sediments. Plants exposed to algal mats had δ 34S signals closer to the δ 34S of sediment sulfide, whereas plants with no mats present had δ 34S signals closer to the δ 34S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to algal mats. The δ 34S varied between the plant tissues with the leaves having more positive δ 34S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS concentrations were higher in plants exposed to algal mats suggesting that sulfur derived from sediment sulfide accumulated in the plants. Fsulfide showed that up to 50% of the sulfides in the plants were derived from sedimentary sulfides. The combined effect of water column anoxia in the lower parts of the meadow and high sulfide invasion into the plants lead to significantly reduced growth rates after 3 weeks and the below-ground tissues showed signs of degradation suggesting that algal mats invasion in to Zostera marina meadows can result in seagrass decline. [Copyright &y& Elsevier]

Sulfate reduction rates and biogeochemical parameters of fish farm sediments across the Mediterranean were investigated in the order to evaluate the potential effects of organic matter inputs on habitat quality for the common seagrass Posidonia oceanica. Four study sites were selected in Spain, Italy, Greece and Cyprus to represent the Mediterranean basin. P. oceanica was found in immediate vicinity of all the farms, which were located at physically exposed sites about 1 km from the shore lines. Organic matter accumulation, sulfate reduction rates and sulfur pools were measured in depth profiles along transects from the farms in both bare and vegetated sediments. Results show that although the organic matter accumulation was minor at the sites (POC < 2.8% DW), the sulfate reduction rates were high, in particular at the largest farm in Italy (up to 212 mmol m−2 d−1), similar to rates found at shallower, temperate fish farm sites, where higher sedimentation rates can be expected. Sulfate reducing bacteria in these low-organic, carbonate-rich Mediterranean sediments respond strongly to organic matter loadings and cause habitat degradation. Sulfate reduction rates measured in the P. oceanica sediments were among the highest recorded (7.8–42.0 mmol m−2 d−1) similar to rates found in degrading meadows impacted by organic matter loadings. As sulfate reduction rates were correlated with the sedimentation rates along the transects rather than organic matter pools this suggests mineralization processes were controlled by organic matter loading in fish farm sediments. The vegetated sediments near the net cages were more reduced due to accumulation of sulfides compared to control sites, which is a possible contributing factor to the observed seagrass decline in the farm surroundings. It is recommended that Mediterranean fish farms are placed in areas with rapid dispersal of particulate waste products to minimize organic matter loading of the sediments and thereby preserve habitat quality for benthic fauna and flora. [ABSTRACT FROM AUTHOR]

The spatial and seasonal changes of total non-structural carbohydrate (TNC) reserves in above- and belowground tissues in eelgrass ( Zostera marina) were assessed at four different sampling sites in Danish waters. The study consisted of annual field sampling, where sites differed in water depth and nutrient content in sediments. The TNC concentrations varied considerably in all tissues, and rhizomes generally represented the major storage organ. Sucrose was the main sugar contributing to TNC whilst starch represented the minor (<15%) reserves. Trehalose, which is reported in Z. marina here for the first time, due to the use of a recently developed HPLC method, was found in low concentrations (up to ∼2%) and showed a reverse seasonal variation compared to other sugars, with maximum concentrations in winter. Plants growing near depth limits (5.5 m) had significantly higher reserves, whereas no significant effect of sediment nutrient contents on the TNC concentrations was observed, reflecting the ability of Z. marina to tolerate a range of different growth conditions. [ABSTRACT FROM AUTHOR]

Seagrass distribution and sediment biogeochemical conditions were measured around Libong Island in the Andaman Sea, Thailand. Seagrass diversity was moderate for a tropical site (four species found: Cymodocea rotundata, Enhalus acoroides, Halophila ovalis and Thalassia hemprichii), with the diminutive species H. ovalis particularly abundant and perhaps serving as important food for dugongs present in the area. Although the sediment organic matter content was low in the seagrass meadows, the sulfate reduction rates were high (48–138 mmol m-2 d-1). A significant positive relationship between belowground biomass and sulfate reduction rates across the seagrass species examined indicates that stimulation of microbial activity in the rhizosphere sediments is controlled by the biomass of roots and rhizomes, rather than by specific seagrass characteristics (e.g., morphology, plant activity). The low iron contents in the seagrass tissues were within the range suggested for nutrient limitation. Sediment sulfur pools, in particular pyrite pools, were high and most of the available iron was bound in sulfides, which may cause iron limitation in seagrass tissues. High sediment sulfide production and sulfide accumulation in concert exert sulfide stress on the seagrasses, as indicated by an accumulation of elemental sulfur in the plants examined, suggesting that tropical seagrasses are prone to sulfide intrusion, similar to findings for temperate seagrasses. [ABSTRACT FROM AUTHOR]

Here we demonstrate, through experimental iron additions to a Mediterranean seagrass meadow, that iron plays a pivotal role in seagrass systems on carbonate sediments, directly through its role as a limiting nutrient, and indirectly by stimulating phosphorus recycling through the activity of the enzyme alkaline phosphatase and by buffering the development of reduced conditions in sediments. Iron additions were performed throughout the active root zone (30 cm depth) to two Posidonia oceanica meadows, one on organic-enriched sediments and one on organic poor sediments (Reference). Seagrass growth, nutrient incorporation and sediment biogeochemical conditions were followed for four months. Iron additions had positive effects on seagrass growth (leaf production increased with 55%) and nutrient incorporation (increased 46–91%) in the organic-enriched site, increasing to levels found at the Reference site. There was no effect of iron additions in the Reference seagrass meadow suggesting that iron was not the most important controlling factor at this site. The iron pools were about two times higher compared to the organic-enriched site. The main effect on the sediment biogeochemical conditions at the organic-enriched site was a suppression of sulfate reduction activity to the levels encountered at the Reference site (6.7 mmol m−2d−1 vs. 4.7–5.9 mmol m−2d−1). This suggests that the sulfide stress on the seagrasses was removed and that the iron availability increased due to reduced precipitation of iron-sulfides and thus improving seagrass growth conditions in these organic-enriched sediments. [ABSTRACT FROM AUTHOR]

Abstract: The distribution and composition of neutral carbohydrates in the solid phase and porewater, and their role in carbon cycling were investigated in contrasting marine sediments of the Baltic-North Sea region. Depth-invariant profiles of particulate carbohydrates (PCHO) and low PCHO yields (PCHO/organic carbon) indicated that a small inert carbohydrate fraction deposits on the sediment at the deeper stations in the northern Kattegat and Skagerrak compared to the shallower stations further South. This was supported by long-term sediment incubations, in which the PCHO concentrations remained unchanged during 480 days, revealing that neutral carbohydrates play a minor role in carbon mineralization at the deeper sites. In contrast, the reactivity of PCHO was high (first-order rate constant of 3.2 yr−1) at one shallow site in the Belt Sea. Monosaccharide spectra were uniform with sediment depth and between sites with the exception of the shallowest site in the middle of Kattegat, where glucose dominated the polymers at the surface. This was likely due to benthic diatoms. Addition of fresh algae to surface sediment from the deeper sites resulted in a preferential mineralization of particulate glucose polymers. The addition of algae also resulted in an initial pulse of glucose in the porewater pools of total hydrolyzable carbohydrates (THCHO), indicating a faster hydrolysis of glucose polymers in the particulate phase than the subsequent hydrolysis and bacterial consumption of oligo- and polymers of glucose in the porewater. This study shows that some carbohydrates such as glucose polymers are selectively utilized by heterotrophic bacteria during the settling of organic particles through the water column, and a relatively inert fraction arrives to the sediments where much of it escapes mineralization and becomes permanently buried. In shallow coastal environments, where the degradation in the water column is less extensive and where benthic algae may represent a local carbohydrate source, neutral carbohydrates appear to be more important in organic matter mineralization. [Copyright &y& Elsevier]

Abstract: Eelgrass (Zostera marina) was grown under exposure to high levels of sediment sulfides to examine their ability to reoxidize sulfides intruding into the plants. The plants were kept under full light (control and high sulfide level) and at 10% of light saturation (high sulfide level) for 3 weeks and growth and accumulation of elemental sulfur (S0) in the plants were examined. The growth rate was reduced with ∼75% in the low light treatment, whereas there was no significant difference between the rates at full light saturation. S0 was accumulating in the below-ground structures of the plants exposed to high sulfide concentrations with highest concentration in the youngest roots and oldest internodes. There was no accumulation of S0 in the leaves, suggesting that the intruding sulfides were reoxidized in the below-ground structures before reaching the leaves. The accumulation of S0 was higher in the roots of the low light treatment (up to two times) suggesting a larger intrusion of sulfides. These plants also appeared highly affected by the treatment with rotting meristems and increased mortality after the 3-week growth period. These results are the first to show an accumulation of sulfur compounds internally in seagrasses as a result of reoxidation of sulfides. The reoxidation is facilitated by the internal transport of oxygen and is an example of the advantage of the internal lacunae system in seagrasses. [Copyright &y& Elsevier]

Abstract: The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (&#60;5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6–14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments. [Copyright &y& Elsevier]

The decomposition of different types of organic matter in marine sediments, including eelgrass (Zostera marina) leaves, roots and rhizomes and a macroalga (Polysiphonia spp.), was studied under anoxic conditions using decomposition bags over a period of 24 weeks. The carbohydrate composition of the plant materials and sediment was analyzed using high performance liquid chromatography (HPLC) with pulsed amperometric detection (PAD) and the total sum of individual sugars was taken to represent the total carbohydrate content. The alga was degraded more rapidly than other plant materials, while eelgrass rhizome was recalcitrant to decomposition. The non-structural carbohydrate pool from all plant materials degraded quickly and was gone within the 24 week experiment. A clear reduction in carbohydrate yield for the plant materials provided the most consistent indicator of decomposition status. There was no evident change in carbohydrate amount and composition in the sediment during the decomposition experiments and the non-structural carbohydrates from plant materials were thus more labile and readily available for bacterial decomposition than the bulk of carbohydrates in the sediments. [Copyright &y& Elsevier]

Sulfur cycling was investigated in carbonate-rich and iron-poor sediments vegetated with Posidonia oceanica in oligotrophic Mediterranean around Mallorca Island, Spain, to quantify sulfate reduction and pools of sulfide in seagrass sediments. The oxygen penetration depth was low (< 4.5 mm) and sulfate reduction rates were relatively high (0.7–12 mmol m-2d-1). The total pools of reduced sulfides were remarkably low (< 5 mol S m-2) indicating a fast turnover of reduced sulfides in these iron-poor sediments. The sulfate reduction rates were generally higher in vegetated compared to bare sediments possible due to enhanced sedimentation of sestonic material inside the seagrass meadows. The sulfate reduction rates were positively correlated with the seasonal variation in water temperature and negatively correlated with the shoot density indicating that the microbial activity was controlled by temperature and release of oxygen from the roots. The pools of reduced sulfides were low in these iron-poor sediments leading to high oxygen consumption for reoxidation. The sediments were highly anoxic as shown by relatively low oxygen penetration depths (< 4.5 mm) in these low organic sediments. The net shoot recruitment rate was negative in sediments enriched with organic matter, suggesting that organic matter enrichment may be an important factor for seagrass status in these iron-depleted carbonate sediments. [ABSTRACT FROM AUTHOR]

Effects of mussel dredging on sediment metabolism (oxygen uptake and sulfate reduction rates) and phosphorus dynamics (flux across sediment-water interface and sequential extraction) were examined in Limfjorden (Denmark) during spring (May) and summer (August). Sediment samples were taken during mussel dredging and in addition an experimental simulation of the dredging was performed to investigate short-term changes in phosphorus (P) dynamics. Iron-bound P was reduced by up to 2/3 in the surface layer in the dredging track (from 31 to 8 mmol P m−2), whereas the dissolved P-pools and less reactive particulate pools were not affected by dredging. Sediment oxygen consumption was enhanced immediately after dredging, but returned to the initial level after 4 days (20–40 mmol m−2 d−1). The enhanced consumption was attributed to reoxidation of reduced compounds released during dredging. Sulfate reduction rates were high in the area (13–15 mmol m−2 d−1) and sulfides competed with P for oxidized iron resulting in low iron-bound pools in the area (<4% of total P pools). Sulfate reduction rates were stimulated by the resuspension of sediments, especially in August, where a subsurface maximum was found, possibly due to a mixing of labile organic matter into these layers. In contrast sulfate reduction rates were reduced in the dredging track due to removal of labile organic matter from the surface layers. The loss of P during dredging was to some extent counteracted by regeneration of iron-bound P pools in the surface layers. The release of P due to mussel dredging was estimated to be in the same order of magnitude as the annual loading from the catchments and point sources to Limfjorden. [ABSTRACT FROM AUTHOR]

Eelgrass Zostera marina was collected in spring and autumn from a light-saturated environment with low-organic sediments and a light-limited environment with organic-rich sediments in Denmark. The eelgrass and sediment responses to reduced light conditions were studied in 2-week shading experiments. Z. marina responded to reduced light conditions by decreasing growth rates and a loss of above-ground biomass. The spring plants were most sensitive to light reductions and the relative leaf elongation rates were reduced with up to 58% and the shoot densities with 33–36%. There was no difference in light response in relation to sediment organic matter contents. The sulfate reduction rates were reduced in the shaded low-organic sediments with up to 67%, whereas there was no effect of shading on rates in the organic-rich sediments. The lack of effect of shading in the organic-rich sediments was attributed to a limited coupling between Z. marina production and sediment bacterial carbon cycling. In contrast to the sulfate reduction rates, the pools of reduced sulfur were increased with up to 89% in the shaded, low-organic sediments, suggesting that the reoxidation of sulfides was reduced. Shading had no effect on the pools of sulfides in the organic-rich sediments due to much larger pools of sulfides. The enhanced sensitivity of spring plants to shading was probably due to a low above- to below-ground ratio compared to the autumn plants, which limited the plant-mediated oxidation of the sediments and thus the reoxidation of sulfides. The shaded plants were possibly more exposed to anoxic and sulfidic conditions affecting their growth and survival. [Copyright &y& Elsevier]

1. The concentration of sulphate is low in lakes and sulphur cycling has often been neglected in studies of organic matter diagenesis in lake sediments. The cycling of sulphur is, however, both spatially and temporally dynamic and strongly influences many biogeochemical reactions in sediments, such as the binding of phosphorus. This review examines the control of sulphate reduction and sulphur cycling in sediments of lakes with different trophic status. 2. The factors that control the rate of sulphate reduction have not been identified with certainty in the various environments because many factors are involved, e.g. oxygen and sulphate concentrations, temperature and organic matter availability. 3. Sulphate reduction is less significant under oligotrophic conditions, where mineralization is dominated by oxic decomposition. The supply of organic matter may not be sufficient to support sulphate reduction in the anoxic parts of sediments and, also, sulphate availability may control the rate as the concentration is generally low in oligotrophic lakes. 4. There is a potential for significant sulphate reduction in eutrophic lakes, as both the availability of organic matter and sulphate concentration are often higher than in oligotrophic lakes. Sulphate is rapidly depleted with sediment depth, however, and methanogenesis is generally the most important process in overall carbon mineralization. Sulphate reduction is generally low in acidic lakes because of low sulphate availability and reduced microbial activity. 5. It is still unclear which of the forms of sulphur deposits are the most important and under which conditions burial occurs. Sulphur deposition is controlled by the rate of sulphate reduction and reoxidation. Reoxidation of sulphides occurs rapidly through several pathways, both under oxic and anoxic conditions. Only a few studies have been able to examine the importance of reoxidation, but it is hypothesized that most of the reoxidation takes place under anoxic conditions and that disproportionation is often involved. The presence of sulphide oxidizing bacteria, benthic fauna and rooted macrophytes may substantially enhance oxic reoxidation. Deposition of sulphur is generally higher in eutrophic than in oligotrophic lakes because of a number of factors: a higher rate of sulphate reduction, enhanced sedimentation of organic sulphur and less reoxidation as a result of reduced penetration of oxygen into the sediments, a lack of faunal activity and rooted macrophytes. [ABSTRACT FROM AUTHOR]

Saltmarshes are known for their ability to act as effective sinks of organic carbon (OC) and their protection and restoration could potentially slow down the pace of global warming. However, regional estimates of saltmarsh OC storage are often missing, including for the Nordic region. To address this knowledge gap, we assessed OC storage and accumulation rates in 17 saltmarshes distributed along the Danish coasts and investigated the main drivers of soil OC storage. Danish saltmarshes store a median of 10 kg OC m−2 (interquartile range, IQR: 13.5–7.6) in the top meter and sequester 31.5 g OC m−2 yr−1 (IQR: 41.6–15.7). In a global context, these values are comparatively low. Soils with abundant clay (> 20%), older and stable saltmarshes in mesohaline settings, and with low proportion of algal organic material showed higher OC densities, stocks, and accumulation rates. Grazing led to significantly higher OC stocks than neighboring ungrazed locations, likely due to trampling modifying soil abiotic conditions (higher erosion‐resistance and higher clay content) that slow carbon decay. Scaling up, Danish saltmarsh soils, comprising about 1% of the country's area, have the potential to yearly capture up to 0.1% of Denmark's annual consumption‐based CO2 emissions. Our research expands the baseline data needed to advance blue carbon research and management in the Nordic region while highlighting the need for a more comprehensive approach to saltmarsh management that considers the full range of services of these ecosystems and does not only focus on climate benefits. [ABSTRACT FROM AUTHOR]

Harmful algal blooms (HABs), which are promoted by eutrophication and intensified by global warming, occur worldwide. Allelochemicals, which are natural chemicals derived from plants or microbes, are emerging weapons to eliminate these blooms. However, the cost and technical challenges have limited the discovery of novel antialgal allelochemicals. Herein, the decomposition of agricultural straws is manipulated by white‐rot fungi and achieved elevated antialgal efficiency. The transcriptomic analysis reveals that nutrient limitation activated fungal decomposition. By using a comparative nontarget metabolomics approach, a new type of allelochemical sphingosines (including sphinganine, phytosphingosine, sphingosine, and N‐acetylsphingosine) is identified. These novel natural algaecides exhibit superior antialgal capability, with as high as an order of magnitude lower effective concentration on blooming species than other prevalent allelochemicals. The co‐expression relationship between transcriptomic and metabolomic results indicate that sphinganine is strongly correlated with the differentially expressed lignocellulose degradation unigenes. The algal growth suppression is triggered by the activation of programmed cell death, malfunction of algal photosystem and antioxidant system, the disruption on CO2 assimilation and light absorption. The sphingosines reported here are a new category of allelochemicals in addition to the well‐known antialgal natural chemicals, which are potential species‐specific agents for HABs control identified by multi‐omics methodology. [ABSTRACT FROM AUTHOR]

A preface for the journal "Hydrobiologia" is presented.

The ability of seagrass meadows to filter nutrients and capture and store CO2 and nutrients in the form of organic carbon (OC) and nitrogen (N) in their sediments may help to mitigate local eutrophication as well as climate change via meadow restoration and protection. This study assesses OC and N sediment stocks (top 50 cm) and sequestration rates within Danish eelgrass meadows. At four locations, eelgrass‐vegetated and nearby unvegetated plots were studied in protected and exposed areas. The average OC and N sediment 50 cm stocks were 2.6 ± 0.3 kg OC m−2 and 0.23 ± 0.01 kg N m−2, including vegetated and unvegetated plots. In general, OC and N stocks did not differ significantly between eelgrass meadows and unvegetated sediments. Lack of accumulation of excess 210Pb suggested sediment erosion or low rates of sediment accumulation at most sites. OC accumulation rates ranged from 6 to 134 g m−2 yr−1 and N from 0.7 to 14 g m−2 yr−1. Generalized additive models showed that ≥ 80% of the variation in sediment OC and N stocks was explained by sediment grain size, organic matter source, and hydrodynamic exposure. Long cores, dated with 210Pb, showed declining OC and N densities toward present time, suggesting long‐term declines in eelgrass OC and N pools. Estimates of potential nation‐wide OC and N accumulation in eelgrass sediments show that they could annually capture up to 0.7% ± 0.5% of CO2 emissions and 6.9% ± 5.2% of the total terrestrial N load. [ABSTRACT FROM AUTHOR]

Our goal was to understand the mechanisms behind the impact of nutrient enrichment at intermediate distances from aquaculture on the interactions of a subtidal macroalgae community with its main grazer, the sea urchin Paracentrotus lividus. We assessed the diversity and cover of the macroalgal community, the abundance and biometrics of the sea urchins, the carbon and nitrogen elemental and isotopic compositions, and their metabolome in two stations, at an intermediate distance (station A) and away (station B) from a fish cage facility in the Aegean Sea (Greece), during the warm and cold seasons. The nutrient input at station A favored a shift to a macroalgal assemblage dominated by turf-forming species, depleted of native-erected species and with a higher abundance of invasive algae. A stable isotope analysis showed fish-farm-associated nitrogen enrichment of the macroalgae and trophic transfer to P. lividus. A decrease in metabolites related to grazing, reproduction, and energy reserves was found in P. lividus at station A. Furthermore, the metabolomic analysis was able to pinpoint stress in P. lividus at an intermediate distance from aquaculture. The chosen combination of traditional ecology with omics technology could be used to uncover not only the sublethal effects of nutrient loading but also the pathways for species interactions. [ABSTRACT FROM AUTHOR]

Temporal and spatial variation of δ34S, total sulfur (TS) concentration, and elemental sulfur concentration (S0) in leaves, roots, and rhizomes of Zostera marina was followed between June 2002 and May 2003 at four locations in Roskilde Fjord and Øresund, Denmark. These were related to temporal changes in sediment sulfide concentrations, sulfur pool size, and sulfur pool δ34S. The δ34S of Z. marina was most negative in the roots, followed by rhizomes and leaves, indicating that roots were mostly affected by sulfide. A significant relationship between decreasing δ34S and increasing TS in the plant tissues indicated that sulfide accumulated in the plant and, furthermore, a positive relation between TS and S0 in the plant suggests that part of the sulfide is reoxidized to S0. There were marked temporal changes in all variables at all sites, but the pattern of change varied between sites. The temporal and spatial heterogeneity in plant δ34S, TS, and S0 depended on a variety of factors, such as sediment sulfide concentrations and the below : aboveground biomass ratio of the plants. This suggests that mechanisms of sulfide invasion are complex, and several factors (plant morphology, environmental variables) acting in concert or against each other need to be considered to successfully predict sulfide invasion in seagrasses. [ABSTRACT FROM AUTHOR]

We investigated the response, adaptation and tolerance mechanisms of the temperate seagrass Zostera marina to water column anoxia. We exposed Z. marina to a diurnal light/dark cycle under anoxia and assessed the metabolic response by measuring the metabolome with gas chromatography coupled to mass spectrometry (GC-MS). During anoxia and light exposure the roots showed an altered metabolome whereas the leaves were only marginally affected, indicating that photosynthetically derived oxygen could satisfy the oxygen demand in the leaves but not in the roots. Nocturnal anoxia caused a biphasic shift in the metabolome of roots and leaves. The first phase, after 15 h under anoxia and 3 h of darkness showed a fast increase of lactate, pyruvate, GABA (γ-aminobutyric acid), succinate, alanine and a decrease in glutamate and glutamine. The second phase, after 21 h under anoxia and 9 h of darkness showed a decrease in lactate and pyruvate and an increase in alanine, GABA and succinate. This reprogramming of the metabolome after 21 h under anoxia indicates a possible mitigation mechanism to avoid the toxic effects of anoxia. A pathway enrichment analysis proposes the alanine shunt, the GABA shunt and the 2-oxoglutarate shunt as such mitigation mechanisms that alleviate pyruvate levels and lead to carbon and nitrogen storage during anoxia. This work demonstrates the applicability of metabolomics to assess low oxygen stress responses of Z. marina and allows us to propose an anoxia recovery model. [ABSTRACT FROM AUTHOR]

Highlights: [•] Seasonal δ34S measured within root, rhizomes and leaves of Halophila ovalis. [•] Summer seagrass growth likely inhibited by sediment quality in Swan-Canning Estuary. [•] A new ‘sediment-stress’ indicator relating seagrass growth with sulfur dynamics. [Copyright &y& Elsevier]

Constraints on the availability of freshwater and land plants and animals to feed the 9.2 billion humans projected to inhabit Earth by 2050 can be overcome by enhancing the contribution the ocean makes to food production. Catches from ocean fisheries are unlikely to recover without adequate conservation measures, so the greater contribution of the oceans to feeding humanity must be derived largely from mariculture. For the effort to be successful, mariculture must close the production cycle to abandon its current dependence on fisheries catches; enhance the production of edible macroalgae and filter-feeder organisms; minimize environmental impacts; and increase integration with food production on land, transferring water-intensive components of the human diet (i.e., production of animal protein) to the ocean. Accommodating these changes will enable the oceans to become a major source of food, which we believe will constitute the next food revolution in human history. [ABSTRACT FROM AUTHOR]

The effects of organic enrichment on sediment biogeochemistry was studied in diffusion controlled sediment mesocosms, where labile organic matter (OM) (fish feed) pulses were added once a week to the sediment surface. Two types of sediments, differing mainly in content of reactive Fe, were used. The aim of this experiment was two-fold, (1) to evaluate the importance of Fe-driven sulfide buffering for sulfide accumulation in surface enriched sediments, and (2) to estimate the diagenetic capacity for degradation of labile OM near the sediment surface. The simulated OM loading rate of 375 mmol C m−2 day−1 led to a 5–6 times increase in CO2-production and a 4–5 times increase in O2-uptake. Sulfate reduction estimated by radiotracer experiments and CO2-release was 105–131 mmol m−2 day−1, but accumulation of porewater sulfide was low in both sediment types. Instead 99% of sulfide was oxidized with O2 at the sediment water interface in the low Fe treatment, whereas 46% of produced sulfide precipitated as Fe-S compound in the high Fe treatment resulting in significantly lower O2-uptake. Furthermore, the accumulation of up to 30% of added OM by the end of the experiment indicated a saturation of the heterotrophic microbial communities in the upper enriched surface layer. These results suggest a maximum diagenetic capacity for OM degradation in the range of ~25 μmol C cm−3 day−1 or 260 mmol m−2 day−1 for the present sediment types. [ABSTRACT FROM AUTHOR]

Abstract: Despite the fact that iron plaque formation is ubiquitous in aquatic macrophytes and has been known for several decades, there are few reports of plaque occurrence in seagrasses to date. Herein we present the first microscopical observation and chemical quantification of iron (Fe) plaques on the shoots, rhizomes and roots of the seagrass Cymodocea serrulata (R. Brown) Ascherson collected from intertidal seagrass beds in Thailand. Plaques were observed on shoot bases, rhizomes and roots with the highest concentrations of iron in the plaques from the roots, reaching an average of 509μmolgDW−1. Interestingly, the most negative stable sulphur isotope (δ34S) values, indicating H2S intrusion into the plants occurred in the sampling site with the most intense root oxidizing capacity, as indicated by a greater Fe plaque formation. These apparently contradictory findings may be attributed to oxidizing capacity of root tips and root hairs sufficient to promote Fe(III) deposition in the rhizosphere, preceding deposition of plaques on the roots. While this rhizosphere oxidation may result in a more efficient sulphide detoxification during the day photosynthetic phase, root tips and hairs may serve as vulnerable sites for sulphide intrusion at night. The presence of Fe plaque on C. serrulata roots and rhizomes reveals the complexity of seagrass–sediment interactions and deserves further attention to understand if this is a local phenomenon or a newly discovered adaptive mechanism in seagrasses. [Copyright &y& Elsevier]

Abstract: Fish farming impact on decomposition and loss of carbon, nitrogen and phosphorus fixed in seagrass litter were studied in a Posidonia oceanica meadow (Aegean Sea, Greece) using in situ incubation of senescent seagrass leaves collected under (station: cages) and away (station: control) from fish cages and deployed in a cross design of origin/station. Decomposition rate and loss of carbon and nitrogen fixed in seagrass litter were pronounced under the cages while loss of phosphorus was less evident. Decomposition was related to nutrient availability in seagrass tissue and pore water, sediment organic matter and origin of seagrass litter. When incubated under the cages, litter originated from the control decomposed faster than litter originated from the cages since the former was qualitatively better substrate for decomposers and the nutrient conditions in that station were enriched in the pore water and sediment. The lower decomposition of litter originated from cages suggests that seagrass tissues under the cages accumulate chemical deterrents, possibly in order to confront high grazing pressure, which on the other hand reduces the rate of decomposition. [Copyright &y& Elsevier]

Abstract: Fish farming impact on the seasonal biomass, carbon and nutrient (nitrogen and phosphorus) balance of the endemic Mediterranean seagrass Posidonia oceanica was assessed in the Aegean Sea (Greece) in order to detect changes in magnitude and fate of seagrass production and nutrient incorporation with organic loading of the meadows. Phosphorus concentration in the leaves, rhizomes and roots was enhanced under the cages throughout the study. Standing biomass was diminished by 64% and carbon, nitrogen and phosphorus standing stock by 64%, 61% and 48%, respectively, under the cages in relation to those at the control. Seagrass production decreased by 68% and element (C, N, P) incorporation by 67%, 58% and 58%, respectively, under the cages. Leaf shedding was reduced by 81% and loss of elements (C, N, and P) through shedding by 82%, 74% and 72%, respectively, under the cages. Leaf and element (C, N, P) residual loss rate, accounting for grazing and mechanical breakage of leaves, was decreased by 79%, 85%, 100% and 96%, respectively, at the control station. At the control station, 13.98gCm−2 yr−1, 1.91gNm−2 yr−1 and 0.05gPm−2 yr−1 were produced in excess of export and loss. In contrast, under the cages 12.69gCm−2 yr−1, 0.31gNm−2 yr−1 and 0.04gPm−2 yr−1 were released from the meadow. Organic loading due to fish farm discharges transformed the seagrass meadow under the cages from a typical sink to a source of organic carbon and nutrients. [Copyright &y& Elsevier]

Abstract: The effect of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds was studied from June 2004 to July 2005 in Flensborg fjord, Denmark. The field experiments were conducted at two stations, one with only Z. marina (Eelgrass station) present and one where M. edulis were present in the Z. marina beds (Mixed station). Zostera marina parameters were measured (growth of leaves, shoot density, leaf length, and nutrient content) in combination with epiphyte cover and sediment parameters (sulphate reduction rates, sediment nutrient fluxes, organic content, C, N and P content) to examine possible positive and negative effects of the mussels on eelgrass performance. The fluxes of ammonium from the sediments were stimulated at all sampling dates at the Mixed station, and possibly stimulated epiphyte growth at this station. Further 15N signals in epiphytes from the Mixed station suggested that excretion products from the mussels were important nitrogen sources at this station. Sulphate reduction rates were enhanced at the Mixed station and also sediment sulphide concentrations increased under mussel influence, which may have resulted in sulphide toxicity and decreased growth of Z. marina at this station. The study indicates that for Z. marina beds in Flensborg Fjord the effects of M. edulis in seagrass beds are primarily negative, and raises the question whether this leads to negative effects on the stability and expansion of Z. marina beds. [Copyright &y& Elsevier]

Abstract: The relationship between sedimentary Fe inputs and net seagrass population growth across a range of Posidonia oceanica meadows growing in carbonate Mediterranean sediments (Balearic Islands, Spain; SE Iberian Peninsula, Spain; Limassol, Cyprus; Sounion, Greece) was examined using comparative analysis. Sedimentary Fe inputs were measured using benthic sediment traps and the net population growth of P. oceanica meadows was assessed using direct census of tagged plants. The meadows examined ranged from meadows undergoing a severe decline to expanding meadows (specific net population growth, from −0.14yr−1 to 0.05yr−1). Similarly, Fe inputs to the meadows ranged almost an order of magnitude across meadows (8.6–69.1mgFem−2 d−1). There was a significant, positive relationship between sedimentary iron inputs and seagrass net population growth, accounting for 36% of the variability in population growth across meadows. The relationship obtained suggested that seagrass meadows receiving Fe inputs below 43mgFem−2 d−1 are vulnerable and in risk of decline, confirming the pivotal role of Fe in the control of growth and the stability of seagrass meadows in carbonate sediments. [Copyright &y& Elsevier]

Abstract: A large-scale mesocosm (sixteen 500L tanks) experiment was conducted to investigate the effects of hypersalinity (45–65psu), porewater sulfide (2–6mM) and nighttime water column hypoxia (5–3mgL−1) on the tropical seagrass Thalassia testudinum Banks ex König. We examined stressor effects on growth, shoot survival, tissue sulfur (S0, TS, δ 34S) and leaf quantum efficiencies, as well as, porewater sulfides (∑TSpw) and mesocosm water column O2 dynamics. Sulfide was injected into intact seagrass cores of T. testudinum exposing below-ground tissues to 2, 4, and 6mMS2−, but rapid oxidation resulted in ∑TSpw <1.5mM. Hypersalinity at 65psu lowered sulfide oxidation and significantly affected plant growth rates and quantum efficiencies (F v/F m <0.70). The most depleted rhizome δ 34S signatures were also observed at 65psu, suggesting increased sulfide exposure. Hypoxia did not influence ∑TSpw and plant growth, but strengthened the hypersalinity response and decreased rhizome S0, indicating less efficient oxidation of ∑TSpw. Following nighttime hypoxia treatments, ecosystem level metabolism responded to salinity treatments. When O2 levels were reduced to 5 and 4mgL−1, daytime O2 levels recovered to approximately 6mgL−1; however, this recovery was more limited when O2 levels were lowered to 3mgL−1. Subsequent to O2 reductions to 3mgO2 L−1, nighttime O2 levels rose in the 35 and 45psu tanks, stayed the same in the 55psu tanks, and declined in the 65psu tanks. Thus, hypersalinity at 65psu affects T. testudinum''s oxidizing capacity and places subtle demands on the positive O2 balance at an ecosystem level. This O2 demand may influence T. testudinum die-off events, particularly after periods of high temperature and salinity. We hypothesize that the interaction between hypersalinity and sulfide toxicity in T. testudinum is their synergistic effect on the critical O2 balance of the plant. [Copyright &y& Elsevier]

Abstract: Impacts of organic enrichment and a modified benthic fauna community (caused by fish farming) on benthic mineralization rates and nutrient cycling were studied in sediments at one Danish and one Cypriote fish farm. Sediment organic matter concentration and macrofauna community composition were manipulated in microcosms and changes in total benthic metabolism (oxygen consumption, TCO2 production), anaerobic metabolism (sulfate reduction rates), nutrient fluxes and sediment parameters were followed for a period of 3 weeks. Mineralization rates were found to be highly correlated with irrigation velocities and largest fauna effects were found in the Danish sediments with the large and active irrigating climax species (Nereis diversicolor and Macoma balthica). Eastern Mediterranean climax species (Glycera rouxii and Naineris laevigata) also stimulated mineralization rates but to a smaller extent due to lower irrigation, whereas the opportunistic species (Capitella in Danish sediment and Hermodice carunculata in Cypriote sediment) showed less effect on mineralization. Ammonium and phosphate release increased with increasing irrigation velocities, but much less in Cyprus indicating higher nutrient retention at the ultra-oligotrophic location compared to eutrophic Danish site. Irrigation velocities, and thus mineralization rates, increased by organic matter loading, indicating larger fauna-induced oxidation in enriched environments. The result implies that a change in fauna structure in fish farm sediment towards smaller opportunistic polychaete species with lower irrigation will result in slower mineralization rates and potentially increase accumulation of organic waste products. [Copyright &y& Elsevier]

For five shallow-water eelgrass stands in Denmark, changes in area distribution over the period 1940/50s–1990s were assessed from aerial photographs. In areas where the wasting disease had decimated eelgrass meadows in the 1930s, this included determination of the time scale of recolonization. In addition, we aimed at evaluating whether fluctuations in eelgrass area distribution were related to various natural and human-induced disturbances, namely storm events, ice-cover, water temperature and nutrient loading. We found that populations affected by the wasting disease exhibited a time lag of more than 10 years before substantial recolonization began, probably reflecting long distances to seed-producing populations and extreme climatic events in the period. After the initial time lag, eelgrass area distribution increased rapidly, and a substantial recovery had taken place in the 1960s. All eelgrass populations showed marked inter-annual fluctuations. Declines were often rapid with reductions of about 60% occurring in less than 6 years. Recoveries occurred over similar time scales and documented that recolonization may take place relatively fast when suitable environmental conditions are present. Fluctuations in eelgrass area distribution tended to be larger in enclosed, protected bays as compared to open coasts, probably because enclosed sites are often more eutrophic. Changes in shallow-water eelgrass area distribution did not correlate with the available long-term records of natural and human-induced disturbance parameters. Thus, while deep-water eelgrass populations have declined markedly over the last century in response to eutrophication, long-term changes in shallow-water populations are less equivocal and seem more stochastic. [Copyright &y& Elsevier]

The distribution of eelgrass (Zostera marina) was mapped from aerial photographs at three Danish sites with different levels of physical exposure by digital image analysis at 1 m × 1 m resolution. A set of indices from landscape ecology were used to quantify various components of the eelgrass landscapes such as patch shapes and landscape aggregation in photographs from 1995. Furthermore, the long-term fluctuations in the spatial distribution of eelgrass were investigated during the period 1954–1995/1999. Eelgrass landscapes exposed to wave dynamics were less aggregated with more elongated patch shapes than the sheltered eelgrass areas. The outline of the larger patches also tended to be more complex at exposed sites. Patch size distribution at all sites was highly skewed with 75–95% of the patches being <10 m2. Although the majority of the patches were small, they only constituted a small proportion of the total eelgrass area because most of the eelgrass-covered area was contained in a few, large patches. Large fluctuations in the spatial distribution of eelgrass occurred at both exposed and protected sites in the course of the investigation period. In the relatively short intervals of ∼7 years between two consecutive photographs, on average 39–62% of the total eelgrass area covered in the photographs had changed (i.e., disappeared or recolonized) and the largest changes occurred at exposed sites. Overall, this study showed that shallow eelgrass populations form characteristic landscapes with a configuration that is highly related to the level of physical exposure and that the size and position of eelgrass beds changes substantially among years. [Copyright &y& Elsevier]

The lack of interest among pre-university students to choose STEM subjects for their higher education is a heavily debated issue in many western-world countries. To boost Danish students' interest in biology, a study event on artificial reefs was introduced when teaching marine biology in lower secondary school and upper secondary school (student age 13–20 years). The purpose was to investigate if the focus on artificial reefs could generate an increased interest in natural science and marine biology among the students. The students' interest in science was evaluated using electronic questionnaires before and after they had completed the teaching programme. The students were significantly more interested in natural sciences and marine biology after than before the teaching program. The development in situational interest in science and in the oceans was different for males and females with females being most interested. Thus, it is possible to trigger a situational interest for science and marine biology by teaching about artificial reefs, but the way interest is triggered differs between different age groups and sexes. [ABSTRACT FROM AUTHOR]