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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 (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.

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.

The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ∼8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0-250 μmol photons m-2 s-1, but the leakage rate saturated at the maximum irradiance of 500 μmol photons m-2 s -1. Oxygen leakage decreased by ∼60% from light to darkness and Z. marina was able to maintain an oxic zone around the root tip even in darkness as long as oxygen in the overlying water was at 100% air saturation (280 μmol L-1). O2 leakage from the root tips stopped at 25% air saturation (70 μmol L-1) and the oxic microniche rapidly disappeared. Increasing the oxygen concentration above 100% air saturation induced oxygen leakage from zones that otherwise appeared impermeable to oxygen. The roots on average grew by 8.7 mm d-1, and a series of O 2 images documented the high spatial and temporal dynamics of the oxic microniches around the root tips. The estimated total oxygen release to the rhizosphere of Z. marina beds was 2.3 mmol m-2 d-1, which only corresponded to 12% of the diffusive oxygen uptake at the primary sediment-water interface. Rhizospheres of seagrass are thus probably of minor importance for total benthic O2 uptake rates.

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.

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

Holmer, Marianne et al., This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows. © 2008 Elsevier Ltd. All rights reserved.

Here we studied the effects of adding organic carbon and nutrients to sediment on the physiology and survival of the seagrass Posidonia oceanica in a field experiment in the Medes Islands (NE Spain). Nine randomly selected plots were established at a depth of 10 m; three were enriched with organic carbon (OM treatment), three with organic carbon and nutrients (OMN treatment), and three were kept as controls (CON). The experiment was performed over 5 months and sampling of plants and sediments was done in March, May and July 2002. Sediment sulfide pools and pore water ammonium concentrations increased significantly in OM and OMN plots, both treatments showing increased reducing conditions in the sediment. Plants in these two treatments showed higher mortality and lower biomass compared to plants from CON plots. The greatest effects on seagrass occurred in the OMN plots, indicating a synergistic effect of organic carbon and nutrient additions. Treatments had significant effects on plant nitrogen (N) and phosphorous (P) metabolism, shown by an increase in free amino acid (FAA) content, a change in FAA composition and a lack of increase in N and P tissue content. Treated plants showed higher g -aminobutyric acid (GABA) and malate concentrations and lower concentrations of non-structural carbohydrates compared to CON, indicating that anaerobic respiration in below-ground tissues occurred. Several of the physiological changes shown by P. oceanica can be interpreted as adaptations to anoxia exposure. However, the increased mortality in treated plots demonstrates that this seagrass does not tolerate highly reduced sediments.

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., Sí

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.

Temporal and spatial variation of d34S, 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 Oresund, Denmark. These were related to temporal changes in sediment sulfide concentrations, sulfur pool size, and sulfur pool d34S. The d34 So fZ. 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 d34S 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 d34S, 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.