Search

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]

Quantifying the dynamics of the early stages in the life cycle of mangroves is essential to predict the distribution, species composition and structure of mangrove forests, and their maintenance and recovery from perturbations. The growth and population dynamics of two stands of the mangrove Kandelia candel in Halong Bay (Viet Nam) were examined for 1 year. Growth was highly seasonal, with high growth rates and fast internode formation in the summer, dropping to extremely low growth during January–February, the coldest and driest months in the year. In addition, growth and internode formation rates showed important inter-annual variability during the last decade. The complete reproductive period required 7–8 months. Flower initiation was maximal in June and peak propagule maturity occurred in December–January. Only one mature propagule developed for every 67 and 127 inflorescence buds formed at Site 1 and Site 2, respectively. Kandelia candel propagules begun to sink 10 days after being released, and after 18 days all propagules had negative buoyancy. The propagules developed roots within 19–68 days, depending on whether they were held on the water or sediment, and were capable of long range dispersal, for 15–20% of them dispersed more than 100 m within 1 day. The median age of K. candel plants ranged between 8.7 and 5.6 years, with a density of 1900 and 470 plants ha−1, in Sites 1 and 2. Plant mortality was high, with 64 and 74% of the plants surviving after a year at Sites 1 and 2. Life expectancy (i.e. median age-at-death) of only 2.2 and 2.7 years at Sites 1 and 2, respectively, indicates that mortality of young K. candel plants was specially high. Recruitment occurred in early spring, and did not suffice to balance the mortality within the annual period examined. These results suggest that the K. candel stands in Halong Bay might be maintained by a few years of high recruitment which would compensate for generally high mortality rates. [Copyright &y& Elsevier]

Across the Earth, mangroves play an important role in coastal protection, both as nurseries and carbon sinks. However, due to various human and environmental impacts, the coverage of mangroves is declining on a global scale. The Red Sea is in the northern-most area of the distribution range of mangroves. Little is known about the surface covered by mangroves at this northern limit or about the changes experienced by Red Sea mangroves. We sought to study changes in the coverage of Red Sea mangroves by using multi-temporal Landsat data (1972, 2000 and 2013). Interestingly, our results show that there has been no decline in mangrove stands in the Red Sea but rather a slight increase. The area covered by mangroves is about 69 Km 2 along the African shore and 51 Km 2 along the Arabian Peninsula shore. From 1972 to 2013, the area covered by mangroves increased by about 0.29% y −1 . We conclude that the trend exhibited by Red Sea mangroves departs from the general global decline of mangroves. Along the Red Sea, mangroves expanded by 12% over the 41 years from 1972 to 2013. Losses to Red Sea mangroves, mostly due to coastal development, have been compensated by afforestation projects. [ABSTRACT FROM AUTHOR]

Blue carbon ecosystems, particularly seagrass meadows, provide a myriad of critical ecosystem services through their role in supporting biodiversity, protecting shorelines, and sequestering and storing carbon. Despite their increased recognition for advancing global marine conservation efforts, our knowledge of the location and spatial extent of seagrass ecosystems globally remains relatively poor, due to the logistical and operational challenges of accurately mapping seafloor habitat using remote sensing techniques. Here we develop and validate an aerial LiDAR approach for mapping tropical seagrass blue carbon habitat in The Bahamas, by integrating full waveform bathymetric LiDAR with multi-spectral imaging. The results demonstrate accuracies between 92 and 98% for predicting seagrass types, over an extensive (e.g., >1000 km2) study area whilst utilizing robust ground-truthing techniques. These findings demonstrate the value of the aerial LiDAR approach used here for ongoing bathymetric surveys of seagrass habitat in The Bahamas and throughout similar ecosystems in the wider Caribbean, which should provide key insights for coastal management and conservation efforts. • Innovations in the mapping of blue carbon ecosystems are needed to properly their conservation potential • We developed a full waveform bathymetric LiDAR and multi-spectral imagining solution for mapping tropical seagrass meadows. • We focused our efforts in the most extensive seagrass ecosystem on Earth, located in The Bahamas. • Results suggest unprecedented accuracies in identifying all four seafloor class types, ranging from 92 to 98% for seagrass. • These findings provide a new solution for the high-resolution mapping of expansive and extensive seagrass habitat, which may be confidently expanded to other regions and blue carbon ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: The available information from experimental and descriptive studies on the effects of sediment burial and erosion on seagrasses was compiled to synthesize the information regarding the species-specific impacts and to relate them to plant characteristics. Burial thresholds (i.e. the burial levels causing 50% and 100% shoot mortality) and mortality-burial curves were estimated for the 15 seagrass species where the effects of experimental burial have been tested. All the species investigated reached 50% shoot mortality at burial levels ranging from 2cm (Halophila ovalis) to 19.5cm (Posidonia australis). P. australis was the most tolerant seagrass species to burial, while Thalassia testudinum was the most tolerant species to erosion. The relationships among plant size, growth, biomass and density with burial thresholds were examined. There were significant relationships between the burial thresholds and the shoot mass, the rhizome diameter, the aboveground biomass, the horizontal rhizome elongation and the leaf length of seagrass species. The leaf size and the rhizome diameter are the best predictors of the capacity of seagrasses to withstand burial. The burial thresholds estimated for seagrass species were in many cases in agreement with the burial impacts described by field observations (bioturbation), while in some cases was related to the species long-term colonization capacity (dune migration). Most human-induced impacts result in important changes of the sedimentary environment, with permanent negative effects on seagrass meadows (regression and complete destruction), whereas natural events, whether extreme (hurricane) or regular (dune migration), allow the recovery and/or adaptation of seagrasses to the burial/erosion sediment dynamics. The extent of the effects of burial and erosion on seagrasses is species-specific and strongly size-dependent. [Copyright &y& Elsevier]

Abstract: In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6μM) to those growing over higher sulfide conditions (33.5μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10μmols H2SL−1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea. [Copyright &y& Elsevier]

Abstract: Posidonia oceanica is a slow growing seagrass species that extends via growing rhizomes that grow only centimetres both horizontally and vertically each year. Posidonia oceanica forms topographically complex biogenic reefs of dead rhizome and sediments that are up to 4m in height that are called “matte”. This study investigates the role of slow horizontal and vertical growth of rhizomes in the formation of topographic complexity in P. oceanica matte using agent-based modelling. The simulated infilling of landscapes by P. oceanica was run over 600 iterations (years) for 10 random starts of 150 agents each. Initial infilling rates were very slow and P. oceanica had limited cover after a century of growth. Growth accelerated after 100years but plateaued after 400years such that after 600years only two-thirds of the landscape was occupied by P. oceanica. The pattern of spread of agents was initially random in direction but after larger patches were formed spread was radial from these patches. The seagrass landscape was initially highly fragmented with many small separate patches made up of a few agents each, with a Landscape Division index close to 1. Between 300 and 600years Landscape Division declined sharply to 0.42, indicating patches had coalesced into larger more continuous meadows forming a less fragmented landscape. Perimeter to area ratio of seagrass patches declined exponentially from >1 to approximately 0.2 over 600years of simulation. The matte developed from growth of patches and its greatest height occurred in more continuously occupied cells of the grid. The topography of the reef that occupied two-thirds of the landscape after six centuries of growth could be described as a pattern of channels between reef plateaus elevated 1–2m above channels. These results demonstrate that development in P. oceanica meadows of three-dimensional structure, in the formation of biogenic reefs, can be explained by, and is an emergent property of, slow horizontal and vertical rhizome growth rates combined with the time it takes for the accumulation of rhizomes in any region of the landscape. As such, the model provides a parsimonious explanation for the development of complex matte topography. [Copyright &y& Elsevier]

Abstract Coastal salt marsh, one of the blue carbon ecosystems that can adapt and mitigate climate change influence, is drawing global attention due to its high carbon sequestration capability. In China, however, coastal salt marsh has suffered great losses. Nation-wide analysis of salt marsh trends and management is critical to ecosystem protection and restoration. Thus, by analyzing previous coastal salt marsh studies, we found that the extent of coastal salt marsh varied greatly among the Liao River Delta, the Yellow River Delta, the middle coast of Jiangsu Province, Chongming Dongtan and Jiuduansha in Shanghai, with a 59% overall loss of salt marsh extent from the 1980s to the 2010s. The rate of salt marsh loss slowed down after the year 2000. Coastal land-claim (reclamation) is the most dominant driver of salt marsh loss. Climate change and coastal erosion, invasive species, and vegetation dynamics driven by competition and succession have also led to various effects on salt marsh extent and the ecological services they provide. Sea level rise, reclamation pressure and environmental pollution are the main factors, as negative drivers, together with conservation and restoration policies, as positive ones, affecting future trends in salt marshes. China has implemented several measures to protect and restore salt marshes, such as setting up protected areas, drawing marine ecological redline, and making strict regulations on reclamation. However, stronger legal protection for wetlands, more effective enforcement, and participation by local communities can further enhance salt marsh restoration, conservation and management. Graphical abstract Image 1 Highlights • Great losses (59%) of salt marshes occurred in China from the 1980s to the 2010s due to a few main drivers. • Salt marshes in China face natural and anthropogenic threats. • China has taken even tougher measures to conserve and restore salt marshes. • Legal basis for wetland protection, more effective enforcement and public participation are needed. [ABSTRACT FROM AUTHOR]

Abstract: The usefulness of vertical rhizome growth as an early indicator of fish farm impacts to Posidonia oceanica meadows was tested by comparing annual estimates of vertical rhizome growth, quantified retrospectively, at distances ranging between 5 and 1200m from fish cages at four Mediterranean locations (Cyprus, Greece, Italy and Spain). The studied fish farms had been operating since, at least, 1997, producing between 150 and 1150tonsyr−1 of sea bream (Sparus aurata) and sea bass (Dicentrachus labrax), and, at Italy, also sharpsnout sea bream (Diplodus puntazzo). The reconstructed vertical rhizome growth spanned from 19 to 25 years of growth, depending on sites, and the average vertical rhizome growth before the onset of fish farm operations ranged between 4.48 and 8.79mmyr−1. The vertical rhizome growth after the onset of farming activities declined significantly (t-test, P <0.05) from the control station (at >800m from the farm; vertical growth rate averaged 6.79, 5.52, 3.89 and 3.70mmyr−1 at Cyprus, Greece, Italy and Spain control stations, respectively) to the impacted one (at 5–300m from the farm; vertical growth rate was 4.82, 3.52, 2.77 and 1.92mmyr−1 at Cyprus, Greece, Italy and Spain impacted stations, respectively) at each farm. Moreover, vertical growth significantly (t-test, P <0.05) declined by about twofold following the onset of fish farm operations for the extant meadow nearest to the cages, as well as those supporting intermediate impacts at distances 35–400m from the cages. Vertical rhizome growth was not significantly affected after the onset of fish farm operations for the meadows located more than 800m from the farm, except in those from the Italian site, the largest farm. Examination of the time course of vertical growth for individual rhizomes in the areas of the meadow nearest to the farms, except for those at Cyprus, showed that the decline in vertical growth was initiated within the year of the onset of farming activities, suggesting a negligible resistance of seagrass meadows to fish farm impacts. The results obtained confirm that fish farm activities strongly affect seagrass health on the surrounding meadows, and clearly demonstrate the value of reductions in vertical rhizome growth as an early warning symptom of stress and impacts to P. oceanica meadows. [Copyright &y& Elsevier]

Mangrove ecosystems are highly productive and provide important ecosystem services. However, in the Red Sea mangroves are under severe nutrient-limiting conditions, reflected in dwarf plants. The nutrient limitation is especially acute for iron, as verified experimentally, although the low carbon-to-nutrient stoichiometric ratios reported for Red Sea mangrove leaves are indicative of general nutrient depletion. Therefore, atmospheric nitrogen (N 2) fixation in mangrove sediments might be particularly important considering the minimal nitrogen inputs from land. Here, we tested the effect of temperature and crab density on sediment N 2 fixation rates in mature and juvenile mangrove (Avicennia marina) stands in the central Red Sea. The average N 2 fixation rates (from 0.002 ± 0.002 to 0.46 ± 0.12 mg N m−2 d−1) fall in the low range of N 2 fixation rates reported in mangroves elsewhere, which is in agreement with the small size of the mangrove plants. Mature mangrove sediments hold higher N 2 fixation rates than the juvenile mangrove sediment, related to a higher sediment organic matter and carbon content. We found a detrimental effect of temperature and crab density on sediment N 2 fixation rates. Maximum N 2 fixation rates were detected at 28 °C with a sharp decrease at 35 °C. Similarly, high crab-density reduced N 2 fixation, likely due to the sediment oxygenation or the grazing of cyanobacteria by crabs. This is supported by i) previously reported higher oxygen concentration and redox around burrows compared to undisturbed sediment and ii) lighter sediment carbon isotopic composition in high crab-density than in low crab-density sediments, indicating a higher contribution of microphytobenthos in the mature sediments supporting low crab-density. Our data document temperature and crab density as factors affecting N 2 fixation in the Red Sea mangrove sediments. • Sediment N2 fixation in Central Red Sea mangroves fall in the low range. • Higher N2 fixation rates in mature than juvenile mangrove sediments. • Temperature and crab density are important interacting drivers of N2 fixation. • Temperature and crab density had detrimental effects on mangrove N2 fixation. • High crab-density reduced N2 fixation, likely due to grazing of cyanobacteria. [ABSTRACT FROM AUTHOR]