Posidonia oceanica meadow: a low nutrient high chlorophyll (LNHC) system?

BACKGROUND: In spite of very low nutrient concentrations in its vicinity – both column and pore waters-, the Posidonia oceanica of the Revellata Bay displays high biomass and productivity. We measured the nutrient fluxes from the sediment into the water enclosed among the leaf shoots ("canopy water") to determine if it is possible source of nutrients for P. oceanica leaves. RESULTS: During the summer, the canopy water appears to act as a nutrient reservoir for the plant. During that period, the canopy water layer displays both a temperature 0.5°C cooler than the upper water column, and a much higher nutrient content, as shown in this work using a very simple original technique permitting to sample water with a minimal disturbance of the water column's vertical structure. Despite low nutrient concentrations in pore water, mean net fluxes were measured from the sediment to the canopy water. These fluxes are sufficient to provide 20% of the mean daily nitrogen and phosphorus requirement of the P. oceanica shoots. CONCLUSION: An internal cycling of nutrients from P. oceanica senescent leaves was previously noted as an efficient strategy to help face low nutrient availability. The present study points out a second strategy which consists in holding back, in the canopy, the nutrients released at the water-sediment interface. This process occurs when long leaves, during poor nutrient periods in the water column, providing, to P. oceanica, the possibility to develop, high biomass, high chlorophyll quantities in low nutrient environment (a Low Nutrients High Chlorophyll system)

Natural patches in Posidonia oceanica meadows: the seasonal biogeochemical pore water characteristics of two edge types

peer reviewedSeagrass meadows can be assimilated to seascape matrixes encompassing a mosaic of natural and anthropogenic patches. Natural patches within the Mediterranean Posidonia oceanica meadows show a structural particularity which consist in a duality of their edge types. One edge is eroded by bottom currents, while the adjacent meadow colonizes the bare sediments. This study aims to study the dynamics of these two edges through the investigation of the biogeochemistry (pH, total alkalinity, dissolved inorganic carbon, CO2, CH4, N2O, H2S, dissolved inorganic nitrogen, PO4 3−) within vegetated and unvegetated sediments. These observations are compared with the adjacent meadow to have a better understanding of the colonization processes. Our results reveal that the P. oceanica matrix shows differences from the vegetated edges of sand patches, especially with regard to nutrient availability, which is generally more important at the colonized edge (dissolved inorganic nitrogen up to 65.39 μM in June). A clear disparity also occurs between the eroded and colonized edge with both a seasonal and bathymetrical variation of leaf biomass with higher disparities at 10 m in June (colonized edge 1415 gDW m−2; eroded edge 1133 gDW m−2). Themost important contrasts during this study were assessed in June, suggesting that the warm period of the year is more suitable for sampling to highlight disparate characteristics in temperate seagrass meadows. These findings put into light the potential importance of biogeochemical processes in the dynamics of natural patch edges. We hypothesize that they may influence the structural dynamics of P. oceanica seascapes

From mechanical to chemical impact of anchoring in seagrasses: the premises of anthropogenic patch generation in Posidonia oceanica meadows

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20 m depth (42.6 µM and 18.8 µM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20 m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures.STARE-CAPME

Seascape ecology in Posidonia oceanica seagrass meadows: Linking structure and ecological processes for management

Seagrass meadows constitute marine habitats in shallow water temperate and tropical coastal areas worldwide that have a high ecological and economic importance. Amongst the 60 or so seagrass species, the endemic Mediterranean species Posidonia oceanica forms meadows that are arguably the most important shallow water coastal habitat in the region but which are subjected to high anthropogenic pressures. Because of the relatively large size of the plant, the meadows formed by this seagrass have high architectural and morphological complexity, which results in different morphotypes or seascapes. While numerous studies of P. oceanica architectural characteristics for continuous meadows of the seagrass are available, few works have addressed seascape ecological features and the influence of environmental factors (natural and anthropogenic) thereon. In the present review, we give an overview of P. oceanica meadow architectural and morphological characteristics and how these contribute to Mediterranean landscapes and seascapes. Studies addressing the influence of natural and anthropogenic factors on morphometric features of different meadow types and landscape ecological characteristics of P. oceanica habitat are also reviewed, as well as their influence on ecosystem processes. Finally, by considering the available data and tools for seascape studies, we present a discussion on methods to assess seagrass seascapes within the framework of coastal management. Our review highlights several gaps in P. oceanica seascape ecology knowledge such as the lack of data on the spatial distribution of this engineer species, and the possibility to use modern techniques and procedures for analysing structural and ecosystemic data

Indices to determine the status of marine coastal water bodies: Development, applications and support to management

peer reviewedQuatre indices de qualité des masses d’eau marine ont été mis au point dans une baie-type de référence en Méditerranée: un indice de qualité biologique pour la Directive-Cadre sur l’Eau (PREI), un indice de colonisation par une espèce invasive (ICar), un indice de valeur patrimoniale du paysage (LIMA) et un indice de pollution (TEPI). Ces indices ont été validés scientifiquement. La qualité environnementale qu’ils expriment peut être facilement visualisée par des codes simples de couleurs ou de combinaisons lettres-chiffres positionnés sur une carte. Ils ont ensuite été appliqués à plus larges échelles spatiales et/ou temporelles ; leur principe de calcul peut être transposé vers d’autres régions et/ou écosystèmes moyennant la mise en place de bornes adaptées à l’écosystème, à la région investigués.Four marine water quality indices have been developed in a reference bay in the Mediterranean: a biological quality index for the European Water Directive (Posidonia Rapid and Easy Index, PREI), an invasive species colonization index (Indice Caulerpa cylindracea, ICar), a landscape heritage value index (indice Littoral Marin, LIMA) and a pollution index (Trace Element Pollution index, TEPI). These indices have been validated scientifically. The environmental quality they express can be easily visualized by simple codes of colors or combinations of figures-letters positioned on a map. These indices were then applied to larger spatial and/or temporal; their calculation principle can be transposed to other regions and/or ecosystems by setting up reference conditions for the ecosystem, the region investigated.STARECAPMEDFRFC 2.4.502.08; ARC RACE 05 / 10-33

Belgique

Posidonia oceanica is a magniolophyte endemic to the Mediterranean Sea, in which flowering can vary depending on water temperature and nutrient concentration (C,N,P). Along corsican coast, in the Northwestern Mediterranean Sea, the flowering was unusual but in the last 20 years it flowered regularly. The aim of our study was to determine the effect of the frequency of flowering episode on the shoots by measuring the dynamics of elementary (C, N, P) and the trace element contents (TE) in different tissues (leaves, inflorescences and rhizomes) of flowering and non-flowering shoots. This study also focused on the biometry of these shoots. Keywords: Posidonia, North-Western Mediterranean, Trace elements, Monitoring, BiometricsThès

Fish farm impacts on Posidonia oceanica meadows: interest of the microbenthic loop.

Posidonia oceanica, the seagrass endemic to the Mediterranean Sea, is a valuable tool to assess the environmental quality in coastal zones. However, only few studies have attempted to use characteristics of its sediment compartment as an indicator of environmental perturbations. In this study, the impact of a fish farm on the microbenthic loop (organic matter, bacteria, microphytobenthos and meiofauna) of P. oceanica meadows will be described. Samples were taken under an aquaculture situated in Calvi Bay (Corsica, France) at a depth of 22 m, in March and June 2008. The control site was the meadow situated in front of the research station STARESO (Calvi Bay, Corsica, France), sampled at the same periods and depth. Results concerning bacteria, microphytobenthos and organic matter will be presented here. For both seasons, differences between sites exist. For example, biomasses of organic matter, microphytobenthos and bacteria are higher in the fish farm than in the control site, indicating that the microbenthic loop has potential to be a good early indicator of pollution in this sea.Utilisation de l'exergie pour mesurer et détecter les effets des perturbations affectant les herbiers à Posidonia oceanica (L.) Delile: caractérisation au sein du compartiment sédimentaire

Courants et vents

peer reviewedConnaissances, prévisions, stratégie, lacun