Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning. The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels. Enhanced primary production results in an accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. Dead zones have now been reported from more than 400 systems, affecting a total area of more than 245,000 square kilometers, and are probably a key stressor on marine ecosystems.

Supporting Online Material for Spreading Dead Zones and Consequences for Marine Ecosystems

Trabajo presentado en el 39th CIESM Congress, celebrado en Venecia, Italia, del 10 al 14 de mayo de 2010

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Biodiversity of the Mediterranean Sea: estimates, patterns and threats

The Mediterranean Sea is a marine biodiversity hot spot. Here we combined an extensive literature analysis with expert opinions to update publicly available estimates of major taxa in this marine ecosystem and to revise and update several species lists. We also assessed overall spatial and temporal patterns of species diversity and identified major changes and threats. Our results listed approximately 17,000 marine species occurring in the Mediterranean Sea. However, our estimates of marine diversity are still incomplete as yet—undescribed species will be added in the future. Diversity for microbes is substantially underestimated, and the deep-sea areas and portions of the southern and eastern region are still poorly known. In addition, the invasion of alien species is a crucial factor that will continue to change the biodiversity of the Mediterranean, mainly in its eastern basin that can spread rapidly northwards and westwards due to the warming of the Mediterranean Sea. Spatial patterns showed a general decrease in biodiversity from northwestern to southeastern regions following a gradient of production, with some exceptions and caution due to gaps in our knowledge of the biota along the southern and eastern rims. Biodiversity was also generally higher in coastal areas and continental shelves, and decreases with depth. Temporal trends indicated that overexploitation and habitat loss have been the main human drivers of historical changes in biodiversity. At present, habitat loss and degradation, followed by fishing impacts, pollution, climate change, eutrophication, and the establishment of alien species are the most important threats and affect the greatest number of taxonomic groups. All these impacts are expected to grow in importance in the future, especially climate change and habitat degradation. The spatial identification of hot spots highlighted the ecological importance of most of the western Mediterranean shelves (and in particular, the Strait of Gibraltar and the adjacent Alboran Sea), western African coast, the Adriatic, and the Aegean Sea, which show high concentrations of endangered, threatened, or vulnerable species. The Levantine Basin, severely impacted by the invasion of species, is endangered as well.

This abstract has been translated to other languages (File S1).

/pdf/the-biodiversity-of-the-mediterranean-sea-estimates-patterns-38y4s7jond.pdf

The Biodiversity of the Mediterranean Sea: Estimates, Patterns, and Threats

Eutrophication: A new wine in an old bottle?

Marine ecosystems' responses to climatic and anthropogenic forcings in the Mediterranean

Modelling nitrogen, primary production and oxygen in a Mediterranean lagoon. Impact of oysters farming and inputs from the watershed

Nutrient and oxygen exchanges at the water–sediment interface in a shellfish farming lagoon (Thau, France)

This study was based on in situ experiments conducted in a shellfish lagoon (Thau, France) to determine the interactions between suspended oyster Crassostrea gigas Thunberg cul- tures and their environment at a seasonal scale. Three groups of descriptors were considered simul- taneously during the course of the study: the taxonomic composition of an oyster culture unit (OCU), the nutrient and oxygen exchanges at the OCU-water interface (fluxes) and the water-column char- acteristics (water). We used a multiple factorial analysis (MFA) to assess in a single reference frame: (1) the seasonal organisation of the taxonomic groups which compose the OCU and (2) the interac- tions between this multispecific assemblage and the water column. We found strong seasonal vari- ability in the OCU composition and the OCU interactions with the water column. Taxonomic richness of the biofouling was at a maximum in spring and minimum in August after a bottom anoxia. Among biofouling species, ascidians always constituted the dominant group. The annual pattern of coexis- tence observed between ascidians and oysters might be explained by resource partitioning, and also by a beneficial trophic interaction between these 2 groups. The structure of the OCU was based on a complex system of interactions, particularly in spring. During this season, the development on the OCU of macrofauna of soft substratum such as polychaetes reflected the presence of a suspended sediment compartment. The influence of the OCU on the water column was maximum in summer and minimum in winter. In the shellfish zone, the OCU plays a central role in nutrient renewal, particu- larly in summer when benthic fluxes are insufficient to satisfy phytoplankton nitrogen requirements. We estimate that the multispecific assemblages occurring on the oyster ropes (oysters and biofouling) have a potential annual DIN production of 2 × 10 7 mol yr -1 , and thus oyster cultures could have a determining influence on nitrogen recycling in the water column in the Thau lagoon.

/pdf/composition-of-biofouling-communities-on-suspended-oyster-1jog2u26g2.pdf

Composition of biofouling communities on suspended oyster cultures: an in situ study of their interactions with the water column

Experimental trawl surveys were performed in the Gulf of Lions. A multicompartmental approach was used to study the spatial distribution of demersal assemblages and to estimate their associations with two other components of the ecosystem: the benthic macrofauna and substratum type. At the Gulf scale, we found that these components explained 38 and 19% of the total variability of the spatial organization of groundfishes, respectively. Our results showed that groundfish and benthic macrofauna species were primarily distributed along a coast–open sea gradient and secondarily along a longitudinal axis. The longitudinal variability of the spatial organisation of species was maximal in the shelf area, whereas the upper-slope and coastal areas were characterized by strong homogeneity. Although no partitioning could be made based on their spatial distribution, the coastal groundfish assemblages were split into two groups. In the first group, groundfishes were strongly associated with both benthic macrofauna and type of substratum; in the second group, they were strongly independent of these two components. Furthermore, in the Gulf of Lions, the presence of productive zones associated with the Rhone river plume or with shelf-break upwelling seemed to attract some groundfishes. This study illustrates how useful such a multicompartmental approach can be for both fisheries management and community ecology.

Spatial structure and habitat associations of demersal assemblages in the Gulf of Lions: a multicompartmental approach

The present study was designed to quantify the potential influence of oyster cultures (Crassostrea gigas, Thunberg) on dissolved inorganic nitrogen (DIN) renewal in a shallow coastal lagoon (Thau, France), where nitrogen is the factor limiting primary production. The approach was based on a comparative in situ analysis of the seasonal patterns of nitrogenous nutrient exchanges at both the water sediment and oyster cultures water interfaces. The experiments were performed at a station under the influence of oyster cultures (UC) compared with benthic exchanges in a station outside their direct influence (OC) which was used as the reference site for benthic fluxes. In contrast to what it is usually reported for shallow ecosystems, I found evidence of a key influence of sus- pended oyster culture units (OCU) on the regeneration in the water column of nitrogen available to the phytoplankton, with benthic remineralisation being only of secondary importance. The results also showed that the greatest impact of oyster cultures was on DIN renewal (e.g. nitrogen transfor- mation) in the water column, not on particulate organic nitrogen transfer to the sediment. In the sec- tor studied, it also appeared that nitrogen recycling by both sediments and OCU could satisfy the nitrogen requirements of the primary producers except in summer when other internal nitrogen sources were necessary. Conversely, during most seasons, pelagic primary production could not meet the demand for particulate organic nitrogen of the cultured assemblages (oysters and epibiota). This emphasises the importance to the diet of cultured assemblages of other nitrogen sources such as heterotrophic populations of bacteria, cillates or flagellates.

https://www.int-res.com/articles/meps2004/276/m276p103.pdf

Nabila Mazouni

Papers

Modelling nitrogen, primary production and oxygen in a Mediterranean lagoon. Impact of oysters farming and inputs from the watershed

Nutrient and oxygen exchanges at the water–sediment interface in a shellfish farming lagoon (Thau, France)

Composition of biofouling communities on suspended oyster cultures: an in situ study of their interactions with the water column

Spatial structure and habitat associations of demersal assemblages in the Gulf of Lions: a multicompartmental approach

Influence of suspended oyster cultures on nitrogen regeneration in a coastal lagoon (Thau, France)