Relationships between biodiversity and the stability of marine ecosystems: comparisons at a European scale using meta-analysis.

The relationship between biodiversity and stability of marine benthic assemblages was investigated using existing data sets (n = 28) covering various spatial (m-km) and temporal (1973-2006) scales in different benthic habitats (emergent rock, rock pools and sedimentary habitats) through meta-analyses. Assemblage stability was estimated by measuring temporal variances of species richness, total abundance (density or % cover) and community species composition and abundance structure (using multivariate analyses). Positive relationships between temporal variability in species number and richness were generally observed at both quadrat (<1 m2) and site (100 m2) scales, while no relationships were observed by multivariate analyses. Positive relationships were also observed at the scale of site between temporal variability in species number and variability in community structure with evenness estimates. This implies that the relationship between species richness or evenness and species richness variability is slightly positive and depends on the scale of observation, suggesting that biodiversity per se is important for the stability of ecosystems. Changes within community assemblages in terms of structure are, however, generally independent of biodiversity, suggesting no effect of diversity, but the potential impact of individual species, and/or environmental factors. Except for sedimentary and rock pool habitats, no relationship was observed between temporal variation of the aggregated variable of total abundances and diversity at either scale. Overall our results emphasise that relationships depend on scale of measurements, type of habitats and the marine systems (North Atlantic and Mediterranean) considered

Calcium carbonate production of a dense population of the brittle star Ophiothrix fragilis (Echinodermata : Ophiuroidea): role in the carbon cycle of a temperate coastal ecosystem

The production of calcium carbonate by a dense Ophiothrix fragilis population was calculated in order to investigate its role in the carbon budget of a temperate coastal ecosystem (Dover Strait, eastern English Channel). Production, calculated using monthly data of population density, demographic structure and a size/CaCO3 conversion, was 682 g CaCO3 m(-2) yr(-1). Assuming that the molar ratio of CO2 released to CaCO3 precipitated varied with temperature between 0.66 and 0.73, this production would result in the release of 4.8 mol CO2 m(-2) yr(-1). This calculation reinforces the suggestion that this coastal system is a source of CO2 to the atmosphere

Long-term changes (1979-1994) in two coastal benthic communities (English Channel): analysis of structural developments

The long-term variations of two coastal benthic communities (Eastern English Channel, France) were studied between 1979 and 1994 by means of factor correspondence analysis (FCA), trophic structure and rank-frequency diagrams (RFD). FCA identified periods of relative homogeneity and periods of strong variation in the structure of the two communities. The first community, established on heterogeneous sand, showed a multi-annual cycle Linked to grain-size variations in the sediment. As passive supplies of organic matter were not important and suspension feeders could hardly live in these conditions, the community remained poor and stable and was dominated by deposit-feeders and carnivores. The other one, a rich and diverse mussel (Mytilus edulis) bed, showed great variations in the abundance of the main species which generated the enrichment of the community by its suspension-feeding behaviour (biodeposition). Beyond these variations, the community remained stable until 1990 as no continuous trend or permanent change could be seen. Since 1990, no recruitment of M. edulis occurred and the relative dominance of suspension feeders, as well as the number of species, strongly decreased. During the following years, several species of tunicates (sessile suspension feeders with gregarious recruitment) recruited and partly took the ecological niche of M. edulis but they did not induce an increase of biodeposition and the previous richness was not restored. After a strong and rapid shift in the structure, a new equilibrium has been established within the community.Les variations à long terme de deux peuplements benthiques côtiers ont été étudiées entre 1979 et 1994 au moyen d'analyses factorielles des correspondances (AFC), de l'étude de la structure trophique et des diagrammes rang-fréquence. Les AFC ont mis en évidence des périodes relativement homogènes et des périodes de variations importantes pour les deux peuplements. Le premier peuplement, établi sur des sables hétérogènes, montre une variation pluri-annuelle de sa structure fortement liée à celle de la granulométrie du sédiment. Les apports de matière organique étant assez faibles et les suspensivores étant peu abondants dans ce type de peuplement, celui-ci reste pauvre, relativement stable et dominé par les déposivores et les carnivores. Le deuxième peuplement, une moulière àMytilus edulis, présente de grandes variations de l'abondance relative de l'espèce principale M. edulis, à l'origine de l'enrichissement du peuplement et de la modification du sédiment par biodéposition. Au-delà de ces variations, la structure globale du peuplement paraissait stable jusqu'en 1990, quand aucun recrutement de M. edulis n'a été constaté. Après cette année-là, il semble qu'il n'y ait plus de recrutement de moules, ce qui s'est d'abord traduit par une baisse sensible du nombre d'espèces et de la dominance relative des suspensivores. Depuis, plusieurs espèces de Tuniciers (espèces sessiles suspensivores à recrutement grégaire) occupent la niche de M. edulis, mais leur activité suspensivore n'induit pas de bio-déposition susceptible d'enrichir le peuplement ou de modifier le sédiment et la richesse spécifique n'a pas été entièrement restaurée. Le peuplement a ainsi brusquement changé de structure pour constituer un nouvel équilibre stable à l'échelle de quelques années

A closed-chamber CO2-flux method for estimating intertidal primary production and respiration under emersed conditions

This paper describes a closed-chamber method for measuring CO2 fluxes in intertidal soft sediments during periods of emersion. The method relies on closed-circuit incubations of undisturbed sediment and measurement of CO2 exchanges using an infrared gas analyser. The method was assessed during field experiments, both in light and dark conditions, on an exposed sandy beach and in an estuary. The rates of gross community production measured under moderate diance (4.2 mg C m(-2) h(-1) on the exposed sandy beach and 35 mg C m(-2) h(-1) in the estuary) are in good agreement with rates reported in the literature. In conjunction with appropriate sampling strategies, this method can be useful for estimating and comparing production of intertidal areas or for assessing factors that influence production