Influence of seagrass beds and Oyster Park on the abundance and biomass patterns of meio- and macrobenthos in tidal flats

In Arcachon Bay, on the south-west coast of France, the intertidal area is mainly occupied by sandbanks, oyster parks (Crassostrea gigas) and mud flats covered with seagrass beds (Zostera noltii). In order to estimate the relative contribution of meio- and macrofauna to the benthic ecology of these tidal flats, seven stations were studied seasonally for a year. Relationships between faunal density and biomass, and external factors such as sediment structure, benthic chlorophyll and seagrass debris were investigated. A comparison was made between bare sands, oyster beds and vegetated sediment in semi-exposed conditions and in sheltered areas.

Using a stepwise method of multiple linear regression it was shown that high densities of macrobenthos are mostly explained by high quantities of plant debris. For meiofauna, together with plant debris, other effective variables are involved: silt content, organic carbon, chlorophyll pigments. On an average, a correlation between macro- and meiofaunal abundances could be found. However, this general pattern is modified by the biogenic structure created by the oysters and seagrass. When compared to the adjacent sandbanks, oysters clearly enhanced meiofaunal abundance (from 1130–4170 individuals 10 cm−2) but depressed macrofaunal densities (from 640-370 individuals 400 cm−2). The organic-rich oyster biodeposits probably favour meiofauna by an increase of the trophic resources but do not favour macroinfauna by inducing low oxygen concentrations. Moreover, it is likely that macrofauna is more sensitive to predation than meiofauna both in sandbanks and in oyster parks. For both meio- and macrofauna the highest incidences (7200 individuals 10 cm−2 and 2470 individuals 400 cm−2, respectively) are recorded in seagrass bed sediments. Zostera induces an enhancement of organic detritus and provides a refuge against predation. In terms of biomass, the macrofauna/meiofauna ratio is 25·1 in sandbanks, 1·5 in oyster parks and 4·2 in seagrass bed sediments. Macrofaunal biomass is more variable both spatially and temporally than meiofauna biomass. It is likely that the macrofauna is more sensitive to external factors such as predation, anoxia, exposure, than the meiofauna. Meiofauna abundance and biomass are more usually a function of food abundance and physical properties of the sediment.