Spatial variability and dynamics of macrobenthos in a Mediterranean delta front area: The role of physical processes

Benthic communities in delta fronts are subject to burial risk because of high riverine sediment discharges and to substrate instability due to deposition of fine sediments at shallow depths. This study examines the spatial distribution of macroinfauna in the subaqueous deltaic depositions of a small river in the eastern Mediterranean (the Spercheios river, Maliakos Gulf, Aegean Sea) in relation to environmental variables in the water column and sediment. Samples were taken at eight stations in January, May, August and November 2000. From late winter to spring enhanced phytoplanktonic biomass, elevated suspended load and poorly sorted sediments showed a simultaneous influence of riverine discharges and hydrodynamics on the benthic system. In contrast, from summer to autumn oligotrophy in the water column and low hydrodynamic regime were observed. Total abundance, biomass and numbers of benthic species were positively correlated with distance from the river but negatively correlated with suspended inorganic particles and sediment skewness. Species from different functional groups, ranging from surface-living opportunists to burrowers and predators, coexisted at each station. However, suspension feeders were numerically suppressed near the river mouth. Non-parametric multivariate regressions showed that the variance in the species data was explained by environmental variables to a level ranging from 53 to 69%. This indicated a strong link between the macrofauna and the delta front environment. The variables used as measures of hydrodynamics and turbidity (i.e. sediment skewness and sorting, suspended material and transparency) displayed great explanatory power. The results of the present study show that the distribution of species is related to fluctuations in hydrodynamic regime that influence substrate characteristics. The study also demonstrates that sediment discharges of small temperate rivers can determine species composition in the delta front and have a detrimental impact on the community at short distances from river outflows.