Water chemistry and substrate type as major determinants of molluscan feeding habit and life-mode in lagoon sediments

Few organisms can live in harsh transitional waters due to highly variable and rapid changeable physicochemical conditions. Fortunately, connecting the community structure of such fauna to specific environmental parameters can be used in tracing past and recent environmental changes. Herein, we analyzed the benthic communities in the Manzala lagoon (Nile-Delta), a region very impacted by the construction of the High-Dam at Aswan, along the inshore-offshore gradient. Approx. 3000 specimens belonging to 31 species sampled in 54 sampling sites were identified. The environmental and biological data were subjected to multivariate data analyses. Hierarchical clustering and non-metric multidimensional scaling identified three biotopes along a gradient of water salinity and sediment grain-size. The epifaunal/infaunal ratio was negatively correlated with salinity (R2 = - 0.72, p < 0.01). The low salinity-stations with a high content of silt and clay sediments were dominated by deposit-feeders, opportunistic gastropods. In contrast, the suspension-feeding bivalves dominated the more saline water and sand-sized sediments (R = 0.83, p < 0.001) near the Mediterranean outlets. The general low diversity and high dominance in the lagoon indicate high environmental stress and biodiversity loss as the background freshwater species have declined and only a few opportunistic taxa have flourished. In addition, a specific salinity-range was estimated at both species and community level. The results confirm that salinity and substrate type are major determinants for the structure of the molluscan community (feeding habit and life-mode) in marginal marine environments. Consequently, the feeding habit and life-mode of the mollusks, which have a continuous and very complete fossil record, can be used as a robust indicator for paleosalinity and paleo-hydrodynamics.