Sedimentary dynamics and benthic macrofauna distribution: Insights from the shoreface in southern Portugal

This study examined temporal distributions of macrobenthic communities in relation to bottom depth and specifically addressed whether variation in sedimentary dynamics affects species composition and functional structure. Sampling was carried in 2014 (February, May and September) off Tavira Island (southern Portugal). In each sampling period, three replicates of sediment samples were collected at 7, 12, 17 and 22 m deep for both macrobenthic and sediment characterisation. Distributional patterns of macrofauna communities were a function of the interaction between depth and temporal variability, influenced by a gradient in hydrodynamics and species recruitment. The duration of sediment remobilisation and the organic matter content in sediments were the main features of the shoreface sedimentary dynamics responsible for differences in macrobenthic species composition and feeding habits. At low depths (7 m), under intense hydrodynamic conditions and longer period of sediment remobilisation, sediments were characterised by slightly gravelly sand with low contents of organic matter and calcium carbonate. Macrobenthic communities inhabiting shallow depths displayed low abundance and species richness, being dominated by grazers and suspension feeders. In contrast, at the highest studied depth (22 m), subject to low sediment remobilisation, sediments were composed by gravelly sand with high contents of organic matter and calcium carbonate. This depth (22 m) was characterised by high abundances and species richness, reflecting a more stable macrobenthic community composed mainly by surface deposit feeders. According to the benthic communities, the seaward limit of significant wave effects on the seabed is around 7 m in low-energy periods and over 17 m in high hydrodynamic energy periods.