Environmental factors affecting benthic macrofauna along a gradient of intermediate sandy beaches in northern Spain

Abstract

Ten sandy beaches along the north coast of Spain were studied during September 1999 to analyze number of species, abundance and biomass of macroinfauna along a gradient of intermediate beach types and exposure range. Faunal samples were collected with metallic cylinders (25 cm diameter,15 cm depth) at 10 equally spaced shore levels along six replicated transects separated randomly and extending from above the drift line to the low tide swash zone. Exposure rate, Dean’s parameter (Ω), beach state index (BSI) and relative tidal range (RTR) were estimated at each beach. Length and width of the beach, intertidal slope, sorting and median grain size and also swash amplitude and wave characteristics were measured. Number of species was between 10 and 29. Macrofaunal abundances ranged between 4962 and 71228 ind. m-1 and between 31 and 329 ind. m-2, while biomass (ash free dry weight) per square meter of beach ranged between 0.027 and 0.278 g m-2 and between 3 and 61 g. m -1. Results show some significant trends: number of species is the biotic variable more affected by physic and morphodynamic factors, increasing linearly with relative tidal range and decreasing with increasing average grain size; the same trend was observed from exposed to very exposed beaches; the biomass decreased exponentially with increasing average grain size. These trends agree with previous studies in different coasts in the world where coarse sands limit the benthic macrofauna. Morphodynamic parameter as Dean’s parameter and Beach State Index did not show predictive value. Results suggest that different characteristics of benthic macrofauna communities in intermediate beaches are affected in different ways by the physical processes involved in beach morphodynamic.