Wrack breakdown on sandy beaches—its impact on interstitial meiofauna

Abstract

The impact of the breakdown of stranded macroalgae on interstitial fauna was investigated over a one-year period on two exposed sandy beaches. On one beach (wrack-loaded beach, WLB), regular heavy deposition caused the formation of an anoxic black layer close to the sand surface. On the other (sparse-wrack beach, SWB) the sediment remained well oxidized with wrack scattered infrequently onto the midshore. The algae Hypnea rosea and Plocamium corallorhiza, made up 58% and 21% of the wrack, respectively. Groundwater nutrient analyses showed high NH 4 levels below the black layer at WLB. No significant difference in sediment bacteria densities was found between beaches. Meiofauna was dominated by nematodes, harpacticoid copepods, oligochaetes and turbellarians at both sites. The toxic effects of H 2S and other reduced compounds associated with the black layer inhibited meiofaunal colonization at WLB. Total meiofaunal biomass at this beach was only 9% of that at SWB. Nematode feeding types were chiefly predators, selective and non-selective deposit feeders. The nematode/copepod ratio, an indicator of environmental stress, did not reflect any adjustment of the interstitial communities on either beach as a response to the deposition of wrack. It is suggested that high meiofaunal densities occur when the balance between organic input and oxygen availability approaches an optimum.