Physical factors in benthic ecology:effects of changing sand particle size on beach fauna

This paper reports on the disposal of diamond mine tailings on a Namibian sandy beach. Coarse sand in the tailings greatly increases the grain slze of the affected parts of the beach and thereby provides the opportunity to assess the effects of changing sand grain size on a beach when other physical vanables are kept constant. Elizabeth Bay (Namibia) is 4 km long and was originally composed of fine sand which, exposed to moderate to heavy wave acbon, produced a log spiral bay with a disslpative beach. Tailings disposal in the centre of the bay has increased mean sand particle size from original values of 110 to 160 pm to present values of 500 to 800 pnl w t h a concomitant conversion of beach morphodynanlic state from dissipative to intermediate. Surveys of the 2 ends of the bay, which are relatively undffected by disposal, and of an undisturbed similar bay nearby revealed intertidal benthlc macrofauna communities with 15 to 20 species occurring in h ~g h abundance (24 120 to 129276 m -' ] . In 3 transects in the affected area, species nchness was 8 to 12 per transect and abundance was 640 to 4710 m -' Beds of the large sand mussel Donax serra have d ~s a p p e a r e d from the affected sector of the bay and peracarids typical of finer sands have been replaced by a more robust species. Regression analysis revealed signlf~cant correlations between community parameters (species richness and abundance) and both beach slope and part~cle size; ANOVA confirmed the significantly lower abundances of fauna in the affected areas Smothering effects appeared to be localised and 11mited. This study has supported the hypothesis that a n increase In sand particle size (on a beach where tide range and wave energy have remained constant) results in a change in beach state and a decrease In species richness and abundance KEY WORDS.