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

Abstract

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Donax serra have d~s a p p e a r e d from the affected sector of the bay and peracarids typical of finer sands have been replaced by a more robust species.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 11m-ited. 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

Spatial distribution of mollusks in the intertidal zone of sheltered beaches in southeastern of Brazil

“…In the Barra Velha, Araçá I and Araçá II sectors, diversity increased from the upper toward the lower levels, confirming the results found by many investigators (MCLACHLAN 1990, RIZZO & AMARAL 2001. Olivella minuta showed clear zonation patterns, being more abundant in the lower levels, especially in the Araçá I and Araçá II sectors.…”

“…Other researchers conceived of the resurgence zone as part of the midlittoral of Dahl (MCLACHLAN 1990, RAFAELLI et al 1991, JARAMILLO & GONZALEZ 1991. According to MCLACHLAN (1990), resurgence zone species are intertidal forms that require wet sand during low tide, while retention zone species are also intertidal forms, but are capable of remaining in sand that is only slightly damp during the low tide period. In the sectors studied, the resurgence zone species (lower zone) occurred nearest to the water line, and their distribution may extend to the sublittoral.…”

“…The distribution of the macrofauna is the result of the relationships among environmental factors and species characteristics. The community structure and intertidal distribution of the macroinfauna have been related to abiotic factors such as grain size (JARAMILLO & GONZALEZ 1991, MCLACHLAN 1996, wave action (BROWN & MCLACHLAN 1990), water content (WENDT & MCLACHLAN 1985), and temperature and salinity of the interstitial water (KOH & SHIN 1988). Biological factors such as recruitment, predation and competition are also of key importance in the establishment and maintenance of zonation (MCLACHLAN & JARAMILLO 1995).…”

Diversity and distribution of crustaceans from 13 sheltered sandy beaches along São Sebastião Channel, south-eastern Brazil

“…Molluscs are second in abundance to polychaetes, while crustaceans are least and also have the fewest species. Although crustaceans may be proportionally more abundant in sandy beaches subjected to strong, frequent wave action (Dexter, 1983(Dexter, , 1984McLachlan et al, 1993), their species richness is still generally low (up to 16 species) (McLachlan et al, 1981;Dexter, 1984;McLachlan, 1990;Defeo et al, 1992;Jaramillo et al, 1993;McLachlan, 1996). Studies in sheltered environments are rare and generally conducted in estuaries, where salinity varies widely and where input of sediment and nutrients is constant.…”

“…In general, the di¡erent macrofaunal groups show particular responses to environmental factors, related mainly to substrate composition (Gray, 1974;Whitlatch, 1981;Snelgrove & Butman, 1994;Hall, 1994;McLachlan, 1996). Substrate heterogeneity may also directly in£uence species richness and diversity (Gray, 1974;Abele, 1976;Hendrickx, 1996).…”