Changes in epilithic biomasses and invertebrate community structure over a deposit metal concentration gradient in upland headwater streams

Abstract

Stream bed metal deposits affect the taxon richness, density and taxonomic diversity of primary and secondary producers by a variety of direct or indirect abiotic and biotic processes but little is known about the relative importance of these processes over a deposit metal concentration gradient. Inorganic matter (IM), algal and non-photosynthetic detrital (NPD) dry biomasses were estimated for 10 monthly samples, between 2007 and 2008, from eight sites differing in deposit density. Invertebrate abundance, taxon rich- ness and composition were also determined. Relations between these variables were investigated by canoni- cal correspondence analysis (CCA), generalized esti- mating equation models and path analysis. The first CCA axis correlates with deposit density and inver- tebrate abundance, with lumbriculids and chironomids increasing in abundance with deposit density and all other taxa declining. Community structure changes significantly above a deposit density of approximately 8m g cm -2 , when algal biomass, invertebrate richness and diversity decline. Invertebrate richness and diversity were determined by direct effects of NPD biomassand indirecteffects of IM. Algal biomassonly had an effect on invertebrate abundance. Possible pH, oxygen, food and ecotoxicological effects of NPD biomass on the biota are discussed.