The effects of acid sulphate run-off on a subtidal estuarine macrobenthic community in the Richmond River, NSW, Australia

Abstract

The Richmond River is periodically exposed to acid sulphate run-off (ASR). Leaching of acidified water into this system commonly arises from infiltration of rainwater through acid sulphate soils that have been oxidized during construction of flood mitigation drains. Leaching events have been linked to major fish kills and can also lead to sublethal effects in fish, such as red spot disease. The effects of ASR on subtidal estuarine macrobenthic assemblages in the region of the Richmond known as the Tuckean Broadwater are examined here. Canonical correspondence analysis revealed that water transparency, pH, and soluble aluminium concentration were the most significant contributors to variation in macrobenthic community structure. Partial canonical correspondence analysis was performed to examine the effects of factors associated with ASR on the relative abundance of macrobenthic species, in isolation from the influence of other environmental variables. Variation in relative abundance was not significantly related to the individual effects of either soluble aluminium or pH. However, their combined effect on species abundance was significant, reflecting that macrobenthos at this site responded to chemical speciation of aluminium at certain pH ranges, rather than directly to either pH or soluble aluminium concentration. Results also showed that two species of sub-surface deposit-feeding polychaetes, Nephtys australiensis and Notomastus torquatus, were the most sensitive to the chemical speciation of aluminium and have tolerances for aluminium species that occur at different levels of acidification. The feeding activities of these two species may have removed the iron floc formed on the sediment surface under acidified conditions, thereby providing suitable conditions for other macrobenthic species to recolonize the Tuckean Broadwater.