Ecophysiology in environmental impact assessment: implications of spatial differences in seasonal variability of bivalve condition

Abstract

Nucleic acid ratios (RNA:DNA, RNA:protein or total RNA content) have been success- fully used as indirect measures of short-term growth in a range of marine organisms. However, using biochemical and physiological indicators in environmental impact assessment requires changes to be evaluated against a background of natural variability (e.g. spatial differences in the seasonal variabil- ity and size-dependence in bivalve condition). The natural temporal and spatial variability in nucleic acid ratios and condition indices in adult cockles Austrovenus stutchburyi was investigated along a gradient of increasing sediment grain size and wave exposure in an estuary on the east coast of New Zealand. The results were examined in relation to bivalve reproductive state, water temperature, and sediment and hydrodynamic characteristics of the estuary. The magnitude of seasonal variability in RNA was site-specific and increased towards more exposed sites, likely due to enhanced food avail- ability at these sites. In addition, investigation of the effect of bivalve size class on nucleic acid ratios and condition indices revealed site-specific patterns. Thus, by integrating information over a range of spatial and temporal scales, we can increase our understanding of the interaction between the phys- iology of estuarine soft-sediment bivalves and their spatially and temporally complex environment to provide a sound basis for assessing the effects of added environmental stressors.