MEPS 326:175-186 (2006)  -  doi:10.3354/meps326175

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

J. Norkko1,2,3,*, S. F. Thrush1

1National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand
2School of Biological Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand
3Present address: Finnish Institute of Marine Research, PO Box 2, 00561 Helsinki, Finland

ABSTRACT: Nucleic acid ratios (RNA:DNA, RNA:protein or total RNA content) have been successfully 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 variability 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 availability 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 physiology 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.

KEY WORDS: Impact assessment · Nucleic acid ratios · RNA · Condition index · Natural variability · Size class · Austrovenus stutchburyi

Full text in pdf format