Indicators of short-term growth in bivalves: Detecting environmental change across ecological scales

Abstract

Marine benthic communities function within the constraints of changing environmental conditions. To better understand the dynamics of this relationship, increased knowledge of physiological mechanisms that link environmental conditions to ecological responses is imperative. Here we develop a synthesis demonstrating the potential to link biochemical/physiological and population/community level responses based on the effects of increased sedimentation, hypoxia, food availability, and habitat characteristics on bivalve physiology using two soft-sediment, suspension-feeding species, Austrovenus stutchburyi and Paphies australis. 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. Bivalve nucleic acid ratios were analysed over several temporal and spatial scales in a combination of laboratory experiments, field experiments, transplants and surveys, and only by synthesising results from several different studies were clear patterns in bivalve RNA dynamics revealed. The magnitude of response in RNA content (RNA) varied seasonally, along environmental gradients, and with bivalve species and size class. The major factor affecting RNA was enhanced food availability, which appeared to have the potential to mask any negative effects of stressors. Over the short-term (10–14 days), elevated suspended sediment concentrations or single deposition of terrigenous clay did not affect RNA, but over the longer term (months), transplantation of bivalves to turbid sites and repeated deposition of clay had a slight negative effect on RNA. In the laboratory, hypoxia did not affect the fast increase in RNA in response to added food or the slow decrease in RNA in response to starvation. In addition, a species-specific baseline RNA content was indicated, possibly representing the basal metabolism of a species. Thus, RNA was more sensitive to factors facilitating growth than to factors inhibiting growth, indicating that RNA could be a good indicator of factors related to energy acquisition, while not being as sensitive to stressors. By assessing the variability in response across time and space scales, and considering the context and time scale of both the stressor and the response, organism-level measures may be used as part of an integrated approach in impact assessment and ecological experiments.