Regional differences driving organic matter and trace metal signatures reflected in temperate reef bivalve communities on the South Island, New Zealand.

Abstract

Increases in anthropogenic activity along coastal zones has led to an influx of terrestrial particulate matter containing high levels of nutrients and contaminants to coastal ecosystems. In order to manage these detrimental impacts, it is imperative that we understand the factors that drive the spatial variation in pollutants at both regional and local scales. Bivalves are key marine players in the processing of organic matter providing a link between benthic, pelagic, and terrestrial habitats. To this effect, bivalves are recognized as sentinel organisms in evaluating marine ecosystem function as well as detecting pollutants associated with land-based inputs. Therefore, changes to the basal organic matter source pools supporting these species will result in inherent changes in organic matter sources supporting species further up the food chain. Comparisons between Marlborough Sounds, Tasman Bay, and Fiordland on New Zealand’s South Island, provide a unique study system containing gradients in anthropogenic impacts influencing the uptake of organic matter and accumulation of pollutants in bivalves.

The present study aimed to identify the influence of anthropogenic stressors in driving organic matter flux and trace element concentrations within bivalve communities in Marlborough Sounds, Tasman Bay, and Fiordland. We evaluated direct interactions between catchment modifications and salmon farming on feeding strategies, organic matter source pools, and trace element signatures of five individual bivalve species.

We established that feeding strategies reveal interspecific differences among sites, while intraspecific differences are reflective of small-scale differences in organic matter resource pools. Further, organic matter source pools shaped by catchment modification and salmon farms revealed the observed differences in proportions of macroalgae supporting primary consumers. Lower proportions of organic matter were correlated with a high degree of catchment modification. Salmon farms influenced nutrient delivery to the surrounding benthos to all subtidal species. However, the intertidal (Mytilus galloprovincialis) was uninfluenced.

Spatial difference in trace element concentrations tracked through basal organic matter source pools revealed the influence of catchment modification on the transmission of trace elements into bivalves. Further, aquaculture systems influenced the use of alternate basal organic matter sources (macroalgae versus phytoplankton) which were coincident with trace metal concentrations reflected in sediment-dwelling species.

Lastly, as mercury concentrations have not been well resolved for commercially important bivalves in South Island fisheries, using current mercury analysis methodology, we reported baseline mercury concentrations. Further, we illustrated the influence of catchment modification from forestry on spatial variability in mercury concentrations.

The contrast of the relatively developed catchments surrounding Marlborough Sounds and Tasman Bay with Fiordland’s relatively pristine forested catchment provides an important test of the links between anthropogenic activity and the contaminant loads reflected in coastal bivalves. Monitoring these coastal bivalves provided essential insight into the movement of particulate organic matter throughout food webs and their implications for ecosystem health.