Bivalve communities of Wellington Harbour and their relationship with sediment type

Abstract

Studies of benthic macrofauna in soft sediment are numerous and are conducted for many reasons. The benthic bivalve molluscs at eight sites within Wellington Harbour, New Zealand were surveyed six times over an eighteen-month period. Site location was determined by sediment type. The aim of the study was to describe and quantify spatial and temporal variation of bivalve species and assemblages in the harbour, and to determine if the variations could be related to sediment type. Temporal variation of sedimentary characteristics was also examined. Univariate and multivariate analyses were used to investigate biological variation between sites and within sites (temporal variation), and to determine if organic carbon of the sediment varied as a function of time. Sampling identified a total of 46 bivalve species from the harbour. Analysis of the bivalve fauna determined that significant differences in the structure and composition of bivalve assemblages between the eight sites existed. Therefore each of the sites can be viewed as a separate community and not as a geographical sub-section of a natural unit formed by the entire harbour. Although each of the communities was significantly different from one another, differences varied in magnitude between sites. Four groupings of sites with similar structure and composition were identified. Sites in each group shared biological and abiotic similarities. It was therefore concluded that species distribution is in some way affected by sediment type. Mean grain size and silt (which was highly correlated with carbon) were the environmental variables that best explained the spatial patterns of bivalves identified by multivariate analyses, suggesting these factors determine species composition. The percentage of silt in the sediment appeared to determine whether deposit feeders or suspension feeders dominated a community. Temporal variation of community structure and species composition was detected for six of the eight sites; however, few distinct seasonal trends could be discerned. In the context of these findings, this study provides a knowledge base regarding the bivalve fauna of the harbour, which may be used for future benthic research in Wellington Harbour.