The influence of runoff on intertidal mudflat benthic communities

Abstract

Intertidal mudflats receiving runoff from different sub-catchments were sampled to assess the effects of runoff on surficial sediments and benthic communities in Manukau Harbour, New Zealand. High sediment concentrations of hydrocarbons, cadmium, copper, lead and zinc indicated that runoff was causing significant sediment contamination. The mudflat fauna was dominated by a combination of small, opportunistic species, such as tubificid oligochaetes, the polychaete Heteromastus filiformis and the amphipod Proharpinia hurleyi and large, long-lived, hardy species, such as the crabs Helice crassa and Macrophthalmus hirtipes and the pulmonate gastropod Amphibola crenata. Within site variability in species numbers was low, probably mainly as a result of a high level of species mobility, although large scale disturbances may have also been implicated. Comparisons between stations revealed low numbers of species and individuals at only the most heavily contaminated site. However, patterns of community structure revealed by multivariate analysis were correlated with gradients of various sedimentary variables, including several contaminants. It also seems likely that other factors were influencing between-station differences in community structure, including stochastic variation in recruitment and various species interactions. The latter is supported by the fact that significant correlations (both negative and positive) existed between densities of several species. The study highlights the difficulty of detecting runoff effects at the benthic community level. This being despite the fact that the Manukau Harbour has a large city within its catchment and compared with other urbanised estuaries has only relatively small point sources of pollution.