Nearshore subtidal soft-bottom macrozoobenthic community structure in the western sector of Algoa Bay, South Africa

The objectives of this study were to characterise macrozoobenthic community structure of the western sector of Algoa Bay, to identify the drivers of community structure and to develop a long-term monitoring framework. Data were collected from six study sites stratified along-shore. Each site comprised three stations; most sites were located in areas directly influenced by anthropogenic activities such as inflow from storm water drains and areas where dredged spoil was dumped. Other sites included areas in close proximity to estuary mouths. Physico-chemical parameters of the water column were measured with a YSI instrument, sediment for faunal and physico-chemical analyses was sampled with a Van Veen grab, and collected macrofauna were sedated and preserved pending analysis. In the laboratory, macrofauna were identified to finest taxonomic resolution possible under dissecting and compound microscopes, and enumerated. Sediment samples for physico-chemical analyses were kept frozen pending analysis. Up to 187 species belonging to 137 genera and 105 families were identified. Univariate community parameters such as abundance and number of species varied significantly along-shore, generally increasing towards less wave-exposed sites. Multivariate analyses revealed that community assemblages were heterogeneously distributed along-shore, corresponding to areas where anthropogenic influences such as effluent discharge and commercial harbour activities prevailed. During the 2008 survey, species assemblages separated into six groups corresponding to the six sites but xvii during the 2009 survey, species assemblages separated into four groups probably due to changes in environmental parameters such as the hydrodynamic regime. In both surveys the assemblage opposite a drainage canal (Papenkuils outfall) was distinct as it was dissimilar to all other assemblages. This site was also heterogeneous over relatively small spatial scales. Important physico-chemical variables influencing community structures during the 2008 survey included bottom measurements of temperature, salinity, dissolved oxygen, coarse sand and mud. During the 2009 survey, only bottom temperature and mud content were identified as important physico-chemical variables structuring community assemblages. The principal variable was probably the hydrodynamic regime, driving community structure at a larger scale in Algoa Bay. On a localised scale, communities were probably structured by other factors such as effluent discharges, influence of estuary mouths and activities associated with the harbour. With a lack of information on keystone species (regarded as good monitoring species) in Algoa Bay, it was proposed that groups that cumulatively comprise 50–75 percent of total abundance within communities be monitored annually. Included are amphipods, polychaetes, cumaceans, ostracods, tanaids and bivalves. It was also proposed that areas opposite estuary mouths, effluent outfalls and the dredged spoil dumpsite be monitored. This routine monitoring programme should be accompanied by periodic hypothesis driven research to assess the importance of stochastic events (e.g., upwelling) on macrozoobenthic community dynamics. Keywords: macrozoobenthos, soft-bottom, community assemblages, spatial distribution patterns, environmental drivers, long-term monitoring framework