The effects of suspended bivalve culture on benthic community structure and sediment fluxes in Saldanha Bay, South Africa

This study provides a follow-up to prior measurements of the impact of mussel Mytilus galloprovincialis farming on benthic macrofauna and biogeochemistry and includes the first measurements of the effect of oyster Crassostrea gigas farming in Saldanha Bay, South Africa. The Shannon–Wiener index was significantly lower for farm than for control sites, indicating reduced diversity at farm sites. The lowest values of the Warwick statistic were recorded at the established farm site (R-old). Nested PERMANOVA showed no significant difference in community composition between farm and control sites. Suspension feeders (the mudprawn Upogebia capensis, clam Venerupis corrugata, and holothurian Thyone aurea) dominated at control sites and were important contributors to dissimilarity. Deposit feeders and predatory/scavenging gastropods tended to be more abundant at the farm sites. nMDS ordination showed a clear separation of the more exposed Big Bay stations. The proportions of mud and very coarse sand were moderately associated with community structure. Biogeochemical fluxes showed insignificant differences in sediment oxygen demand between the farm and control sites. Ammonium (NH4) regeneration dominated nitrogen (N) fluxes and was greater at the farm (313 µmol N m−2 h−1) than at the control (187 µmol N m−2 h−1) sites, and maximum rates were restricted to the established mussel raft (R-old) during winter. In contrast, fluxes of oxidised N were less at farm sites relative to the controls. There was no significant difference in phosphate (PO4) regeneration at farm and control stations, leading to higher inorganic N to PO4 regeneration ratios at farm sites.