Impact of mussel (Mytilus galloprovincialis) raft-culture on benthic macrofauna, in situ oxygen uptake, and nutrient fluxes in Saldanha Bay, South Africa

Culture of the mussel Mytilus galloprovincialis in a South African bay crea ted organic enrichment and anoxia in sediments. Particulate organic matter (POM) was high under rafts versus the references, especially in the first 1 0 cm (C = 7.5 versus 0.4%, N = 0.7 versus 0.08%). Total reducible sulphides (TRS) increased threefold downcore (from 0.04 to 0.12%). High C:N ratios ( 12-15) indicated accumulation of refractory POM, derived mainly from faeces and decaying mussels and foulers. Although O-2 uptake by raft sediments wa s the lowest, rates could not conclusively be separated from the references . Ammonium dominated N efflux, the highest and most variable rates being un der mussels (825 +/- 500 mu mol NH4.m(-2).h(-1)). Phosphate efflux (25-140 mu mol.m(-2).h(-1)) could not be ascribed to culture biodeposition, but the re was an inconclusive trend for the molar N:P ratio to be highest in these sediments. Macrofauna biomass was reduced and trophic groups and taxa alte red. Under rafts, macrofauna and organic debris were linked to O-2 uptake r ates, whereas at the reference sites, macrofauna appeared to be the major O -2 consumer. It was concluded that POM and TRS in sediment as well as macro fauna biomass, and potentially molar N:P ratios, were more sensitive indica tors of benthic impact from mussel culture than O-2 uptake rates or nutrien t fluxes.