Characterisation of water quality in effluents of land-based abalone farms in the Western Cape, South Africa

Abstract

Effluent water quality was measured at 9 abalone (Haliotis midae) farms in 2 regional nodes (west and south) along the South African coastline. For most farms, effluent total suspended solids (TSS) exceeded the background reference level (80th percentile), and 3 did not comply with the 5 mg l-1 standard. Total ammonia nitrogen (NH4+) concentrations were mostly greater than reference levels but well below the 43 μmol N l-1 standard. Inflow-corrected concentrations of nitrate, nitrite and phosphate were low compared to NH4+ and would not pose a significant eutrophication risk. Similarly, the biochemical oxygen demand measured at 3 of the farms was low (median 1.31 mg l-1). Abalone production-specific annual loads of TSS (334 kg per metric tonne [mt]), total N (20.3-38.1 kg N mt-1) and total P (3.2-7.5 kg P mt-1) agree with what has been found for different land-based aquaculture operations. These figures translate to N-based human population equivalents of 5.4-10.6 persons mt-1 for both regions. At the broader ecosystem level, the annual TSS loads calculated from 2013 production data of 43 mt yr-1 (west) and 369 mt yr-1 (south) are, respectively, 0.35 and 2.8% of that estimated for kelp erosion. Similarly, the dissolved inorganic N loads of 1.9 mt N yr-1 (west) and 9.4 mt yr-1 (south) are trivial by comparison with nitrate advected during upwelling. Local abalone farms have a relatively high specific C footprint-conservatively ~44 kg CO2 kg-1 production. Our findings support a relatively low potential impact of farm effluents in this coastal upwelling environment.