Bioavailability of phosphorus in fish farm effluents to freshwater phytoplankton

Abstract

Water samples were collected from the inflow and untreated effluents of six landbased rainbow trout, Oncorhynchus mykiss (Walbaum), and Atlantic salmon, Salma salar L., farms in Ireland. These samples were analysed for general water quality, phosphorus (as total, total reactive, total soluble and soluble reactive) content, phosphatase activity and bioavailable phosphorus (BAP). The latter was determined by a phytoplankton (Selenastrum capricornutum and Oscillatoria redekei) bioassay. Nitrogen and trace elements did not appear to limit phytoplankton growth, but phosphorus did in all samples except for two taken during the cleaning of fish tanks. The growth of O. redekei and S. capricornutum was similar in almost all samples, and when significantly different, the former species showed the greater growth. The phosphorus fractions, phosphatase activity and BAP were greater in effluents compared to inflows, and BAP was greater in unfiltered than in filtered samples. Phosphatase activity was not correlated with any phosphorus fractions or BAP, and was lower than in previous studies of phosphatase activity in fresh waters. On average, 33% of total phosphorus in farm inflow water, and 82% in effluents was BAP. This may reflect that there was 12-15% more phosphorus in the reactive form in effluent than inflow samples. Total and total soluble phosphorus were significantly correlated to BAP in inflows, but no single phosphorus fraction was consistently related with BAP in all instances. Hence, in monitoring fish farm effluents, at least total phosphorus should be determined.