Fish as sources and sinks of nutrients in lakes

Summary 1. The release of total phosphorus (TP) and nitrogen (N in ammonium) was measured for the five most abundant fish species (>85% of biomass) in Mouse and Ranger Lakes, two biomanipulated, oligotrophic lakes in Ontario. 2. The specific release rate of both nutrients was significantly related to fish mass; log 10 TP release rate ( μ g h −1 ) = 0.793 (±0.109) [log 10 wet mass (g)] + 0.7817 (±0.145), and log 10 N release rate ( μ g h −1 ) = 0.6946 (±0.079) [log 10 wet mass (g)] + 1.7481 (±0.108). 3. When fish nutrient release was standardized for abundance (all populations, 1993–95) and epilimnetic volume, fish were estimated to contribute 0.083 (±0.061) μ g TP L −1 day −1 , and 0.41 (±0.17) μ g N L −1 day −1 in Mouse L., and 0.062 (±0.020) μ g TP L −1 day −1 and 0.31 (±0.08) μ g N L −1 day −1 in Ranger L. 4. In comparison, concurrent rates of total planktonic P regeneration were 1.02 (±0.45) μ g L −1 day −1 (Mouse L.) and 0.85 (±0.19) μ g L −1 day −1 (Ranger L.). Fish represented 8% of planktonic P release in Mouse L. and 7% in Ranger L. 5. Fish dry mass had mean elemental body compositions of 39.3% carbon, 10.9% nitrogen, and 4.0% phosphorus (all fish combined), with a mean molar C : N : P ratio of 27 : 6 : 1. This comprised about 55% and 23% of the total epilimnetic particulate P and N respectively. 6. Turnover times of P and N in fish were approximately 103 and 48 days respectively. In comparison, planktonic turnover times of particulate P in Mouse and Ranger Lakes were 4.3 and 4.4 days respectively. Given their high P content and low turnover rates, fish appear to be important P sinks in lakes.