Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka, Florida

Abstract

Summary1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993–2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation.2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass‐specific rates of excretion than smaller fish.3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature.4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year−1 (3.46 kg N ha−1 year−1) and 7700 kg P year−1 (0.62 kg P ha−1 year−1) on average. The actual impact on the P cycle varied substantially from year to year (range 7900–78 800 kg N year−1; 1200–14 800 kg P year−1), primarily because of fluctuations in the catch.5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993–2005.6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.