Effects of Fertilization and Planktivorous Fish on Epilimnetic Phosphorus and Phosphorus Sedimentation in Large Enclosures

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Abstract
Enclosure experiments were conducted during 1986 and 1987 to test the hypothesis that predation-induced shifts in plankton size-structure affect epilimnetic total phosphorus (TP) through changes in sedimentation rates, both with and without external nutrient loading. Fertilization and/or planktivorous fish were applied to eight enclosures in a 2 × 2 factorial design. Epilimnetic TP was higher with fish in the unfertilized enclosures and lower with fish in the fertilized enclosures; that is, fertilization increased epilimnetic TP only in enclosures without fish, and much less than expected based on the amounts added. The fraction of TP sedimenting per unit time, and the spring to summer decline in TP, was consistently lower with fish. Sedimentation rates were poorly correlated with epilimnetic TP, but were more strongly correlated with TP decline rates and particulate phosphorus larger than 20 μm. Presence or absence of planktivorous fish can alter the trophic status of planktonic systems, and the impact of external nutrient loading by changing the size-distribution and biomass of plankton.