The Effects of Fertilization and Fish on Community Structure and Biomass of Aquatic Macrophytes and Epiphytic Algal Populations: An Ecosystem Experiment

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Abstract
Eighteen similar ponds (0.07 ha area, 1.3 m water depth) were fertilized at 3 levels with a fertilizer containing N, P and K compounds. Two ponds were left as controls. Simultaneously similar blue gill sunfish (Lepomis macrochirus) populations were added to 9 of the ponds. The effects of fertilization and the presence of fish which eat invertebrates, including grazers on the aquatic macrophyte and epiphytic algal populations, were investigated. Medium fertilization (0.23 g N m-2 wk-1, 0.02 g P m-2 wk-1) did not change the macrophyte biomass or species composition, or the biomass and species composition of the epiphyte community. Its sole effect was to increase diversity of the epiphytic diatom community slightly. There was no apparent influence of fish at low and medium fertilization levels. High levels of fertilization (0.91 g N m-2 wk-1, 0.09 g P m-2 wk-1) caused major changes in the limnology of the ponds, in the macrophyte community composition and biomass, in the epiphyte biomass and diatom community composition and diversity. Some of the reasons for change in the diatom community could be directly ascribed to fertilization and some to a change in the nature of the host macrophyte. There was a reduction in biomass at high fertilization levels which was probably due to shading by increased phytoplankton populations. Fish had measurable influence at high fertilization levels. They caused increases in biomass of certain macrophytes and of the epiphytes, probably through predation on grazing invertebrates, and they changed slightly the species composition of the epiphytic diatom community, perhaps because of modification of water chemistry, perhaps because of predation on grazers which ate the epiphytes selectively. Overall the effects of fertilization on the macrophyte-epiphyte community were much greater than those of the presence of fish. Some comments are made on the advantages and drawbacks of large-scale ecosystem experimentation.

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