The Impact of Nutrient Pulses on Trophic Interactions in a Farm Pond

Abstract

We placed eight 1500 L mesocosms in a 0.2 ha eutrophic cattle pond during summer 1991 to determine if zooplankton grazing, nutrients, or both control algal biomass and productivity. The three treatments: + zooplanktivorous fish (39 bluegill, mean total length = 36 mm); + zooplankton (10x ambient); and + N + P (160 μM NH₄ ⁺ and 10.0 μM PO⁴ _3-) were duplicated and compared to ambient pond conditions and two control mesocosms. In the + N + P treatment, chl a concentrations increased 700% in four days and then decreased to initial levels; further nutrient enrichments failed to create an algal response, probably because of grazing associated with an eightfold increase in large cladocerans. After nutrients were added to the + fish treatment, the NH₄ ⁺ and soluble reactive phosphorus concentrations rose and then decreased rapidly, whereas chl a concentrations and rotifer numbers increased. When nutrients were added to the + zooplankton treatments, chl a increased, but less than when either fish or nutrients alone were added. In small eutrophic ponds, trophic manipulations may have little effect during equilibrium conditions but do alter algal responses during nutrient pulses. An increase in large cladocerans in response to a nutrient pulse may control nuisance algal blooms, even with subsequent nutrient additions.