Document: WAY-3-72-6

Fish-mediated nutrient transfer in lakes: Is it important for phytoplankton?

GALLO, K.* and W.A.WURTSBAUGH

Utah State University, Logan, UT 84322 USA ¹

Abstract:
Ecologists have spent considerable effort studying consumer excretion and related effects on nutrient recycling; however, transfer of nutrients by consumers has received less attention. Fish may move nutrients from the littoral zone of a lake to the pelagic zone through their feeding and excretion activities, thereby providing nutrients for phytoplankton that were previously unavailable. We present a pond study using stable isotopes to examine fish-mediated nutrient transfer. The pond was divided into 8 parallel lanes, each of which received one of two treatments: fish (brook trout) present or absent. ¹⁵N-labelled periphyton grown on tiles was placed in the littoral zone of each lane. Movement of ¹⁵N was traced through the pond food web, comparing the fish and fishless lanes. Nutrient concentration and biomass of phytoplankton, periphyton, zooplankton, and benthic invertebrates were also quantified. ¹⁵N values in phytoplankton and periphyton (new growth during the experiment) were significantly higher in the lanes with fish, indicating transport of nutrients from periphyton in the littoral zone. However, total nitrogen in the water column was not increased by the presence of fish. Although fish diet consisted of 90 % littoral prey, and fish excreted/egested over 30 % of N consumed, fish contributed only 1.2 % of TN in the water column. Fish did not have significant effects on phytoplankton or zooplankton biomass, however fish did significantly reduce benthic invertebrate biomass and increase periphyton biomass. We conclude that although fish did transfer significant quantities of N, transport was not as important as other sources of N in the pond. Availability of N from the littoral zone was apparently not important for phytoplankton, since biomass was not increased in the presence of fish.

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This abstract is being presented at: 9:30 AM in session:
Oral Session #70: Aquatic Ecology.