Responses of rainbow trout (<i>Oncorhynchus mykiss</i>) and their prey to inorganic fertilization of an oligotrophic montane lake

Abstract

We used two montane lakes (4.5 and 7.8 ha) in a before-after control-impact pair experimental design to measure the responses of rainbow trout (Oncorhynchus mykiss) and their prey to 5 years of inorganic phosphorus and nitrogen (liquid ammonium polyphosphate and urea ammonium nitrate) additions. Epilimnetic total phosphorus (TP) concentration, euphotic zone chlorophyll a concentration, limnetic macrozooplankton biomass, and annual trout yield increased significantly in the fertilized lake. Chlorophyll a concentration and zooplankton biomass varied directly with epilimnetic TP. Trout yield varied as TP 0.5 . Fertilization increased trout reproductive output, growth, and yield but did not alter yearling survival or the mean age of mature fish. Prey utilization by trout did not change. DeltaC data indicated that trout in the fertilized lake obtained 65% of their body carbon from benthic insects and 35% from zooplankton. Increased biomass of Hesperodiaptomus accounted for most of the zooplankton response. Benthic insect emergence decreased with increasing water depth but was greater at a given depth in the fertilized lake. Resources accounted for smaller proportions of the variance of consumer biomass with increasing trophic level.