Influence of changing food web structure on nutrient limitation of phytoplankton in a highly humic lake

Abstract

We determined the potential nutrient limitation of summer phytoplankton production using 2 × 2 × 2 factorial design of enrichment bioassays both before and after the introduction of planktivorous whitefish (Coregonus lavaretus) in a polyhumic lake with an abundant population of the large cladoceran Daphnia longispina. After the fish introduction, Daphnia disappeared and was substituted by the rotifer Keratella cochlearis. During the dominance of Daphnia, additions of nutrients had little or no effect on primary production of phytoplankton in short-term (1-day) incubations. However, 5-day incubations suggested that P most likely was the limiting nutrient. After the fish introduction, both 1- and 5-day enrichment bioassays indicated potential N limitation of phytoplankton production. The strong response of phytoplankton production to combined enrichments with P and N also suggested colimitation by P and N, but this was probably forced by the relatively high additions of nutrients. Consistently low primary production after the combined addition of P and glucose suggests that in the presence of a labile organic C source, epilimnetic bacteria were superior to algae in the uptake of P. Our results suggest that changes in the food web structure may determine the limiting nutrient also in a highly humic lake.