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

We determined the potential nutrient limitation of summer phytoplankton pro duction using 2 x 2 x 2 factorial design of enrichment bioassays both befor e and after the introduction of planktivorous whitefish (Coregonus lavaretu s) in a polyhumic lake with an abundant population of the large cladoceran Daphnia longispina. After the fish introduction, Daphnia disappeared and wa s substituted by the rotifer Keratella cochlearis. During the dominance of Daphnia, additions of nutrients had little or no effect on primary producti on of phytoplankton in short-term (1-day) incubations. However, 5-day incub ations suggested that P most likely was the limiting nutrient. After the fi sh introduction, both 1- and 5-day enrichment bioassays indicated potential N limitation of phytoplankton production. The strong response of phytoplan kton production to combined enrichments with P and N also suggested colimit ation by P and N, but this was probably forced by the relatively high addit ions of nutrients. Consistently low primary production after the combined a ddition 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. O ur results suggest that changes in the food web structure may determine the limiting nutrient also in a highly humic lake.