Spring phytoplankton phenology – are patterns and drivers of change consistent among lakes in the same climatological region?

1. Advancement of phytoplankton peaks has been reported from a large variety of aquatic systems, attributed mostly to climate warming. Most studies have used chlorophyll a as a proxy for phytoplankton biomass. Here, we investigated phenology of three phytoplankton taxa (Asterionella formosa, Aulacoseira spp. and Cryptomonas spp.) in four lakes of the English Lake District situated within the same catchment over a 58-year period (19452003). 2. We used two methods (the centre of gravity method and a fitted Weibull-type function) to quantify the timing of the spring bloom. Both methods gave similar results for the diatoms, but not for Cryptomonas spp.. 3. The timing of the spring bloom advanced over the study period for A. formosa but was delayed for Aulacoseira spp. and Cryptomonas spp.. These changes were consistent in all four lakes. 4. We tested whether surface water temperatures prior to the time of phytoplankton peak abundance, winter soluble reactive phosphorus (SRP) and silicate concentrations, winter taxon inoculum, Schmidt stability or a combination of these affected phytoplankton phenology in the four lakes. Results were remarkably consistent: each driver either advanced or delayed the peak of each taxon in all four lakes. However, the different drivers affected the taxa differently. 5. Contrary to our expectations, the regional driver, temperature, did not have a consis-tent effect on the three phytoplankton taxa, significantly advancing A. formosa and Aulacoseira spp. in the North Basin of Windermere but significantly delaying Cryptomonas spp. peaks in three lakes. Overall, the locally variable driver, SRP concentration, had a more consistent effect upon phenology, affecting all taxa in nearly all lakes.