Long Time Series from the English Lake District: Irradiance-Dependent Phytoplankton Dynamics During the Spring Maximum

We analyzed rates of phytoplankton increase and decline during the spring maximum via long-term (25 yr) records of biomass (Chl a) and abundance of the dominant diatom, Asterionella formosa, sampled from the surface waters of Windermere (English Lake Districk). Average rates of net increase in early spring (i.e. up until the end of March) are best correlated with the logarithm of surface irradiance $(r^2 = 0.71-0.87)$. Average spring growth rates in late February and March (weeks 6-13 of the year) defined by this analysis significantly exceed rates predicted from calculations of integrated production with existing physiological data for A. formosa but are similar to the predicted growth rates at the mean irradiance in the mixed layer. In late spring (April-May), stratification becomes well established and the rate of population increase lessens. This decrease occurs before dissolved silicate is depleted to growth-limiting concentrations during increasing average irradiance in the surface layer. Near exhaustion of dissolved silicate and rapid loss of A. formosa from the surface layer follow. P limitation, enhanced sedimentation, and photoinhibition are factors that may slow net diatom accumulation before the onset of silicate limitation.