The effect of extreme rainfall on summer succession and vertical distribution of phytoplankton in a lacustrine part of a eutrophic reservoir

Abstract.

During the summer of 2006, phytoplankton succession and vertical distribution were studied in the eutrophic Římov Reservoir (Czech Republic). Disturbances in the form of dramatic summer storms had a crucial effect on phytoplankton succession and its vertical distribution. Two extreme rainfalls substantially increased flushing rate, yielding a fairly short retention time within the reservoir that disrupted established thermal stratification. The first storm initiated the development of summer phytoplankton, while the second storm reversed the phytoplankton succession to an earlier stage. The measurement of vertical profiles of chlorophyll a showed that the phytoplankton were heterogeneously distributed in the water column and formed subsurface diatom maxima (SDM) over most of the summer. Biogenic silica deposition studied by PDMPO (2-(4-pyridyl)-5{[4-dimethylaminoethyl-aminocarbamyl)-methoxy] phenyl}oxazole) was measured to compare diatom growth rates over the season and between surface and depth of SDM. Diatoms deposited 20 times more silica at the surface than at the SDM depth, although only a half of diatom population was involved in the silica deposition. Maximum rates of silica deposition were measured in samples just after both summer storms events. When diatoms dominated the phytoplankton, the amounts of available Si and P in water were significantly correlated (r ² = 0.81, p < 0.001, df = 42). Outliers from the linear relationship were found only during a period when substantial picocyanobacterial population developed in the reservoir. Due to their lack of Si requirements, ambient Si concentration markedly increased while P was depleted to growth limiting levels. Temporal variations in Si:P ratio therefore illustrate the importance of resource competition in phytoplankton seasonal succession.