Phytoplankton blooms in late winter or spring occur regularly in temperate lakes and transport significant amounts of nutrients to the sediment when they sink. However, information on the transfer of nutrients between the two seasons remains scarce. Here we studied the phytoplankton community composition and how it affects vertical nutrient fluxes in Rappbode Reservoir, a deep, dimictic, clear-water lake in central Germany. We conducted a sampling campaign from November to July spanning one winter circulation and one spring period and collected bi-weekly samples for phytoplankton, water chemistry, and physical variables, and monthly measurements of sedimentation fluxes using sediment traps. Phytoplankton, with special attention to diatoms, was quantitatively investigated. The reservoir mixed from mid-December until stratification onset in mid-April. Cyanobacteria were the most dominant group during mixing with peak biomass in March. The diatom bloom started at the onset of stratification in April and peaked in early June. The most abundant diatom species were Aulacoseira granulata, Tabellaria fenestrata, and Fragilaria spp. The particulate phosphorus content per unit of carbon in the sediment traps peaked in March and April, but the highest vertical phosphorus flux was in May during the diatom bloom. Our results show that winter-spring phytoplankton, particularly diatoms, and cyanobacteria can store substantial amounts of nutrients in the biomass. Moreover, the winter-spring conditions influence the phytoplankton composition, which in turn affects nutrient cycling in lakes.

Primary Presenter: Valeria Fárez-Román, Helmholtz - UFZ (claudiafarez@gmail.com)

Authors:

Valeria Fárez-Román, Helmholtz-Centre for Environmental Research, Department of Lake Research, Magdeburg, Germany  (valeria.farez@ufz.de)

Karsten Rinke, Helmholtz-Centre for Environmental Research, Department of Lake Research, Magdeburg, Germany  (karsten.rinke@ufz.de)

Tom Shatwell, Helmholtz-Centre for Environmental Research, Department of Lake Research, Magdeburg, Germany  (tom.shatwell@ufz.de)