Dynamics of silica and phytoplankton population under altered conditions of river flow in the Daugava River, Latvia

摘要

Three dams affect the flow of the lower Daugava River. The water reservoirs created by damming are relatively small with residence times from three to nine days. However, the water flow alteration is sufficient to cause a decrease in the phytoplankton biomass from an average of 246 to 96?mg?m–3. The diatom community was dominated by single-cell and small (d?=?5–10?μm) centric diatoms under the unaltered flow conditions. Under the altered flow conditions, chain-forming fresh-water planktonic diatoms and singe-cell pennatae diatoms dominated the diatom community. The concentrations of dissolved silica varied between 51 and 121?μmol?dm–3 and of biogenic silica from 1 to 12?μmol?dm–3. In both cases the lowest values were observed during summer months. Furthermore, dissolved and biogenic silica concentrations were higher at stations representing unaltered flow conditions in comparison to those representing altered flow conditions during summer.