Phosphorus cycling in a dimictic reservoir - the Sec Reservoir (Czech Republic)

The origin of accelerated eutrophication in the SeC Reservoir, a reservoir with highly elongated morphology, was studied in 1994-1995. External source s and internal cycling of phosphorus, characteristics of phosphorus in sedi ments, and phytoplankton concentration and composition were evaluated. The seasonal pattern of phytoplankton had two maxima: the spring peak dominated by diatoms and the summer-to-autumn peak of cyanobacteria and cryptomonads . The phosphorus content in the sediments was lower at the inflow (2.6 mg g (-1) DM) than in the lacustrine part of the reservoir (6 mg g(-1) DM). The dominant farm of phosphorus was fraction combined with Al and Fe oxides (up to 60%). The second most important fraction, dissolved and loosely bound p hosphorus, accounted for less than 25% of total phosphorus content. The pri ncipal inflow was the main source of phosphorus to the reservoir during the whole study period. The role of the sediments in the internal phosphorus c ycle became more important during the summer.