NUTRIENT METABOLISM (C, N, P, AND SI) IN THE TROPHOGENIC ZONE OF A MEROMICTIC LAKE

The dynamics of seston and dissolved elements in a meromictic lake wit h high concentrations of manganese and iron in the monimolimnion were studied through an annual cycle. This publication presents results for assimilation, sedimentation and recovery of nutrients (C, N, P, and S i) in the trophogenic zone. Phosphorus deficiency kept the productivit y of the diatom dominated phytoplankton at an oligotrophic level. High concentrations of iron in influent streams and redistribution followe d by precipitation of iron during periods of partial turnover removed phosphorus from the water. High concentrations of manganese and sulfat e did not have the anticipated fertilizing effect, and recovery of nut rients from the depth of the lake was negligible. Mass balance calcula tions indicate that liberation of phosphorus from the sediments in the trophogenic zone was most important for the maintenance of primary pr oduction. 75% of carbon, 80% of nitrogen and 25% of phosphorus assimil ated by the phytoplankton was mineralized in the trophogenic zone. Sil ica was effectively regenerated from the littoral zone during the decl ine of diatom blooms. Nitrogen and silica retention was 45% of the ext ernal load compared to 66% for phosphorus.