Characterisation and quantification of phosphorus release from profundal bottom sediments in two dimictic lakes during summer stratification

Phosphorus release from the sediment in two dimictic lakes was compared during the summer stratification. Soluble reactive phosphate (SRP) and also NH4 + were released from the anoxic sediment into the water column in Lake Dudinghausen and Lake Tiefer. The sediment released NH4 + and SRP only when oxygen and nitrate/nitrite were absent. Besides the SRP concentration in the hypolimnion, P-binding in the upper sediment changed during summer stratification in both lakes. In Lake Dudinghausen a decrease in the redox-sensitive bound P was observed in the upper sediment. Only in this lake does the classic anoxic P-release by reduction of the FeOOH-phosphate complex play a dominant role. In both lakes, however, the contents of organically bound P increased in the upper 0.5 cm layer during the summer stratification. P-release was calculated using four different methods. The calculated P-release during summer stratification varied between 15 and 207 mg P m-2 y-1 in Lake Dudinghausen and between 22 and 55 mg P m-2 y-1 in Lake Tiefer.