THE ROLE OF GLOETRICHIA-ECHINULATA IN THE TRANSFER OF PHOSPHORUS FROM SEDIMENTS TO WATER IN LAKE ERKEN

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migation to the water during summer were estimated. The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 mug P (mg dw)-1 or 81 ng P colony-1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies. The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m-2 or 2.5 mg p m-2 d-1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken. The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 mug P (g dw)-1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.