ON THE SEASONAL MIGRATION OF THE CYANOBACTERIUM GLOEOTRICHIA-ECHINULATA IN LAKE ERKEN, SWEDEN, AND ITS INFLUENCE ON THE PELAGIC POPULATION

The quantity and distribution of colonies of Gloeotrichia echinulata m igrating from the sediments into the water column were studied in Lake Erken by the use of inverted traps. The epilimnetic population and th e phosphorus status of the epilimnetic colonies were quantified. The n umbers of colonies in the traps were greatest in the shallower areas. As the stratification became weaker and the epilimnetic population dec lined, the amount of colonies in all the traps increased markedly. Mos t of the colonies in the traps were due to redistribution within the w ater column. On the assumption that no epilimnetic uptake of phosphoru s occurred in G. echinulata, the internal loading of phosphorus due to migrating colonies was calculated to be between 0.4 and 0.6 mg P m(-2 ) day(-1) during July and August. The proportion of newly migrated col onies in the epilimnetic G. echinulata population was calculated to be around 50% for the period before the epilimnetic population maximum. The increasing phosphorus content of the epilimnetic colonies after th e population maximum indicates that the migration continued even after this.