Cyanobacterial blooms in a shallow lake: a largescale enclosure assay to test the importance of diurnal stratification

Abstract

The importance of diurnal stratification for the formation of cyanobacterial blooms was examined using large enclosures (180 m 3 ) in a shallow bay of Lake Biwa, Japan in 1995 and 1997. Three different mixing regimes were set up: enclosures covered with transparent plastic sheet to prevent wind from causing turbulence in the water column and loss of heat from radiation were continuously stratified, those artificially mixed with pumps each day to disrupt diurnal stratification were never stratified, and controls which stratified diurnally, Concentration of dissolved inorganic nitrogen showed similar changes among enclosures despite the differences in mixing regimes. Concentrations of dissolved inorganic phosphorus tended to increase during the experimental period in all the enclosures. Chlorophyll levels in the diurnally stratified enclosures were high relative to those in the continuously stratified and unstratified ones, suggesting that a stable water column and nutrient upwelling were responsible for the high phytoplankton growth. It proved difficult to detect completely consistent patterns in the occurrence of major groups of phytoplankters. There was greater similarity between the two non-stratified replicates and the two continuously stratified replicates than there was among the four diurnally stratified replicates. Only Cryptomonas and Chlorophytes formed substantial proportions of the phytoplankton biomass in all enclosures in both years. Ceratium was significant in all but one of the non-stratified enclosures. Among the cyanobacteria, Microcystis was a very small proportion of the biomass in the non-stratified enclosures but was frequently dominant in both the diurnally and continuously stratified enclosures. Anabaena occurred in all the diurnally stratified enclosures but to a greater extent in 1995 than in 1997. Thus, although thermal stratification was a prerequisite for the growth of Microcystis, it was still unclear whether diurnal or continous stratification was more important for cyanobacterial growth. Anabaena was clearly more susceptibile to aspects of the chemical environment, such as nitrogen availability.