Vertical Separation of Light and Available Nutrients as a Factor Causing Replacement of Green Algae by Blue-Green Algae in the Plankton of a Stratified Lake

(1) A change from the green alga, Dictyosphaerium pulchellum to the blue-green algae Microcystis aeruginosa and Anabaena spiroides occurred in the plankton of Mt Bold reservoir, South Australia, during November and December 1978. (2) The reservoir was 30-34 m deep, thermally stratified with water mixed to 6-7 m, and a euphotic zone of less than 2.7 m. Measurements of photosynthesis and light penetration showed that photosynthesis was restricted probably to the upper 2.5 m. (3) Growth experiments with eight cultured species, and the natural phytoplankton assemblage, showed that during this change there was a decline of the potential of the water for algal growth to undetectable levels at a depth of 8 m. This could be restored by the combined addition of nitrogen, phosphorus and Fe-EDTA. (4) Continuous measurement of concentration of chlorophyll a in the water column demonstrated that the blue-green algal community was able to migrate to a depth of 12 m despite substantial density barriers. (5) In situ growth experiments, using dialysis bags, showed that populations which were artificially circulated between 0.2 and 10 m, were able to grow, whereas those maintained at a single depth (0-2 m or 10 m) were not. (6) The ability of blue-green algae to overcome successfully this spatial separation between light and nutrients was proposed as the probable cause of the change from green to blue-green algae.