Diurnal mixed layers and the long-term dominance of Microcystis aeruginosa

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Abstract
The population dynamics of the cyanobacterium Microcystis aeruginosa in a hypertrophic, subtropical lake (Hartbeespoort Dam, South Africa) were followed over 4.5 years. We examined the hypothesis that M.aeruginosa dominated (>80% by volume) the phytoplankton population up to 10 months of each year because it maintained itself within shallow diurnal mixed layers and was thus ensured access to light, the major limiting resource. Wind speeds over Hartbeespoort Dam were strong enough to mix the entire epilimnion through Langmuir circulations only 12% of the time. At other times solar heating led to the formation of diurnal mixed layers (z1) that were shallower (cu; mean = 3.5 m, range: 0.45–8.4 m) while the seasonal mixed layer (zm) was always deeper than zcu (range: 7–18 m). By means of its buoyancy mechanism M.aeruginosa maintained the bulk of its population within z1, while non-buoyant species sank into dark layers. Adaptation to strong light intensity was implicated from low cellular chlorophyll a content (0.132 μg/106 cells) and high Ik (up to 1230 μE m−2 s−1). Ensured access to light, the post-maximum summer populations persisted throughout autumn and winter, despite suboptimal temperatures, by sustaining low losses. Increased sedimentation losses caused a sharp decline of the population at the end of winter each year, and a short (2–3 months) successional episode followed, but by late spring M.aeruginosa was again dominant. The data from Hartbeespoort Dam point out the importance of distinguishing between zm and z1, and show the profound effect that the daily pattern of z1 as opposed to the seasonal pattern of zm, can have on phytoplankton population species composition.

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