Sammendrag
We investigated the factors influencing cyanobacterial biomass and microcystin (MC) concentrations in several
Ugandan lakes from September 2008 to February 2009. We characterized thermal structure, light availability,
nutrient concentrations, chlorophyll
a
, phytoplankton
d
13
C (as an indicator of CO
2
limitation), and
phytoplankton community composition and abundance as well as MC concentrations. We used these data to
test several hypotheses based on previous research in temperate lakes regarding the factors that encourage high
cyanobacterial biomass and MC concentrations. Site characteristics that appeared to favor high cyanobacterial
biomass (especially
Microcystis
) included: high total phosphorus concentrations, low total nitrogen to total
phosphorus (TN:TP) ratios, and possibly low CO
2
availability. Light availability, total nitrogen concentrations,
and thermal structure of the water column were not related to cyanobacterial biomass. MC concentrations were
strongly related to
Microcystis
biomass (and were not related to the biomass of any other cyanobacterial taxa),
which was positively correlated with total phosphorus and chlorophyll
a
concentrations. MC cell content may be
moderated by CO
2
availability, with MC cell quotas tending to be lower where the potential for C-limitation of
photosynthesis was higher. In these phosphorus-rich tropical lakes, the shallowest study sites were most conducive
to the development of large standing crops of
Microcystis
and high MC concentrations. The environmental
conditions that appear to favor high cyanobacterial biomass and MC concentrations in our Ugandan study lakes
are similar to what has been observed for temperate lakes
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