Abstract_ Mosepele

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					 “Boom and burst” in a highly fluctuating aquatic system; monitoring dynamic food
                   web interactions in Lake Ngami, Botswana

                 Mosepele, K., Mosepele, B., Rose, A and Dikhai, S

Seasonally flooded aquatic environments provide a terrestrial/ aquatic interface that is
readily utilized by fish as a breeding and feeding habitat. Standardized techniques
were used to collect fish and aquatic invertebrates in the lake over a three month
period. Species succession was observed in the lake where two months after it filled
with water, there was a rapid colonisation by various fish species whose diversity
peaked in October. Thereafter, there was a gradual decline in diversity towards
November when the water levels gradually decreased in the lake. This essentially
characterized “boom and burst” scenarios experienced in similar systems around the
world. The top-five most important species in the lake (based on an index of relative
importance) were Barbus paludinosus, Marcusenius macrolepidotus, Clarias
gariepinus, Oreochromis andersonii, and Schilbe intermedius respectively. Species
such as Clarias gariepinus showed dramatic changes in mean size over time where
larger fish migrated out of the lake immediately after spawning. Dramatic decreases
in relative abundance and species diversity were also observed as a consequence of
decreasing water levels in the lake. There were changes in the feeding ecology of
some fish species over time in the lake. A higher percentage of S. intermedius
specimens had empty guts during the water filling phase, while stomach fullness was
observed during the high water phase in the lake. Contrary to published research, O.
andersonii fed on prey items different from to other systems, while S. intermedius
exhibited an opportunistic feeding behaviour that has been documented before in
other systems. Aquatic invertebrates also showed a dramatic decline in relative
abundances over time in the lake. Hemiptera was the most abundant macro-
invertebrate order in the lake and showed a dramatic decline over time possibly due to
declining water levels. Conversely, the Notonectidae (family) increased gradually
over time in the lake. Cleary there is a need to conduct a more deliberate study aimed
at tying fish, aquatic macro-invertebrates and plankton in highly dynamic systems like
this one. The role of plankton, its abundance, its dynamics such as diapause stages
(especially the egg stages) in lake sediments, is a critical gap which needs to be
studied within a full systems approach. These preliminary results, though based on a
short time frame, nonetheless give a snap-shot of boom and burst dynamics in a
highly fluctuating aquatic system. The study also highlights the need to initiate long
term monitoring of this system, which would ultimately reveal the role it plays in the
dynamics of aquatic communities.

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