Ke. Havens et al., EXPERIMENTAL STUDIES OF ZOOPLANKTON-PHYTOPLANKTON-NUTRIENT INTERACTIONS IN A LARGE SUBTROPICAL LAKE (LAKE-OKEECHOBEE, FLORIDA, USA), Freshwater Biology, 36(3), 1996, pp. 579-597
1. Over a 1-year period, twenty controlled experiments were performed
using small mesocosms (20-1 clear plastic carboys) and plankton commun
ities collected from four sites in shallow, subtropical Lake Okeechobe
e, Florida. In replicated treatments, macrozooplankton grazers were ex
cluded by size fractionation (115 mu m), and/or nutrients (N and P) we
re added, and impacts on phytoplankton biomass and productivity were m
easured after 3-day incubations. 2. In most experiments (fifteen out o
f twenty), there was no significant effect of zooplankton exclusion on
phytoplankton biomass or productivity, but there were significant inc
reases in those attributes due to nutrient additions. The magnitude of
the responses was a function of light availability at the collection
sites. 3. In three experiments, zooplankton exclusion led to declines
in phytoplankton biomass and productivity, suggesting that animals may
sometimes have net positive effects on the phytoplankton, perhaps via
nutrient recycling. 4. In only two experiments was there evidence of
net negative impacts of grazers on the phytoplankton In both instances
, cladocerans (Daphnia ambigua and Eubosmina tubicen) were dominant in
the zooplankton. However, the increases in chlorophyll a due to zoopl
ankton exclusion were small (5-20%), probably because of the small siz
e and relatively low grazing rates of the cladocerans. 5. The results
support the hypothesis that phytoplankton biomass in Lake Okeechobee i
s little affected by herbivorous macrozooplankton. This may be a commo
n feature of lowland tropical and subtropical lakes.