1. The hypothesis is tested that large daphnids are able to suppress their own and other spedes' feeding and reproduction by means of excreting an inhibitory chenucal (or chemicals). 2. In laboratory experiments with an Australian spedes, Daphnia carinata, water preconditioned with 3-67 daphnids I"' for 30 h had the effect of redudng feeding rates of D. carinata and D. lumholtzi provided with Selenastrum capricornutum. 3. For the two Daphnia spedes, there were highly significant negative correlations between feeding rate and the preconditioning density of D. carinata. 4. Water preconditioned with 20-30 daphnids T' for 1-2 weeks reduced the grazing rates of Daphnia, Moina, and Diaphanosoma 2-3-foId. 5. Moina kept in such water for 2 days stopped feeding. Conditioned water kept for 3 days without animals still inhibited grazing by Moina. Heating to 1(X)°C removed the inhibitory effect. 6. Given excess food, and in non-renewed water, a gradient of D. carinata densities developed a strong negative correlation between clutch size and daphnid density after a 6-day time lag. This result may help explain the direct density-dependent regulation of dadoceran reproduction observed earlier in a subtropical lake.
The properties of Daphnia carinata King as a grazer for use in biomanipulation trials were investigated. Mesocosm experiments suggested that in water from a lake where D. carinata was scarce, phytoplankton was nutrient-limited and the manipulated biomass of zooplankton had no effect on total chlorophyll a, whereas in water from a lake where D. carinata was dominant, nutrients were not limiting and total chlorophyll a was negatively correlated with the manipulated biomass of zooplankton. When offered lake phytoplankton in feeding trials, D. carinata consumed all items present, including colonies of cyanobacteria and long filaments of diatoms. In large outdoor tanks with natural plankton, the biovolume of prokaryotic ultraplankton (possible predecessors of cyanobacterial blooms) was consistently reduced in the presence of D. carinata. There was no evidence of an adverse effect of single-celled Microcystis aeruginosa containing the peptide toxin microcystin-LR on D. carinata grazing rates or survival. Different concentrations of microcystin-LR in solution covering the range of toxicities observed during M. aeruginosa blooms (5-500 nM) had no effect on D. carinata grazing. The suppression of phytoplankton biomass by D. carinata grazing is one of several possible mechanisms that might be considered for biomanipulation in Australia.
1. Grazer and nutrient controls of phytoplankton biomass were tested on two reservoirs of different productivity to assess the potential for zooplankton grazing to affect chlorophyll/phosphorus regression models under Australian conditions. Experiments with zooplankton and nutrients manipulated in enclosures, laboratory feeding trials, and the analysis of in‐lake plankton time series were performed. 2. Enclosures with water from the more productive Lake Hume (chlorophyll a = 3–17.5 μg l–1), revealed significant zooplankton effects on chlorophyll a in 3/6, phosphorus limitation in 4/6 and nitrogen limitation in 1/6 of experiments conducted throughout the year. Enclosures with water from the less productive Lake Dartmouth (chlorophyll a = 0.8–3.5 μg l–1), revealed significant zooplankton effects in 5/6, phosphorus limitation in 5/6 and nitrogen limitation in 2/6 of experiments. 3. While Lake Hume enclosure manipulations of the biomass of cladocerans (Daphnia and Diaphanosoma) and large copepods (Boeckella) had negative effects, small copepods (Mesocyclops and Calamoecia) could have positive effects on chlorophyll a. 4. In Lake Hume, total phytoplankton biovolume was negatively correlated with cladoceran biomass, positively with copepod biomass and was uncorrelated with total crustacean biomass. In Lake Dartmouth, total phytoplankton biovolume was negatively correlated with cladoceran biomass, copepod biomass and total crustacean biomass. 5. In both reservoirs, temporal variation in the biomass of Daphnia carinata alone could explain more than 50% of the observed variance in total phytoplankton biovolume. 6. During a period of low phytoplankton biovolume in Lake Hume in spring–summer 1993–94, a conservative estimate of cladoceran community grazing reached a maximum of 0.80 day–1, suggesting that Cladocera made an important contribution to the development of the observed clear‐water phase. 7. Enclosure experiments predicted significant grazing when the Cladocera/Phytoplankton biomass ratio was greater than 0.1; this threshold was consistently exceeded during clear water phase in Lake Hume. 8. Crustacean length had a significant effect on individual grazing rates in bottle experiments, with large Daphnia having highest rates. In both reservoirs, mean crustacean length was negatively correlated with phytoplankton biovolume. The observed upper limit of its variation was nearly twice as high compared to other world lakes.
In a small lake in northern Argentina pelagic water mite Piona sp. had the maximum of population density in January following with a five-day delay after the peak of zooplankton dominant - Daphnia laevis. The mite density was highly predicted by the previous variations of Daphnia density during 4 months of observation (December-March). Daphnia density was a negative delayed function of the predator density but only in December-January when Piona was abundant. During that period Daphnia death rate, d was also correlated with the mite density (r =0.80, P<0.005). In laboratory experiments water mites killed 1-7 Daphnia · predator · hour in a broad range of prey density. Another zooplankton component, Diaphanosoma birgei, was consumed at the same rates. The mite hardly consumed any copepods. In the pelagium during 24 hours the mite was more associated with Daphnia, than with Diaphanosoma, probably, because of the coincidence in photoreactions with Daphnia. Piona contribution to the death rates of its prey estimated by using the data on functional and numerical responses as well as by means of Edmondson-Paloheimo model, could reach 53% for Daphnia and 40% for Diaphanosoma. A computer experiment on the reconstruction of prey dynamics after subtraction of predator influence showed that the mite could have caused a depression in Daphnia numbers observed in the lake, but the declines in Diaphanosoma population were caused by other factors. After the "removal" of mite pressure model Daphnia population increased its average density 10-fold. Experiments on Piona feeding revealed a strong effect of interference among predators. This was eliminated by putting one mite per experimental vessel, which led to a 20-fold increase in predation rate. The effect explains the low feeding rates of Piona obtained by the previous authors who ignored the possibility of interference.
Patterns of zooplankton-phytoplankton interactions in subtropical lakes of the Southern Hemisphere may deviate from those established for north-temperate lakes. We tested the responses of phytoplankton growth to different community structures of zooplankton and nutrient enrichment in a subtropical Australian reservoir for the prediction of potential outcomes of lake biomanipulation. Two zooplankton communities were created in lake enclosures over 4 weeks: a rotifer-dominated community developed in the presence of planktivorous fish (Hypseleotris spp.) and a Ceriodaphnia-dominated community developed in the absence of fish. Biomass gradients of both communities were established in 20 L containers and several separate containers received no additions (controls) or were enriched with nitrogen and/or phosphorus. The growth rate of total phytoplankton significantly increased in response to nutrient enrichment, indicating nutrient limitation. Most phytoplankton taxa were not markedly affected by grazing of either zooplankton community. However, both communities had significant stimulatory effects on the growth of inedible chlorophytes. The ability of zooplankton grazing to negatively affect phytoplankton growth during the summer was counteracted regardless of zooplankton community structure, possibly by nutrients regenerated by zooplankton. We hypothesise that in the subtropical system studied, changes in food web nutrient recycling may be more important for the outcome of biomanipulation than grazing impacts.
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