Pelagic Food Web Structure of Lake Hume and Dartmouth

"December 1996".

Project Number: Pelagic Food Web Structure of Lakes Hume and Dartmouth - M/03/5089; LWRRDC project MDR9.

MDFRC item.

9 pages.

1. Grazing by zooplankton significantly affects phytoplankton biomass in reservoirs. Its relative influence is comparable to nutrient limitation in the frequency of seasonal occurrence. Grazing varies considerably in time and between lakes. It is assumed to be efficient when Cladocera/Phytoplankton biomass ratio exceeds 0.1, large Cladocera predominate over Copepoda and mean crustacean length is > 1.0 mm. 2. Total biomass of zooplankton may be irrelevant for predicting algal biomass. Different zooplankton may have opposing effects. Thus, copepods can stimulate while cladocerans suppress phytoplankton with zero net effect for total algal biomass. Zooplankton community structure but not biomass is the key determinant of the overall influence. 3. Grazing rates are higher and feeding niches are wider in larger zooplankton. Largest zooplankton, such as Daphnia are the best grazers. 4. At moderate to high relative biomass of Daphnia, mean crustacean length is strongly correlated with algal biomass in Lakes Hume and Dartmouth. This supports models for US lakes, which predict the potential for biomanipulation from crustacean length . In Australian reservoirs, the potential may be higher because daphnids are larger. 5. Biomass of zooplankton- feeding fish varies between sites. It is considerably higher in reservoirs with algal blooms than in unproductive waters. Therefore. it is likely that fish effects on zooplankton grazers are higher in eutrophic waters. 6. Some reservoirs should be considered as huge breeding grounds for introduced fish. L. Hume, for instance, was estimated to contain 1.7 x 1010, juveniles of redfin perch (Perca fluviatilis) in spring, 1996. This is expected to have implications for inland fisheries and native fish conservation, as well as for the ecology of the whole river system. 7. Long-term structural changes in food webs may accompany population declines of endangered native fish in reservoirs such as the decline of Macquarie perch (Macquaria australasica) in L. Dartmouth. 8. For a gradient of reservoirs with varying nutrient concentrations (TP=8-40 µg/l) and algal abundance (from clear water to algal blooms), Daphnia biomass was significantly negatively correlated with algal biovolume, while fish biomass was negatively correlated with Daphnia biomass, suggesting the impact of top-down trophic interactions. Algal biovolume was not correlated with total P, while the correlation with Iota I N was marginally significant. 9. Existing literature suggests that the success rate for algal control in lakes by the reduction of phosphorus input rate is similar to the success rate for control through enhanced zooplankton grazing. 10. It is desirable to conduct biomanipulation experiments in Australia designed to test for the concepts suggested at a scale of whole-lakes. 11. Management options for biomanipulation include stocking predatory (piscivorous) fish into reservoirs. As existing fisheries programs already perform stocking, inadvertent "biomanipulation" in Australia is underway. However, it is highly desirable to supplement it with proper food web analysis which should include nutrients, plankton and forage fish biomass assessments prior to and after the stocking to monitor the consequences for ecosystem health. 12. Other management options for biomanipulation may include: eradication, where possible, of undesirable fish (such as carp) with biodegradable chemicals, catch restrictions on predatory fish , management of fish breeding sites, manipulation of reservoir water level during fish reproductive period, and planting of macrophytes to create refuges for micro-grazers.