A MODEL OF MUSSEL FILTRATION IN A SHALLOW NEW-ZEALAND LAKE, WITH REFERENCE TO EUTROPHICATION CONTROL

Filtration by the freshwater mussel (Hyridella menziesi) population of Lake Tuakitoto (New Zealand) was modelled by determining the populati on biomass, filtration capability, and proportion of time spent filter ing. The mean mussel biomass in the lake was 12.3 g/m(2) AFDW, the mea n filtration rate of 32 mussels was 1.9Lhr(-1)g(-1) at 19-21 degrees C , and mussels actively filtered for 93 % of the time. It was calculate d that the mussel population of Lake Tuakitoto filters a volume of wat er equal to that of the lake once every 32 hours, which represents an exponential grazing rate on the phytoplankton of 0.29/day. Mussel graz ing may account for the observed suppression of chlorophyll-a concentr ations to about 10 % of those predicted by phosphorus-chlorophyll rela tionships in this lake. Hyridella was found to filter dinoflagellate c ells from a nuisance bloom in another hypereutrophic lake at about 35 % of their rate when feeding on the green algae, Choricystis coccoides . These results suggest that Hyridella has potential as a biomanipulat ion tool for control of phytoplankton in eutrophic lakes.