Does the impact of nutrients on the biological structure and function of brackish and freshwater lakes differ?

The effects of nutrients on the biological structure of brackish and freshwater lakes with different nutrient loadings were compared. The lakes were generally shallow and had total phosphorus levels of 0.034-1.64 mg per litre and salinities of 0.19-35.6 ppt. Densities of fish, mysids, zooplankton and macrophytes were determined and the extent of crustacean grazing calculated. Fish biomass increased with increasing phosphorus concentration in freshwater lakes. Fish biomass in brackish lakes was reduced at phosphorus concentrations above 0.4 mg per litre. Mysids were found at a salinity greater than 0.5 ppt and increased with phosphorus concentration. Zooplankton in freshwater lakes were most abundant at intermediate phosphorus levels. Daphnia was only found at salinity levels below 2 ppt. Above 2 ppt zooplankton was dominated by Eurytemora and Acartia. Predation pressure on zooplankton was high and algal grazing capacity lower in brackish eutrophic-hypertrophic lakes than in freshwater lakes. The salinity threshold dividing freshwater and brackish lakes was lower than the conventionally defined 5 ppt. There are 57 references.