Interrelations between phytoplankton, submerged macrophytes, black swans (Cygnus atratus) and zooplankton in a shallow New Zealand lake

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Abstract
Different features of the limnology of Tomahawk Lagoon, a shallow, hypertrophic New Zealand lake were studied for varying periods between 1964 and 1986. Surveys in 10 years revealed a strong positive correlation between the winter black swan population and the biomass of macrophytes which in turn in a shorter study was inversely related to the abundance of phytoplankton. A regression model was developed relating the winter maximum swan population to phytoplankton productivity in the previous summer. The model worked well for another very shallow lake, but less well for a third lake with a mean depth greater than the feeding reach of swans.No general mechanism could be established for suppression of phytoplankton when macrophytes were abundant, though the effect was large. Suppression of macrophytes in other years is attributed to shading by phytoplankton. The decline of one algal bloom was related to nitrogen deficiency followed by intense zooplankton grazing. Another was ended by the optical effects of a large flood‐borne silt inflow and flushing of phytoplankton. Empirical models relating chlorophyll a to phosphorus or nitrogen, and zooplankton biomass to chlorophyll a or phosphorus do not work well for this lake, and the general problems of modelling shallow highly eutrophic lakes are discussed.