Eutrophication and black swan (Cygnus atratus Latham) populations: tests of two simple relationships

In an earlier study, regression models were developed which related black swan populations to macrophyte biomass and to phytoplankton productivity in Tomahawk Lagoon No 2, a small South Island, New Zealand, lake in which phytoplankton and macrophytes vary widely from year to year, in an irregular inverse cycle. Two years of further study confirmed that winter swan populations are directly correlated with macrophytes, for samples taken on a single day in winter each year. They also confirmed that winter maximum swan populations can be predicted from measurements of phytoplankton productivity in the previous summer. This relationship is inverse. For 6 other lakes, 9 of the 11 data points lay within the 95% confidence intervals of the macrophyte-swan relationship, with the only outlier being a lake in which the macrophytic vegetation is dominated by filamentous algae. Further analysis, giving equal weight to each of the 7 lakes in the sample, confirmed that there is a significant correlation in winter between swan populations and macrophyte biomass for these lakes which show a wide range of size and trophic status. There was no such significant correlation in summer. All of the 8 data points from 7 other lakes lay within the rather broad 95% confidence intervals of the phytoplankton- swan relationship. The 4 lakes which fitted most closely to the predictions of this relationship have silty sediment. The other 3, which all had lower swan populations than predicted, have sediment predominantly of sand or fine clay, and macrophyte biomass and swan populations may be constrained by the nature of the sediment, rather than by shading effects from phytoplankton.