GRAZING BY BLACK SWANS (CYGNUS-ATRATUS LATHAM), PHYSICAL FACTORS, ANDTHE GROWTH AND LOSS OF AQUATIC VEGETATION IN A SHALLOW LAKE

Hawksbury Lagoon, a shallow coastal New Zealand lake, alternates betwe en a clear-water, macrophyte-dominated state, and a turbid, phytoplank ton-dominated state. The potential role of black swans (Cygnus atratus Latham) in stabilising the phytoplankton dominated state by grazing o n macrophytes was investigated during a period of increase and decline in the benthic vegetation (Nitella) in 1993-94. The swan population d ensity was closely correlated with plant biomass (r(2) = 0.95). Althou gh the swan population became as high as 25 ha(-1) direct grazing cons umption was slight. The grazing rate was 0.007 day(-1), by comparison with plant growth rates of 0.06-0.10 day(-1), and loss rates in period s of decline of 0.07-0.18 day(-1). Indirect effects of the swans on th e plants through nutrient recycling and bioturbation, are also unlikel y to have been important. Concentrations of suspended solids and phyto plankton, and light attenuation, remained high throughout the study. P lant biomass normally increased when the benthic photon irradiance exc eeded 7% of that at the surface, and decreased when it was lower than that. We conclude that lack of light was far more important than swan grazing for plant decline. When light or other conditions for macrophy te growth are marginal, the cumulative effect of waterfowl grazing con sumption might well be critical, however, for keeping macrophyte bioma ss below the threshold for macrophyte dominance, in spite of the consu mption being small.