Aquatic botany ELSEVIER Aquatic Botany 55 (1996) 205-215 Grazing by black swans (Cygnus atratus Latham), physical factors, and the growth and loss of aquatic vegetation in a shallow lake Stuart F. Mitchell *, Robert T. Wass Department of Zoology, Universityof Otago, P.O. Box 56, Dunedin, New Zealand Accepted 9 July 1996 Abstract Hawksbury Lagoon, a shallow coastal New Zealand lake, altemates between a clear-water, macrophyte-dominated state, and a turbid, phytoplankton-dominated state. The potential role of black swans (Cygnus atratus Latham) in stabilising the phytoplankton dominated state by grazing on macrophytes was investigated during a period of increase and decline in the benthic vegetation (Nitella) in 1993-94. The swan population density was closely correlated with plant biomass (r 2 = 0.95). Although the swan population became as high as 25 ha t direct grazing consumption 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 periods of decline of 0.07-0.18 day-i. Indirect effects of the swans on the plants through nutrient recycling and bioturbation, are also unlikely to have been important. Concentrations of suspended solids and phytoplankton, and light attenuation, remained high throughout the study. Plant biomass normally increased when the benthic photon irradiance exceeded 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 macrophyte growth are marginal, the cumulative effect of waterfowl grazing consumption might well be critical, however, for keeping macrophyte biomass below the threshold for macrophyte dominance, in spite of the consumption being small. Keywords: Herbivory;Waterfowl; Phytoplankton; Macrophytes; Light 1. Introduction Shallow eutrophic lakes may exist in two alternative stable steady states. In one, macrophytes are dominant, and in the other phytoplankton dominate, and macrophytes * Corresponding author. Fax + 64 03 479 7584; email: stuart.mitchell@stonebow.otago.ac.nz. 0304-3770/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304-3770(96)0 1077-7