Primary production in a shallow eutrophic lake dominated alternately by phytoplankton and by submerged macrophytes

Abstract

Tomahawk Lagoon No. 2, a shallow eutrophic New Zealand lake, has shown irregular cyclic variations between phytoplankton and macrophyte dominance since 1963. During a 4-year study, which encompassed a change from phytoplankton to macrophyte dominance, there was a large decline in phytoplankton biomass and production, and an increase in macrophyte biomass. However, there was little increase in the macrophyte production, and the total primary production of the lake declined sharply, by a factor of 4. The effects of phytoplankton having better access to the available light and a longer growing season, and the more rapid nutrient cycling expected among planktonic communities, clearly outweighed those of macrophytes having access to sediment nutrients in this lake. Grazing by black swans was calculated to have removed about 15–25% of the production of macrophytes during their growing season, or 20–50% per annum.