Seasonal changes in macrophyte biomass in South Island lowland streams, New Zealand

Abstract

Seasonality in stream macrophyte biomass can have a profound effect on the flood risk in stream reaches as hydraulic resistance of macrophytes increases with increasing biomass. We studied the seasonality in total macrophyte biomass, and of eight common plant species, in eight streams over 1 year in the South Island, New Zealand. Biomass was generally highest in summer and lowest in winter, with biomass in winter being 40% less than the annual mean and 50% higher in summer. The season of maximum biomass varied among the eight species studied. Ranunculus trichophyllus and Veronica anagallis-aquatica were most abundant only in summer; Elodea canadensis, Myriophyllum triphyllum, Rorippa nasturtium-aquaticum, Potamogeton cheesemanii, and Potamogeton crispus were most abundant in summer or autumn, whereas Lagarosiphon major did not show any seasonal variations. These results can help predict the season of highest flood risk in South Island streams based on the predominant species in a stream reach, and thus when the most efficient time for weed cutting might be. © 2003, Copyright Taylor & Francis Group, LLC.