Ultraphytoplankton biomass and production in some New Zealand lakes

Abstract

Abstract Size‐fractionation by Nuclepore filters shows that about one‐third of the biomass of freshwater phytoplankton is 0.2–3 μm in size. Autoradiographs of natural water from North and South Island lakes of various trophic states, incubated with ‘'C‐bicarbonate, reveal the contribution of this ‘ultraplankton’ to primary production. The lakes sampled in 1975–76 were Rotongaio, Rotorua, Tarawera, and Taupo (North Island); Brunner, Johnson, Kangaroo, Ohau, Pukaki, Rotoiti, and Wakatipu (South Island). In general, oligotrophic and mesotrophic lakes contain most ultraplankton, forming 11–35% of the biomass and 33–76% of the primary productivity. In both New Zealand and North America the importance of ultraplankton seems grossly underestimated, probably because it normally escapes detection in standard Utermohl and direct nitration examinations.