Seasonal changes in metabolic rates of zooplankton and their importance to phytoplankton production in two New Zealand lakes of contrasting trophic state

Abstract

Abstract Zooplankton and water were sampled from Lakes Taupo and Rotongaio from July 1985 to June 1986. Zooplankton excretion and respiration rates were measured for the copepod Boeck‐ella propinqua and excretion rate also for the zooplankton community. Photosynthetic 14C uptake was measured to estimate phytoplankton nitrogen requirements. Highest metabolic rates were found in spring and summer in both lakes. In eutrophic Lake Rotongaio, metabolic rates were closely correlated with temperature. In Taupo, a large oligo‐trophic lake, food supply was also an important variable. The atomic O: N ratio (oxygen consumption: nitrogen excretion) indicated protein utilisation for B. propinqua from Lake Rotongaio but an alternation between protein and stored lip‐ids for Lake Taupo specimens. The potential contribution of nitrogen regeneration by zooplankton excretion to phytoplankton nutrient requirements was greatest during early stratification in both lakes when zooplankton biomass was greatest (up to 160% of estimated requirements in Lake Rotongaio and 214% in Lake Taupo). Regenerated nitrogen is a major source of nitrogen for Lake Taupo phytoplankton but in Lake Rotongaio other sources are more important.