PHYTOPLANKTON BIOMASS AND PRODUCTIVITY IN 2 OLIGOTROPHIC LAKES OF SHORT HYDRAULIC RESIDENCE TIME

Phytoplankton biomass and productivity at two sites in the newly impou nded Lake Dunstan and, upriver, in Lake Wakatipu (Frankton Arm) showed ranges throughout 1 year of 0.2-2.0 mg chlorophyll a m(-3) and 0.9-6. 0 mg C m(-3) h(-1) (maximum rate of carbon fixation). Peaks in phytopl ankton abundance occurred in spring and summer in both lakes, but rela tionships of biomass and productivity vs light and nutrients differed between the lakes. In Lake Dunstan, P-vs-I parameters indicated that p hytoplankton were photo-acclimated to prevailing light conditions. In the Frankton Arm, P-vs-I parameters indicated that phytoplankton were poorly adapted to light conditions. Phytoplankton biomass and producti vity were only depressed at the shortest observed hydraulic residence times (< 3 days). Phytoplankton biomass and productivity in Lake Dunst an were predicted from dissolved and particulate nitrogen concentratio ns and mean mixed-layer light intensity. Annual mean chlorophyll a lev el was predicted satisfactorily using a published empirical model base d on total phosphorus and inorganic suspended solids; a published dete rministic model developed for reservoirs was tested and did not predic t chlorophyll a concentrations accurately. The results are discussed i n the context of the paradigm of trophic upsurge which is commonly obs erved in newly impounded reservoirs.