Temporal and vertical dynamics of phytoplankton net growth in Castle Lake,California

The impact of nutrient additions, zooplankton grazing and light intensity o n phytoplankton net growth with depth and season was studied with five micr ocosm experiments in meso-oligotrophic, subalpine Castle Lake, California, during the period of summer stratification in June-September 1994. The incu bations (4 day) were performed at 5 m intervals from the surface to the bot tom using natural phytoplankton and zooplankton assemblages, with enrichmen ts of phosphorus and nitrogen. The phytoplankton community was only limited by nutrients in the upper 5 m (epilimnion), as indicated by change in chlo rophyll concentration. Nutrient enrichments had the greatest effect on the phytoplankton net growth in June and July. High light inhibited the phytopl ankton net growth at the surface. Low light intensities limited phytoplankt on at 20 m and below, and at the end of the growing season already around 1 0-15 m. A deep chlorophyll maximum in the hypolimnion in June-August was no t limited by either light or nutrients. The results showed variation in gra zers' impact on phytoplankton. These results suggest the importance of nutr ient limitation only in the epilimnion with light inhibition at the surface , light limitation in the hypolimnion, and varying impact of zooplankton gr azing in influencing the development of the phytoplankton in Castle Lake.