Castle Lake Limnological Research Station

In a 3-year study of Castle Lake, California, potential nutrient (N,P) limitation of phytoplankton growth occurred rapidly (within 1-4 d of ice-out). Both N and P acted as potential limiting factors to phytoplankton growth in short-term (4-5 d) bioassays. Phytoplankton responded strongly to single additions of N or P in 1990 and 1992 but weakly so in 1991. This difference was associated with low inorganic N concentrations during spring 1991. In 1990 and 1991, variation of the primary limiting element correlated with the N:P ratio df the zooplankton community; phytoplankton tended to be N limited when the zooplankton was dominated by species with high N:P ratios (Diaptomus novamexicanus and Diacyclops thomasi: N:P ratios, by mass = 10.6-12.5) but limited by P when low N:P taxa (Daphnia rosea, N:P = 4.7) dominated. However, N vs. P limitation and zooplankton elemental data for 1992 did not fit the 1990-1991 pattern and there was no correlation for the 3-year data set. A field experiment demonstrated that the inorganic N:P ratio (NH4/SRP) increased dramatically with elevated Daphnia grazing but declined significantly with increased Diaptomus; this supported the 1990-1991 -correlation between phytoplankton N/P limitation status and zooplankton community elemental ratio.