Castle Lake Limnological Research Station

We measured the concentrations, as well as lake-wide amounts, of nitrogen (N) and phosphorus (P) in dissolved, seston and zooplankton pools throughout the water column of Castle Lake, California, during summer, 1991. This allowed us to determine the stoichiometric ratios of important elements in each pool (C:N, C:P, N:P) as well as for the entire lake. Dissolved and seston pools were the predominant storage compartments for both N and P; zooplankton never contained >5% of N or 10% of P lake wide. However, by late summer, the concentrations of P in seston and in zooplankton were similar in the upper portions of the water column, suggesting that changes in food web structure that alter zooplankton biomass and community composition (and hence elemental storage in the zooplankton) may produce significant shifts in nutrient storage among pelagic pools. Lake-wide levels of dissolved N were largely constant over the study period; however, lake-wide dissolved P increased. These dynamics suggested that the Majority of nutrients stored in dissolved pools were unavailable for phytoplankton growth. N:P and C:P ratios indicated that Castle Lake phytoplankton became severely deficient in P during the course of our observations. These ratios also greatly exceeded recently reported threshold values for elemental constraints on growth and reproduction for several species of zooplankton. The ratio of N to P in the zooplankton pool was relatively constant and consistently lower than that in the sestion. As a result, the predicted N:P ratio of zooplankton-regenerated nutrients exceeded the N:P ratio of the seston, implying that zooplankton nutrient regeneration further skewed N and P supply ratios, and potentially enhanced P limitation of phytoplankton in Castle Lake.