MICROCONSUMER GRAZING AND SOURCES OF LIMITING NUTRIENTS FOR PHYTOPLANKTON GROWTH - APPLICATION AND COMPLICATIONS OF A NUTRIENT-DELETION DILUTION-GRADIENT TECHNIQUE/

A series of eight nutrient-deletion/dilution-gradient experiments was performed in Castle Lake during summer 1993 to quantify and characteri ze microconsumer grazing and contributions of nutrient supply sources (external, cell quota, recycling) supporting phytoplankton production. Responses of net chlorophyll production rate to dilution under nutrie nt-saturated conditions were frequently nonlinear, indicating saturati on of micrograzer feeding at low biomass levels within the dilution gr adient (dilutions of < 10-30% whole lake water). Despite feeding satur ation, micrograzers exerted substantial grazing pressure on phytoplank ton: microconsumer grazing coefficients (0.05-0.22 d(-1), mean:0.14 d( -1)) exceeded previous measures of crustacean grazing in this system. Nonlinear feeding kinetics required that piecewise multiple regression be used to estimate contributions of external, cell quota, and recycl ing to N and P supply. In deep-water layers, phytoplankton were growin g at nutrient-saturated rates, indicating that phytoplankton growth wa s more likely light limited. In the epilimnion, recycled and internal sources were important for both N and P in different experiments, but the importance of various supply sources did not systematically differ for N and P. Tn epilimnetic experiments, there was strong experiment- to-experiment variation in contributions of recycling sources of N and P, suggesting that resupply of N and P via grazers was decoupled. Com parison of phytoplankton responses to nutrient deletion in undiluted v s. highly diluted treatments indicated that inferences regarding frequ ency and magnitude of nutrient limitation, as well as identity of the primary limiting nutrient, depended on dilution.