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 characterize microconsumer grazing and contributions of nutrient supply sources (external, cell quota, recycling) supporting phytoplankton production. Responses of net chlorophyll production rate to dilution under nutrient-saturated conditions were frequently nonlinear, indicating saturation of micrograzer feeding at low biomass levels within the dilution gradient (dilutions of < 10-30% whole lake water). Despite feeding saturation, micrograzers exerted substantial grazing pressure on phytoplankton: 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 recycling to N and P supply. In deep-water layers, phytoplankton were growing at nutrient-saturated rates, indicating that phytoplankton growth was 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. Comparison of phytoplankton responses to nutrient deletion in undiluted vs. highly diluted treatments indicated that inferences regarding frequency and magnitude of nutrient limitation, as well as identity of the primary limiting nutrient, depended on dilution.