Relationship between Phytoplankton Volume and Rainbow Trout—Daphnia pulex Interactions after Phosphorus Inactivation, Medical Lake, Washington

ABSTRACT

Fisheries and limnological investigations in post-restoration Medical Lake, Washington, suggest that establishing a successful rainbow trout Oncorhynchus mykiss fishery depends on the maintenance of a healthy Daphnia pulex population and that phytoplankton volume is related to interactions between rainbow trout and D. puluex. As rainbow trout abundance increases, D. pulex standing crop decreases. This change results in an increase in phytoplankton standing crop. Multiple regression analyses on eight years of data was used to determine the statistical relationship between rainbow trout—D. pulex interactions, total phosphorus levels, and phytoplankton volume. Seasonal variation in phytoplankton volume correlated more closely to rainbow trout—D. pulex interactions than did monthly variations in algal biomass. Accounting for the lag in response time between the independent variables and phytoplankton volume improved the percentage of explained variance. The results indicate that the variance in seasonal phytoplankton standing crop is primarily a function of rainbow trout—D. pulex population dynamics.