Loss rates from a lake phytoplankton community1

Measurements of primary productivity and phytoplankton biomass in Castle Lake are presented for an 8‐month period. Primary productivity and phytoplankton carbon content under a square meter of lake surface exhibit similar seasonal trends and achieve maximum values in July. Phytoplankton loss rates are calculated by subtracting the rate of change of carbon content from the primary productivity, and specific loss rates are estimated by dividing loss rates by phytoplankton carbon. Specific loss rates range from more than 0.80 day −1 in May to less than 0.20 day −1 in midsummer and in December. Loss rates in the spring cannot be attributed to water transport, sinking, or grazing. Cell mortality and decomposition when environmental tolerances are exceeded may be significant causes of phytoplankton loss in the lake.