Lake Elsinore is a shallow, polymictic lake in the southwest corner of Riverside County in Southern California. It has a history of poor water quality (algal blooms, low DO, and periodic fish kills) and was listed on the State of California's 303d list for impairments due to nutrients and other factors. Water column measurements and nutrient concentrations have been closely monitored since 2000. Less is known about the biological condition of the lake. A 1-yr biological monitoring study was conducted in 2010 to quantify the abundance, diversity, and richness of phytoplankton, zooplankton, and benthic invertebrates in Lake Elsinore. This report summarizes the results of that study and also includes findings from an experiment to determine the response of the phytoplankton community to nutrient inputs.

The phytoplankton community exhibited very strong variation in Simpson's diversity (~0.2 -0.8) and biomass (~2,000 -30,000 mg m-3) during the study period. The lowest diversities and highest biomasses occurred during the 2010 summer bloom of Pseudanabaena limnetica. The highest diversities and lower overall phytoplankton biomasses occurred during winter and spring of 2010 when green algae and/or diatoms were a prominent component of the biomass.

The zooplankton community was comprised primarily of smaller zooplankters with rotifers dominating summer through fall and cyclopoid copepods being more prominent during cooler seasons. Large cladocerans were scarce and only found early in 2010 after heavy rainfall caused Canyon Lake to spill over into Lake Elsinore.

The benthic community was the most impoverished, consisting predominantly of organisms (Chironomids and Ilyocryptus) with red hemoglobin-like pigments that aid in the binding and transfer of oxygen. This reflects the low DO concentrations in the bottom waters and sediments of Lake Elsinore during much of the year. Invertebrate densities varied strongly between sites and sampling dates and were consistently higher at the shallower sites with higher DO and lower organic carbon concentrations in the sediments.

Results from the nutrient addition experiment suggest that phytoplankton were mainly N-limited, but that the diatoms were limited mostly by P. They also suggest that an excess of N relative to P contributes to the formation of Pseudanabaena limnetica blooms.