Laguna Watershed Knowledgebase

The Laguna de Santa Rosa is the focus of many management activities that seek to improve its flood-control and ecological functions. In order for management activities to succeed, their effects on the aquatic community must be both known and measurable. However, with the exception of a study performed in 1988, there is little information on the composition of the Laguna de Santa Rosa's fish and aquatic-invertebrate communities. As a result, we surveyed a number of parameters (water quality, aquatic invertebrates, and fishes) and analyzed the gut contents of fishes in August 2008 to provide baseline data for management activities, to see if the composition of the biotic communities had changed relative to the 1988 study, and to explore the trophic relationships among fishes, invertebrates, and primary producers. Water-quality measurements revealed that the Laguna de Santa Rosa is a very eutrophic waterway, although orthophosphate levels were lower in 2008 than in 1988. Invertebrate and fish communities were similar to those found in 1988. The invertebrate community was dominated by families (e.g., the Chironomidae) that are both resistant to pollution and common in still-water habitats. Similarly, the fish species caught in 2008 can withstand low dissolved oxygen concentrations and spawn on or in aquatic vegetation, suggesting that water quality and lack of riffle habitat are the major abiotic factors structuring the fish community. Gut-content analyses showed that small and juvenile fishes (e.g., western mosquitofish Gambusia affinis, fathead minnows Pimephales promelas) fed most heavily on zooplankton, while the majority of larger fishes (e.g., bluegill Lepomis macrochirus, common carp Cyprinus carpio) depended more on aquatic insect larvae. As a result, smaller fishes rely on a food source based on phytoplankton, whereas larger fishes are more dependent on food derived from aquatic plants and detritus.