Aquatic Ecology of San Felipe Lake, San Benito County, California

This two-year study examined San Felipe Lake environmental factors and food-web relationships. The lake is shallow, warm, productive, and generally well-mixed. High turbidity resulted from wind mixing of bottom sediments, carp feeding activity, and plankton blooms and excluded submerged aquatic plants. Emergent vegetation was restricted by cattle grazing. Summer temperatures reached 26-28°C throughout the water column, and dissolved oxygen was occasionally low on the bottom overnight and during windless daytime periods. Phytoplankton, fine organic matter, and detritus formed the base of the food web, supporting a zooplankton community dominated by copepods, the water column macroinvertebrate Neomysis, and several open water fish species. Fine organic matter and detritus supported benthic macroinvertebrates (chironomids and oligochaetes), which in turn supported benthic, omnivorous fish species. Carp, hitch, and Sacramento suckers were the most common fish captured by gill net. Predatory fish, including Sacramento pikeminnow, brown bullhead, and largemouth bass, formed the top of the aquatic food web, although observed abundance was low. Fish predation on juvenile steelhead was not a likely threat, and turbid water and greater relative spring depth likely limited avian predation. San Felipe Lake is too warm for steelhead summer rearing but is suitable for spring growth before and during steelhead smolt out-migration