Salinity and fish effects on Salton Sea microecosystems: benthos

The Salton Sea, the largest lake in California, has a surface elevation 69 m below sea level which is maintained predominantly by the balance of agricultural runoff and evaporation. The lack of outflowing streams is resulting in a gradual buildup of salts in the lake, increasing the salinity. A 15 month microcosm experiment was conducted to determine the effects of salinity and tilapia ( Oreochromis mossambicus) on an assemblage of benthic and planktonic Salton Sea algae and invertebrates. This article reports the responses of the benthic invertebrates. Microcosms (312 l fiberglass tanks) were set up without tilapia at 30, 39, 48, 57, and 65 g · l-1. Additional microcosms were set up with tilapia at 39 and 57 g · l-1. In the absence of fish Gammarus mucronatus dominated the benthos at the lower salinities, and Trichocorixa reticulata and the larvae of Ephydra riparia were most abundant above 48 g · l-1. The most abundant meiofaunal species included the harpacticoid copepod. Cletocamptus deitersi, three nematodes, the rotifer Brachionus plicatilis, ciliates, including Condylosoma sp. and Fabrea salina, two foraminiferans including Quinqueloculina sp., and a large flagellate. Most meiofaunal species responding to salinity were most abundant at 65 g · l-1, especialy after 6 months when Gammarus dominated the lower salinities. The tilapia reduced the abundance of macrofaunal species, especially at 39 g · l-1, and generally increased the abundance of meiofaunal species and ciliates. The microcosm benthic macro- and meiofaunal communities were most likely structured by Gammarus, salinity and tilapia. Gammarus reduced the other species by predation and changing the detritus from an algal base to a fecal pellet base. Gammarus was itself reduced by tilapia and by reduced reproductive success above 39 g · l-1. More species were therefore able to compete at higher salinities and in the presence of tilapia. Tilapia also affected the benthos by depositing loosely packaged fecal material which may support more meiofaunal species than either the robust Gammarus fecal pellets that were abundant at 39 g · l-1 or the algae-fecal pellet mix at 57 g · l-1.