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 agric
ultural runoff and evaporation. The lack of outflowing streams is resulting
in a gradual buildup of salts in the lake, increasing the salinity. A 15 m
onth microcosm experiment was conducted to determine the effects of salinit
y and tilapia (Oreochromis mossambicus) on an assemblage of benthic and pla
nktonic Salton Sea algae and invertebrates. This article reports the respon
ses 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 3
9 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 C
ondylosoma sp. and Fabrea salina, two foraminiferans including Quinquelocul
ina sp., and a large flagellate. Most meiofaunal species responding to sali
nity were most abundant at 65 g.l(-1), especialy after 6 months when gammar
us dominated the lower salinities. The tilapia reduced the abundance of mac
rofaunal species, especially at 39 g.l(-1), and generally increased the abu
ndance of meiofaunal species and ciliates.
The microcosm benthic macro- and meiofaunal communities were most likely st
ructured by gammarus, salinity and tilapia. Gammarus reduced the other spec
ies by predation and changing the detritus from an algal base to a fecal pe
llet base. Gammarus was itself reduced by tilapia and by reduced reproducti
ve 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 mo
re meiofaunal species than either the robust Gammarus fecal pellets that we
re abundant at 39 g.l(-1) or the algae-fecal pellet mix at 57 g.l(-1).