Salinity controls on trophic interactions among invertebrates and algae of solar evaporation ponds in the Mojave Desert and relation to shorebird foraging and selenium risk
Three saline evaporation ponds formed by wastewater from a solar energy-generating facility near Harper Dry Lake in the Mojave Desert of California, USA were compared for differences in the communities of benthic and planktonic invertebrates and algae present along with avian visitation and foraging activity. Salinity of the ponds ranged from near 90 to over 200 g L-1 total dissolved solids. During the period of study (1997-1999), the lowest salinity pond averaged 98 g L-1, the intermediate salinity pond 112 g L-1, and the high salinity pond 173 g L-1. Differences in the biological communities, abundance of invertebrates and algae, and avian foraging were examined in relation to these differences in salinity. Only three aquatic invertebrate species were present in substantial numbers, a water boatman (Trichocorixa reticulata), a brine shrimp (Artemia franciscana), and a brine fly (Ephydra gracilis). An abundance of the predator Trichocorixa under low salinity conditions appeared to reduce algae-grazing Artemia, and so released phytoplankton growth, but this was observed only in surveys later in the growth season when populations were mature and had greatest potential for efficient consumption of resources. Brine fly larvae were also fed upon by Trichocorixa and were least abundant in the low salinity pond. At highest salinities where Trichocorixa could not survive, Artemia were abundant and waters were usually clear, becoming dense with phytoplankton only during the winter dormancy of brine shrimp. Intermediate salinity levels supported some water boatmen, often coexisting with dense brine shrimp and phytoplankton populations, and the greatest dry mass of benthic brine fly larvae and pupae. The high salinity pond produced abundant but small Ephydra larvae and pupae, accompanied by reduced emergence success of adult flies. Birds appeared to forage primarily on benthic brine fly larvae and were most successful in the intermediate salinity pond, possibly because lower salinity resulted in loss of this preferred prey to water boatman predation, and high salinity produced prey of poor quality. These observations suggest that reduced salinity may at times mediate a trophic cascade within a simple food chain, where an invertebrate predator may reduce primary consumers and permit enhanced algal density, but the predation control becomes uncoupled as salinity increases. In the case of the ponds studied here, there appeared to be minimal risk associated with selenium poisoning of water birds because Se was not detected in brine fly larvae or pupae and was found only occasionally in low content in the brine shrimp and corixids and mostly in locales where few birds were found feeding.