Population dynamics of benthic invertebrates at Walker Lake (Nevada) under changing lake levels and potential food web alteration

Authors: Ian D. Bell, David B. Herbst, R. Bruce Medhurst, Scott W. Roberts

Year: 2012 (xliv)

Abstract

Nearly one hundred and fifty years of agricultural diversions have dropped Walker Lake, Nevada over thirty vertical meters, and drastically increased the total dissolved solids levels content in the lake. The benthic invertebrates of Walker Lake are an important part of the food web that sustains populations of native fish, so understanding invertebrate population dynamics as salinity and lake level changes is vital to understanding the food web linking nearshore benthic habitat to the pelagic fish and migratory water birds. We studied the population dynamics of the primary benthic invertebrates of Walker Lake: the damselfly Enallagma clausum, the Chironomid midges Cricotopus ornatus and Tanypus grodhausi, the alkali fly Ephydra hians, and the dytiscid beetle Hygrotus masculinus. Over the past five years, we regularly sampled cobble rock substrates from nearshore littoral habitat to determine benthic invertebrate densities. We also conducted lake bottom sampling using an Ekman Dredge, used stable isotopes of nitrogen and carbon to construct food webs for the lake ecosystem, and surveyed aquatic vegetation beds where many invertebrates reside. Population trends over the period of 2007 to 2010, as the lake declined 4 meters and lost 20% of its volume, showed a decline in the abundance of the nearshore midge Cricotopus, little change in the abundance of damselfly Enallagma, and an increase in the salt-tolerant alkali fly, Ephydra hians. After declining to a historic low lake level in 2010, the lake rose almost four feet in 2011 following a winter of >50% above average Sierra snowpack, which corresponded to an increase in peak densities and biomass of both Enallagma and Cricotopus as lake levels came up and salinities declined. Stable isotope ratios from samples collected in 2010 and 2011 show that the primary food source of the Tui chub, Siphateles bicolor appears to be the damselfly Enallagma. Damselfly nymphs in turn feed mainly on larvae of Cricotopus, and large nymphs appear also to feed on smaller nymphs. Lake levels have steadily declined since 2011. Walker Lake is currently at a historic low and salinities are approaching 20 g/L. As the lake level continues to change, documenting past, present and future shifts in the benthic invertebrate community will show how the food web is altered, the potential for supporting recovery of Walker Lake fish and birds, and contribute to our understanding of the limits and transitions of biological communities in desert lake ecosystems.