Studies on a saline lake ecosystem

Abstract

Saline lakes are in many respects simple natural models of ecosystem structure and function. In this regard, a descriptive study was made of a saline crater lake over 3 years (1968-1970). Lake Werowrap is associated with the Red Rock scoria cone, western Victoria. It is small in area (~21 .6 ha), shallow (~1.4 m), and of variable salinity (23-56 g/l.). The local climate is typically cool temperate, but evaporation exceeds precipitation over most of the year. Meteorological data were used to compute the hydrological and analytical energy budgets for 1969 and 1970. The main determinant of variations in lake salinity and volume was the evaporation/rainfall balance, although groundwater seepage also was implicated. The average rate of water renewal was 0.725 times per year. The annual heat budgets of the lake (mean, 1712 g cal/cm² and the lake sediments (mean, 1218 g cal/cm²) were very small fractions of the total energy fluxes within the system. Energy fluxes were governed mainly by incident radiation, evaporation, back-radiation, and sensible heat transfer. Chemically, the lake water is highly alkaline (pH 9. 8), predominantly of sodium "chlorocarbonate" composition, and showed stable ionic proportions despite salinity fluctuations. Inorganic nitrogen and phosphorus concentrations were high and influenced seasonally by a gull population breeding on a small islet in the lake. Partial information on trace elements was obtained. Optical measurements indicated rapid light attenuation due to dissolved organic material and dense plankton populations. The trophic structure of the ecosystem was taxonomically very simple. The blue-green alga Anabaena spiroides predominated in summer and autumn 1969, but subsequently was displaced by the dinoflagellate Gymnodinium aeruginosum and a blue-green alga tentatively identified as Chroococcus. The change in species composition probably was due to a progressive increase in salinity which occurred during the study. G. aeruginosum, unlike the other species, persisted throughout the seasonal cycle, although changes were observed in the size and structure of the species. Summer and autumn generally were the seasons of most intensive photosynthetic activity. The total phytoplankton production in 1970, measured by the carbon-14 method, was 435 g C/m², evidently the highest figure recorded for any unpolluted lake. Carbon-14 and oxygen productivity data showed good correspondence only in summer, due to effects of non-photosynthetic respiration. Changes in cell and pigment concentrations also were monitored. Radiation and temperature appeared to be the most influential factors governing primary productivity. Inorganic nitrogen was the only chemical constituent to show significant depletion, and may also have been important. Considerable bacterial activity was indicated by high rates of "dark" carbon-14 assimilation. Periphytic productivity, although not measured, was likely to have been significant only in spring 1968. Attention was given to the population dynamics of the rotifer Brachionus plicatilis, the dominant zooplankter, which attained peak densities in summer and autumn. Variations in rotifer population parameters were attributed mainly to salinity, temperature, and food supply. Other zooplankton species included the rotifer Hexarthra jenkinae, two ciliates, and larvae of aquatic and terrestrial insects, notably the dytiscid Necterosoma penicillatum, which occurred around the lake margins. The benthic community consisted solely of the chironomid Tanytarsus barbitarsis. Incomplete productivity data suggest that the lake had the most productive benthic community yet recorded. Caloric measurements for some species in the lake are recorded.