Studies on a saline lake ecosystem
Authors: Walker KF 
Published: November 2005

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.