by Dave Grant
"He will watch from dawn till gloom
The lake-reflected sun illume" (Shelley)
Solar Lake, on the eastern shore of the Sinai Peninsula, is
a natural solar collector with peculiar hydrographic, chemical,
and biological properties. This small hypersaline pond is 16
feet deep and became isolated from the Red Sea as littoral sediments
closed off an embayment between two rocky headlands.
In this subtropical climate of 320 or more cloudless days per
year, evaporation is almost continuous and isolated waters are
quickly lost to the atmosphere. Most precipitation here is virga,
evaporating before it hits the ground. Conditions in Solar Lake
concentrate the already high-salinity waters that infiltrate
through the pebble and cobble barrier between it and the Gulf
of Aqaba (Eilat).
By gaining and storing solar energy, and losing water through
evaporation, Solar Lake becomes stratified in the winter into
extremes of temperature and salinity. The upper layer temperature
is 20ºC (68ºF), somewhat cooler than the adjacent waters
of the Gulf. Salinity can reach 80 ppt (more than twice the world
ocean average).
At night the top meter of surface water loses heat to the cold
desert air but insulates the lower layer. The insulated lower
layer continues to gain solar energy each day and to accumulate
heavy brine from above. Temperatures as high as 60ºC (140ºF)
and salinities of 180 ppt have been recorded.
In the summer, evaporation from the surface exceeds the infiltration
rate of seawater and as the temperature rises the density layering
disappears. This results in a turnover of the water mass. The
homogenous mixing reduces the heat-storing capacity of the lake,
but the resulting mass at 35ºC (95ºF) and salinity
of 100 ppt still ranks as some of the hottest and saltiest surface
water on the planet.
As you might expect, these environmental extremes severely limit
the types of organisms that can survive here. But with nature
as vigorous as it is, some creatures have evolved unique strategies
to thrive in this thermal and osmotic battleground. The shallow
sides of the lake support a 40-inch thick carpet of cyanobacteria,
sulphur bacteria, and diatoms. These creatures combat the osmotic
stress without accumulating toxic salts by producing organic
solutes like glycerol inside their cells. Seasonal growth of
the mat results in an annual accumulation of about 1/25 inch
of carbonate-rich biogenic sediments; inferring that the pond
first formed about 1,000 years ago.
This place must be like Hades
to most invertebrates, but a few species have adapted, including
beetles, copepods, flatworms, and ciliates. Toward the center
of the pond the mat disappears and is replaced by a precipitate
crust of gypsum and anaerobic mud.
The only open water creatures are brine shrimp, restricted to
the upper layer. Below it the hot hypolimnion is anoxic, rich
in deadly hydrogen sulfide, and lacks higher organisms, but is
ideal for sulphur bacteria.
Humans, as is their nature, have experimented with the solar
pond concept. In Israel, researchers exploiting the temperature
extremes in large scale experimental solar ponds, are operating
mini-OTEC (Ocean Thermal Energy Conversion) turbines to generate
electricity. An engineer in New Mexico has patented a more efficient
gel-layered pond that can accumulate enough heat energy to almost
boil water after a few months of operation. In New Jersey there
has been some work on solar ponds to supplement domestic hot
water needs.
Heliothermal ponds may become an attractive and reliable source
of energy even in temperate regions, as our future needs evolve.
Even the lowly algal inhabitants growing in Solar Lake may prove
to be a source of commercially valuable chemicals if culture
and extraction methods are developed.
NOTE: Grant worked on aquaculture projects in the Mideast
in 1980.
Vol. 16, Number 1: August 1986
Something to think about:
- Describe the conditions responsible for the high salinity of Solar Lake and speculate where else similar bodies of water may form.
- Calculate the approximate age of this lake based on the rate of accumulation of the biogenic mat and its present thickness.
- What adaptations might the invertebrate inhabitants have developed to survive the salinity and temperatures of Solar Lake?
- Design and describe the adaptations of a vertebrate you might
"invent" to withstand the conditions of Solar Lake
or similar bodies of water. Draw your animal and explain its
life style and life cycle.