Groundwater - an unknown habitat
The River Styx is
one of the five rivers in the underworld of Greek mythology and a synonym
for all subsurface waters. In the past it was unknown that these waters
are teeming with life.
Only recently it was realised that groundwater is a habitat for a diverse
fauna that evolved special features for life within the dark interstices
between the grains of sand. Refering to the River Styx the invertebrates
which spend their whole lifes in subsurface waters are called stygobionts.
They are found not only in porous groundwater aquifers of river valleys
but also in karstic areas where caves of different sizes may be found.
Stygobionts are able to penetrate into man made habitats such as wells
and slow sand filters.
The stygobionts show a remarkable set of morphological and physiological
characteristics which can be interpreted as adaptations to life in
groundwater. The most apparent insignia are the depigmented and vermiform
body and the lack of eyes. Compared with related organisms in surface
waters the stygobionts have very long lifes (15 times longer and some
even more) and all developmental stages are prolonged.Their metabolism
is reduced and the reproductive output is small compared with surface
water organisms. But due to the long life span of stygobionts the absolute
number of offspring may be similar to that of surface water organisms.
The main property of the groundwater ecosystem is the paucity
of vitally important resources. Stygobionts are able to withstand adverse
situations very well by adopting an appropriate strategy, the so called
'adversity-selection model' or A-type stArategy. They are resistant against
starvation and low oxygen conditions. Due to the lack of light no primary
production takes place in the groundwater ecosystems. As a result the
groundwater food webs are almost heterotrophic. The production depends
on the input of organic matter from the surface. Unpolluted groundwater
systems have scarce trophic resources and their biocoenoses are delineated
by simple and few trophic links. Most of the organic matter entering the
groundwater habitats is mineralised by microorganisms (e.g. bacteria,
fungi). The microbes, on the other hand, serve as food for secondary consumers
such as protozoans, nematodes and crustaceans. Predatory invertebrates
mark the end of the food web. Although differently spezialised feeding
types and/or feeding habits may be found it is important to realise that
most stygobionts are omnivores and not spezialised feeders.