Abstract:
The influence of natural short-term fluctuations in
environmental parameters on three components of transient
benthic invertebrate community structure: abundance of
individuals and species, biomass of individuals, and
species diversity, were investigated in this study. The
effect of low dissolved-oxygen on transient benthic
community structure was studied with samples from
Golfo Dulce, an intermittently anoxic basin off the
west coast of Costa Rica and the Posa de Cariaco, an
anoxoic trench off the north coast of Venezuela.
Periodic fluctuations in oxygen concentration were
accompanied by a community numerically dominated by a
single polychaete species and low species diversity.
As the frequency of fluctuations in oxygen concentration
decreased, the number of species and individuals in
the community increased with a corresponding increase
in species diversity.
In contrast to fluctuating oxygen conditions which
eliminated many species from the community, fluctuating
amounts of suspended matter in the bottom water allowed
one species to proliferate while maintaining the total
species list length. High rates of terrigenious
sedimentation occurring naturally off the Spanish
Sahara coast produced conditions which apparently
hampered the feeding mechanisms of a spionid
polychaete. Further offshore, where the diversity
should be expected to increase, the spionids were able
to flourish. The result was greater numerical abundance
and biomass offshore and a lower transient diversity
value. Results of simulation of catastophic burial by
in situ burial of small isolated portions of Buzzards Bay
sediment indicated that sedimentation rates recorded off
Spanish Sahara would not eliminate species by burial.
However, the small size of the organisms found off
Spanish Sahara is probably a result of the constant
expenditure of energy for escape.
In regions of fluctuating environmental conditions,
diversity values are low, principally because of dominance
by a single species. Increasingly stable conditions, even
though stressful, result in a more even distribution of
individuals among the species present and a correspondingly
high transient value.
