442 
Any factor, such as chronic turbidity that limits the availability of nu- 
trients or light will be reflected in a reduced production of phytop] ank- 
ton. If this basic food supply is reduced, starvation occurs at various 
points higher up in the food chain, and the quantity and diversity of 
marine animals that can survive in the area becomes correspondingly re- 
stricted. 
Ecosystems Under Stress 
All life on this planet is adapted to survive under the particular set of 
environmental conditions within which it normally lives and to which it has 
become genetically and evolutionarily adjusted through eons of time. Mark- 
ed deviation of an environmental factor from the normal may place a burden 
upon the biological system, a condition referred to as stress. Biological 
response to stress takes many forms, and it occurs at all levels of biolog- 
ical organizations. Mild stress or stress of short duration may produce 
little permanent biological effect, but severe stress or stress of long 
duration inevitably takes a significant biological toll. Plankton popula- 
tions are known to exhibit changes in species composition under mild stress 
(Mosser et al. 1970), and to be locally wiped out by severe stress. How- 
ever, due to the dilution capacity of large volumes of seawater, the move- 
ment and mixing capacities of turbulent diffusion and water currents, and 
the widespread distribution of most plankton species, permanent damage to 
plankton populations by dredging stress agents appears remote. 
Most nektonic species are sufficiently mobile that they may avoid areas of 
severe stress if it can,be sensed as being dangerous. Unfortunately this 
is not always the case. Subtle stress agents, such as heavy metals, which 
become concentrated up food chains, may not induce avoidance reactions, and 
these could, in sufficient concentrations, damage the nektonic popula- 
tions. 
Benthic environments, however, cannot provide for dilution and transport. 
Many benthic species are of somewhat limited distribution, and most are 
relatively immobile. Hence, widespread and long-term damage to benthic 
