Case studies UK

Case studies elsewhere

The deposition of organic matter (acting like silt) is increased in the vicinity of sewage outfalls and this can exert a number of detrimental influences on marine benthic algal communities (Fletcher, 1996). The sediment can:

• cover all available substrata interfering with the processes of spore (and larval) attachment and recruitment
• smother young germlings resulting in their growth and development being inhibited.
• in combination with water movement, abrasively scour surfaces settled with spores.
• (as deposits on algal thalli) reduce photosynthetic activity while at the same giving competitive advantages to species and life-forms of algae that are better adapted to life in areas of siltation

All the above-described detrimental influences of increased sedimentation on benthic communities have been well-recognised and have been offered as explanations for the decline of kelp beds off the coast of southern California. Devinny & Volse (1978) showed that sediments interfere with Macrocystis gametophyte development. A similar detrimental effect of silt on zoospore development was demonstrated for Laminaria saccharina by Burrows (1971). The kelp Saccorhiza polyschides is tolerant of sediment layers, however, when the substratum is not too steeply sloping (Santos, 1993). Changes in algal zonation patterns and depth distributions as a result of increased sediment loading have been reported (Fletcher, 1996). For example, a reduction in the kelp depth range has been reported in the vicinity of sewage outfalls with plants generally restricted to shallow water and very few new kelps present under the canopy of the older plants.

There are sediment traps at Skomer and in the Menai Straits, where long-term monitoring programmes have been initiated. The results of these continuing studies are not yet available.

Schroeter et al. (1993) examined the ecological effects of the cooling water discharge from a coastal nuclear power plant in southern California on Macrocystis forest. Relative to control populations, there were statistically significant reductions in density of snails, sea urchins, and starfish, all of which occurred primarily on rocky substrata. All of the reductions were larger at the impact station about 0.4 km from the discharge than at a second impact station 1.4 km away. The most plausible mechanisms for the declines seem to be linked to the turbidity plume created by the power plant and the resultant increase in suspended inorganic and organic materials (+46% at the impact site nearest to the discharge). Any associated flux of fine particles on rocks would have deleterious effects on many of the species inhabiting the hard substrata. Populations of two filter-feeding species, a gorgonian coral and a sponge, showed relative increases in density.

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