Despite their small size in both areal extent and volume,
the waters over continental
shelves are usually rich in nutrients, which in turn make them
among the most biologically productive areas of the oceans. Accordingly, about 90%
of the world's fisheries
production is harvested over the continental
shelves.
One of the reasons for this higher productivity is the increased nutrient
loads via runoff from the continental
landmasses (mostly by rivers), however many shelf areas
receive significant nutrients from upwelling
of deeper ocean waters.
Because continental
shelves
are usually less than about 200 meters deep, they tend to be less vertically stable, which also
brings more nutrients into the euphotic
zone. The algal cells over shelf areas
are often larger diatom
and dinoflagellate
species, rather than the smaller picoplankters
and nanoplankters
typically found in oceanic waters.
Because continental
shelf waters are so shallow, most of the biomass
produced in the water
column does not have far to sink before it is intercepted by the bottom and the benthic
biota.
There is therefore considerably more trophic
interaction between the benthic
and water column communities
compared to
the deeper oceanic areas, where the great distance to the ocean floor effectively
separates the pelagic
and benthic
communities. Continental
shelf waters also tend to have
food chains with fewer trophic
levels, and on average
support larger fish.
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