Shelf regions of the oceans

The nutrient budgets of coastal regions of oceans, like estuaries, are strongly influenced by the nature of catchment areas that supply the water, via rivers, to the sea. Concentrations of nitrogen or phosphorus may limit productivity in these areas as in other water bodies, but a further human-induced effect on the chemistry of riverwater has special significance for planktonic communities in the oceans. Today, more than 25% of the world's rivers have been dammed or diverted (for hydroelectric generation, irrigation and human water supply). Associated with damming is the loss of upper soils and vegetation through inundation, loss of soil through shoreline erosion, and underground channeling of water through tunnels. These effects reduce the contact of water with vegetated soil and, therefore, reduce weathering. Figure 18.13 illustrates the patterns of export of dissolved silicate, an essential component of the cells of planktonic diatoms in the sea, for a dammed river and a freely flowing river in Sweden. The export of silicate was dramatically lower in the dammed case. The possible ecological effects of silicate reduction to nutrient fluxes and productivity in the sea may become particularly significant in East Asia, where major rivers are being dammed at accelerating rates (Milliman, 1997).

Another important mechanism of nutrient enrichment in coastal regions is local upwelling, bringing high nutrient concentrations from deep to shallow water where they fuel primary productivity, often producing phytoplankton blooms. Three categories of upwelling have been described and studied off the east coast of Australia: (i) wind-driven upwellings in response to seasonal north and northeasterly breezes; (ii) upwelling driven by the encroachment of the East Australian Current (EAC) onto the continental shelf; and (iii) upwelling caused by the separation of the EAC from the coast. Figure 18.14 provides examples of the distribution of nitrate concentrations associated with each mechanism. Wind-driven upwellings (generally considered to be the dominant mechanism globally) are not persistent or massive in scale. The highest nitrate concentrations are generally associated with encroachment upwellings, whilst separation upwellings are the most widespread along the coast of New South Wales.

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