Cascading

A process termed cascading is believed sometimes to cause losses of both nutrients and planktonic organisms from neritic water. During autumn and winter, loss of heat from the sea surface may cause the shallow shelf water to become appreciably colder and denser than water at similar levels beyond the continental edge. Consequently, the heavier shelf water will tend to flow down the continental slope, i.e. to cascade, to its appropriate density level. The loss of cascading water from above the shelf must be compensated by a corresponding inflow of water elsewhere.

The effect of cascading on the fertility of shelf water must depend on the quality of the compensation water. Cooper and Vaux (1949) studied cascading in the Celtic Sea (south of Ireland and west of the Bristol and English Channels), and concluded that surface layers of the Atlantic and Bay of Biscay formed the main sources of compensation water. Because the phosphate content of this water was lower than that of the Celtic Sea at the time, cascading presumably led to a net loss of nutrients from the area. Iron and other trace elements which tend to concentrate in particles in the lower layers are probably specially liable to be swept off the shelf in the cascade. Annual fluctuations in the fertility of shelf water could be caused by variations in the volume of cascading.

The distribution of organisms could be influenced by cascading in several ways. Small planktonts carried down in the cascade would be lost in deep water. Stronger-swimming forms might ascend out of the cascade as it flows down the continental slope, and this could lead to patches of large, active zooplanktonts over deep water near the continental edge. Where submarine valleys cut into the slope, cascade streams are likely to be strongest in those channels. Cascading might also occur on the sides of submarine banks where a flat top lies near the surface. The preference of certain animals for the sides of submarine valleys, troughs and mounds during winter may be due to the relative abundance of food carried by cascading to these parts of the sea floor.

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