Discussion
The amount of plankton to be found in the ocean is not determined by .................
Answer: D
Of the four factors, length of day and night is the one that does not decide how much plankton the ocean holds.
Plankton, above all the plant plankton at the base of the food chain, is limited by light and by nutrients. Depth sets how far sunlight reaches, so plankton is concentrated in the upper, lit layer of the water. Ocean currents carry nutrients and set the pattern of upwelling, which is why cold nutrient-rich waters are the richest plankton waters. Temperature and salinity control which species live in a water mass and how fast they grow.
Day length changes only with season and latitude and acts mainly by altering the light received, which depth and currents already govern, so among the options it is the weakest control and the one the key selects.
Note that day length does influence seasonal plankton blooms in high latitudes, so the item is a relative one: the key reads 'not determined by' as 'least determined by'.
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