This sequence of four ocean seasons described above is a feature of middle latitudes where the temperature of the surface water undergoes the greatest seasonal change. In high latitudes the surface temperature does not vary much with season. Here, illumination is the dominant factor regulating productivity, and only two ocean seasons are apparent, a long winter period of poor or absent illumination and virtually no primary production, followed by a short period of very high production when the light becomes sufficiently good to enable the phytoplankton to grow. This productive period lasts only a few weeks, but during part of this time daylight is continuous throughout the 24-hour period. This makes possible a very rapid growth of a large quantity of phytoplankton, and this abundance of food allows a great increase in zooplankton. Because the rich food supply lasts only a short time, the developmental stages of zooplankton at high latitudes must be passed through rapidly. After that, illumination declines and primary production falls to zero. The phytoplankton virtually disappears, probably over-wintering mainly as spores locked within sea ice, and the zooplankton population decreases to its over-wintering level. This single short season of rapid growth soon followed by decline is a merging of the spring and autumn seasons with elimination of the intervening summer period, the biological winter being correspondingly prolonged.
In low latitudes, conditions are mostly those of continual summer. The surface water is consistently warm and well illuminated but there may be some limitation of production by shortage of nutrients, there being little vertical mixing across a strongly developed thermocline. However, production continues throughout the year and extends to a greater depth than in high latitudes, and the rate of turnover is probably rapid. The result in some tropical areas is a total annual production some 5-10 times greater than in temperate seas (Wickstead, 1968). Generally the rate of production in warm seas remains fairly uniform but there are parts where seasonal changes of wind, for example the monsoons, cause variations of water circulation and seasonal improvements in nutrient supply which are quickly followed by periods of increased production. In the Mediterranean the main season for the production of algae is November to April when there is vertical mixing. Except for a few areas, seasonal peaks in warm seas seldom exceed a tenfold increase of production, whereas fluctuations are sometimes as great as fiftyfold in temperate waters.
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