Seasonal variations in temperature, illumination and availability of nutrients in the surface layers of the sea impose a pronounced seasonal cycle in production and composition of the plankton between winter and summer, particularly in temperate latitudes (Figure 5.11). Both geographical and year-to-year patterns of fluctuation have their origins within the dynamics of the seasonal cycle. For example, continuous plankton recorder studies have shown that the copepod Centropages hamatus in the North Sea is most abundant in the south and in coastal areas, and the seasonal cycle in the west is about a month later than in
the south and east. This pattern can be related to hydrographic features (Colebrook et al, 1991).
5.4.1 Seasonal features of middle latitudes
The four seasons of the sea in middle latitudes have the following general features (Bogorov, 1960).
A period when the surface of the sea is losing heat to the atmosphere, consequently the surface water is becoming progressively colder. Convectional and wind mixing extends deep into the water column, bringing nitrates, phosphates and other inorganic nutrients to the surface. By late winter the surface water reaches its annual minimum temperature and its annual maximum of inorganic nutrients. Short day-length and the low angle of the sun results in poor or absent illumination within the water. The quantity of both phytoplankton and zooplankton are minimal, except at the end of winter when many animals start their spawning, so timing their reproductive period that their larvae have the advantage of the rapidly increasing food supplies that are shortly to follow in spring.
In neritic areas from late autumn through to the end of winter the water column is often virtually fully mixed throughout its entire depth, surface and bottom temperatures being almost the same. Beyond the continental edge convectional mixing in winter usually extends to between 100 and 200 m, depending on the temperature of the deeper levels.
A period when increasing insolation is causing the surface water temperature to rise, the water column is becoming stabilized by thermal stratification, illumination is increasing and the critical depth becomes lower than the zone of wind mixing. The concentration of nutrients in the surface layer is initially high, but begins to decrease sharply due to rapid absorption by the phytoplankton which now begins to multiply very quickly in this favourable combination of temperature, light, nutrient supply and stable water column. The rate of primary production soon becomes very high and there is an enormous increase in the quantity of phytoplankton, especially diatoms, which soon reach their greatest abundance for the year (the spring diatom peak). The zooplankton undergoes a more gradual increase, but during late winter and early spring it becomes augmented by the spawning of innumerable marine animals, contributing great numbers of eggs and larvae which by late spring have developed to more advanced larval or juvenile stages. As the zooplankton increases in amount, the quantity of phytoplankton declines rapidly.
A period during which the surface water is warm and well illuminated. The concentration of inorganic nutrients at the surface is now low because they have been taken up by the phytoplankton, and there is little replenishment from deeper water because vertical mixing is restricted by a sharp thermocline. The dinoflagellates are at their greatest numbers, but the phytoplankton as a whole has declined in amount and primary production is reduced due to grazing by zooplankton and shortage of inorganic nutrients. Diatoms are often quite scarce at this time. During midsummer the numbers and production of phytoplankton are often greatest within the discontinuity layer, usually at some 15-20 m, where nutrients are to some extent available from the deeper mixed layers. The zooplankton, mainly holoplankton, now reaches its greatest amount for the year, and after that diminishes. The concentration of DOM (see Section 4.3.2) is usually highest during the summer.
During this season the surface water is cooling and illumination is becoming less. The deeper layers are still getting slightly warmer until eventually the thermocline breaks and convectional mixing is re-established. This leads to rapid replenishment of nutrients in the surface layer, and a consequent increase in primary production, both diatoms and dinoflagellates becoming more numerous. This autumn increase of phytoplankton is always less than the spring peak. It is often followed by a slight increase in zooplankton, but these increases are short-lived. Vertical mixing disperses much of the phytoplankton below the critical depth and the size of stock falls quickly. As temperature and illumination decrease further, the quantities of both phytoplankton and zooplankton gradually reduce to their winter levels, and their over-wintering stages appear.
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