Seasonal Plankton Cycle: Productivity, Grazing & Vertical Mixing
Phytoplankton production follows seasonal cycles that differ according to geographical regions. As phytoplankton production is largely determined by nutrient availability, wind action plays an important role in primary production in the oceans, as it not only mixes surface waters with deeper nutrient rich waters, but also influences distribution of phytoplankton, which are carried in currents produced by strong wind action.
In the north Atlantic ocean, strong winter winds mix the surface waters with deeper nutrient rich waters, destroying the thermocline and bringing nutrients up to the euphotic zone. In spring phytoplankton flourish due to the availability of nutrients and the increase in sunlight. In summer the winds are weaker, allowing phytoplankton to take full advantage of the available light and nutrients, which they rapidly deplete. The nutrients are not replenished from below as the winds are weak, and mixing is minimal, and the reduction in nutrient availability eventually inhibits phytoplankton growth. But, as autumn approaches, wind activity increases, mixing the surface waters with the deep nutrient rich waters, and nutrients are brought up to the euphotic zone and made available to phytoplankton, resulting in a second peak in phytoplankton production. However, with the increased mixing of surface and deep waters, phytoplankton are carried out of the euphotic zone into deeper water where they soon die off as they cannot photosynthesize in the absence of light. Nutrient depletion is the primary cause of the reduction in the first peak of phytoplankton growth, while vertical mixing is the primary cause in the reduction of the second peak in phytoplankton growth, as both nutrients and sunlight are critical for phytoplankton production. Zooplankton graze on the phytoplankton, and thus, their numbers fluctuate in accordance with food availability, and they also play a large role in reducing the phytoplankton in each of the peaks.
Upwelling: Effect on Productivity & Fisheries
In areas that experience strong offshore or cross-shore winds, the surface waters are displaced, and are replenished with nutrient rich waters from the ocean depths. Areas that experience upwelling of cold nutrient-rich water, such as the Arctic, Antarctic, and the west coasts of Africa and America, also experience high productivity, often resulting in dense blooms of phytoplankton. Following a similar trend as above, as the winds drop, an upper layer of sunlit water, rich in nutrients, allows the phytoplankton cells to multiply. As their numbers increase, the phytoplankton cells, pigmented with chlorophyll that is essential for photosynthesis, reduce the amount of light that is able to penetrate the euphotic zone, thus making the layer that they are able to actively grow in, shallower. While the bloom is at its peak, the phytoplankton are grazed on by zooplankton, which in turn are preyed on by carnivorous zooplankton, invertebrates and fish. Nutrients in the euphotic zone are depleted within a week, whereupon the remaining phytoplankton cells that have not been grazed, sink and die off due to lack of nutrients in the upper layers, or lack of light in the deeper layers where nutrients are available, allowing sunlight to penetrate to greater depths once again. This is usually followed by another upwelling event injecting nutrients into the euphotic zone to fuel a new phytoplankton bloom. This continuous stream of productivity sustains zooplankton, invertebrates, fish, and higher predators such as seabirds and sea mammals, which establish large breeding colonies around areas of upwelling as it provides abundant resources essential to successfully raise their young. These are also important areas for fisheries, providing an important source of protein for human populations around the world.