The UK is a temperate country that undergoes the four seasons; spring, summer, autumn and winter. The waters surrounding the UK are also temperate and undergo a seasonal cycle too. The seasonality of the sea is linked to the annual light and nutrient cycles, that also play a role in terrestrial (land) seasonality, these seasonal affects are first seen in primary producers.
Microscopic phytoplankton cells are the main primary producers found in the sea. Phytoplankton populations in the seas surrounding the UK undergo a seasonal cycle of growth and decline. There are two periods during the year when phytoplankton growth increases and ‘blooms’ occur. A larger bloom occurs in the spring and is aptly named the spring bloom. A smaller bloom then occurs in the autumn. A range of factors control the beginning of these phytoplankton growth periods and the following decline, including light and nutrient availability, and grazing by zooplankton (little animals that drift in the ocean). In the spring, nutrients has been mixed from deeper waters and added to the sea water from rivers and washed off from the coast. It is the light availability and the strength of mixing water (during the winter the water is too turbulent and mixes the phytoplankton deeper than light can penetrate) that controls the beginning of the accelerated growth of phytoplankton and the spring bloom. Once the bloom sets in, the phytoplankton utilise the light and nutrients in the surface of the water. Zooplankton also benefit from the increase in their food source and their abundances grow as well. The nutrients begins to become exhausted slowing the growth rate of the phytoplankton. Combining this with grazing by zooplankton the bloom begins to disappear as more phytoplankton die than grow and reproduce. The next ‘season’ is then entered,which is a summer low amount of phytoplankton. By autumn, the nutrients available in the water has increased enough to allow the phytoplankton to bloom again, though in a smaller quantity. This bloom lasts until the winter light is too little to allow fast growth and the storms mix any remaining phytoplankton below the euphotic zone (the depth the sun can reach).
Phytoplankton bloom in the seas around the UK (GNN)
Why does this phytoplankton seasonality matter? Just like plants on land, phytoplankton are the basis of nearly every food web in the ocean. Different phytoplankton can thrive in different conditions, which means not only the quantity but the nutritional value of food for the primary consumers varies throughout the year. The timings off the phytoplankton blooms, in temperate seas, are often reflected further along the food web. The emergence of juvenile fish from the larval stages is often timed to coincide with the phytoplankton and zooplankton blooms. Further up the food web, seabird chicks are often born to match the increase in the population of the small fish.
A simplistic ocean food web (Seafriends.org)
Just as we are seeing less ‘traditional’ weather during our seasons, such as warmer and wetter winters, so our seas are beginning to experience differences too. The timing of the phytoplankton spring bloom is changing and this can have knock on effects to health of fish stocks, marine mammals and sea birds.
Cod (August Linnmann)
One of the more well known effects of this is the impact on the declining cod stocks in the North Sea. Cod stock stability is threatened by several factors. Overfishing, the movement of the stocks to deeper and more northern waters as the central North Sea warms, and the timing of the phytoplankton spring bloom and the juvenile cod appearance becoming disjointed. As the spring bloom has moved forward in the year, the emergence of juvenile cod has not. This means that the baby cod miss much of the nutritional phytoplankton and end up with less nutritional food, leading to a lower likelihood of surviving to adulthood.
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