Plankton are microscopic organisms that are found at the bottom of most marine food chains. They lack the strength to freely swim and are therefore passively transported by the oceans currents. Plankton are typically split into two different groups; phytoplankton and zooplankton. Although they can be found within the water column all year round, there are times during the year when the population can explode and it is this increase that is known as a planktonic bloom. These blooms can be so large that it can be seen from space. So much so that satellites are often used to calculate concentration and monitor temporal biomass changes.

What causes these blooms?

There are three main drivers for phytoplankton growth, the availability of sunlight for photosynthesis, the abundance of nutrients for bio-chemical reactions and the stability of the water column. When all three of these factors are met, phytoplankton can reproduce to great amounts. With this, it is found that there is also an increase in zooplankton, due to the fact zooplankton feed upon the phytoplankton. These blooms most commonly occur within temperate and polar regions during spring. Within the Atlantic, studies have previously shown that phytoplankton blooms occur when temperatures reach around 9-10 °C, typically in late April/May. During Winter months the water column is stratified, meaning it is divided into two layers, the surface and the bottom layers. The bottom layer, having restricted access to sunlight, cannot sustain photosynthesis and this allows for the build up of nutrients within this layer. On the other hand, the surface layer has an abundance of sunlight, yet lacks the the resupply of nutrients from the deeper ocean. Its suggested that planktonic blooms occur due to these two separate water layers mixing within the spring months, allowing for the greater amount of primary production.

Satellite image of Chlorophyll distribution

What’s significant about these blooms?

These blooms provide most of the primary production for the year, with the small variations in phytoplankton populations throughout the rest of the year providing the rest. With such a large amount of available food, a variety of fauna and even mega fauna, like gray whales, migrate to these blooms annually. However, the plankton doesn’t only supply nutrients to the surface, but also the deep sea in the form of marine snow. When the surface layers run out of nutrients, it can no long sustain such a large population, meaning, most of the plankton die.  Their bodies begin to sink towards the bottom, this is what is known as marine snow. This process transfers nutrients that are stored within the bodies of the plankton to the sea floor, where other organisms feed on them, recycling the nutrients. Without this transfer many organisms, such as deep sea corals, would struggle to live within the deep sea.

The Killer Bloom-

Although most planktonic blooms are good there are some that can cause significant harm to ecosystems and even humans. These are referred to as harmful algal blooms or sometimes known as ‘red tides’. These occur when toxin producing algae grow out of control, releasing large amounts of their toxins into the water column. These toxins can kill fish, make shellfish dangerous to eat and even make the surrounding air difficult to breathe. One of the most well know harmful algal blooms occurs off Florida’s gulf coast each summer. The algae that causes these ‘red tides’ is called Karenia brevis. Although there is no direct link between nutrient pollution and the harmful algae bloom that occurs every year off Florida, most of these blooms that occur around the world are caused by nutrient pollution that’s associated with urban or agricultural run offs.

Although planktonic blooms provided a source of nutrients for many fauna throughout the water column, unnatural planktonic blooms caused by pollution, can have detrimental effects on ecosystems. An example of this can be seen at the outlet of the Mississippi river. The run off from farm land and urban areas have run into the ocean causing an increase in nutrients. The increased amount of nutrients caused an rise in primary production. However, the primary production within the water column used up a large majority of the oxygen creating a hypoxic dead zone, were very little life can live.

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