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 plankton can be found within the water column all year round, there are time during the year were the population can explode, and it is this increase that know 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. Its suggested that the increased concentration of nutrients is due to the mixing of layer depths and the build-up of nutrients due to the restricted planktonic growth in the winter months, thus creating the perfect conditions.

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 of the primary production. 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 is 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 of 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 to urban or agricultural run offs.

Although, planktonic blooms provided a source nutrients for many fauna throughout the water column. Unnatural planktonic blooms caused by pollution, could have detrimental effects to ecosystems.

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