Spring algal blooms are just beginning

As the day lengthens noticeably in late March and early April, the intensity of sunlight increases, and with it the temperature, signs of spring can be seen not only on land, but also in the water. The proliferation of photoautotrophic planktonic algae – phytoplankton – is taking off in aquatic ecosystems of the temperate and polar zones. A surge in their abundance, is commonly referred to as a bloom. Mass gain would not be possible without nutrient supply. Changes in the density of water and its mixing to the bottom, intensified by spring winds, cause water movement (spring circulation) and the carrying away towards the upper layers of nutrients accumulated in winter at the bottom. An additional significant source of nutrients, in the case of snowy winters, is their delivery with meltwater. As a result, the water, which is clear in winter even in relatively fertile bodies of water, becomes more turbid and takes on the color produced by the pigments found in the cells of the dominant algae.

Algae that can cope with early spring conditions belong to various taxonomic groups, such as goldilocks or dinoflagellates, exceptionally cyanobacteria (especially in shallow, fertile reservoirs after mild winters and at high temperatures in spring). However, diatoms are usually the winners in this competition. This success is assured by their lower temperature optimum and rapid growth rate. With sufficient abundance of nitrogen and phosphorus compounds in the reservoir, as well as silica (necessary for the construction of the armor characteristic of this group), and with significant turbulence of the waters (providing access to light), the rate of multiplication far exceeds the loss of population, so the abundance and biomass of algae increase. Due to the presence of fucoxanthin and other yellow-orange carotenoids in the cells, the color of the water takes on various shades of brown. However, there is no accumulation of biomass in the form of floating dross, often observed during summer blooms.

The spring water bloom is a common and natural stage in the annual succession of phytoplankton, not only in inland waters, but also in the seas and oceans, where it accounts for most of the primary production. It appears every year in the North Atlantic and is often presented as the most important biological phenomenon visible from space.

The timing and intensity of the spring bloom have consequences for the entire trophic network of the ecosystem. As a result of the intensive multiplication of algae, the amount of carbon dioxide absorbed in photosynthesis decreases and oxygenation increases, but at high intensity the light conditions for the development of underwater vegetation deteriorate.

Later in the spring, pressure from consumers (zooplankton), parasites and viruses, as well as nutrient depletion, primarily nitrogen, regulate algal abundance and the population collapses, the bloom ends and water clarity increases again. The ecosystem enters what is known as the clean water phase, which lasts until algae abundance begins to increase again with rising temperatures and nutrient availability, often culminating in a summer bloom. But that’s a topic for another story.