The chlorophyll-α content of a water sample indicates the amount of algae contained within it

Chlorophyll or chlorophyll-α is a green pigment used by phytoplankton and algae in photosynthesis. Its concentration reflects the levels of phytoplankton present in the water. Indeed, the total amount of phytoplankton in water is most commonly measured by analysing the chlorophyll-α content of a water sample.

The average chlorophyll concentration in the open sea during the summer is 2–3 µg/l and 3–6 µg/l, respectively, in the Gulfs of Bothnia and Finland. In coastal waters, concentrations are generally higher. During the spring algal maximum and the occurrence of summer cyanobacterial blooms, short-term concentrations of chlorophyll can be up to ten times higher compared to average summer levels.

In the Gulf of Finland and the northern part of the Baltic Proper, algal abundances increased until the early 2000s due to eutrophication. The levels of algae began to increase again towards the end of 2010s, which was indirectly related to the salt pulses of that decade. These salt pulses pushed waters which were low in oxygen and high in phosphorus from the main basin of the Baltic Sea into the Gulf of Finland.

Learn more about the fluctuations in algal abundances in the Baltic Sea: long-term trends, annual cycles and vertical depth variations!