Nutrients and their effect on the Baltic Sea

The level of nutrients in seawater is directly reflected in the state of eutrophication of the sea. The more nutrients there are in the water, the more algae and aquatic plants grow also. Such growth is particularly affected by both nitrogen and phosphorus.

The productivity of the sea increases with nutrients

The concentrations of nitrogen and phosphorus in seawater reflect the development of eutrophication. The availability of these nutrients determines the productivity of the sea.

Here, productivity refers to the amount of energy produced by marine primary producers, i.e. aquatic plants and algae, such as phytoplankton, epiphytic algae, and macroalgae. All of the marine primary producers generate organic material through photosynthesis. 

Long-term trends in eutrophication are monitored using surface nutrient levels in winter

The nutrient levels in the surface layer of water during the winter is commonly used to describe the long-term evolution of eutrophication in the sea. At this time of year, the water mass is mixed and there is very little phytoplankton in the water to use up the nutrients. When there are no species to consume them, the nutrient levels are at their highest.

Vesinäytteenotinta lasketaan alukselta osin jäiden peittämään mereen.
Wintertime sampling onboard research vessel Aranda.

The inorganic compounds of nitrogen and phosphorus are directly available to algae

Thus, both nitrogen and phosphorus are nutrients that regulate the growth of aquatic plants and algae. These dissolved inorganic compounds are directly available to algae. Inorganic compounds of nitrogen include ammonia, nitrite, and nitrate, while the inorganic form of phosphorus is phosphate.

When speaking about long-term changes in nutrient levels, this specifically means changes in the concentrations of nitrates and phosphates.

The nitrogen dissolved in the Baltic Sea is mainly in the form of organic compounds. The usefulness of these to algae is not well known. By contrast, most of the total phosphorus in the Baltic Sea is inorganic phosphate, which is directly available to algae.

Nutrient ratios also affect eutrophication

In addition to direct nutrient levels, nutrient ratios also regulate the growth of primary producers. If one of the essential nutrients is depleted, algae growth will be inhibited even if other nutrients are available. The first nutrient to be depleted is called a growth-limiting or minimum nutrient.

The minimum nutrient can be identified by the ratio of nitrogen to phosphorus. Phytoplankton uses soluble inorganic nitrogen and phosphorus for their growth, in an atomic ratio of 15-20 to 1, i.e. 15 to 20 units of nitrogen per unit of phosphorus. A significantly higher ratio than these values indicates phosphorus restriction. A lower ratio, on the other hand, describes nitrogen limitation.

There are large differences in the nitrogen and phosphorus concentrations in Finnish waters

In general, oceans are nitrogen-limited while inland waters are phosphorus-limited. In the brackish water of the Baltic Sea, the minimum nutrient changes from nitrogen to phosphorus.

The Bay of Bothnia is heavily phosphorus-limited, while the Bothnian Sea ranges from mild nitrogen-limitation to total-limitation. Except for its easternmost area, the Gulf of Finland is nitrogen-restricted.

Blue-green algae are efficient at taking advantage of nutrients 

In recent decades, the amount of filamentous blue-green algae (cyanobacteria) in the Baltic Sea has increased. Cyanobacteria can bind atmospheric nitrogen into a soluble form in water. This means that the production of cyanobacteria is phosphorus-limited.

During the growing season in the Gulf of Finland and the northern Baltic Proper, the minimum nutrient may change from nitrogen to phosphorus. This occurs when the blue-green algae decompose, and the nitrogen bound by the cyanobacterial blooms is released into the water to become available for use by other groups of algae.