The open sea and the invisible inhabitants of the water column

The open or high sea means a large open marine area in which the horizon is bounded in at least one direction by the sea and not by land. The outer reaches of the Finnish coast are naturally located in the outer archipelago and areas of the open sea. The high sea is an important distribution route for both species and habitats.

Although the water column forms most of the open sea’s ecosystem, its functions are also affected by organic matter sinking to the seafloor, as well as upwelling currents which bring bottom material back into the water column.

The species and food web structure of the Baltic Sea’s open sea ecosystem is mainly determined by the physical environment, such as salinity, depth, and the stratification of water.

The open sea covers a wide area

The open sea areas along the Finnish coastline cover a distance of 1,300 kilometres, and there are great differences between them. The deepest and most saline and water is found in the Archipelago Sea and the Western Gulf of Finland. Correspondingly, due to river runoff, the waters in northern reaches of the Bay of Bothnia and the eastern Gulf of Finland are almost fresh. The salinity of the Finnish open sea areas ranges from about 3 to 6.5 ‰.

The large distances between the southern and northern waters also lead to seasonal variations in light, water temperature, and ice cover.

Vertebrate predators form the apex of the food web of the high seas

In addition to fish and seals, the water column in the high seas is a habitat for many tiny organisms. Primary production relies on phytoplankton, which in turn serves as food for zooplankton. These tiny invertebrates are eaten by larger invertebrates, such as crustaceans, mysid shrimps, mussels and barnacles, which in turn end up as prey for fish, birds, and seals.

In the open seas, dead organic matter either sinks to the bottom and accumulates there or is released from the bottom sediments back to the water column by the actions of decomposer organisms. The production levels of the high seas determine the amount of organic matter sinking to the seafloor, which in turn affects the amount of food available for benthic organisms, as well as the oxygen conditions of the sediments.

The open sea. A north route marker is visible in the foreground of an open sea vista, with a lighthouse in the background.
In the water column of the open seas, organisms swim, float, and drift freely, often regulating their depth according to the ambient light levels and temperature.

The abundance of biota varies both regionally and seasonally

The peak of phytoplankton production, i.e. the spring bloom, occurs in the Archipelago Sea and the Gulf of Finland generally between April and May, in the Bothnian Sea by mid-May and in the Bothnian Bay by mid-June. The predominant species of the spring bloom are diatoms and dinoflagellates.

As the water warms, the abundance of zooplankton increases markedly, reaching its peak in late summer. The most abundant groups are rotifers, water fleas, and copepods.

In late summer, the cyanobacteria, commonly known as blue-green algae also proliferate, forming large surface rafts or blooms almost every summer. In the Bay of Bothnia, diatoms (Bacillariophyta) flourish throughout the growing season.

As the water cools in the autumn, the abundance of plankton in the water column decreases as many species form a resting phase which sinks to the bottom sediments. In spring, these will develop into a new plankton population.

A diver floats on the surface with the support boat in the background.
Deep water areas are researched by various methods, such as scuba diving.

Eutrophication and climate change are changing algae communities and the open seas

Eutrophication can change algae communities to the detriment of zooplankton, which affects the rest of the food chain. Also, the decrease in salinity due to climate change, in turn, is reducing the numbers of large marine species of zooplankton, which also undermines the quality of the available food in the food chain.