Baltic Sea

Many rivers pour fresh water into the Baltic Sea. Only a little salty ocean water gets in.

So the water is much less salty than the ocean. It is least salty far in the north and east.

The land that drains into the Baltic is four times larger than the sea itself, so rivers pour in a lot of fresh water. The only way to the ocean is through the narrow, shallow Danish straits. The shallowest strait, the Öresund, has a minimum depth of only 7 to 8 meters.

So the Baltic's water is brackish. Its salinity is about one-fifth of the ocean's, and it is freshest at the far ends of the Gulf of Bothnia and the Gulf of Finland.

Brackish conditions follow from the water budget: a catchment of about 1.6 million km², four times the sea's area, delivers fresh water through numerous rivers, while exchange with the ocean is restricted to the narrow, shallow Danish straits (the Öresund and the Belt Sea), with a minimum depth of 7-8 m in the Öresund. Mostly, low-salinity Baltic water flows out through the straits.

Salinity averages about 7 per mille and falls towards the Gulf of Bothnia and the Gulf of Finland. Because water exchanges slowly, substances entering the sea stay there for a long time, and the small volume dilutes them poorly.

Sometimes, during winter storms, a big rush of salty ocean water flows into the Baltic Sea. It brings fresh oxygen to the deep water.

In the far north, the land is rising out of the sea.

Only rarely, when the weather is just right for weeks, does a very large amount of salty North Sea water flow in. This is called a salt pulse. Large ones happen perhaps only once or twice a decade and bring oxygen-rich water to the deep basins.

In the north, the land is still rising after the weight of the ice-age glacier melted away. UNESCO says the High Coast in Sweden has risen about 285 meters since the ice left, 9,600 years ago.

A major Baltic inflow, or salt pulse, carries 200-300 km³ of North Sea water into the sea over a few weeks, with conditions most favorable during winter storms; only a strong pulse renews the oxygen-poor deep water. Large or significant pulses arrived in 1951, 1975-1976, 1993, 2003 and 2014 (the third largest ever), with medium-sized ones in 2015, 2016 and 2023; the portal notes that deep-water oxygen deficiency had become so severe that these improved it only slightly and temporarily.

The High Coast (Sweden) and Kvarken Archipelago (Finland), a World Heritage Site since 2000 (extended 2006), record glacio-isostatic uplift of about 285 m since the ice retreated 9,600 years ago, which UNESCO calls the highest known rebound.

In winter, ice covers the north of the Baltic Sea.

Baby ringed seals are born in snow caves on the ice. Gray seals live here too, and tiny algae even grow inside the ice!

The Baltic is the only brackish sea where a big part of the surface freezes every winter. The ice matters: Baltic ringed seals give birth to their pups, about five kilograms each, in lairs usually dug into snowdrifts on the ice.

The gray seal is the largest seal in the Baltic; in 2024 the estimate was just under 49,000. Tiny algae live in salty pockets inside the ice, and the first algae bloom of spring happens in and under the ice.

Ice cover varies, from mild winters (under 115,000 km² at most) to extremely severe ones (over 345,000 km²), and the Finnish Environment Institute's portal reports it is shrinking by an average of 3,400 km² per decade, especially since the early 1990s. The Baltic ringed seal (Pusa hispida botnica) depends on snow and ice for breeding, so the portal counts climate change among its greatest near-future threats.

The gray seal (Halichoerus grypus) is the largest and most abundant: just under 49,000 in 2024, a minimum estimate from aerial surveys. Brine pockets in the ice host algae that bloom first within and under the ice.

In late summer, the water can have green or yellow bits in it. These are tiny living things called cyanobacteria.

Don't swim in water that has them. Ask a grown-up.

In July and August, when the water is warm, cyanobacteria (blue-green algae) can grow into big blooms. They look like green, blue-green or yellowish bits in the water, or a thick layer like paint near the shore.

Some of them make poisons, so the Finnish Environment Institute's Baltic Sea portal says to treat every bloom with caution and not to swim in water that has cyanobacteria in it. Don't use that water for sauna steam either.

The Finnish Environment Institute's portal explains that Baltic cyanobacterial blooms peak in July and August and are formed mainly by Aphanizomenon flos-aquae and Nodularia spumigena, the latter producing the liver toxin nodularin. Because confirming toxicity requires detailed tests, every bloom should be approached with caution: swimming in water containing cyanobacteria is not advisable, and such water should not be used for sauna steam.

Blooms appear as green, blue-green or yellowish particles or a dense surface layer on calm days; yellow pine pollen in early summer can look similar. Visitors can report what they see, including 'no algae', through Finland's vesi.fi map service.