Natural features of the Baltic Sea
The Baltic is one of the largest brackish seas in the world. It lies in northern Europe and is a young sea: it took its present form after the last Ice Age, some thousands of years ago.
- Shallow: average depth is only about 55 m. The deepest point, in the Landsort Deep, is about 459 m.
- Almost closed: it is joined to the North Sea only through the narrow Danish straits (the Øresund and the Belts).
- Slow renewal: it takes about 25 to 35 years to replace the water. Salty, oxygen-rich water comes in only during rare strong "inflows".
- Brackish: ocean water has about 35 g of salt per litre. In the central Baltic it is only about 7 g, and in the northern and eastern gulfs even less.
- Layers: heavier salty water lies below, lighter fresh water above. The two seldom mix.
- Winter ice: the northern parts freeze each winter.
State of the sea today
The Baltic is one of the most stressed seas in the world. Large areas of the deep bottom have almost no oxygen. Fish such as cod have declined, and harmful algae can cover large patches in summer. Some old chemicals and sunken war material still lie on the seabed. Things are improving where sewage treatment and farming rules are strict, but recovery is slow because the water renews slowly.
Why the Baltic needs protection
The cause-and-effect chain:
- Farms (fertiliser) and towns (sewage) release nitrogen and phosphorus.
- Rivers such as the Vistula and Oder carry them to the sea.
- Extra nutrients make algae grow very fast: eutrophication.
- Dead algae sink and rot; bacteria use up the oxygen.
- Fish and bottom animals cannot live in the oxygen-free dead zone.
Other threats: oil spills from ships, plastic, toxic chemicals, overfishing and invasive species. Because the sea is shallow, almost closed and brackish, it cannot clean itself quickly.
Protection: the countries around the sea work together in HELCOM (Helsinki Commission). Actions include better sewage plants, lower fertiliser run-off, limits on ship waste and fuel, and protected marine areas.
Poland's maritime economy
Poland has about 500 km of Baltic coast, with sandy beaches, spits (like the Hel Peninsula) and lagoons.
- Ports and shipping: Gdańsk, Gdynia, Szczecin and Świnoujście. Gdańsk handles very large container ships. Ferries link Poland with Scandinavia.
- Shipbuilding and repair: a long tradition in Gdańsk, Gdynia and Szczecin.
- Fishing: cod, herring, sprat and flatfish, under catch limits.
- Tourism: beaches, resorts, sailing and spa towns.
- Energy: an LNG terminal in Świnoujście, and offshore wind farms that are being built in the Polish part of the sea.
All these jobs depend on a clean, healthy sea, so protection and the economy go together.
Try it at home
Make "brackish" water: stir 1 teaspoon of salt into a glass of water (ocean-like is about 7 teaspoons per litre; the Baltic is about 1 teaspoon per litre). Taste a tiny drop of each if your teacher allows, and notice how the Baltic is almost fresh. Next, put a drop of food colour in a jar of still water and see how slowly it spreads. That slow mixing is why the Baltic cleans itself slowly.
Key formulas and definitions
- Average depth ≈ 55 m; deepest ≈ 459 m
- Salinity: ocean ≈ 35 g/L, central Baltic ≈ 7 g/L
- Water renewal time ≈ 25–35 years
- Eutrophication chain: nutrients → algae bloom → dead algae → oxygen used up → dead zone
- Key terms: brackish, eutrophication, dead zone, HELCOM, halocline (salinity layer)
Worked examples
1. Why is the Baltic less salty than the North Sea?
Salty ocean water enters only through narrow straits, and many rivers pour in fresh water. So the salt is diluted, especially in the east.
2. Explain how fertiliser on a farm can create a dead zone in the sea.
Rain washes fertiliser into rivers. Rivers carry the nutrients to the sea. Algae grow very fast, then die and sink. Bacteria feeding on them use up the oxygen, so the bottom becomes a dead zone.
3. Water in the Baltic is renewed once in about 30 years. How many times is it renewed in 90 years?
90 ÷ 30 = 3 times.
4. Give two ways Poland earns money from the Baltic and one thing that could harm it.
Ports and shipping, and tourism (also fishing, energy). Pollution and algae blooms could harm beaches, fish and tourism.
Common mistakes
- Thinking the Baltic is as salty as the ocean. It is brackish.
- Thinking algae blooms come from "too little" nutrients. They come from too many.
- Saying the Baltic is deep. It is shallow, about 55 m on average.
- Mixing up the Baltic Sea with the North Sea, which is the open-ocean side.