On a coastal zone land changes to seabed and it is called a continental shelf. The continental shelf is in a sense a continuation of the continent. It is relatively shallow and flat, and in the Atlantic, for example, its width can be up to 500 kilometers. Continental shelves therefore reach far into the ocean, and only when they end starts a continental slope, where the ocean begins to deepen rapidly.

The Baltic Sea is a part of the large continental shelf of the eastern North-Atlantic, which is why determining the extent of the coastal zones in the Baltic Sea is complicated. Unlike the oceans, the Baltic Sea does not have a clear continental shelf.

The simplest definition of a coastal zone is an area, where the depth of water is less than for example 20 meters. With this method most of the islands in the Baltic Sea belong to the coastal zone. Regions where the water depth is less than 20 meters cover around one fourth of the whole area of the Baltic Sea. In wintertime the coastal zone can be determined to be where fast ice can exist.

Archipelagos

The most important archipelago areas in the Baltic Sea are the Archipelago Sea, Stockholm Archipelago, Vaasa Archipelago, Archipelago of the Northern Bay of Bothnia, and the Belt Sea. Islands determine the circulation in these areas very strongly. Modelling and forecasting these areas can be difficult on these areas due to the many small islands.

Phenomena on the coastal zones

The most important phenomena in the coastal zones are for example the variations in sea level and upwelling. Sea level changes over a short time range can be seen as flooding on the land areas or dry seabed during low water periods. In a long range, land uplift increases land area in the northern Baltic Sea. On the other hand, in the southern Baltic Sea water level rises and coastal land areas will be under water.

It has a difference if the shoreline is strait or very curvy. Curvy shoreline with many islands makes the biodiversity of the sea richer. Strait and bare shore is more open to waves, for example.

Shallow water also warms up and cools down more effectively than deeper water in the open sea.