During millions of years, salts from the crust, and its loose layers of soil and minerals, has dissolved into the water of the oceans. At the same time, salts are precipitated into shallow seas. It is considered to be in balance today, so that the salinity of the oceans is constant at about 35 grams per kilogram of seawater. In coastal areas in higher latitudes, the salinity is significantly lower than 35‰, since the water is diluted with water from rivers and precipitation. The ecosystems in Skagerrak and Kattegat (salinity 20-34‰) has an species-rich fauna while the ecosystems in the Baltic (salinity 8-14‰) is characterized by a few species with many individuals of each species. The water quality of an isolated part of the ocean is determined by a number of factors, particularly those that control the water circulation in the area. The most important factors are the inflow of fresh water, the bottom type, winds, currents and tides. Every coastal sea and estuary is unique. There are great variations.
The composition of seawater
What does 35 g of dissolved substances in a kilogram of seawater consist of? All of the currently known elements have been detected in seawater, most of them in very low concentrations. The special properties of the water molecule cause all salts in the ocean to dissolve into ions. The composition of the sea salt gives the oceans a good capacity to resist changes in pH as opposed to most freshwater bodies of water.
Trace elements make up less than 0.5 percent of the dissolved substances. Many of these elements are essential for the biological production in the ocean. It is first and foremost the inorganic nutrients in the form of ammonium, nitrates and phosphates, which control primary production. Some trace metals, such as copper and iron, are also required for biological production.
The gases of the atmosphere, carbon dioxide (CO2), oxygen(O2), nitrogen(N2) and noble gases, are dissolved into the surface water of the oceans and are then included in the marine biogeochemical cycles. The solubility of gases in water increases as temperature and salinity decreases. A small portion of oxygen is supplied through photosynthesis.
The only consumption of oxygen in the deep sea results from aerobic bacterial activity during the decomposition of settling dead organic material (detritus).
Seawater in different layers
The supply of freshwater around the coasts through precipitation and river run-off causes the surface water to have a lower salinity than deep water, which is brought in laterally from the ocean by bottom currents. A halocline (a strong, vertical salinity gradient) is formed between surface water and deep water. Heating and cooling over the seasons is mainly limited to the layer above the halocline. During the summer, the surface is heated through solar radiation and this creates a thermocline (a thermal distinct layer in which temperature changes more rapidly) whose depth often coincides with that of the halocline. The salt also causes the water to freeze at a lower temperature. Water seeks stable stratification. Therefore, the water's density increases with increased depth.
Normally, a lighter surface water mass lies on top of a significantly heavier deep water mass, separated by a more or less thick pycnocline (a layer where the density gradient is greatest ). If the surface water is cooled off during the winter, or if the salinity increases due to evaporation, the water will get heavier it will sink. Thus, the surface water is mixed downwards. However, the pycnocline is an effective border, which is why mixing is limited only in the surface water layer.