Temperature varies with season. Temperature affects all physiological processes and most organisms grow faster in warmer waters. In summer, a so-called thermocline often forms between the warmer, lighter surface water and the cooler, heavier bottom water. In shallow areas, temperature can change rapidly with the weather.
Salinity sets the limit for which species can live in the Baltic Sea. The Baltic Sea is the second largest brackish water basin in the world, which means that it is a mixture of fresh and salty water. The salty water enters the Baltic Sea through the Danish straights, while the freshwater enters the Baltic Sea via the rivers. Most species prefer either fresh or salty water, not the brackish water that we have in the Baltic. Only relatively few species have adapted to the brackish conditions and therefore the species diversity in the Baltic Sea is low.
Oxygen is crucial for life in the sea. Oxygen is produced by plants and algae, and consumed through respiration and decomposition. The Baltic Sea suffers from oxygen deficiency. Eutrophication leads to more algal blooms and when these sink to the bottom and decompose, the process uses up a lot of oxygen. The limited water exchange in the Baltic also means that the oxygen concentrations can be very low. Oxygen concentrations below 4 mg/litre are starting to negatively affect the behaviour of animals. When the oxygen drops below 2 mg/litre, the water is hypoxic and many species start dying, and when it drops to zero, the water is anoxic and only anaerobic microbes can thrive.
Clear water has low turbidity, while murky water has high turbidity. The turbidity can increase when there are algal blooms, when heavy rainfall brings sediment via the rivers and when storms mix up sediment from the seafloor. In clear water, light can penetrate deeper, which means that, for example, seagrasses and bladder wrack can grow deeper. This can be used as an indicator of the health of the sea – the clearer the water is, the deeper the plants can grow.
pH describes the acidity of the water. The pH of the open, fully marine ocean is around 8, while in the brackish Baltic Sea it is a bit lower. In shallow waters pH is also usually higher than in deeper waters. Ocean acidification (lower pH) is a problem linked to climate change. When we burn fossil fuels, carbon dioxide is released into the atmosphere. Some of this carbon dioxide is dissolved in the seawater and there it makes the water more acidic. Low pH makes it hard for mussels to build their shells and it is like the marine organisms are suffering from osteoporosis.