Temperature and salinity, two essential structuring factors of biodiversity in the Baltic Sea

Table 1. Relationship between number of taxa and salinity. At lower salinity, fewer macrobenthis species are present.

Area	Macrobentic taxa	Salinity
Oderhaff	130	7-8
Mecklenburg Bight	c.350	20-10
Kiel Bight	c.500	25-20
Kattegat	c. 1,000	25-30
North Sea	c. 1,400	32-34

The Baltic Sea is a relatively enclosed brackish water body with substantial salinity and temperature gradients. Both, the changeable salinity conditions and the above-given gradient patterns are the main factors influencing the distribution of the species in the Baltic Sea. Besides these natural factors, human impact - mainly pollution/eutrophication has been revealed as playing a significant role in distribution and abundance of the various organisms.

The Baltic Sea is inhabited by natural immigrants of mainly three different origin: marine boreal species (e.g. soft-shell clam Mya arenaria, ocean quahog Arctica islandica, cod Gadus morhua callarias, herring Clupea harengus membras, Temora longicornis, Pseudocalanus elongatus), freshwater species (e.g., pike Esox lucisus, perch Perca fluviatilis, Daphnia spp., Keratella spp.) and glacial relicts (e.g., fourhorned sculpin Myoxocephalus quadricornis, Limnocalanus macrurus, Mysis relicta). Their preferences for the environment and, therefore, spatial distribution differ: marine species dominate in the open Baltic and southwestern basin, freshwater species prevail in the northeastern Baltic and coastal areas and glacial relicts are more abundant in communities in deeper areas of the northeastern part of the sea. Below we give two examples how salinity and temperature affect distribution of the Baltic biota.

Effect of the salinity gradient (Mecklenburg Bight)

In the Western Baltic Sea (Belt Sea), the transition area between the North Sea and the Baltic Proper, high saline North Sea water mixes with Baltic Sea water, which usually has a lower salinity due to the strong freshwater input from the Baltic Sea catchment area. This results in a salinity gradient (25 -10) and a pronounced halocline. Salinity as well as oxygen content in the near bottom layer varies seasonally and between years, depending and the saltwater inflow from the North Sea or the river runoff. Owing to these hydrographical features this part of the Baltic Sea forms a natural border in regard to the distribution of many marine euryhaline organisms, as shown in Table 1 for macrobenthic species. 

Fig. 1. Benthic biodiversity in the Mecklenburg Bight (Zettler et al., 2000, and Zettler, unpubl.).

Different studies have revealed that biodiversity in the Mecklenburg Bight is representative for the whole southern Baltic Sea (Zettler et. al., 2000, and unpubl. data). However, even within the Mecklenburg Bight benthic biodiversity is not uniform, alternating between relatively high (more than 50 taxa) and low values (less than 20 taxa) (Fig. 1). 

What are the reasons for this? As first investigations have shown, sediment characters and oxygen availability appear to be important factors beside salinity determining the composition of the marine benthic communities in this area.

At present, the function of these "biodiversity islands" for recruitment and resettlement of areas effected by oxygen depletion or human activities (dumping, dredging) are yet not fully understood.

Effect of the seasonal thermocline (Gulf of Riga)

In spring, when water temperature starts to increase, the seasonal themocline is formed. In the Gulf of Riga basin (max. depth ca 60 m) it is the main factor governing the distribution of organisms due to the lack of a halocline. The location of the thermocline moves between spring and autumn from ca 5-10m to 25-30m water depth. In late autumn the thermocline is less pronounced and the convective mixing in winter results in a vertical homothermium. 

Fig. 2. Seasonal dynamics of the general distribution pattern of fish of different origin in relation to the seasonal thermocline on the coastal slope of the Gulf of Riga.

Of the three different groups of organisms, freshwater fish species stay above the thermocline by following its seasonal course (descend to deeper layers from spring to autumn) (Fig. 2).

The same distribution pattern is valid for most of the zooplanktonic and benthic taxa of freshwater origin. Many of them tend to prefer warm water. Distribution of marine eurytherm (pelagic) species is closely connected with the temperature front. They are located around the thermocline and in the area marked by the coastal slope and the projection of the thermocline, called 'mixing zone'. 

Except during their reproduction period in winter, glacial relicts are confined to areas below the seasonal thermocline where the temperature is substantially lower. However, some of them (e.g., smelt Osmerus eperlanus and the mysid Mysis mixta) can also be found in the mixing zone area: they probably face shortage of food in deeper areas (with preferred temperature conditions) and are forced to move to warmer water in order to find sufficient amounts of food.

Bibliography

Zettler, M.L., Bönsch, R., Gosselck, F. 2000. Verbreitung des Makrozoobenthos in der Mecklenburger Bucht (südliche Ostsee) - rezent und im historischen Vergleich. Meereswissenschaftliche Berichte 42: 144pp.