9.6 Marine areas
Salinity, temperature and oxygen are physical parameters that act to constrain the biodiversity in semi-enclosed water bodies. The biology in the Baltic Sea, which is such a water body, is therefore influenced by both the physical and chemical environment. As described in Section 9.2, the Baltic Sea is a particularly brackish sea, with significant gradients in both salinity and temperature. In addition, the pycnocline (thermo- and haloclines) defines the water column profile of the Baltic Sea (see Section 9.2). In general, biodiversity and species richness increases with increasing salinity. Therefore diversity is generally lowest in the Gulf of Finland and increases towards Germany.
The ecosystem is composed of species or species groups, communities and habitats, and the interaction between the different trophic levels (feeding position in the food web). For the Baltic Sea, the relevant species or species groups (i.e. receptors) are plankton, benthic flora and fauna, fish, marine mammals and birds. The habitats are influenced by the specific combination of abiotic and biotic conditions which determine both the individual species and communities as well as the assemblages of species supported by them. For a further description of the overall ecosystem functions and biodiversity, see Section 9.6.8.
In the following sections, the terrestrial flora and fauna onshore at the landfall areas and the marine biology receptors, together with the protected areas of the Baltic Sea, are described in detail. Key areas used to describe the biological baseline are shown in Figure 9-1 (sub-basins) and Figure 9-17.
Figure 9-17 Key areas of the Baltic Sea used for the biological baseline description; see also Figure 9-1.
Plankton 9.6.1
Plankton comprises small organisms such as phytoplankton and zooplankton that live in the water column.
9.6.1.1 Phytoplankton
Phytoplankton comprises a group of microscopic photosynthetic organisms (microalgae; e.g.
diatoms, dinoflagellates and cyanobacteria). They are the main source of primary production in the Baltic Sea and form the base of the marine food web. Therefore phytoplankton are essential to the ecosystem function, as they provide the basis for the productivity of higher trophic levels (zooplankton, fish, etc.). Phytoplankton also perform a vital role in the biogeochemical cycles of many important chemical compounds (especially Carbon, Nitrogen, Phosphorous, Silicium-cycles), particularly the carbon cycle of the ocean. Carbon fixed by phytoplankton enters the food web, where it is consumed by mostly zooplankton. Detritus (dead organic material) subsequently sinks, often in areas away from the coast, which leads to the transport of carbon from the surface waters to the deep water. This process, known as the ‘biological pump’, is one of the reasons that the oceans constitute the largest (active) pool of carbon on earth.
Owing to its high dependency on light for growth, phytoplankton are restricted to the upper part of the euphotic zone, which in the Baltic Sea ranges from a few metres in coastal areas to 35 m in the central areas. The vertical and horizontal distribution of phytoplankton is also dependent on the turbidity of the water and the availability of nutrients (nitrogen and phosphorus), which are essential for growth, as well as climatic conditions and currents. A high nutrient load due to eutrophication can give rise to substantial increases in phytoplankton biomass, which leads to an increased detritus load to the seabed. The degradation of detritus in turn results in high oxygen consumption and a potential oxygen deficiency on the seabed, which can impact the benthic
communities (species living on the seabed), as discussed in Section 9.2.2.5 on eutrophication dynamics and the status of the Baltic Sea.
Chlorophyll a is the most abundant photosynthetic pigment among all photosynthetic organisms and can therefore be used to estimate phytoplankton biomass, and hence its horizontal distribution. The surface chlorophyll a concentration in European waters is measured continuously by the Joint Research Centre of the European Commission using satellite mapping (Ocean colour remote sensing). Surface chlorophyll a is shown for each of the months of 2012 (Figure 9-18, Atlas Map PE-02-Espoo) and for the month of July for the period 2004-2012 (Atlas Map PE-01-Espoo). This indicates that plankton is distributed throughout the entire Baltic Sea with the biomass in general highest in the summer months (June-August), with the highest levels occurring in the Gulf of Finland and in the eastern Gotland Basin (Figure 9-18, representing the year 2012) /110//111/.
Figure 9-18 Surface chlorophyll a concentration (mg/m3) for each month in 2012 /110/.
Phytoplankton also exhibit significant cyclical changes in response to seasonal variations in sunlight and temperature. In general, there are three annual phytoplankton blooms in the Baltic Sea /110/, /111/, /112/, /113/. The timing of the blooms in the different areas depends on the above-mentioned factors. Although seasons vary slightly between regions, in general the timing of the phytoplankton blooms can be described as follows:
• In spring, when nutrients and light become available, the biomass of phytoplankton increases massively. The spring bloom typically consists mostly of diatoms and/or dinoflagellates. When the dissolved nitrogen is depleted, the algal biomass in the upper part of the water column decreases until it reaches the summer minimum.
• In summer, recurrent blooms of cyanobacteria usually dominate the coastal areas and surface waters /112/. Cyanobacteria blooms depend on the available amounts of
phosphate in the surface water and favourable weather conditions. Some cyanobacteria are capable of nitrogen fixation, i.e. uptake of nitrogen from the atmosphere, and can form massive visible surface accumulations of several weeks’ duration throughout large parts of the Baltic Sea /114/.
• In autumn, as temperatures decrease and winds increase, water mixing typically increases the supply of nutrients from the nutrient-rich bottom water, which may lead to a third, minor naturally occurring autumn bloom.
As a result of the brackish conditions of the Baltic Sea, phytoplankton communities differ in composition from those in other marine areas, with the low salinity resulting in lower species richness compared with other areas. Approximately 1,700 phytoplankton species have been recorded in the Baltic Sea /112/, although many of these are only represented in very low numbers. Species diversity for phytoplankton does not follow the general pattern of low species diversity in areas with lowest salinity, as the most phytoplankton diverse areas in the Baltic Sea are in the Gulf of Finland, which has low salinity /112/. This is due to the influence of freshwater species. In the more saline waters (southern Baltic), phytoplankton are dominated by diatoms and dinoflagellates (marine species). The diversity is lowest in the Bornholm and Gotland Basins (central Baltic) owing to unfavourable salinity conditions for both marine and freshwater species.
There are no records of species of phytoplankton on the HELCOM Red List or the IUCN Red List.
Bloom-forming cyanobacteria occur throughout the Baltic Sea (Atlas Map PE-03-Espoo). Some of these species are potentially toxic to fish, mammals and humans. Dominant bloom forming and potentially toxic species are Aphanizomenon (occurring primarily in the northern parts of the Baltic), Nodularia (occurring primarily in the central and southern parts of the Baltic) and Dolichospermum (which occurs in all regions) /113/, /114/.
The production of plankton can be very high due to a very low turnover time, which is on average 2-6 days for phytoplankton.
9.6.1.2 Zooplankton
Zooplankton is a group of small planktonic animals that constitute a food source for zooplanktivorous fish. Therefore zooplankton is a key link in the food chain.
The zooplankton communities in the Baltic Sea comprise a mixture of freshwater, brackish and marine species. Approximately 1,400 species of zooplankton ranging from microzooplankton to macrozooplankton (0 µm to more than 20 mm) have been recorded for the entire HELCOM area (Baltic Sea, Danish Straits and Kattegat) /112/. The species richness increases with salinity.
Again, the brackish conditions restrict the diversity of the marine species and as a consequence of the salinity gradient in the Baltic Sea, the marine species dominate the southern Baltic Sea /115/. Microzooplankton is the most diverse group, dominated by cilates and rotifers. The meso- and macrozooplankton is dominated by calanoid copepods (Pseudocalanus, Temora longicornis and Acartia spp.) and cladocerans (Evadne nordmanii). There are no records of species of zooplankton on the HELCOM Red List or the IUCN Red List.
Although zooplankton can occur throughout the water column, the temporal variation in vertical and horizontal distribution depends on the eco-physiological tolerances (e.g. salinity, oxygen level and temperature preferences) of particular species and the availability of food resources (e.g. phytoplankton and bacteria) /112/, /116/. The pycnocline (see Section 9.2.2.1) constrains the vertical distribution of zooplankton species and is thus a key determinant of the vertical assemblage patterns in the different layers of the water column /112/.
Zooplankton biomass is closely linked to the food source, i.e. phytoplankton and microzooplankton (ciliates and smaller flagellates). As a consequence, the zooplankton blooms follow the timing of phytoplankton blooms, with their intensity being linked to, but lower than, the blooms of phytoplankton. Therefore mid-summer (exact timing depending on region) is the