The open water column is the key setting for productivity in the Baltic Sea. Microscopic primary producers support the growth of zooplankton, which all fish species depend upon during at least some part of their life. The status of pelagic habitats is affected by human induced pressures such as eutrophication and hazardous substances, as well as by natural and human-induced changes in climate. Primary producers generally show not good status in the Baltic Sea region, except in the Kattegat. Zooplankton were only assessed north of the Gotland Basin, indicating good status in the Gulf of Bothnia but not in the other assessed areas.
Phytoplankton form the base of the pelagic food web and support the growth of zooplankton, either directly as food, or by a more complex route including the microbial loop. Phytoplankton blooms are a natural phenomenon in the Baltic Sea ecosystem, with blooms in late summer dominated by nitrogen-fixing cyanobacteria. Due to eutrophication, however, the phytoplankton blooms have become more frequent and extensive (Vahtera et al. 2007).
Zooplankton consist of small crustaceans and several other animal groups. Cladocerans and copepods are the dominating groups of crustaceans, and a key food base for pelagic fish. Since larger zooplankton are often more nutritious, and a strong production of zooplankton is important for the productivity of higher trophic levels, the biomass and size distribution of the zooplankton community is a useful measure of the status of the pelagic food web (Gorokhova et al. 2016).
Indicators for assessing pelagic habitats
The status of the pelagic habitats in the open sea was assessed using the core indicator ‘Zooplankton mean size and total stock’ in the northern part of the Baltic Sea (Gulf of Bothnia, Gulf of Finland and the Northern Baltic Proper (Core indicator report: HELCOM 2017w), and the two eutrophication indicators ‘Cyanobacterial bloom index’ and ‘Chlorophyll-a’ in order to represent changes in primary producers (Core indicator reports: HELCOM 2017g, i). The indicator ‘Chlorophyll-a’, gives a general measure of the level of primary productivity, via variation in the biomass of phytoplankton, and responds strongly to eutrophication.
Coastal areas were assessed using national indicators on chlorophyll-a and phytoplankton bio-volume as defined for assessments in relation to the Water Framework Directive, focusing on eutrophication, which is a major pressure impacting the status of pelagic habitats. However, particularly in coastal waters, the results of the biodiversity assessment may differ from the results of the eutrophication assessment in coastal areas (Chapter 5.1 Benthic habitats), which uses a different set of indicators. Further work to develop indicators representing the pelagic habitat is foreseen to strengthen the reliability of the assessment. The use of national indicators varied among geographical areas and hence, the results for coastal areas are not directly comparable between countries but provide an indication on the status of the coastal micropelagic system at Baltic regional scale.
Integrated status assessment of pelagic habitats
Good status was not achieved in any open sea sub-basin, with the exception of Kattegat (Figure 5.2.1). The integrated results reflect a deteriorated status according to all assessed core indicators in most cases (Figure 5.2.3).
The indicator ‘Cyanobacterial bloom index’ did not achieve the threshold value in any of the open sea sub-basins where it was assessed. Based on satellite data, the frequency and coverage of cyanobacterial blooms have oscillated since the 1970s (Kahru and Elmgren 2014). The total area of cyanobacterial accumulations has been above the earlier values since 1999.
The core indicator ‘Chlorophyll-a’ achieved the threshold value only in the Kattegat. It showed particularly deteriorated status in the Bornholm Basin, Northern Baltic Proper and Gulf of Finland. Chlorophyll-a concentrations have increased since the 1970s in most sub-basins east of the Bornholm Basin, but the increase has levelled off since the late 1990s. In the Kattegat and Danish Straits the chlorophyll-a concentrations have decreased since late 1980s.
The zooplankton community indicator achieved the threshold value in the Bothnian Bay and Bothnian Sea, but not in the Åland Sea, Northern Baltic Proper or Gulf of Finland. In the Northern Baltic Proper, both the mean size and the biomass of zooplankton have decreased from the 1970s to the present (see also Figure 5.2.4). Coastal areas showed higher variability, with the results of integrated assessment indicating good status in 24 out of 114 assessed coastal areas, corresponding to 19% of the area of the Baltic Sea region (Figure 5.2.2). The confidence in the assessment was between moderate and high in the open sea and low in coastal areas.
|Bay of Mecklenburg|
|Eastern Gotland Basin|
|Western Gotland Basin|
|Gulf of Riga|
|Northern Baltic Proper|
|Gulf of Finland|
Figure 5.2.3. Summary of core indicator results in the open sea areas, showing the proportion of assessment units achieving good status. White represents areas not assessed as the indicator is not relevant or applicable (Cyanobacterial blooms) or due to lack of threshold values (Zooplankton mean size and total stock).
Changes in the species and size structure
The function of the pelagic food web is not only dependent upon levels of productivity, but also upon changes in the relative abundance of different species and species groups. Diatoms and dinoflagellates are the dominating groups of phytoplankton during the spring bloom, and both are important food for higher trophic levels. Shifts in the relative abundance of diatoms and dinoflagellates occurred primarily in the late 1980s when a series of mild winters occurred (Wasmund et al. 2013). These fluctuations may affect the nutrition of zooplankton and lead to subsequent changes in other parts of the food web. For example, diatoms produced in the pelagic habitat are also important for the benthos as they sink quickly after the bloom, whereas dinoflagellates stay longer in the water column.
In the Eastern Gotland Basin an indicator based on the ratio of diatoms to dinoflagellates has been tested, showing that good status was not achieved in the assessment period (Wasmund et al. 2017, Figure 5.2.5).
In zooplankton, changes among taxa and species groups varied among the sub-basins. In the Gulf of Finland, changes observed in the core indicator were largely attributed to a decline in the groups of cladocerans over time, whereas the decline in total zooplankton biomass in the Northern Baltic Proper and the Bornholm Basin was mostly attributed to a decline in copepods (see also Figure 5.2.4). Regardless of this variability, an increase in the proportion of small-sized taxa and groups was observed in all basins that did not achieve the threshold value.
Figure 5.2.5. Trend over time in the ‘Diatom/Dinoflagellate index’ in the Eastern Gotland Basin. The green line shows the minimum threshold value, which is set at 0.5 in this basin (Pre-core indicator report: HELCOM 2017ah).
Impacts and recovery
The status of pelagic food-webs is highly dependent on nutrient levels. Surplus nutrients elevate phytoplankton growth, but the pelagic phytoplankton and zooplankton are also highly influenced by other factors in their environment, such as temperature and acidity (pH). These factors affect both the productivity and species composition of the pelagic community.
The abundance, but also the species composition of pelagic primary producers and zooplankton, is important for their quality as food for higher trophic levels. Blooms of cyanobacteria can include species that are toxic and induce alterations in the species composition of the grazing zooplankton. An increase in small-sized zooplankton with simultaneous decrease in total zooplankton biomass is likely to result in poorer food quality for pelagic feeding fish, such as herring, sprat and juvenile cod (Rönkkönen et al. 2004, Gorokhova et al. 2016).
The decreased size structure may also lower the level of grazing by zooplankton on phytoplankton, potentially affecting their abundances. Surplus primary productivity also decreases the recreational value of the sea, and enhances oxygen consumption and the extension of hypoxic conditions in benthic habitats (Vahtera et al. 2007).
The improvement of the status of the pelagic habitat in the Baltic Sea depends to a large degree on the success in reducing eutrophication but also on maintaining the structural integrity of the Baltic Sea food web. Both primary producers and zooplankton are also directly affected by changes in temperature and seasonality, leaving the pelagic system responsive to changes in climate (Dippner et al. 2001, Möllman et al. 2005).