This indicator and its threshold values are yet to be commonly agreed in HELCOM. The indictor is included as a test indicator for the purposes of the 'State of the Baltic Sea' report, and the results are to be considered as intermediate.
The status of biodiversity and food webs can be assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the "Seasonal succession of dominating phytoplankton groups", this indicator will in the future also contribute to an overall food webs assessment, along with the other biodiversity core indicators.
The proposed core indicator is among the few indicators able to evaluate the structure of the Baltic Sea food web, since phytoplankton have known links between environmental conditions (e.g. nutrient conditions) and higher trophic levels. Furthermore they have an important influence on other environmental or ecosystem components such as the supplementation of the microbial food web and possible consequences for oxygen conditions. Climate-induced changes in phenology can have consequences for the productivity of certain phytoplankton groups as well. Assessments on the structure and functioning of the marine food web are requested by the Baltic Sea Action Plan (BSAP) and the EU Marine Strategy Framework Directive (MSFD).
The BSAP ecological objective 'Thriving and balanced communities of plants and animals' calls for balanced communities, which has a direct connection to the food web structure. The background document to the Biodiversity segment of the BSAP describes a target for this ecological objective as 'By 2021 all elements of the marine food webs, to the extent that they are known, occur at natural and robust abundance and diversity'.
The EU MSFD lists a specific qualitative descriptor for the food webs: 'All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity.'
Phytoplankton are the main primary producers in the marine ecosystem. These organisms occur in vast numbers and capture sunlight via photosynthesis to build biomass. These primary producers are commonly autotrophic and photosynthetic (though some can be mixotrophic) and they form a direct link between the environmental conditions (e.g. nutrient status) and the marine food webs. Phytoplankton biomass represents the base of the classical marine food web, forming the carbon and energy (and nutrient) source for grazers and predators such as zooplankton, which in turn are eaten by fish. Furthermore, phytoplankton can also play a role in the regulation of secondary basal producers (i.e. bacteria) that classically rely on exudates, and the degradation of phytoplankton biomass has consequences for biochemical cycles, such as oxygen consumption, and thus the status of the marine environment.
In aquatic ecosystems, a hierarchical response across trophic levels is commonly observed. That is to say that, higher trophic levels may show a more delayed response or a weaker response to eutrophication than lower ones. Measurements of biomass (rather than abundance) were used to develop this indicator, since they can readily be translated into understanding biogeochemical cycles, they link to eutrophication, and are considered to give a more accurate depiction of the phytoplankton community. The succession of phytoplankton has a rather regular pattern and the initial event like spring bloom may also influence the formation of summer communities. Firstly, the dominance of either diatoms or dinoflagellates in the spring period determines the rate of sinking organic matter and subsequent oxygen consumption in bottom sediments. The diatoms settle out quickly and may cause oxygen depletion, which may in turn launch the release of phosphorus from sediments. This favours those phytoplankton which benefits from excessive P, especially diazotrophic (nitrogen fixing) cyanobacteria that bloom (e. g. Eilola et al., 2009).
The succession of dominant groups can provide an index that represents a healthy planktonic system, with a natural succession of dominant functional groups throughout the seasonal cycle. Deviations from the normal seasonal cycle, such as a too high or too low biomass, absence or appearance of some dominating groups at unusual time periods of the year, may indicate impairment in environmental status.
"the most important anthropogenic threat to phytoplankton is eutrophication"
Input of nutrients — diffuse sources, point sources, atmospheric deposition.
Input of organic matter — diffuse sources and point sources.
(introduction of non-native species)
The shift in the plankton community is most probably due to complex interactions between warming (climate change impacts), eutrophication and increased top-down pressures due to overexploitation of resources, and the resulting trophic cascades. Eutrophication is commonly noted as being the major driver behind current impacts on the phytoplankton community. A shift in functional groups may affect ecosystem function in terms of the carbon available to higher trophic levels or settling to the sediments. The examination of seasonality shows the broad temporal variability of phytoplankton populations. Succession of dominant groups can potentially provide an index that represents a healthy planktonic system, with a natural progression of dominant functional groups throughout the seasonal cycle. Alterations in the seasonal cycle may be related to nutrient enrichment. Expert judgement must be used when alterations in the seasonal cycle, and their causes, are interpreted.