Relevance of the Indicator

This pre-core 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.

Food web assessment

The Dia/Dino index is the only indicator that analyses the pathway of pelagic nutrients and biomass in the food web. Diatoms tend to sink quickly down after the bloom and feed the benthos whereas dinoflagellates stay in the pelagial for a longer time and contribute to the pelagic food web (see below). Changes in the Dia/Dino index indicate changes in the conditions for the nutrition of higher trophic levels. According to historical data, a high Dia/Dino index, i.e. diatom dominance, indicates a good status. While sinking, diatoms remove nutrients from the open water and therefore contribute to mitigation of eutrophication.


Policy relevance

Assessments on the structure and functioning of the marine food web are requested by the Baltic Sea Action Plan (BSAP) and the MSFD. This indicator may be applied to the Marine Strategy Framework Directive (MSFD), primarily for descriptor 4 (Food web). As it deals with the principal pelagic food basis in spring, it establishes a link to the higher trophic levels both in the pelagic and the benthic communities. A sort of diatom/dinoflagellate-ratio is already considered by OSPAR, as described in Results. It may have importance in the whole ICES area. The ICES-Working Group on Phytoplankton and Microbial Ecology (WGPME) discussed this ratio and will consider it in the planned "ICES phytoplankton and microbial plankton status report". The recent status report is available at http://wgpme.net/status-report-now.

This indicator may also contribute to descriptor 5 (Eutrophication) in the case on ongoing eutrophication that may lead to silicate limitation. It will react very sensitively to that limitation. Eutrophication is one of the four thematic segments of the HELCOM Baltic Sea Action Plan (BSAP) with the strategic goal of having a Baltic Sea unaffected by eutrophication.

 

Role of diatoms and dinoflagellates in the ecosystem

Phytoplankton are an important component of the food web in aquatic ecosystems and influences the global carbon cycle significantly (e.g. Smetacek 1999). Diatoms and dinoflagellates are the main components of the phytoplankton community not only in the Baltic Sea but also in the oceans. Their biomass can reach 6 g/m3 or more in the Baltic Proper.

Strong changes in the diatom/dinoflagellate ratio in spring blooms in the southern Baltic Proper were discovered by Wasmund et al. (1998) and identified by Alheit et al. (2005) as regime shifts. Such regime shifts are of high concern as they impact the whole food web. A dinoflagellate to diatom ratio has already been suggested "to reflect ecosystem state and the quality of the phytoplankton community as food for zooplankton" in the GES-REG project final report on food web indicators, September 2013 (Uusitalo et al. 2013, p.9).

The phytoplankton spring bloom does not only feed the pelagic food web, but sinks partly to the bottom where it feeds the benthic food web. Diatoms are much more susceptible to sedimentation than dinoflagellates and therefore the main contributor of organic matter to the benthos (Heiskanen 1998). Thus the Dia/Dino index may indicate whether the food substances stay primarily in the pelagial or are exported to the benthal. An indicator of the pathway of the food is of high interest for assessing the status of the environment.

Phytoplankton reacts directly to eutrophication by biomass increase, reflected in an increase in chlorophyll- a concentrations, which is already approved as a core indicator. The search for indicator species for eutrophication was not successful. However, the Dia/Dino index may have an indicator function based on the silicate requirement of diatoms. Eutrophication is mainly caused by anthropogenic input of nitrogen and phosphorus but not silicate. As silicate concentrations decrease with eutrophication, this nutrient may become the limiting nutrient for diatom growth (Danielsson et al. 2008). The Dia/Dino index will be able to indicate severe silicate limitation provoked by eutrophication.

 

Human pressures linked to the indicator

‚ÄčGeneral MSFD Annex III, Table 2a
Strong linkEutrophication by phosphorus and nitrogen leads to relative silicate shortage that may limit diatom growth and support unwanted flagellates. However, recently no Si limitation is expected.

Inputs of fertilisers and other nitrogen and phosphorus-rich substances (e.g. from point and diffuse sources, including agriculture, aquaculture, atmospheric deposition).

Weak linkThe Dia/Dino index is related to the minimum winter temperature: mild winters inhibit diatoms.Significant changes in thermal regime (e.g. by outfalls from power stations).