Abstract
The diatom/dinoflagellate index (Dia/Dino index) serves as an indicator in the assessment of the ecological status of the Baltic Sea within the scope of the Marine Strategy Framework Directive. It describes the dominance patterns in the phytoplankton spring bloom. Implementation of this indicator requires a definition of the conditions describing good environmental status (GES). The aim of this study was to determine thresholds for GES for the Dia/Dino index on the basis of historical phytoplankton data from different regions of the Baltic Sea. Data from the first half of the 20th century, corresponding to the pre-eutrophication period, provide an unadulterated reference, as exemplified by nutrient data. Early phytoplankton data showed high dominance of diatoms over dinoflagellates in spring blooms. Diatom dominance relates to a Dia/Dino index > 0.5, which allowed GES threshold of the Eastern Gotland Basin to be set at a Dia/Dino index of 0.5. The consistently very high Dia/Dino index in Kiel and Mecklenburg Bays supported a previously suggested GES threshold value of 0.75. Recent monitoring data revealed a sudden decrease of the Dia/Dino index, especially between 1984 and 1991, and thus a worsening environmental status. This deterioration could be attributed to warming rather than to eutrophication.
Highlights
According to the Marine Strategy Framework Directive (MSFD) of the European Union (EU), “good environmental status” (GES) must be reached in European marine waters by the year 2020 (European Commission, 2008)
This paper focuses on a phytoplankton indicator, the diatom/dinoflagellate index (Dia/Dino index) suggested for the Baltic Sea by Wasmund et al (2017) primarily as an indicator for descriptor 4
Together with the phytoplankton data, we made use of the few historical nutrient data available in order to identify the period of most severe eutrophication
Summary
According to the Marine Strategy Framework Directive (MSFD) of the European Union (EU), “good environmental status” (GES) must be reached in European marine waters by the year 2020 (European Commission, 2008). GES “requires that all relevant human activities are carried out in coherence with the requirement of protecting and preserving the marine environment and the concept of sustainable use of marine goods and services. As a first step some selected ... Indicators for an overall screening of the environmental state...” can be applied (European Commission, 2010). A set of indicators have to be combined in a holistic assessment of the overall environmental status. A specific threshold value (i.e., GES boundary) has to be defined. The Working Group on Good Environmental Status (European Commission, 2015) recommended a common approach, based
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