This indicator illustrates the geographical distribution and trends in mean summer (May to September) concentrations of chlorophyll-a (µg/l) in the upper 10m of the water column in European seas. Chlorophyll-a concentrations are used as a biological indicator of the indirect effects of nutrient enrichment and, consequently, eutrophication. The objective of the indicator is to demonstrate the effects of measures taken to reduce discharges of nutrients on phytoplankton concentrations.
Under the WFD, target chlorophyll concentrations/ranges have been defined for different water types and water categories, including coastal and transitional water bodies in Commission Decision (2018/229). Under the MSFD, threshold values for coastal waters are set in accordance with the WFD, and beyond coastal waters threshold values must be consistent with those under the WFD. Member States establish those values through (sub)regional cooperation.
The indicator presents the geographical distribution and trends in mean summer (May to September) concentrations of chlorophyll-a (µg/l) in the upper 10m of the water column in European seas.
The two main sources of data for this indicator are the International Council for the Exploration of the Sea (ICES) and the European Marine Observation and Data Network (EMODnet) data sets. Data kept by the ICES are collected through the Eionet Central Data Repository (Eionet CDR) from the marine conventions and are therefore sub-samples of national data assembled for the purpose of providing comparable indicators of the state of and impacts on transitional, coastal and marine waters (TCM data) on a Europe-wide scale. Data supplied by EMODnet are combined with ICES data. In cases where both ICES and EMODnet data are available for the same station (defined by position and time), ICES data were used.
The primary aggregation involves:
· identifying (clusters of) stations and assigning them to marine (sub)regions;
· creating statistical estimates for each combination of station and year.
Geographical classification: sea region, coastal or offshore and station
Stations are defined geographically by position, given as longitude and latitude in decimal degrees. All geographical positions defined in the data are assigned to marine (sub)region by coordinates. The reported data do not always contain reliable and consistent station identifiers, which may fragment the time series. To improve the aggregation into time series, data are aggregated into squares of 0.01 degree × 0.01 degree for coastal stations (within 20km of the coastline) and 0.05 degree × 0.05 degree for open water stations. These squares are considered ‘locations’.
Raw concentration values are log-transformed to calculate averages and trends. Results are re-transformed to a linear scale to plot the outcoming results as concentration values. Average concentrations by location and by day are first calculated to flatten observations made at different depths and hours or even minutes in some cases. These daily averages are then aggregated by year and location. Only stations with data for at least 5 years and with data for at least the last 5-year period (2015-2019) were considered. Averages and temporal trends are calculated for each selected location.
Data with bad quality flags (bad value, probably bad value) or of uncertain quality (value in excess, uncertain value, missing value) are discarded. Only data from depths of <10m and for the months May to September are considered.
Trend analysis
Trends are estimated with the non-parametric Mann-Kendall test. A p-value of <0.05 is considered to indicate a statistically significant trend. Gap filling in a time series is not necessary for trend analysis that uses the Mann-Kendall test.
Trend analysis was carried out for each station in regions for which there were at least some data from the past 5-year period (2015 or later) and data for 5 or more years from the period since 1980. Trends were detected in each time series using the non-parametric Mann-Kendall trend test. The tests reported here are two-sided (testing for both positive and negative trends). Data series with p-values of <0.05 are reported as significantly positive or negative. The test analyses only the direction and significance of the change, not the size of the change.
Methodology for gap filling
Not applicable
The analysis of chlorophyll-a values and their change over time is key to assessing progress towards better marine and coastal water quality in line with EU policy objectives, such as those under the MSFD and WFD. The objective of the indicator is to demonstrate the effects of measures taken to reduce discharges of nutrients on phytoplankton concentrations. Measurements of chlorophyll-a, used as an estimate of phytoplankton biomass, represents the biological eutrophication indicator with the best geographical coverage at the European level and are included in most eutrophication monitoring programs. The primary effect of eutrophication is excessive growth of plankton algae, which increases the concentration of chlorophyll-a. The negative effects of excessive phytoplankton growth are 1) changes in species composition and functioning of the pelagic food web; 2) increased sedimentation of organic material and 3) increase in oxygen consumption that may lead to oxygen depletion and the consequent changes in community structure or death of benthic fauna. Eutrophication can also promote harmful algal blooms that may cause discoloration of the water, foam formation, death of benthic fauna and fish, or shellfish poisoning of humans. The marine regions of Europe have different sensitivities to eutrophication, determined by their physical characteristics. The Baltic and Black Seas have high sensitivity to eutrophication due to limited water exchange with connecting seas.
The main aim of the marine indicators set is to support and evaluate efficiency of the Marine Strategy Framework Directive (MSFD) and the UN Agenda 2030 (SDG 14), as well as the Maritime Spatial Planning Directive (MSPD) and other EU and international environmental policies. The objective is to illustrate long-term trends where possible.
The WFDrequires the achievement of good ecological status or the good ecological potential of transitional and coastal waters across the EU and the MSFD requires the achievement or maintenance of good environmental status in European sea basins by the year 2020. The MSFD’s objective to achieve good environmental status for the EU’s marine waters establishes, under descriptor D5 on eutrophication, a primary criterion (D5C2) to ensure that ‘chlorophyll concentrations are not at levels that indicate adverse effects of nutrient enrichment’. Threshold values should be set to assure Good Environmental Status (GES) and be harmonised with the WFD. Commission Decision (EU) 2018/229 summarises the results of the third phase of the WFD intercalibration exercise, providing different threshold values for chlorophyll-a depending on the regional sea and water typology.
Other EU directives are also related to the control of eutrophication by aiming to reduce the loads and impacts of the nutrients. These include the Nitrates Directive, aimed at reducing nitrate pollution from agricultural sources; the Urban Waste Water Treatment Directive, aimed at reducing pollution from sewage treatment works and certain industries; and the Integrated Pollution Prevention and Control Directive, aimed at controlling and preventing the pollution of water from industry. In addition, the EU biodiversity strategy 2030, Farm to Fork and Zero Pollution Action Plan are main policies under the European Green Dea setting ambitious targets for reducing nutrients from agriculture. Also, under the EGD, and with respect to climate change, the European Commission adopted the Climate Law and a new EU Adaptation Strategy with the overall aim of contributing to a more climate-resilient Europe.
EU policies and legislation also support the implementation of the Regional Seas Conventions and Action Plans (RSCAPs) — the Oslo Paris Convention (OSPAR), the Helsinki Convention (HELCOM), the Barcelona Convention (UNEP-MAP) and the Bucharest Convention, which also outline measures that aim to reduce the loads and impacts of nutrients.
Methodology uncertainty
In the current application, two growing seasons are distinguished, one for the northern part of the Baltic Sea (June-September) and one for the southern part of the Baltic Sea, the North-East Atlantic, the Mediterranean Sea and the Black Sea (May-September). It is questionable whether using one growing season for all waters that range geographically from the Mediterranean and the Black Sea to the North Sea and the Baltic Sea is appropriate. Moreover, currently only surface concentrations are considered. However, in the Black Sea, not only do the chlorophyll concentrations show peaks in late winter, late spring and autumn, these peaks not only occur at the surface but also in the subsurface layer.
A station-based analysis may not be the most appropriate approach. An assessment area analysis approach by regional sea where assessment areas have already been identified would provide more consistent data and allow for the use of agreed threshold levels for chlorophyll-a by the regional conventions, as is currently assessed for example by HELCOM and OSPAR.
Geographical comparability
The natural ranges of chlorophyll-a values vary greatly depending on the geographical characteristics of monitored locations. Thus, geographical comparisons of results should be made only within regional seas and between similar water types (i.e. transitional, coastal, or offshore locations).
Comparability over time
The natural ranges of chlorophyll-a values also vary greatly depending on the season. However, as assessments have been made on a yearly basis and only for summer months, temporal comparability is possible. It is also important to note that the temporal range of data at different locations varies depending on the availability of data, although all locations have at least 5-yearly observations and have been updated within the last 5-year period (2015-2019).
