Average concentrations and trends of surface chlorophyll-a in Europe's transitional, coastal and marine waters

Units

Chl-a status in µg/l; trends have no units

Additional information

• Average concentrations of chlorophyll-a (µg/l) and trends were calculated using data from observations in the upper 10m of the water column. Data with bad quality flags (bad value, probably bad value), uncertain (value in excess, uncertain value, missing value) or duplicates were discarded and the 99.9% percentile of the highest values were sorted out.
• Data were aggregated into squares with sides of 1.375km for coastal stations within 20km of the coastline and 5.5km for open water stations more than 20km from the coastline. A masking of 5km inwards from the coast was also applied. The aggregated data were transferred to the official EEA assessment hybrid grid of 20km in coastal areas and 100km for open sea areas.
• Both the summer average between 2018-2023 and the trends were calculated based on yearly averages for each grid cell. The yearly averages were calculated using a combination of estimated marginal means (EMM) and arithmetic means, where the EMM were used for time series containing data from both different years and months and the arithmetic means were used for time-series with yearly observations.
• Temporal trends were estimated with the non-parametric Mann-Kendall test. Only stations having data for at least 5 years were considered. A p-value <0.05 was used to consider a statistically significant trend.
• Growing (summer) season months considered are June to September for stations within the Baltic Sea north of 59 N, and May to September for all other stations.
• The temporal range specified (1980-2023) is the maximum range of dates for raw observations used, but this range varies depending on the availability of observations at each location. Results reflect data availability, which varies significantly across regions and over time.

Temporal coverage
1980-2023