The Danish Environmental Protection Agency initiated a 2-year activity in 2017, where DHI GRAS was in charge of satellite analyses of water quality and benthic habitat mapping for selected Danish water areas.

We examined the potential for using remote sensing technologies in connection with the current Danish national NOVANA (National Monitoring and Assessment Programme for the Aquatic and Terrestrial Environment) monitoring program.

The first part of the project looked at the distribution of eelgrass using data from the Sentinel-2 satellites. Eelgrass is an important water quality indicator and has been included in the Danish water plans to assess the environmental state along the Danish coasts. The eelgrass distribution reflects the clarity of water and nutrient richness and thereby indirectly the quality of the water.

The most common practice for mapping eelgrass meadows is to record the distribution along a transect line with a diver, which is costly and unsuitable to derive the complete temporal and spatial eelgrass cover of the Danish coastal waters.

An updated nationwide map of the eelgrass distribution therefore has several potential uses in the Danish water plans:

1) For further development of eelgrass as an indicator of the coastal areas' condition

2) In connection with land-use planning and concrete protection of eelgrass meadows

3) For quantification of eelgrass's ecosystem services

The second part of the project looked at the chlorophyll concentrations in the Danish surface waters using data from Sentinel-2 and Sentinel-3 satellites.

Chlorophyll concentrations are regularly determined by a chemical analysis of water samples which are collected from a ship and subsequently undergone trial processing in the laboratory. Because this type of traditional sampling is costly, usually measurements are taken at only 1 location per water area, approximately every 14 days.

This limits the ability to document the spread and temporal dynamics of important events such as algae blooms and leads to relatively large deviations in seasonal means.

The combination of Sentinel-2 and Sentinel-3 provided a cost-effective way to supplement national in-situ monitoring of surface chlorophyll in the shallow Danish coastal waters, in order to improve the temporal and spatial distribution of chlorophyll concentration.

Satellite technology cannot replace in-situ sampling completely, but it can optimise sampling design and fill gaps.

By bringing together information from various sources, the confidence in the data can be enhanced and greatly improve national assessments.