Aarhus University Seal / Aarhus Universitets segl

No. 265: Mapping of eelgrass meadows with orthophotos

Ørberg, S.B., Groom, G.B., Kjeldgaard, A., Carstensen, J., Rasmussen, M.B., Clausen, P. & Krause-Jensen, D. 2018. Kortlægning af ålegræsenge med ortofotos - muligheder og begrænsninger. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 68 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 265. http://dce2.au.dk/pub/SR265.pdf


Maps of eelgrass distribution are important for optimal management of eelgrass meadows and also contribute with synergy between e.g. the waterframework directive that uses eelgrass as an indicator of ecological quality, and the birds directive, that has a focus on eelgrass as food for water birds and therefore demands quantification of the distribution area. The many other ecosystem functions and - services of eelgrass also scale directly with the distribution area.

The project tested the possibility of quantifying the area distribution of eelgrass based on the national orthophotos that Aarhus University (AU), in other context, has bought the rights to use. Based on orthophotos from the summers of 2012, 2014 and 2016, in combination with monitoring data on eelgrass, the project developed image analysis techniques and analysed orthophotos from selected, important eelgrass areas: Nibe-Gjøl Bredninger in Limfjorden, Saltholm incl. the Amager coast facing Saltholm, the Southfunen Archipelago and Roskilde Fjord.

The study demonstrates a large potential for mapping eelgrass based on the orthophotos since 1) the analysis methods exhibit relatively high precision, 2) the orthophotos have extensive coverage (in principle national coverage), 3) high spatial resolution (16-20 cm), 4) large temporal coverage (every 2nd summer also back in time), providing the basis for analysing temporal trends, and 5) AU/The Ministry have access to the orthophotos. However, the study also points at important limitations in the current ortophoto deliveries in terms of insufficient image coverage and lacking color-normalization from the coastal zone. These limitations could partly be overcome by entering a different consortium-agreement with the supplier since image data and color normalization already exist for a much larger part of the coastal zone than what the consortium AU is part of has bought access to.

The area distribution of eelgrass can potentially supplement the "eelgrass depth limit" as an indicator of environmental status of coastal areas by supplying important information on the eelgrass area in shallower waters. This, however, requires similar documentation as for the eelgrass depth limit, i.e. analyses of how the eelgrass area responds to changes in water quality/pressures and which eelgrass area corresponds to different categories of environmental status in given water bodies.