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Dataset for: Tracking freshwater browning and coastal water darkening from the Baltic forests to the Barents Sea

Citation

Opdal, Anders Frugård (2022), Dataset for: Tracking freshwater browning and coastal water darkening from the Baltic forests to the Barents Sea, Dryad, Dataset, https://doi.org/10.5061/dryad.xwdbrv1gq

Abstract

The forest cover of Northern Europe has been steadily expanding during the last 120 years. More terrestrial vegetation and carbon fixation leads to more export to surface waters. This may cause freshwater browning, as more degraded plant-litter end up as chromophoric (coloured) dissolved organic matter. Although most freshwater ultimately drains to coastal waters, the link between freshwater browning and coastal water darkening is poorly understood. Here, we explore this relationship through a combination of centennial records of forest and coastal water clarity, contemporary light measurements in lakes and coastal waters, as well as an ocean drift model. We provide a link between forest cover in Northern Europe and coastal water clarity in the Skagerrak Sea, and suggest that brown coloured freshwater from Northern European catchments dictates coastal water clarity across thousands of kilometres, from the Baltic lakes to the Barents Sea

Methods

The dataset is in its entirety based on previously collected and published data.

The forest cover data was retrieved from the Historic Land Dynamics Assessment (HILDA) database provided by the Department of Geoinformation Science and Remote Sensing, Wageningen University, Netherlands (http://www.geo-informatie.nl/fuchs003/#), and the FAO Global Forest Resources Assessment database (https://fra-data.fao.org/WO/fra2020/home/). For Norway, which is not part of the EU, forest cover was drawn from Norway’s national forest inventory available at https://landsskog.nibio, and described in https://doi.org/10.1186/s40663-020-00261-0

Light attenuation estimates for the Norwegian Coastal Waters where calculated based on Secchi disk depth and salinity. A Secchi-salinity pair was defined as samples taken within 2 days and less than 500 m from each other. Data on Secchi disk measurements and salinity were available from the Secchi Disc Archive (https://www.ices.dk/data/dataset-collections/Pages/Secchi-Disc-Archive.aspx) the ICES Oceanography database (https://data.ices.dk/), and the Norwegian Environmental Agency database (https://vannmiljo.miljodirektoratet.no/).

Chlorophyll a concentrations were obtained based on a series of chlorophyll a estimates and measurements over the period 1912 to 2021. Chlorophyll a measurements were drawn from the ICES Oceanography database (https://data.ices.dk/), and the Norwegian Environmental Agency database (https://vannmiljo.miljodirektoratet.no/), while the estimates were based on two additional sources 1) phytoplankton colour indices (PCI) sampled by continuous plankton recorders  and converted to chlorophyll a concentrations, and 2) chlorophyll a concentrations derived from individual cell counts in 1948 and 1912. Detailed methods are found here:  https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1111%2Fgcb.14810&file=gcb14810-sup-0001-Supinfo.pdf

Freshwater discharge into the Norwegian Coastal Water between Skagerrak and the Barents Sea was drawn from The Norwegian Water Resources and Energy Directorate: https://sildre.nve.no/map

Spectral light absorption measurements in 13 western Norwegian lakes are described here https://doi.org/10.1007/s10021-014-9776-2

Funding

Norges Forskningsråd, Award: 287490