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Discussion papers
https://doi.org/10.5194/os-2019-52
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-52
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Jun 2019

Submitted as: research article | 11 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Increasing turbidity in the North Sea during the 20th century due to changing wave climate

Robert J. Wilson1 and Michael R. Heath2 Robert J. Wilson and Michael R. Heath
  • 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK
  • 2Department of Mathematics and Statistics, University of Strathclyde, Glasgow, Scotland, G1 1XH

Abstract. Data on Secchi disk-depth (the depth at which a standard white disk lowered into the water just becomes invisible to a surface observer) show that water clarity in the North Sea declined during the 20th century, with likely consequences for marine primary production. However, the causes of this trend remain unknown. Here we analyze the hypothesis that changes in the North Sea's wave climate were largely responsible, by increasing the concentrations of suspended particulate matter (SPM) in the water column through re-suspension of seabed sediments. First, we analyzed the broad-scale statistical relationships between SPM and bed shear stress due to waves and tides. We used hindcasts of wave and current data to construct a space-time dataset of bed shear stress between 1997 and 2017 across the northwest European Continental Shelf, and compared the results with satellite-derived SPM concentrations. Bed shear stress was found to drive most of the inter-annual variation in SPM in the hydrographically mixed waters of the central and southern North Sea. We then used a long-term wave reanalysis to construct a time series of bed shear stress from 1900 to 2010. This shows that bed shear stress increased significantly across the shelf over this period, explaining more than half of the observed decline in water clarity over this period. Wave-driven processes are rarely included in projections of climate change impacts on marine ecosystems, but our analysis indicates that this should be reconsidered for shelf sea regions.

Robert J. Wilson and Michael R. Heath
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Status: final response (author comments only)
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Robert J. Wilson and Michael R. Heath
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Short summary
The North Sea became much less clear during the 20th century, with potential consequences for primary production. This study analyses the hypothesis that changes in wave regime were a key driver of this change. We hindcast bed shear stress over the 20th Century using a long-term wave reanalysis. Shear stress increased by over 20 % in large parts of the southern and central North Sea during the 20th Century. This can explain the majority of the observed long-term decline in water clarity.
The North Sea became much less clear during the 20th century, with potential consequences for...
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