Turbidity on the European Shelf: 20th Century trends and future changes caused by large-scale tidal energy
Water clarity levels are a key influence on primary production levels in shelf seas. Recent work has concluded that they declined significantly in the North Sea during the 20th Century, and that increases in suspended particulate matter (SPM) concentrations led to a reduction in primary production in the last 25 years. This talk will first analyze the hypothesis that changes in the wave climate were largely responsible, due to an increase in SPM. Second, projections of the impact of large-scale tidal energy on SPM will be outlined. We first analyzed the broad-scale statistical relationships between SPM and bed shear stress due to waves and tides. We used current velocity data from a shelf-wide hydrodynamic model, and a wave reanalysis 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 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, potentially explaining more than half of the observed decline in water clarity over this period. The impact of tidal turbines on SPM was then analyzed for a scenario where tidal arrays in Scottish waters provide approximately 10% of UK electricity demand. SPM is projected to reduce by between 0.5 and 1% in the northern North Sea. This is likely to have a net positive effective on ecosystems. However, the changes due to large-scale tidal energy are small in comparison with those due to changing historical wave climate.