In the previous assessment (HM Government, 2012), no significant trend in suspended particulate matter was identifiable for any of the Charting Progress 2 regions. New analysis using satellite observations for suspended particulate matter over 1998-2015 show significant increases in 5 out of 10 Charting Progress 2 regions that is consistent with a background longer term increase since the beginning of the 20th Century.

Background

Suspended particulate matter in shelf seas determines turbidity in the water column, material transfers to the seabed, and the flux and fate of associated pollutants and contaminants. These in turn constrain primary production; influence air-sea heat transfer; determine sedimentation rates and biogeochemical transfers from the water column to seabed; affect the productivity of the benthos; and determine oxygen levels in bottom waters. Shifts in the fate of suspended particulate matter between export to the pelagic food web or deposition to the benthos are also critical processes for eutrophication.

Different physical and biological processes control suspended particulate matter and turbidity in UK waters. Suspended Sediment Concentration (known as SSC) is the result of the balance between resuspension and deposition. Resuspension is largely due to bed shear stress caused by tidal currents and waves. Deposition depends on settling of suspended particulate matter, mainly governed by the size and density of the particles which is further modulated the aggregation of particles.

Monitoring Turbidity

Monitoring suspended particulate matter in-situ across the scale of the UK seas can be difficult, partly due to their inhomogeneity. Remote ocean colour satellite images are a useful tool to measure the temporal and spatial scales of variability.

In the previous assessment (HM Government, 2012), for which satellite and in situ observations were analysed, no significant trend was observed in any of the Charting Progress 2 regions. For this assessment, only remote sensing observations were considered using daily products generated by Ifremer from multiple sensors (SeaWifs, MODISv, MERIS and VIIRS) and interpolated to a 1.1km grid.

Long-term change

Recent studies have identified a trend of increasing turbidity in UK seas since the beginning of the 20th century (Dupont and Asknes, 2013, Capuzzo and others, 2015), thought to be linked to a combination of increased winds in the second half of the 20th century, changes to land use and river management, draining of wetlands, and increased marine activities such as trawling.

Results

Conclusions

Analysis of time series averaged for the Charting Progress 2 regions identified positive trends in the annual average concentration of suspended particulate matter on the Scottish continental shelf, the Northern North Sea, Minches and Western Scotland, Rockall Trough and South West Approaches. Seasonal trends are evident in winter for the off-shelf regions and for spring to autumn periods on the shelf. The longer-term context for these 18-year trends is the increasing turbidity of UK waters since the beginning of the 20th VCentury (Dupont and Asknes, 2013, Capuzzo and others, 2015).

Knowledge gaps

Further research is needed to confidently attribute the causes of the trends in suspended particulate matter concentration that can now be identified over a wide spatial area from satellites.

There is a need to improve in situ validation for remote sensing estimates, particularly matching the observations to the location and timing of satellite overpasses. Better coverage of sub-surface suspended particulate matter and turbidity would add information that is beyond the reach of satellite sensors.

A latitudinal bias in the satellite estimates due to winter sun-angle limits useful data from December and January north of 57°N when sea condition is normally worse and leads to a local underestimate in mean annual suspended particulate matter values.

Satellite estimates can be biased towards periods of clear skies, which are generally associated with better sea conditions and lower suspended particulate matter values. In coastal areas there can be less data available in winter due to greater cloud cover.

References

The information, figures and all the results presented in this assessment has been extracted from the following reports:

Strategic Environmental Assessment programme Suspended Sediments report:

Silva T, Biermann L, Rees J (2016) ‘Suspended Sediment Climatologies around the UK’ Report for the UK Department of Energy and Climate Change’s offshore energy Strategic Environmental Assessment programme (viewed on 14 January 2019)

Dataset of monthly averaged non-algal Suspended Particulate Matter concentrations on the UK shelf waters from satellite observations:

Silva T (2016) ‘Monthly averages of non-algal Suspended Particulate Matter concentrations’ doi:10.14466/CefasDataHub.31 (viewed on 14 January 2019)

Charting Progress 2 report for Suspended Particulate Matter and Turbidity:

UKMMAS (2010). ‘Suspended Particulate Matter and Turbidity’ Charting Progress 2 Feeder Report: Ocean Processes, Huthnance J (editor). Department for Environment Food and Rural Affairs on behalf of the UK Marine Science Co-ordination Committee (viewed on 14 January 2019)

Other Cited literature

Bakker AM, van den Hurk BJ (2012) ‘Estimation of persistence and trends in geostrophic wind speed for the assessment of wind energy yields in Northwest Europe’ Climate dynamics, 39(3-4): 767-782 (viewed on 30 January 2019)

Capuzzo E, Stephens D, Silva T, Barry J, and Forster RM (2015) ‘Decrease in water clarity of the southern and central North Sea during the 20th century’ Global Change Biology, 21: 2206–2214. doi:10.1111/gcb.12854 (viewed on 14 January 2019)

Dupont N, and Aksnes DL.(2013) ‘Centennial changes in water clarity of the Baltic Sea and the North Sea’ Estuarine, Coastal and Shelf Science, 131:282-289 (viewed on 14 January 2019)

Acknowledgements

Assessment Metadata

Please contact marinestrategy@defra.gov.uk for metadata information