Decrease in water clarity of the southern and central North Sea during the 20th century.

Area of Science:

• Marine science
• Oceanography
• Environmental science

Background:

• Light penetration is a critical factor linking marine physics with biogeochemistry and ecology.
• Reduced light availability due to decreased water clarity limits photosynthesis and alters marine food web energy flow.

Purpose of the Study:

• To investigate changes in water clarity in the central and southern North Sea over the second half of the 20th century.
• To identify the primary drivers of these changes in water clarity and assess potential future impacts.

Main Methods:

• Analysis of published and unpublished data on water clarity (Secchi depth).
• Comparison of Secchi depth measurements from pre-1950 to post-1950 periods.
• Assessment of changes in suspended sediment and phytoplankton concentrations.

Main Results:

• A significant decrease in average Secchi depth (25%-75%) was observed in the North Sea from pre-1950 to post-1950.
• In summer, the seafloor in the East Anglia region was largely within the photic zone pre-1950, but this has changed.
• Increased suspended sediment concentration, not phytoplankton, has been the main driver of reduced clarity in recent decades.

Conclusions:

• Water clarity in the North Sea has substantially declined, impacting light availability for marine life.
• Factors contributing to increased suspended sediments include altered sea-bed communities, weather patterns, reduced estuarine sediment sinks, and coastal erosion.
• Future increases in storminess may further reduce water clarity, affecting phytoplankton, CO2 fluxes, and fisheries.