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

by E. Capuzzo (CEFAS)

Secchi depth_Elisa Capuzzo

Light availability in the water column is a key environmental variable which not only affects the primary producers (phytoplankton, microphytobenthos, macroalgae, seagrass) but also the higher trophic levels of the marine-food web. A simple measurement of water clarity is given by the depth of disappearance of the Secchi disk, a white, 30 cm wide, disk which has been used since the late 1800s (see figure). The Secchi disk is lowered from the side of a vessel (or a pier) into the water and the depth of its disappearance in the water (the Secchi depth) is recorded.
Estimates of Secchi depth measurements, spanning from 1903 until 2010, showed that the southern and central North Sea has become significantly less clear over the second half of the 20th century.
In particular, in some areas, the reduction in Secchi depth from pre-1950 to post-1950 has been of almost 8 m.
This reduction in water clarity could be the result of a combination of different factors such as changes in the sea bed communities (loss/reduction of large molluscs bivalves, including oysters), increased trawling effort, changes in the weather pattern (wind direction and intensity), decreased sink of sediments in estuaries, and increased coastal erosion.

 

Capuzzo E., Stephens D., Silva T., Barry J., Forster RM. (2015). Decrease in water clarity of the southern and central North Sea during the 20th century. Global Change Biology, in press (doi: 10.1111/gcb.12854).