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

Abstract

Light in the marine environment is a key environmental variable coupling physics to marine biogeochemistry and ecology. Weak light penetration reduces light available for photosynthesis, changing energy fluxes through the marine food web. Based on published and unpublished data, this study shows that the central and southern North Sea has become significantly less clear over the second half of the 20th century. In particular, in the different regions and seasons investigated, the average Secchi depth pre-1950 decreased between 25% and 75% compared to the average Secchi depth post-1950. Consequently, in summer pre-1950, most (74%) of the sea floor in the permanently mixed area off East Anglia was within the photic zone. For the last 25+ years, changes in water clarity were more likely driven by an increase in the concentration of suspended sediments, rather than phytoplankton. We suggest that a combination of causes have contributed to this increase in suspended sediments such as changes in sea-bed communities and in weather patterns, decreased sink of sediments in estuaries, and increased coastal erosion. A predicted future increase in storminess (Beniston et al., 2007;Kovats et al., 2014) could enhance the concentration of suspended sediments in the water column and consequently lead to a further decrease in clarity, with potential impacts on phytoplankton production, CO 2 fluxes, and fishery production.

Observing and modelling phytoplankton community structure in the North Sea

“…This demand for detailed information applies to the North Sea, with users and policy makers requiring information about topics including eutrophication and nutrient ratios (Skogen et al, 2014), productivity in relation to fisheries (Chassot et al, 2007), harmful and nuisance algal blooms (Blauw et al, 2010;Kurekin et al, 2014), water clarity (Dupont and Aksnes, 2013;Capuzzo et al, 2015), biodiversity (Brandsma et al, 2013), effects of climate change (van der Wakelin et al, 2015a), effects of trawling (Allen and Clarke, 2007; , and impacts of marine renewable energy generation (van der Molen et al, 2014. In particular, indicators of Good Environmental Status (GES) are required in the context of the Marine Strategy Framework Directive (MSFD; Borja et al, 2013).…”

Stability of marine phytoplankton communities facing stress related to global change: Interactive effects of heat waves and turbidity

“…In addition to other effects of global change, like ocean acidification and sea level rise, increasing windiness and rainfall reduced light availability in the North Sea over the second half of the 20th century (Capuzzo et al, 2015). Further sources for additional sediment input into the ocean are coastal erosion and rivers.…”

“…Further sources for additional sediment input into the ocean are coastal erosion and rivers. Although increasing coastal erosion is related to changes in wind and wave activity (Capuzzo et al, 2015), evidence for increased sediment fluxes from rivers to oceans varies strongly between world regions depending for instance on precipitation and the existence of dams and water reservoirs (Walling, 2009). Yet, the predicted increase in precipitation will likely affect sediment run-off and resuspension.…”

A Review of the Tools Used for Marine Monitoring in the UK: Combining Historic and Contemporary Methods with Modeling and Socioeconomics to Fulfill Legislative Needs and Scientific Ambitions

“…These data are used within the OSPAR eutrophication assessment (Foden et al, 2011). In addition, these data have enabled better design of robust monitoring programmes (Heffernan et al, 2010), validation of models (Große et al, 2016;van der Molen et al, 2016) and satellite marine products (Neukermans et al, 2012), and for studying ecosystem behavior (Devlin et al, 2009;Blauw et al, 2012;Capuzzo et al, 2013Capuzzo et al, , 2015Johnson et al, 2013;Hull et al, 2016). Fixed-point moorings are also used for making measurements of the wave climate.…”