Less Nutrients but More Phytoplankton: Long-Term Ecosystem Dynamics of the Southern North Sea

Abstract

We here assess long-term trends in marine primary producers in the southern North Sea (SNS) with respect to ongoing regional Earth system changes. We applied a coupled high-resolution (1.5-4.5 km) 3d-physical-biogeochemical regional Earth System model that includes an advanced phytoplankton growth model and benthic biogeochemistry to hindcast ecosystem dynamics in the period 1961-2012. We analyzed the simulation together with in situ observations. Coinciding with the decreasing nutrient level at the beginning of the 1990s, we find a surprising increase in phytoplankton in the German Bight, but not in the more offshore parts of the SNS. We explain these complex patterns by a series of factors which are lacking in many state-of-the-art coupled ecosystem models such as changed light availability and physiological acclimation in phytoplankton. We also show that many coastal time-series stations in the SNS are located in small patches where our model predicts an opposite trend than found for the surrounding waters. Together, these findings call for a reconsideration of current modeling and monitoring schemes.

Climate Change Induced Trends and Uncertainties in Phytoplankton Spring Bloom Dynamics

“…This trend magnitude is comparable with the latest findings on chlorophyll-a historical trends in the North-West Shelf regions (0.4-0.96% year −1 ) Hammond et al (2020), noting that this estimate was considering offshore marine waters, not coastal zones. It is also comparable to Xu et al (2020) who found nearly 20-30% chlorophyll increase in the same study area between 1987 and 2012. Various numerical studies using climate models also project moderate increase in daily mean net primary production between 1980-1999 and 2080-2099 in the shallower southern North Sea (Holt et al, 2014(Holt et al, , 2016Pushpadas et al, 2015).…”

“…Various numerical studies using climate models also project moderate increase in daily mean net primary production between 1980-1999 and 2080-2099 in the shallower southern North Sea (Holt et al, 2014(Holt et al, , 2016Pushpadas et al, 2015). We must emphasize that increasing chlorophyll concentration due to climate change is highly region specific (only occurring in some coastal areas) and very much debated (Xu et al, 2020). In fact, some studies only report shift in spring bloom timing and species composition, but not in magnitude.…”

Analysis of 23 Years of Daily Cloud-Free Chlorophyll and Suspended Particulate Matter in the Greater North Sea

“…It is surrounded by heavily populated countries with important industrial and agricultural activities, resulting in large quantities of nutrients and pollutants being added to the North Sea through riverine inputs (Ducrotoy et al, 2000). As a result, the North Sea has suffered from eutrophication issues during several decades (e.g., Desmit et al, 2020;Xu et al, 2020;Friedland et al, 2021). Despite de-eutrophication policies implemented since the 1990s, such as the EU Marine Strategy Framework Directive (MSFD) which aims at reaching a Good Environmental Status (GES) in European waters, the North Sea still receives relatively high nutrient inputs (nitrogen and phosphorous, Van der Zande et al, 2019b).…”

“…This results in intense phytoplankton blooms occurring every year between March and October, with the southern parts of the North Sea, shallower and more affected by industrial and agricultural activities, presenting more intense blooms (Lancelot et al, 2005;Rousseau et al, 2013;Desmit et al, 2015Desmit et al, , 2020. Phytoplankton blooms are at the basis of the marine food web, driving biogeochemical cycles, producing oxygen and acting as a carbon pump (Xu et al, 2020). Phytoplankton spatial and temporal dynamics can be influenced by several factors, including the availability of nutrients and light, water temperature, and grazing (Capuzzo et al, 2017;Xu et al, 2020).…”

“…Phytoplankton blooms are at the basis of the marine food web, driving biogeochemical cycles, producing oxygen and acting as a carbon pump (Xu et al, 2020). Phytoplankton spatial and temporal dynamics can be influenced by several factors, including the availability of nutrients and light, water temperature, and grazing (Capuzzo et al, 2017;Xu et al, 2020). Spring bloom onset in the open ocean typically occurs when turbulent mixing decays, causing convection to stop (Ferrari et al, 2015).…”