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.
Highlights
Marine phytoplankton constitute the fundamental basis of the marine food web and biogeochemical cycles
We have presented and analyzed a long-term simulation of ecosystem dynamics in the southern North Sea
In situ observations match locally deviating trends in our reconstruction, calling into question the representability of station data for highly variable coastal–shelf ecosystems
Summary
Marine phytoplankton constitute the fundamental basis of the marine food web and biogeochemical cycles. Phytoplankton mediates around half of net primary production (NPP) on Earth (Field et al, 1998; Falkowski and Raven, 2007). Changes in primary production impact higher trophic levels, from zooplankton to fish, marine mammals, and seabirds (Chassot et al, 2010; Capuzzo et al, 2018). Coasts and shelf seas generally reveal higher NPP due to shallower water depth and high nutrient influx by upwelling or river inflow. The North Sea, a shallow shelf sea to the eastern North Atlantic is one of the most utilized and highly productive sea areas in the world (Ducrotoy et al, 2000; Emeis et al, 2015). The southern North Sea (SNS) features low water depth, strong tidal mixing, diminished ocean influence, and high riverine nutrient inflow. Nutrient loads were elevated from the 1950s to the 1980s due to increased wastewater discharge and use of fertilizers (eutrophication) but declined in the recent decades (Painting et al, 2013; Burson et al, 2016) due to regulations and better wastewater treatment
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