Modelling bottom-up effects of climate change on primary production in the North Sea

Due to its geographical location, the North Sea is one of the busiest seas worldwide, undergoing increasing pressure due to continuous developments of offshore human activities. These activities affect the North Sea ecosystem, for example by introducing new habitats/species or infrastructure into previously unobstructed environments. At the same time, climate change has already been affecting the North Sea ecosystem, leading to observed changes in species distribution. These changes may also affect physical processes such as stratification. Stratification is one of the main factors influencing primary production, which constitutes the foundation of the marine food web. To be able to mitigate these effects, it is crucial to understand the bottom-up, cumulative impacts of anthropogenic climate change and offshore activities on marine ecosystems. With this goal, we adapted and nested a 3D process-based hydrodynamics and water quality model of the North Sea (3D DCSM-FM) within a global Earth System Model (CMCC-ESM2). We simulated two contrasting future climate change scenarios: one representing the situation of a society focused on global sustainability, with low carbon emissions (SSP1-RCP2.6), and one of a society focused on global markets, with abundant exploitation of fossil-fuels and high carbon emissions (SSP5-RCP8.5). We investigated how the two different scenarios impact important abiotic and water quality variables, up to the end of the century.

As expected, our model results show clear surface water temperature increases for both scenarios, above 3°C by 2100. The Northern part of the North Sea is mainly driven by exchange with the Atlantic ocean. In the Northern and Eastern North Sea and in the Dogger Bank, our model simulates an increase in temperature stratification, leading to decreases in near-surface dissolved inorganic nutrient concentrations, chlorophyll-a and growing-season primary production (~-20-30% by 2100 on average in the Northern North Sea). The Southern part of the North Sea, especially along the coast, is driven by an along-coast current from the English channel and large, nutrient-rich freshwater inputs. The Southern North Sea shows a more spatially-variable response to the simulated scenarios in terms of nutrient concentrations, chlorophyll-a and primary production, with areas of increase and areas of decrease. Overall, the Southern North Sea shows a small increase in growing-season primary production for scenario SSP1-RCP2.6 by 2100 (+6%), while it shows a decrease for SSP5-RCP8.5 (-14%).

Our model offers the resolution to understand the effects of local pressures within a globally changing climate. Such tools are crucial to support the management of future offshore activities, ensure their long-term effectiveness and minimize their impacts on the ecosystem.