While Saccharina latissima seaweed is currently cultivated in the Netherlands at a small scale, there is a major drive towards increasing sustainable offshore cultivation. Future wind farm areas are potential development locations. Harvesting seaweed leads to nutrient removal from the ecosystem. If this is done on a large scale, this can subsequently affect phytoplankton primary production, the basis of the marine food web. In this study, we investigate the spatial variability in ecosystem responses to seaweed cultivation in the Dutch North Sea in designated wind development areas. We focus on the interaction of seaweed and phytoplankton productivity with respect to nutrients and the effect of size, location and distribution of farms. We integrated a module simulating seaweed nutrient uptake and growth dynamics into a fully coupled 3D hydrodynamic-water quality model of the North Sea. We ran different cultivation scenarios over a production year and compared them to a run without seaweed cultivation. Results showed that significant upscaling of seaweed production (around 800 km²₎ in part of the designated areas for offshore wind development would have a strong impact on the ecology, with local decreases in spring phytoplankton production as high as 30%. Depending on locations, seaweed cultivation can also have a very large footprint in terms of impact on phytoplankton primary production. Models such as these benefit the societal debate about acceptable levels of upscaling.

Primary Presenter: Luca van Duren, DELTARES (luca.vanduren@deltares.nl)

Authors:

Lauriane Vilmin, Deltares  (lauriane.vilmin@deltares.nl)

Reinier Nauta, Wageningen Marine Research  (reinier.nauta@wur.nl)

Henrice Jansen, Wageningen University  (henrice.jansen@wur.nl)