Investigating cumulative impacts: coastal primary production in a warmer, darker, and nutrient-enriched future

The highly productive Norwegian coastal marine ecosystem faces increasing pressure due to human activities and climate change. This study uses ‘what-if’ scenarios to investigate the cumulative impact of multiple drivers on lower trophic level productivity in a high-latitude fjord system with intensive aquaculture production. Using a coupled 3D physical, chemical, and biological high-resolution ecosystem model, we simulate potential changes in productivity in a warmer, darker, and nutrient-enriched future. The study focuses on the Hardangerfjord without aquaculture waste release as a base case, and evaluates, individually and cumulatively, the impact of fish farming, water temperature changes, and alterations in underwater light conditions on net primary production. The combined effects of nutrient release from fish farming and warmer temperatures result in a substantial boost in annual primary production, but we find heterogeneous responses within the fjord system: an increase in the inner fjord due to fish farming and a decrease in coastal regions due to warming. Darkening has a moderating effect on production. Despite an overall increase in production, none of the scenarios result in changes to the environmental status defined by management thresholds. The study highlights the spatially heterogeneous nature of primary production responses, underlining the challenges in establishing effective long-term monitoring programs as required by management regulations. To support management decisions, we propose a spatial index that evaluates how well local sites represent the broader ecosystem state, offering a valuable tool for understanding and managing these dynamic environments under both current and future conditions.