Riverine impact on future projections of marine primary 1 production and carbon uptake 2

Riverine transport of nutrients and carbon from inland waters to the coastal and finally the open ocean 12 alters marine primary production (PP) and carbon (C) uptake, not only regionally but also globally. So far, this 13 contribution is represented in the state-of-the-art Earth system models with limited effort. Here we assess changes 14 in marine PP and C uptake projected under the Representative Concentration Pathway 4.5 climate scenario using 15 the Norwegian Earth system model, with four riverine configurations: deactivated, fixed at a contemporary level, 16 coupled to simulated freshwater runoff, and following four plausible future scenarios. The inclusion of riverine 17 nutrients and carbon improves the modelled contemporary spatial distribution relative to observations, especially 18 on the continental margins (5.4% reduction in root mean square error [RMSE] for PP) and in the North Atlantic 19 region (7.4% reduction in RMSE for C uptake). Riverine nutrient inputs alleviate nutrient limitation, especially 20 under future warmer conditions as stratification increases, and thus lessen the projected future decline in PP by 21 up to 0.6 PgC yr-1 (27.3%) globally depending on the riverine configuration. The projected C uptake is enhanced 22 along continental margins where increased PP, due to riverine nutrient inputs, dominates over the CO2 outgassing 23 owing to riverine organic matter inputs. Conversely, where the riverine organic matter inputs dominate over the 24 nutrient inputs, the projected C uptake is reduced. The large range of the riverine input across our four riverine 25 configurations does not transfer to a large uncertainty of the projected global PP and ocean C uptake, suggesting 26 that transient riverine inputs are more important for high-resolution regional studies such as in the North Atlantic 27 and along the continental margins. 28