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Arctic shelf seas receive greater quantities of river runoff than any other ocean region and are experiencing increased freshwater loads and associated terrestrial matter inputs since recent decades. Amplified terrestrial permafrost thaw and coastal erosion is exposing previously frozen organic matter, enhancing its mobilization and release to nearshore regions. Changing terrestrial dissolved organic matter (terr-DOM) loads and composition may alter shelf primary productivity and respiration, ultimately affecting net regional CO2 air–sea fluxes. However, the future evolution of Arctic Ocean climate feedbacks are highly dependent upon the biological degradability of terr-DOM in coastal waters, a factor often omitted in modelling studies. Here, we assess the sensitivity of CO2 air–sea fluxes from East Siberian Arctic Shelf (ESAS) waters to changing terr-DOM supply and degradability using a biogeochemical model explicitly accounting for bacteria dynamics and shifting terr-DOM composition. We find increasing terr-DOM loads and degradability trigger a series of biogeochemical and ecological processes shifting ESAS waters from a net sink to a net source of CO2, even after accounting for strengthening coastal productivity by additional land-derived nutrients. Our results suggest that future projected inputs of labile terr-DOM from peat and permafrost thaw may strongly increase the CO2 efflux from the Arctic shelf sea, causing currently unquantified positive feedback to climate change. [ABSTRACT FROM AUTHOR]

This study investigates the influence of sea ice on eutrophication in the Baltic Sea ecosystem by comparing simulations from 1953 to 2017, with ice and without ice cover. We assessed the impact from ice cover by using eutrophication indicators defined by the Marine Strategy Framework Directive (MSFD), the Dia/Dino index and the newly proposed Trophic Transfer Index (TTI). Five out of six indicators suggest a negative impact of sea ice on the eutrophication status of the Baltic Sea, with a marked increase in ice impact observed in the early 1970s, followed by a decline in the late 1980s. The linear correlation between ice impact on MSFD indicators and nutrient loads suggests that the influence of ice becomes more pronounced under conditions of elevated nutrient loads. Around 1988, both the TTI and Dia/Dino index indicate eutrophication amplification, with ice cover significantly impacting both indicators (approximately 30 %), leading to a shift towards dinoflagellate dominance. While ice influences plankton timing and ecosystem structure, nutrient loads remain the primary driver of the timing of spring and summer blooms. According to our study the reduction in sea ice cover due to climate change, could contribute to the faster achievement of the Baltic Sea's Good Environmental Status., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)