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Inflows of coloured terrestrial organic matter cause seawater browning and reduced phytoplankton production in subarctic coastal ecosystems, potentially deteriorating the nutritional quality of marine food webs. We analyzed the fatty-acid (FA) compositions of seston and the zooplankton taxa Eurytemora affinis and cladocerans at three locations of the northern Baltic Sea. At the coastal and northerly locations, salinity and phosphorus concentrations were low, while concentrations of humic substances (i.e., terrestrial organic matter) were high. The southerly location showed the opposite trend. The ratio between alga-specific [omega]3 polyunsaturated FA and terrigenous monounsaturated FA (MUFA) in Eurytemora decreased from south to north, as did the ratio between the alga-specific docosahexaenoic acid (DHA) and terrigenous MUFA in cladocerans. With increasing humic substances, the biomass of DHA-rich phytoplankton decreased and the zooplankton MUFA content increased. Our results indicate that coloured terrestrial organic matter alters the phytoplankton composition, consequently affecting the zooplankton nutritional quality. Les influx de matiere organique terrestre causent le brunissement de l'eau de mer et reduisent la production du phytoplancton dans les ecosystemes cotiers subarctiques, ce qui peut se traduire par une deterioration de la qualite nutritionnelle des reseaux trophiques marins. Nous avons analyse la composition d'acides gras du seston, du taxon de zooplancton Eurytemora affinis et de cladoceres dans trois sites de la mer Baltique septentrionale. Dans les sites cotier et nordique, la salinite et les concentrations de phosphore etaient faibles, alors que les concentrations de substances humiques (c.-a-d. matiere organique terrestre) etaient elevees. Le site plus au sud presentait des tendances inverses. Le rapport des acides gras polyinsatures [omega]3 specifiques aux algues et des acides gras monoinsatures (AGMI) terrigenes chez Eurytemora diminuait du sud au nord, tout comme le rapport de l'acide docosahexanorque (ADH) specifique aux algues et des AGMI chez les cladoceres. Plus l'abondance de substances humiques etait elevee, plus la biomasse de phytoplancton riche en ADH etait faible et plus le contenu en AGMI du zooplancton etait important. Nos resultats indiquent que de la matiere organique terrestre coloree modifie la composition du phytoplancton et a par consequent une incidence sur la qualite nutritionnelle du zooplancton. [Traduit par la Redaction], 1. Introduction Inputs of coloured terrestrial organic matter to subarctic marine environment, such as the northern Baltic Sea, cause browning of seawater, which enhances light attenuation and reduces phytoplankton production [...]

Warming in the Arctic is predicted to change freshwater biodiversity through loss of unique taxa and northward range expansion of lower latitude taxa. Detecting such changes requires establishing circumpolar baselines for diversity, and understanding the primary drivers of diversity.We examined benthic macroinvertebrate diversity using a circumpolar dataset of >1,500 Arctic lake and river sites. Rarefied α diversity within catchments was assessed along latitude and temperature gradients. Community composition was assessed through region‐scale analysis of β diversity and its components (nestedness and turnover), and analysis of biotic–abiotic relationships.Rarefied α diversity of lakes and rivers declined with increasing latitude, although more strongly across mainland regions than islands. Diversity was strongly related to air temperature, with the lowest diversity in the coldest catchments. Regional dissimilarity was highest when mainland regions were compared with islands, suggesting that connectivity limitations led to the strongest dissimilarity. High contributions of nestedness indicated that island regions contained a subset of the taxa found in mainland regions.High Arctic rivers and lakes were predominately occupied by Chironomidae and Oligochaeta, whereas Ephemeroptera, Plecoptera, and Trichoptera taxa were more abundant at lower latitudes. Community composition was strongly associated with temperature, although geology and precipitation were also important correlates.The strong association with temperature supports the prediction that warming will increase Arctic macroinvertebrate diversity, although low diversity on islands suggests that this increase will be limited by biogeographical constraints. Long‐term harmonised monitoring across the circumpolar region is necessary to detect such changes to diversity and inform science‐based management. [ABSTRACT FROM AUTHOR]