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Abstract. Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibits relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two timeseries from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. There is no standardized method for determining the functional composition of seston and the fractions of POC, PON and PP associated with phytoplankton, therefore any such information has to be obtained by indirect means. In this study, a generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 76 and 55 %. This regression model approach builds on assumptions (e.g. constant POC:Chl-a ratio) and the robustness of the estimates were explored with a bootstrap analysis. In addition the autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were, in general, lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above, or close to, the Redfield ratio. This is due to preferential Correspondence to: H. Frigstad (helene.frigstad@gfi.uib.no) remineralization of nutrients, and the P content mainly governed the difference between the sestonic fractions. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. Sestonic C:N:P ratios close to the Redfield ratios should not be used as an indicator of phytoplankton physiological state, but could instead reflect varying contributions of sestonic fractions that sum up to an elemental ratio close to Redfield.

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Studying more than 3600 observations of particulate organic carbon (POC) and particulate organic nitrogen (PON), we evaluate the applicability of the classic Redfield C:N ratio (6.6) and the recently proposed Sterner ratio (8.3) for the Arctic Ocean and pan-Arctic shelves. The confidence intervals for C:N ranged from 6.43 to 8.82, while the average C:N ratio for all observations was 7.4. In general, neither the Redfield or Sterner ratios were applicable, with the Redfield ratio being too low and the Sterner ratio too high. On a regional basis, all northern high latitude regions had a C:N ratio significantly higher than the Redfield ratio, except the Arctic Ocean (6.6), Chukchi (6.4) and East Siberian (6.5) Seas. The latter two regions were influenced by nutrient-rich Pacific waters, and had a high fraction of autotrophic (i.e. algal-derived) material. The C:N ratios of the Laptev (7.9) and Kara (7.5) Seas were high, and had larger contributions of terrigenous material. The highest C:N ratios were in the North Water (8.7) and Northeast Water (8.0) polynyas, and these regions were more similar to the Sterner ratio. The C:N ratio varied between regions, and was significantly different between the Atlantic (6.7) and Arctic (7.9) influenced regions of the Barents Sea, while the Atlantic dominated regions (Norwegian, Greenland and Atlantic Barents Seas) were similar (6.7-7). All observations combined, and most individual regions, showed a pattern of decreasing C:N ratios with increasing seston concentrations. This meta-analysis has important implications for ecosystem modelling, as it demonstrated the striking temporal and spatial variability in C:N ratios and challenges the common assumption of a constant C:N ratio. The non-constant stoichiometry was believed to be caused by variable contributions of autotrophs, heterotrophs and detritus to seston, and a significant decrease in C:N ratios with increasing Chlorophyll a concentrations supports this view. This study adds support to the use of a power function model, where the exponent is system-specific, but we suggest a general Arctic relationship, where POC = 7.4 PON 0.89. Highlights ► Largest meta-analysis to date of seston C:N ratios in Arctic and pan-Arctic shelves ► Average ratio 7.4, with confidence intervals ranging from 6.4 to 8.8 for different regions ► In general, Redfield ratio (6.6) too low and Sterner ratio (8.3) too high ► Significant difference between regions, recommending the use of regional ratios ► Decreasing C:N ratios with increasing concentrations; non-constant stoichiometry

Organic matter (OM) in aquatic systems is either produced internally (autochthonous OM) or delivered from the terrestrial environment (ter-OM). For eutrophication (or the reverse-oligotrophication), the amount of autochthonous OM plays a key role for coastal ecosystem health. However, the influence of ter-OM on eutrophication or oligotrophication processes of coastal ecosystems is largely unclear. Therefore, ter-OM, or ter-OM proxies are currently not included in most policies or monitoring programs on eutrophication. Nevertheless, ter-OM is increasingly recognized as a strong driver of aquatic productivity: By influencing underwater light conditions and nutrient-and carbon availability, increased ter-OM input may shift systems from autotrophic toward heterotrophic production, but also alter the interactions between benthic, and pelagic habitats. Thus, by changing baseline conditions in coastal zones, ongoing, and predicted changes in inputs of ter-OM due to climate change (e.g., in precipitation) and anthropogenic activities (e.g., reduced sulfate deposition, damming, and coastal erosion) may strongly modify eutrophication symptoms within affected ecosystems, but also hinder recovery from eutrophication following a reduction in nutrient loadings (i.e., oligotrophication). In this review, we aim to shed light upon the role of ter-OM for coastal eutrophication and oligotrophication processes and ecosystem health. Specifically, we (1) discuss the theoretical interactions between ter-OM and eutrophication and oligotrophication processes in coastal waters, (2) present global case studies where altered ter-OM supply to coastal ecosystems has shifted baseline conditions, with implications for eutrophication and oligotrophication processes, and (3) provide an outlook and recommendations for the future management of coastal zones given changes in ter-OM input. We conclude that it is essential to include and target all OM sources (i.e., also ter-OM) in monitoring programs to better understand the consequences of both eutrophication and oligotrophication processes on coastal ecosystems. Our review strongly urges to include ter-OM, or ter-OM proxies in eutrophication monitoring, and policies to safeguard coastal ecosystem health also under changing climatic conditions and globally increasing anthropogenic perturbations of coastal ecosystems.

What is the temporal variability of the elemental stoichiometry of marine microbial communities across ocean regions? To answer this question, we present an analysis of environmental conditions, particulate organic carbon, nitrogen, and phosphorus concentrations and their ratios across 20 time series (3–25 years duration) representing estuarine, coastal, and open ocean environments. The majority of stations showed significant seasonal oscillations in particulate organic elemental concentrations and ratios. However, shorter‐term changes contributed most to overall variance in particulate organic matter concentrations and ratios. We found a correlation between the seasonal oscillations of environmental conditions and elemental ratios at many coastal but not open ocean and estuarine stations. C:N peaked near the seasonal temperature minimum and nutrient maximum, but some stations showed other seasonal links. C:N ratios declined with time over the respective observation periods at all open ocean and estuarine stations as well as at five coastal station but increased at the nine other coastal stations. C:P (but not N:P) declined slightly at Bermuda Atlantic Time‐series Study but showed large significant increases at Hawaii Ocean Time‐series and Arendal stations. The relationships between long‐term changes in environmental conditions and particulate organic matter concentrations or ratios were ambiguous, but interactions between changes in temperature and nutrient availability were important. Overall, our analysis demonstrates significant changes in elemental ratios at long‐term and seasonal time scales across regions, but the underlying mechanisms are currently unclear. Thus, we need to better understand the detailed mechanisms driving the elemental composition of marine microbial ecosystems in order to predict how oceans will respond to environmental changes.

Coastal ecosystems are of high ecological and socioeconomic importance and are strongly influenced by processes from land, sea, and human activities. In this study, we present physical, chemical, and biological observations over two consecutive years from three study regions along the Norwegian coast that represent a broad latitudinal gradient in catchment and oceanographic conditions (∼59-69 • N): outer Oslofjord/southern Norway, Runde/western Norway, and Malangen/northern Norway. The observations included river monitoring, coastal monitoring, and sensor-equipped ships of opportunity ("FerryBox"). The riverine discharge and transports were an order of magnitude higher, and the spatiotemporal extent of this freshwater influence was larger in the coastal region in southern Norway, compared to western and northern Norway. The southern Norway coastal waters had consistently high dissolved organic carbon (DOC) and chromophoric dissolved organic matter (cDOM) fluorescence year-round, connected to the large influence of local riverine input and likely also advected riverine runoff and mixing with water masses from the southern North Sea and Baltic Sea. Meanwhile, the western and northern study regions were more sheltered and characterized by more episodic riverine input of freshwater, DOC, cDOM, and nutrients. The timing of the spring phytoplankton bloom in all three regions generally preceded the periods of high riverine input, which suggested that while the winter nutrient reserve was sufficient to fuel the spring bloom, the input of nutrients during the spring flood could sustain the spring bloom or the input of suspended matter, and DOC/cDOM could result in light limitation of the bloom. This article summarizes the impact of riverine input on three diverse coastal systems in terms timing and duration, as well as the potential consequences for ecosystem function especially as related to rising terrestrial organic matter input into coastal regions over the last decades and the projected increase due to climate change.

Four years go by surprisingly fast, and I am very grateful to have had the privilege to pursue my research interests and be a part of the scientific environment. First of all, I would like to acknowledge my supervisors: Richard Bellerby, Dag O. Hessen and Truls Johannessen. I thank you for believing in me and offering scientific guidance and encouragement. Richard took me in as a master student 7 years ago, and I am very grateful for the opportunities you have given me. Dag has been my introduction to the field of ecological stoichiometry, and I am very grateful for your prompt and thorough feedbacks. Truls has been a continued supporter, and always finds time to read and comment on my work. I also wish to thank Tom Andersen, who was not an official supervisor, but has been instrumental to this thesis and has patiently taught me about statistics and the intricacies of R.I am also grateful to the research group here at GFI and BCCR for a positive working environment, and to the CEES group in Oslo for hosting me during the first year of my PhD. I wish to thank my fellow PhD-students, Anna and Siv, for friendship, countless coffee breaks and the joint arena to share our experiences of the trials of PhD-life. I also wish to thank my office-mate Emanuele, for the continued flow of amazing Italian desserts and cakes, and the occasional mathematical insight.My family and friends, here in Bergen, Oslo, Brigthon, Ål and Arendal are very important to me, and I thank you for being who you are and supporting me and what I do. I very much value the time I can spend with you, and look forward to much more of it in the months and years to come. Finally, I wish to thank my husband Erlend, who always lifts my spirits, and keeps me fed, sane and happy.