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The release of dissolved organic matter (DOM) from sediments serves as an important part of the carbon cycle. Here, we address pore water DOM quality and its release from shallow sediments (0-10 cm) of the central and southern Baltic Sea - Gdansk, Bornholm, and Eastern Gotland Basins - using excitation-emission matrix spectroscopy and size-exclusion chromatography. DOM release from sediments displayed spatial variability, with diffusive DOC fluxes ranging from 0.02 to 1.06 mmol m-2 d-1, and basin-averaged fluxes increased in the order Gotland < Bornholm < Gdansk. DOM qualitative characteristics also varied among investigated basins. In the oxygen-limited study sites from the Gdansk Basin, we observed elevated abundances of low apparent molecular weight DOM fraction and proteinaceous-like fluorescent DOM, while in the Bornholm Basin, pore water DOM was generally more humic-like and of higher apparent molecular weight. Pore waters from the deepest study sites in the Gotland Basin were qualitatively very similar to those of the pore waters from the upper sediment layers (0-4 cm) from all other investigated sites, suggesting little organic matter reworking at depth at those stations. Our results suggest that the spatial variability in the magnitude of DOM release may be linked to the qualitative differences of DOM in sediments. [ABSTRACT FROM AUTHOR]

This study aimed to detect non-conservative processes that affect the distribution of fluorescent dissolved organic matter (FDOM) in the Baltic Sea. An extensive data set comprised of 408 FDOM data, optical and physical profiles, and the development of a water masses balance model allowed us to ascertain the sources of mixing anomalies. These were seen as second-order deviations in the FDOM distribution as a function of salinity in three layers: surface water, Baltic Sea Winter Water, and deep water. The difference between modeled and measured FDOM values at three different excitation/emission wavelengths allowed to show the strength of non-conservative processes, such as photochemical and microbial decomposition (negative residual values) or extracellular release of dissolved organic matter from phytoplankton, heterotrophic uptake and release from anoxic sediments (positive residual values). Humic-like FDOMfractions displayed positive residuals in all seasons for intermediate and deep layers and negative residuals in surface waters. Largest accumulation rates of humic-like fractions were reached in the Gulf of Gdańsk during summer in intermediate and deep layers, while the greatest removal in surface waters was observed during spring in the Bornholm and Gotland Basins and during summer in the Gulf of Gdańsk, probably due to photodegradation. Positive residuals of the protein-like fraction were observed at the surface in summer and autumn in the Gulf of Gdańsk, probably linked to the abundance of phytoplankton and also due to the low molecular weight by-products of photodegradation of humic-like components. Spatial transects revealed an increase in humic-like residuals with depth and a strong correlation with apparent oxygen utilization, increasing with higher fluorescence and exhibiting an asymptotic trend. A relationship was found between the protein-like fractions and phytoplankton biomass proxies. A generalized concept for FDOM cycling in the Baltic Sea was proposed, highlighting photobleaching as the dominant non-linear process determining the efficiency of humic-like FDOM removal. The protein-like component was found to be more efficiently taken up by aerobic prokaryotes at the surface. Microbial utilization and reworking of organic matter, release from sediments, and a decade-long stagnation of bottom water masses, all contribute to the observed accumulation of FDOM in mesohaline deep waters below the permanent pycnocline in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Phytoplankton are responsible for releasing half of the World's oxygen and for removing large amounts of carbon dioxide from the surface waters. Despite many studies on the topic conducted in the past decades, we are still far from a good understanding of ongoing rapid changes in the Arctic Ocean, and how they will affect phytoplankton and the whole ecosystem. An example is the difference in net primary production modeling estimates, which differ two times globally and fifty times when only the Arctic region is considered. Here we aim to improve the quality of Greenland Sea primary production estimates, by testing different versions of primary production model against in-situ data, and then calculating regional estimates and trends for 1998–2022 for those performing best. As a baseline we chose the commonly used global primary production model and tested it with different combinations of empirical relationships and input data. Local empirical relationships were taken from literature and derived from the unpublished Institute of Oceanology of Polish Academy of Sciences measurements across the Fram Strait. For validation we took historical net primary production 14C data from literature, and added to it our own gross primary production O2 measurements to extend the limited validation dataset. The field data showed expected elevated values at the frontal zone together with differences between Arctic and Atlantic-dominated waters, and unexpected good agreement between primary production measured with 14C and O2 evolution methods. From all the model setups, those including local chlorophyll-a profile and local absorption spectrum and using Level 2 photosynthetically active radiation data, reproduced in-situ data best. Our modeled regional annual primary production estimates equal 346 TgC/year for the Nordic Seas region and 342 TgC/year for the Greenland Sea sector of the Arctic defined as 45° W–15° E, 66°33′N–90° N. These values are higher those previously reported. Monthly values show a seasonal cycle with less monthly variability than previously reported, and with peak values observed in May. No significant increase or decrease in primary production was observed when studying regionally averaged trends. The accuracy of the selected here model setups to reproduce the field data in terms of Root Mean Square Difference is poorer than in the related global studies, but better than in the related Arctic studies. [ABSTRACT FROM AUTHOR]

Sampling for dissolved organic carbon (DOC) in the Arctic is challenging given the limited access and because it is not yet possible to measure with instruments deployed in situ. Compared to DOC, colored dissolved organic matter (CDOM) absorption spectroscopy is an easy‐to‐measure, relatively quick and cost‐effective approach which is often closely related to DOC concentrations in water samples. Here we present an algorithm based on quantitative and qualitative metrics of CDOM to provide DOC estimates derived from a Pan‐Arctic data set (n = 3,302) spanning rivers to deep ocean, with DOC ranging between 31 and 1,958 μM. The algorithm provided robust DOC estimates (r2 = 0.94; p < 0.0001) and could reproduce DOC profiles and mixing plots across different locations in the Arctic Ocean. Besides its simplicity, this method is capable of capturing the extremely broad range of DOC within the strong gradients observed between Arctic riverine and marine systems. Plain Language Summary: Surface waters in the Arctic Ocean have high content of dissolved organic carbon (DOC), which plays an important role in CO2 fluxes and absorbing heat, thus warming the ocean. Measuring DOC requires resource‐intensive lab analysis. The optical properties of colored dissolved organic matter (CDOM) are easier, quick and cost‐effective to measure, and can therefore be sampled at a higher frequency or with autonomous sensors. Here we develop an algorithm that uses CDOM optical properties to estimate DOC concentrations across the entire Arctic Ocean, spanning from rivers (high DOC) to the deep waters (low DOC). The algorithm provides reliable estimates and can reproduce regional features, which shows that the method is effective and can be employed to increase sampling efforts for DOC in remote Arctic waters. Key Points: Spectral properties of colored dissolved organic matter can predict dissolved organic carbon concentrations across a range of Arctic aquatic systemsWe developed a pan‐Arctic algorithm from a comprehensive data set spanning from rivers to the deep Arctic watersThe algorithm provides robust estimates allowing for reproduction of regional features such as vertical profiles and mixing lines [ABSTRACT FROM AUTHOR]

Heating rates induced by optically significant water constituents (OSCs), e.g. phytoplankton and coloured dissolved organic matter (CDOM), contribute to the seasonal modulation of thermal energy fluxes across the ocean–atmosphere interface in coastal and regional shelf seas. This is investigated in the western Baltic Sea, a marginal sea characterised by considerable inputs of freshwater carrying nutrients and CDOM and by complex bio-optical and hydrodynamic processes. Using a coupled bio-optical ocean model (ROMS–Bio-Optic), the inherent optical properties of different OSCs are modelled under varying environmental conditions, and the underwater light field is spectrally resolved in a dynamic ocean. We estimate the relative contribution of these OSCs to the divergence of the heat flux and heating rates and find that, while phytoplankton and CDOM both contribute to surface heating in summer, phytoplankton dominates the OSC contribution to heating in spring, and CDOM dominates the OSC contribution to heating in autumn. The study shows that seasonal and spatial changes in OSCs in the western Baltic Sea have a small but noticeable impact on radiative heating in surface waters and consequences for the exchange of energy fluxes across the air–sea interface and the distribution of heat within the water column. In the Pomeranian Bight, where riverine influx of CDOM is strongest, water-constituent-induced heating rates in surface waters in 2018 are estimated to be between 0.8 and 0.9 K m -1 d -1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM. Further offshore, OSC-induced heating rates during the same periods are estimated to be between 0.4 and 0.8 K m -1 d -1. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and longwave) respond to warmer sea surface temperatures, with a small increase in heat loss to the atmosphere of 5 W m -2 during the period April to September. We find relatively good agreement between our modelled water constituent absorption and in situ and satellite observations. More rigorous co-located heating-rate calculations using an atmosphere–ocean radiative transfer model provide evidence of the suitability of the ROMS–Bio-Optic model for estimating heating rates. [ABSTRACT FROM AUTHOR]

Abstract. Heating rates induced by optically significant water constituents (OSCs), e.g. phytoplankton and coloured dissolved organic matter (CDOM), contribute to the seasonal modulation of thermal energy fluxes across the ocean- atmosphere interface in coastal and regional shelf seas. This is investigated in the Western Baltic Sea, a marginal sea characterised by considerable inputs of freshwater carrying nutrients and CDOM, and complex bio-optical and hydrodynamic processes. Using a coupled bio-optical-ocean model (ROMS-Bio-Optic), the inherent optical properties of different OSCs are modelled under varying environmental conditions and the underwater light field is spectrally- resolved in a dynamic ocean. We estimate the relative contribution of these OSCs to the divergence of the heat flux and heating rates and find that phytoplankton dominates the OSC contribution to heating in spring and summer, while CDOM dominates in summer and autumn. The study shows that seasonal and spatial changes in OSCs in the Western Baltic Sea have a small but noticeable impact on radiative heating in surface waters and consequences for the exchange of energy fluxes across the air-sea interface and the distribution of heat within the water column. In the Pomeranian Bight, where riverine influx of CDOM is strongest, water constituent-induced heating rates in surface waters in 2018 are estimated to be between 0.8 and 0.9 K m-1 d-1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM. Further offshore, OSC-induced heating rates during the same periods are estimated to be between 0.4 and 0.8 K m-1 d-1. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and longwave) respond to warmer sea surface temperatures with a small increase in heat loss to the atmosphere of 5 Wm-2 during the period April to September. We find relatively good agreement between our modelled water constituent absorption, and in situ and satellite observations. More rigorous co-located heating rate calculations using an atmosphere-ocean radiative transfer model provide evidence of the suitability of the ROMS-Bio-Optic model for estimating heating rates. [ABSTRACT FROM AUTHOR]

There have been considerable efforts to understand the hydrography of the Storfjorden fjord (Svalbard). A recurring winter polynya with large sea ice production makes it an important region of dense water formation at the scale of the Arctic Ocean. In addition, this fjord is seasonally influenced by freshwater inputs from sea-ice melt and the surrounding islands of the Svalbard archipelago which impacts the hydrography. However, the understanding of factors controlling the optical properties of the waters in Storfjorden are lacking and are crucial for development of more accurate regional bio-optical models. Here, we present results from the first detailed optical field survey of Storfjorden conducted in early summer of 2020. In addition to the expected seasonal contribution from phytoplankton, we find that in early summer waters in Storfjorden are optically complex with a significant contribution from coloured dissolved organic matter (33–64% of the non-water absorption at 443 nm) despite relatively low CDOM concentrations, and in the nearshore or near seabed from non-algal particles (up to 61% of the non-water absorption at 550 nm). In surface waters, the spatial variability of light attenuation was mainly controlled by inorganic suspended matter originating from river runoff. A distinct subsurface maximum of light attenuation was largely driven by a subsurface phytoplankton bloom, controlled by stratification resulting from sea-ice melt. Lastly, the cold dense bottom waters of Storfjorden, from winter sea ice production, which periodically overflows into the Fram Strait, was found to contain elevated levels of both non-algal particles and dissolved organic matter, which is likely caused by the dense flows of the nepheloid layer interacting with the sea bed. [ABSTRACT FROM AUTHOR]