Tracing environmental variability in the changing Arctic Ocean with optical measurements of dissolved organic matter.

The Arctic Ocean plays an important role on the global hydrological and carbon cycles. It contributes 5–14% to the global balance of CO2 sinks and sources. Carbon is also cycled in the Arctic Ocean through the primary producers, with high primary production observed in the marginal ice zones, ice-free zones and melt ponds, with increased biogenic carbon export to the deep layers. Although being the smallest ocean basin, the Arctic Ocean receives ~11% of the global riverine runoff. Along with the freshwater, high loads of organic carbon are introduced in the Arctic Ocean. Most of it is observed in the fraction of dissolved organic matter (DOM). With the ongoing global warming, glacier melt and permafrost thaw are observed and pointed as the main drivers for increasing the freshwater discharge into the Arctic basin. Along side, permafrost thaw coupled with increased coastal erosion lead to an increase in mobilization of carbon from permafrost, which could have critical implications for microbial processes, primary production, terrestrial carbon fluxes to the shelf seas and, thus, carbon cycling in the Arctic. This thesis is focusing on tracing the mixing of DOM along the Siberian shelves and developing potential applications of DOM as an environmental tracer. Four main objectives have been pursed: (1) to quantify, characterize and assess the distribution and transformation of DOM across the river-shelf transition and provide insights into the fate of Arctic riverine DOM; (2) to assess the potential of DOM, especially its fluorescent fraction (FDOM), as a tracer of freshwater in the surface layers in the Arctic Ocean; (3) to characterize the non-water absorption in the surface central and eastern Arctic Ocean and further test whether bio-optical properties (such as absorption and reflectance) can reproduce hydrographical variability; (4) to evaluate the performance of ocean color algorithms frequently applied for studies in the Arctic Ocean using novel data from a central-eastern Arctic expedition. In the first study the fluorescent components of DOM isolated with PARAFAC model were characterized along the river to sea transition in the Laptev Sea and Lena River delta region. Results showed a strong dominance of visible wavelength DOM fluorescence (VIS-FDOM), which is associated to terrestrial signal (or humic-like compounds). The results corroborate previous reports showing strong removal of DOM at low salinity. However, our results showed that the removal occurs preferentially for VIS-FDOM, whereas ultraviolet wavelength FDOM (UV-FDOM, associated to autochthonous marine production) differed in behavior, with an increase during estuarine mixing. DOM removal occurred primarily in the surface layer, under direct influence of the Lena River runoff (salinity <10), which indicates that it was mainly driven by photodegradation and flocculation. The second study explored the potential of VIS-FDOM components isolated with PARAFAC analysis as an environmental tracer in the Fram and Davis Straits. VIS-FDOM was strongly correlated to the fractions of meteoric water (fmw) in polar waters. Furthermore, a pattern allowed the distinction between the sources of polar waters exiting the Fram Strait as being from the Eurasian or Canadian basins. In the bottom waters of the Davis Strait, VISFDOM was correlated to apparent oxygen utilization (AOU), tracing deepwater turnover of DOM and production of VIS-FDOM fluorescence. The findings presented in this study show which wavelengths carry information on sources and mixing of DOM, which therefore can be applied to monitor freshwater and carbon export to the North Atlantic. The third study shows that colored DOM (CDOM) dominates the nonwater absorption in the surface waters of the central and eastern Arctic. Spatial variability observed in the non-water absorbers (phytoplankton, CDOM and non-algal particles–NAP) clustered the sampling sites in agreement with hydrographic variability. Such variability was also detected by the analysis of hyperspectral remote sensing reflectance (Rrs). The empirical and semianalytical ocean color algorithms frequently applied in studies in the Arctic Ocean were applied to in situ measured Rrs to evaluate their performance. The retrievals (chlorophyll-a, and the absorption coefficients of CDOM and phytoplankton) were then validated to the correspondent in situ measurements. The results showed that empirical algorithms have poor performance, whereas the semi-analytical algorithms appeared to be robust for application in the Arctic Ocean; however still with considerable errors embedded to the retrievals. The main findings of this thesis are that bio-optical measurements have strong potential to trace environmental variability in the Arctic Ocean, and those can therefore provide insights on the Arctic hydrological and biogeochemical cycles. These parameters can be monitored by bio-optical sensors (e.g., radiometers, transmissometers, fluorometers, etc.). Such sensors can be further coupled to autonomous platforms such as satellites, gliders, automated underwater vehicles (AUVs) and ice-tethered profilers (ITPs), and significantly increase the amount of biogeochemical data in the Arctic Ocean, filling the gap left by classical sampling methods (i.e., oceanographic expeditions) and ocean color remote sensing, restricted to spring and summer seasons

Dissolved organic matter properties in arctic coastal waters are strongly influenced by fluxes from permafrost coasts and by local meteorology.

Under future climate change scenarios, Arctic coastal waters are believed to receive higher terrestrial organic matter (OM) fluxes. Permafrost carbon is increasingly mobilized upon thaw from rivers draining permafrost terrain and from eroding permafrost coasts. Once received, the coastal waters are the transformation zone for terrestrial OM, although quantities, especially those of dissolved organic matter (DOM) released by coastal erosion, are largely unknown. This nearshore zone plays a crucial role in Arctic biogeochemical cycling, as here the released material is destined to be (1) mineralized into greenhouse gases, (2) incorporated into marine primary production, (3) buried in nearshore sediments or (4) transported offshore. In this presentation, we show data on DOM quantities in surface water in the nearshore zone of the southern Beaufort Sea from two consecutive summer seasons under different meteorological conditions. Colored dissolved organic matter (cDOM) properties help to differentiate the terrestrial from the marine DOM component. Figure 1 shows DOC concentrations and salinities for 23 and 24 days in the summer seasons of 2013 and 2014, respectively. DOC concentrations in the nearshore zone of the southern Beaufort Sea vary between about 1.5 and 5 mg C L-1. In the Lena River Delta, bay water, river water, and permafrost meltwater creeks yielded similar values between 5.8 and 5.9 mg C L-1 (Dubinenkov et al., 2015). Similarly, Fritz et al. (2015) found DOC concentrations in ice wedges between 1.6 and 28.6 mg C L-1. In 2013, the first half of July was characterized by low salinity between 8 and 15 psu and high DOC concentrations of 3.5 to 5 mg C L-1. Then, a sudden change in water properties occurred after a major storm which lasted for at least 2 days. This storm led to strongly decreased DOC (1.5 to 2.5 mg C L-1) concentration and increasing salinity (14 to 28 psu) in surface water, probably due to upwelling In 2014, a more stable situation in both salinity and DOC prevailed, with relatively high salinity (23 to 29 psu) and low DOC concentration (1.5 to 2.5 mg C L-1). This pattern was due to rather windy and wavy conditions throughout the whole season. The water column in 2014 was likely well-mixed and DOC-poor because saline waters have probably been transported from the offshore to the nearshore. We recognized a significant negative correlation between DOC and salinity, independent from varying meteorological conditions. In general, this suggests a conservative mixing between DOC derived from terrestrial/permafrost runoff and marine DOC. The low salinity in July 2013 was probably due to prolonged sea-ice presence in the sampled area. This leads to the assumption that DOC also originates from melting sea ice. Quantitatively more important will be terrestrial runoff which is relatively rich in DOC. A stable stratification in the nearshore zone and calm weather conditions will increase the influence of terrestrial-derived DOM. The strength of the terrestrial influence can be estimated by salinity measures as they directly correlate with DOC concentrations; the lower the salinity the stronger the terrestrial influence. We conclude that the terrestrial imprint of coastal erosion on DOM concentrations in the nearshore zone is significant. We see that DOC concentrations are significantly elevated also compared to riverine input in front of river mouths and deltas. Meteorological conditions play a major role for the strength of the terrestrial DOM signal, which can vary on short timescales. Our approach is different from ship-based oceanography because we study DOM that is directly derived from thawing permafrost coasts, explicitly excluding rivers. When qualifying DOM origin from permafrost landscapes apart from rivers we have to take into consideration the different DOM mobilization pathways. 1) Surface runoff and near-surface groundwater flow mainly drain and flush the active layer. 2) Melting ground ice releases DOM. 3) Ground ice meltwater leaches DOM from sedimentary OM upon permafrost thaw on land. 4) DOM is leached from sedimentary OM upon contact with sea water. The latter three will mobilize old OM which is believed to be highly bioavailable (see Vonk et al., 2013a, b). References: Dubinenkov, I., Flerus, R., Schmitt-Kopplin, P., Kattner, G., Koch, B.P., 2015. Origin-specific molecular signatures of dissolved organic matter in the Lena Delta. Biogeochemistry 123, 1-14. Fritz, M., Opel, T., Tanski, G., Herzschuh, U., Meyer, H., Eulenburg, A., Lantuit, H., 2015. Dissolved organic carbon (DOC) in Arctic ground ice. The Cryosphere 9, 737-752. Vonk, J.E., Mann, P.J., Davydov, S., Davydova, A., Spencer, R.G.M., Schade, J., Sobczak, W.V., Zimov, N., Zimov, S., Bulygina, E., Eglinton, T.I., Holmes, R.M., 2013a. High biolability of ancient permafrost carbon upon thaw. Geophysical Research Letters 40, 2689-2693. Vonk, J.E., Mann, P.J., Dowdy, K.L., Davydova, A., Davydov, S.P., Zimov, N., Spencer, R.G.M., Bulygina, E.B., Eglinton, T.I., Holmes, R.M., 2013b. Dissolved organic carbon loss from Yedoma permafrost amplified by ice wedge thaw. Environmental Research Letters 8, 035023

TRACING THE COMPOSITION OF DOM IN THE ARCTIC OCEAN WITH FLUORESCENCE SPECTROSCOPY

The Arctic Ocean consists of a large pool of dissolved organic matter (DOM), receiving considerable input of terrigenous carbon mobilized from high latitude carbon-rich soils and peatlands. This study aims at characterizing the DOM fluorescent components in two Arctic environments: the Lena River delta region (September 2013) and the Polar (Arctic) waters in the Fram Strait (June 2014). In addition, optical indices of DOM modification were evaluated together with the amount of DOM (expressed as the absorption at 350nm; a350). The colored and fluorescent fractions of DOM (CDOM and FDOM, respectively) were analyzed using fluorescence spectroscopy and PARAFAC modeling. The amount of DOM (a350) decreased with increasing salinity (varying from 15.7m-1 in the Lena delta to 0.34m-1 in the Fram strait), with strong removal at low salinity. Six fluorescent components were identified in the Lena delta region and three of those components were validated in the Fram Strait. The allochthonous humic-like signal was the dominant fraction of DOM within both sampled regions, with the highest relative contributions to total FDOM associated to low salinity. Conversely, autochthonous signal (e.g. protein- and/or marine humic-like) presented higher contribution in relation to total FDOM at high salinity. All the components were inversely related to salinity with the highest removal rates observed at low salinity. Optical indices of DOM modification (CDOM absorption slope, SUVA, fluorescence index, humification index and biological activity index) showed decrease on the humification degree and aromaticity of DOM towards high salinity. Strong removal at low salinity in the Lena delta region is presumed to be driven mostly by photodegradation and flocculation. The lower a350 values observed in the Fram strait indicates low removal through the Arctic Ocean. Further analyses will be conducted to evaluate the main drivers of the DOM removal through the open Arctic Ocean

High colored dissolved organic matter (CDOM) absorption in surface waters of the central-eastern Arctic Ocean: Implications for biogeochemistry and ocean color algorithms.

As consequences of global warming sea-ice shrinking, permafrost thawing and changes in fresh water and terrestrial material export have already been reported in the Arctic environment. These processes impact light penetration and primary production. To reach a better understanding of the current status and to provide accurate forecasts Arctic biogeochemical and physical parameters need to be extensively monitored. In this sense, bio-optical properties are useful to be measured due to the applicability of optical instrumentation to autonomous platforms, including satellites. This study characterizes the non-water absorbers and their coupling to hydrographic conditions in the poorly sampled surface waters of the central and eastern Arctic Ocean. Over the entire sampled area colored dissolved organic matter (CDOM) dominates the light absorption in surface waters. The distribution of CDOM, phytoplankton and non-algal particles absorption reproduces the hydrographic variability in this region of the Arctic Ocean which suggests a subdivision into five major bio-optical provinces: Laptev Sea Shelf, Laptev Sea, Central Arctic/Transpolar Drift, Beaufort Gyre and Eurasian/Nansen Basin. Evaluating ocean color algorithms commonly applied in the Arctic Ocean shows that global and regionally tuned empirical algorithms provide poor chlorophyll-a (Chl-a) estimates. The semi-analytical algorithms Generalized Inherent Optical Property model (GIOP) and Garver-Siegel-Maritorena (GSM), on the other hand, provide robust estimates of Chl-a and absorption of colored matter. Applying GSM with modifications proposed for the western Arctic Ocean produced reliable information on the absorption by colored matter, and specifically by CDOM. These findings highlight that only semi-analytical ocean color algorithms are able to identify with low uncertainty the distribution of the different optical water constituents in these high CDOM absorbing waters. In addition, a clustering of the Arctic Ocean into bio-optical provinces will help to develop and then select province-specific ocean color algorithms. © 2018 Gonçalves-Araujo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Hydrography-driven variability of optically active constituents of water in the South Brazilian Bight : Biogeochemical implications

The South Brazilian Bight (SBB) is a hydrographically dynamic environment with strong seasonality that sustains a diverse planktonic community involved in diverse biogeochemical processes. The inherent optical properties (IOPs; e.g., absorption and scattering coefficients) of optically actives constituents of water (OACs; phytoplankton, non-algal particles–NAP, and colored dissolved organic matter–CDOM) have been widely employed to retrieve information on biogeochemical parameters in the water. In this study conducted in the SBB, a cross-shelf transect was performed for biogeochemistry and hydrographic sampling during a summer expedition. Our research aimed to determine the distribution and amount of the OACs based on their spectral signature, in relation to the distribution of water masses in the region. That allows us to get insights into the biogeochemical processes within each water mass and in the boundaries between them. We observed a strong intrusion of South Atlantic Central Water (SACW) over the shelf, mainly driven by the wind action. With that, phytoplankton development was fueled by the input of nutrients, and increased chlorophyll-a (Chl-a) concentrations were observed within the shallowest stations. Colored dissolved organic matter did not follow the distribution of dissolved organic carbon (DOC). Both CDOM and DOC presented high values at the low salinity Coastal Water (CW), as an indication of the continental influence over the shelf. However, CDOM was inversely correlated with salinity and lowest values were observed within Tropical Water (TW), whereas DOC values within TW were as high as within CW, indicating an autochthonous DOM source. Additionally, a deep Chl-a maximum (DCM) was noticed in the boundary between the TW and SACW. Along with the DCM, we observed the production of fresh, non-colored DOM attributed to the microbial community. Finally, our results suggest that CDOM is photodegraded at the surface of CW. This is mainly due to the Ekman transport effect over the region that traps CW at the surface, making it longer exposed to solar radiation

USING FLUORESCENT DISSOLVED ORGANIC MATTER TO TRACE ARCTIC SURFACE FRESH WATER

Climate change affects the Arctic environment with regards to permafrost thaw, changes in the riverine runoff and subsequent export of fresh water and terrestrial material to the Arctic Ocean. In this context, the Fram Strait represents a major pathway for export to the Atlantic basin. We assess the potential of visible wavelength dissolved organic matter fluorescence (VIS-FDOM) to trace the origin of Arctic outflow waters. Oceanographic surveys were performed in the Fram Strait, as well as on the east Greenland shelf (following the East Greenland Current), in late summer 2012 and 2013. Meteoric (fmw), sea-ice melt (fsim), Atlantic (faw) and Pacific (fpw) water fractions were determined and FDOM components were identified by PARAFAC modeling. In Fram Strait and east Greenland shelf, a robust correlation between VIS-FDOM and fmw was apparent, suggesting it as a reliable tracer of polar waters. However, variability was observed in the origin of polar waters, in relation to contribution of faw and fpw, between the sampled years. VIS-FDOM traced this variability, and distinguished between the origins of the halocline waters as originating in either the Eurasian or Canada basins. The findings presented highlight the potential of designing in situ DOM fluorometers to trace the freshwater origins and decipher water mass dynamics in the region

Towards cost-effective operational monitoring systems for complex waters: analyzing small-scale coastal processes with optical transmissometry.

The detection and prediction of changes in coastal ecosystems require a better understanding of the complex physical, chemical and biological interactions, which involves that observations should be performed continuously. For this reason, there is an increasing demand for small, simple and cost-effective in situ sensors to analyze complex coastal waters at a broad range of scales. In this context, this study seeks to explore the potential of beam attenuation spectra, c(λ), measured in situ with an advanced-technology optical transmissometer, for assessing temporal and spatial patterns in the complex estuarine waters of Alfacs Bay (NW Mediterranean) as a test site. In particular, the information contained in the spectral beam attenuation coefficient was assessed and linked with different biogeochemical variables. The attenuation at λ = 710 nm was used as a proxy for particle concentration, TSM, whereas a novel parameter was adopted as an optical indicator for chlorophyll a (Chl-a) concentration, based on the local maximum of c(λ) observed at the long-wavelength side of the red band Chl-a absorption peak. In addition, since coloured dissolved organic matter (CDOM) has an important influence on the beam attenuation spectral shape and complementary measurements of particle size distribution were available, the beam attenuation spectral slope was used to analyze the CDOM content. Results were successfully compared with optical and biogeochemical variables from laboratory analysis of collocated water samples, and statistically significant correlations were found between the attenuation proxies and the biogeochemical variables TSM, Chl-a and CDOM. This outcome depicted the potential of high-frequency beam attenuation measurements as a simple, continuous and cost-effective approach for rapid detection of changes and patterns in biogeochemical properties in complex coastal environments