Controls on the distribution of fluorescent dissolved organic matter during an under‐ice algal bloom in the western Arctic Ocean

Abstract

In this study we used fluorescence excitation and emission matrix spectroscopy, hydrographic data, and a self‐organizing map (SOM) analysis to assess the spatial distribution of labile and refractory fluorescent dissolved organic matter (FDOM) for the Chukchi and Beaufort Seas at the time of a massive under‐ice phytoplankton bloom during early summer 2011. Biogeochemical properties were assessed through decomposition of water property classes and sample classification that employed a SOM neural network‐based analysis which classified 10 clusters from 269 samples and 17 variables. The terrestrial, humic‐like component FDOM (ArC1, 4.98 ± 1.54 Quinine Sulfate Units (QSU)) and protein‐like component FDOM (ArC3, 1.63 ± 0.88 QSU) were found to have elevated fluorescence in the Lower Polar Mixed Layer (LPML) (salinity ~29.56 ± 0.76). In the LPML water mass, the observed contribution of meteoric water fraction was 17%, relative to a 12% contribution from the sea ice melt fraction. The labile ArC3‐protein‐like component (2.01 ± 1.92 QSU) was also observed to be elevated in the Pacific Winter Waters mass, where the under‐ice algal bloom was observed (~40–50 m). We interpreted these relationships to indicate that the accumulation and variable distribution of the protein‐like component on the shelf could be influenced directly by sea ice melt, transport, and mixing processes and indirectly by the in situ algal bloom and microbial activity. ArC5, corresponding to what is commonly considered marine humic FDOM, indicated a bimodal distribution with high values in both the freshest and saltiest waters. The association of ArC5 with deep, dense salty water is consistent with this component as refractory humic‐like FDOM, whereas our evidence of a terrestrial origin challenges this classic paradigm for this component.

Interannual Variability in the Absorption and Fluorescence Characteristics of Dissolved Organic Matter in the Canada Basin Polar Mixed Waters

“…The four humic‐like (C1‐C2, C4‐C5) and one protein‐like (C3) components have been previously identified in marine studies. The first three components matched aquatic fluorescent components found in the online OpenFluor database depository (95% minimum similarity score; Murphy et al, ) and the last two components were similar to those found in marine studies (Logvinova et al, , ; Mendoza et al, ). For an in‐depth description of the PARAFAC components identified in this study please see supporting information.…”

“…EEMs data were parsed as part of data exploration procedures to include only excitation wavelengths of 280 to 480 nm and emission wavelengths of 300 to 460 nm. This EEM pretreatment was similar to that employed by Mendoza et al () to minimize interference of residual fluorescence produced by scatter, spectral noise, and second‐order peaks to PARAFAC modeling. PARAFAC analysis was carried out in Matlab R2013a (MathWorks) using the drEEM (Murphy et al, ) and DOMFluor (Stedmon & Bro, ) toolboxes.…”

“…It has also been found that newly formed sea ice was composed of smaller DOM molecules (Müller et al, ) with an enriched protein‐like signature (Granskog et al, ; Stedmon et al, ) compared to initial seawater. Improved understanding of the role of sea ice in the storage/release of CDOM/FDOM has supported interpretations of how future changes in sea ice regimes could affect biogeochemical processes in Arctic surface waters (Logvinova et al, , ; Mendoza et al, ). A keen interest was therefore to assess how highly variable sea ice conditions for an interannual data set, including the record low sea ice extent during the polar summer of 2012, influenced DOM absorption and fluorescence properties.…”

“…The distribution and dynamics of CDOM and FDOM in Arctic surface waters are particularly important given its disproportionate share of terrestrially derived organic matter from northern circumpolar freshwater systems (Opsahl et al, 1999). While there have been advances in tracing and characterizing CDOM/FDOM in Arctic surface waters (Dainard & Guéguen, 2013;Fichot et al, 2013;Guéguen et al, 2014;Walker et al, 2009), the role of sea ice has only garnered attention more recently (Jørgensen et al, 2015;Logvinova et al, 2015;Mendoza et al, 2017).…”

Long‐term Trends in Dissolved Organic Matter Composition and Its Relation to Sea Ice in the Canada Basin, Arctic Ocean (2007–2017)

“…An increase of 6,400 km 3 of freshwater was observed in the Beaufort Gyre from 2003 to 2018, the result of persistent wind forcing associated with sea ice melt and redirection of Mackenzie River discharge (Proshutinksky et al., 2019). Brine from ice formation and river runoff also have strong implications for the distribution and the composition of DOM in the interior mixed layers (Dainard et al., 2019; Mendoza et al., 2017; Shen et al., 2016), which can extend to about 150‐200 m in the Canada Basin (Guéguen et al., 2007; Shen et al., 2016).…”