Assessing the potential impacts of declining Arctic Sea ice cover on the photochemical degradation of dissolved organic matter in the Chukchi and Beaufort Seas

A warming and shifting climate in the Arctic has led to significant declines in sea ice over the last several decades. Although these changes in sea ice cover are well documented, large uncertainties remain in how associated increases in solar radiation transmitted to the underlying ocean water column will impact heating, biological and biogeochemical processes in the Arctic Ocean. In this study, six under-ice marine, two ice-free marine, and two ice-free terrestrially influenced water samples were irradiated using a solar simulator for 72 hours (representing ~10 days of ambient sunlight) to investigate dissolved organic matter (DOM) dynamics from the Chukchi and Beaufort seas. Solar irradiation caused chromophoric DOM (CDOM) light absorption at 254 nm to decrease by 48 to 63%. An overall loss in total DOM fluorescence intensity was also observed at the end of all experiments, and each of 6 components identified by parallel factor analyses (PARAFAC) was shown to be photoreactive in at least one experiment. DOM fluorescence (FDOM) also indicated that the majority of DOM in under-ice and ice-free marine waters was likely algal-derived. Measurable changes in dissolved organic carbon (DOC) were only observed for sites influenced by riverine runoff. Losses of CDOM absorbance at shorter wavelengths suggest that the beneficial UV protection currently received by marine organisms may decline with the increased light transmittance associated with sea ice melt ponding and overall reductions of sea ice. Our FDOM analyses demonstrate that DOM irrespective of source was susceptible to photobleaching. Additionally, our findings suggest that photodegradation of CDOM in under-ice waters is not currently a significant source of carbon dioxide (CO2) (i.e., we did not observe systematic DOC loss). However, increases in primary production and terrestrial freshwater export expected under future climate change scenarios may cause an increase in CDOM quantity and shift in quality throughout Arctic Ocean surface waters

The potential role of sea ice melt in the distribution of chromophoric dissolved organic matter in the Chukchi and Beaufort Seas

We investigated chromophoric dissolved organic matter (CDOM) in sea ice and the underlying water column in the Chukchi and Beaufort seas of the Pacific Arctic region and its relationship with both physical and biogeochemical parameters. Sea ice, water and melt pond samples were collected as sea ice melted in June–July 2010 and 2011. CDOM absorption was found to be significantly lower in sea ice compared to under-ice waters. In particular, the average CDOM absorption coefficient at 254 nm was approximately four times greater in the underlying water column than in the overlying ice. This indicates that melting sea ice did not contribute to net CDOM at this point in the melt season, but rather diluted CDOM in the under-ice water column. In the 2011 under-ice water column samples, the average CDOM absorption coefficients at 440 nm were twice as high along a transect associated with high phytoplankton biomass, which may have been contributed through subsequent microbial generation of CDOM. Less extensive sea ice cover with melt ponds may also have increased the presence of CDOM owing to increases in light transmission, leading to under-ice phytoplankton blooms and associated microbial production. However, oxygen isotope analysis of these waters with high under-ice phytoplankton biomass also indicates the presence of prior sea ice melt, including potentially sea ice algae and microbes, which could have also contributed to this anomalously high CDOM. These observations suggest that while melting sea ice may not necessarily contribute to increased CDOM concentrations, there are circumstances where CDOM in underlying waters may be higher than expected, either due to enhanced light transmission and higher under-ice production, and/or prior ice melt that provided significant contributions to under-ice CDOM

Variations in the proportions of melted sea ice and runoff in surface waters of the Chukchi Sea: A retrospective analysis, 1990–2012, and analysis of the implications of melted sea ice in an under-ice bloom

Retrospective analysis of apparent freshwater isotopic end-members through use of salinity–δ18O mixing lines for 15 research cruises from 1990 to 2012 indicates that the freshwater contributed by melted seasonal sea ice does not directly reflect the large change in seasonal sea ice extent in the Chukchi Sea observed over the past several decades. Instead the freshwater that appears to be contributed by melting sea ice relative to runoff is highly dependent upon cruise track (e.g. proximity to runoff) and sampling capabilities in sea ice (icebreakers sample waters with less melted sea ice). Although under certain circumstances, including later seasonal sampling and recurrent cruise tracks between years, increased melted sea ice in surface waters can be readily detected. This suggests a more ephemeral influence of melted sea ice on ecosystem properties despite the significant decadal changes in sea ice extent. As a recent case study, the freshwater component present in waters within an under-ice bloom in the Chukchi Sea reported by Arrigo et al. (2012), included a significant fraction (~10% or more) of freshwater, primarily from melted sea ice. These results suggest that this under-ice bloom, which extended more than 60 km under solid ice still might be reasonably interpreted as being part of a continuum with other ice melt-associated blooms and not independent of sea ice retreat and dissolution

Physical and morphological properties of sea ice in the Chukchi and Beaufort Seas during the 2010 and 2011 NASA ICESCAPE missions

Physical and morphological properties of sea ice in the Chukchi and western Beaufort seas were measured during the 2010 and 2011 June-July ICESCAPE (Impacts of Climate on the Eco-Systems and Chemistry of the Arctic Pacific Environment) missions aboard the USCGC Healy. Observations of ice conditions, including ice thicknesses, types, and concentrations of primary, secondary, and tertiary categories were reported at 2-h intervals while the ship was in transit using the Antarctic Sea ice Processes and Climate (ASPeCt) protocol. On-ice surveys of ice thickness, melt-pond depth, and ice properties (including profiles of internal temperature, salinity, and isotopic composition) were conducted at 21 total ice stations (12 in 2010 and 9 in 2011). Comparison to historical ship-based observations confirms a multi-decadal transition from a multiyear to thinner, first year -dominated seasonal sea-ice pack in the region, with much earlier ice retreat than in past decades. The ice encountered was predominantly (\u3e98%) first year, with un-deformed ice thicknesses ranging from 0.73 to 1.2. m. Pond coverage was extensive, averaging 29% in 2010 and 19% in 2011, resulting in considerable light absorption in the ice and transmission of light to the ocean. Enhanced melting near the ice edge is consistent with transport of Pacific-origin heat and/or ice-albedo feedbacks. Sediment entrainment was visible in 7.5% and 10.9% of the ice in 2010 and 2011, respectively. Overall, the results indicate a regime shift in the characteristics of the ice cover has occurred in the region over recent decades, with substantive implications for thermodynamic forcing, light availability in the upper ocean, and biological and biogeochemical processes in the ice and water column beneath. The results presented have applications for the interpretation of optical and biological measurements in the Chukchi Sea and serve as record of ice conditions for assessing long-term change