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.