Response of the Arctic Marine Inorganic Carbon System to Ice Algae and Under‐Ice Phytoplankton Blooms: A Case Study Along the Fast‐Ice Edge of Baffin Bay

Abstract

Past research in seasonally ice‐covered Arctic seas has suggested that ice algae play a role in reducing dissolved inorganic carbon (DIC) during spring, preconditioning surface waters to low dissolved CO2 (pCO2sw), and uptake of atmospheric CO2 during the ice‐free season. The potential role of under‐ice phytoplankton blooms on DIC and pCO2sw has not often been considered. In this study we examined the inorganic carbon system beneath landfast sea ice starting midway through a bottom ice algae bloom and concluding in the early stages of an under‐ice phytoplankton bloom. During most of the ice algae bloom we observed a slight increase in DIC/pCO2sw in surface waters, as opposed to the expected reduction. Biomass calculations confirm that the role of ice algae on DIC/pCO2sw in the study region were minor and that this null result may be widely applicable. During snow melt, we observed an under‐ice phytoplankton bloom (to 10 mg/m3 Chl a) that did reduce DIC and pCO2sw. We conclude that under‐ice phytoplankton blooms are an important biological mechanism that may predispose some Arctic seas to act as a CO2 sink at the time of ice breakup. We also found that pCO2sw was undersaturated at the study location even at the beginning of our sampling period, consistent with several other studies that have measured under‐ice pCO2sw in late winter or early spring. Finally, we present the first measurements of carbonate saturation states for this region, which may be useful for assessing the vulnerability of a local soft‐shelled clam fishery to ocean acidification.

Modeled Impacts of Sea Ice Exchange Processes on Arctic Ocean Carbon Uptake and Acidification (1980–2015)

“…Ice algae biomass peaks in the spring, generally between 20 and 60 Tg C. This value is 2 orders of magnitude less than the maximum phytoplankton biomass. This large ratio between phytoplankton and ice algae biomass is consistent with Else et al (2019), which finds the under-ice phytoplankton bloom to be the main contributor to water column DIC and pCO 2 changes in the spring. There is no evident reduction in pan-Arctic sea ice algae biomass despite the loss in sea ice area (Figure 2c).…”

“…Likewise, the sympagic and pelagic ecosystems affect the ocean TA at the surface and throughout the water column (although the effect on TA is not entirely analogous to that on DIC Mortenson et al, 2018). While the effect of ice algae on the carbon system is considered to be small compared to that of the pelagic ecosystem (Else et al, 2019), the ice algae bloom begins before that of the pelagic phytoplankton and thus extends the time period for biological impacts on the carbon system. In addition to the direct effect of ice algae on the carbon system, they are suggested to contribute to seeding the pelagic bloom when they get released into the water column (Michel et al, 1993).…”

Estimates of net community production from multiple approaches surrounding the spring ice-edge bloom in Baffin Bay

“…As we do not have measurements of surface pCO 2sw prior to our date of sampling, we make the assumption that surface pCO 2sw would not have changed significantly between the day of retreat (DOR) and our sampling date (i.e., we hold the observed pCO 2sw values constant over DOW). This assumption is supported by the results of previous investigations in Canadian Arctic waters (Shadwick et al, 2011;Else et al, 2012Else et al, , 2019, which show that surface pCO 2sw generally undergoes a rapid decrease associated with an under-ice or ice-edge bloom, quickly reaching its annual minimum value before beginning a slow increase throughout the open water season. The results of Else et al (2019) demonstrate this trend occurring within our study region, showing a rapid drop in surface pCO 2sw and nitrate concentrations associated with the onset of an under-ice bloom in early July.…”

“…This assumption is supported by the results of previous investigations in Canadian Arctic waters (Shadwick et al, 2011;Else et al, 2012Else et al, , 2019, which show that surface pCO 2sw generally undergoes a rapid decrease associated with an under-ice or ice-edge bloom, quickly reaching its annual minimum value before beginning a slow increase throughout the open water season. The results of Else et al (2019) demonstrate this trend occurring within our study region, showing a rapid drop in surface pCO 2sw and nitrate concentrations associated with the onset of an under-ice bloom in early July. We estimate the relative uncertainty of our calculated NCP p CO 2 values to be +20% based on recalculating NCP p CO 2 with variations in TA, DIC W , and DIC 0 within their stated limits of variability.…”

Variability of Surface Water pCO2 in the Canadian Arctic Archipelago From 2010 to 2016

“…Since most of these changes relate to the timing of key processes (e.g., ice breakup, open water duration), we hypothesize that our model should be fairly robust in a changed climate. For example, the onset of under‐ice blooms and their impact on p CO 2sw appears to depend on the timing of snow melt (allowing light transmission to the under‐ice environment), as opposed to the timing of polar sunrise (Else et al, ). Thus, we can expect a similar p CO 2sw environment at “week 0” of open water, regardless of where that week falls in a calendar year.…”