Changes in gross oxygen production, net oxygen production, and air-water gas exchange during seasonal ice melt in Whycocomagh Bay, a Canadian estuary in the Bras d'Or Lake system

Cara C Manning,Rachel H R Stanley,Brice Loose,David P Nicholson,Bruce G Hatcher,Ann Lovely,Peter Schlosser

doi:10.5194/bg-16-3351-2019

Abstract

Abstract. Sea ice is an important control on gas exchange and primary production in polar regions. We measured net oxygen production (NOP) and gross oxygen production (GOP) using near-continuous measurements of the O2∕Ar gas ratio and discrete measurements of the triple isotopic composition of O2, during the transition from ice-covered to ice-free conditions, in Whycocomagh Bay, an estuary in the Bras d'Or Lake system in Nova Scotia, Canada. The volumetric gross oxygen production was 5.4+2.8-1.6 mmol O2 m−3 d−1, similar at the beginning and end of the time series, and likely peaked at the end of the ice melt period. Net oxygen production displayed more temporal variability and the system was on average net autotrophic during ice melt and net heterotrophic following the ice melt. We performed the first field-based dual tracer release experiment in ice-covered water to quantify air–water gas exchange. The gas transfer velocity at >90 % ice cover was 6 % of the rate for nearly ice-free conditions. Published studies have shown a wide range of results for gas transfer velocity in the presence of ice, and this study indicates that gas transfer through ice is much slower than the rate of gas transfer through open water. The results also indicate that both primary producers and heterotrophs are active in Whycocomagh Bay during spring while it is covered in ice.

Highlights

The annual cycle of sea ice formation and melt regulates primary production and CO2 uptake and ventilation in polar regions
The gas transfer velocity was much lower for Injection 1, which was sampled while the basin was essentially full of ice (31 March–10 April), compared to Injection 2, which was sampled when the basin was nearly ice-free (20–23 April)
Using the dual tracer (3He/SF6) technique in the Bras d’Or Lake, we found that at > 90 % ice cover, the gas transfer velocity was 6 % of the open water gas transfer velocity

Summary

Introduction

The annual cycle of sea ice formation and melt regulates primary production and CO2 uptake and ventilation in polar regions. Global warming is causing dramatic reductions in sea ice cover and increases in freshwater inflow and organic carbon supply to the Arctic Ocean, which impacts ecosystems (ACIA, 2004; Vaughan et al, 2013; Macdonald et al, 2015). Measurements at Palmer Station in Antarctica show a strong seasonality in biological productivity and carbon uptake associated with changes in light, physical mixing, and grazing, and demonstrate the benefits of high-frequency sampling for quantifying CO2 uptake in seasonally ice-covered waters (Ducklow et al, 2013; Tortell et al, 2014; Goldman et al, 2015)

Results

Discussion

Conclusion