Space–time dynamics of carbon and environmental parameters related to carbon dioxide emissions in the Buor-Khaya Bay and adjacent part of the Laptev Sea

Abstract. This study aims to improve understanding of carbon cycling in the Buor-Khaya Bay (BKB) and adjacent part of the Laptev Sea by studying the inter-annual, seasonal, and meso-scale variability of carbon and related hydrological and biogeochemical parameters in the water, as well as factors controlling carbon dioxide (CO₂) emission. Here we present data sets obtained on summer cruises and winter expeditions during 12 yr of investigation. Based on data analysis, we suggest that in the heterotrophic BKB area, input of terrestrially borne organic carbon (OC) varies seasonally and inter-annually and is largely determined by rates of coastal erosion and river discharge. Two different BKB sedimentation regimes were revealed: Type 1 (erosion accumulation) and Type 2 (accumulation). A Type 1 sedimentation regime occurs more often and is believed to be the quantitatively most important mechanism for suspended particular matter (SPM) and particulate organic carbon (POC) delivery to the BKB. The mean SPM concentration observed in the BKB under a Type 1 regime was one order of magnitude greater than the mean concentration of SPM (~ 20 mg L⁻¹) observed along the Lena River stream in summer 2003. Loadings of the BKB water column with particulate material vary by more than a factor of two between the two regimes. Higher partial pressure of CO₂ (pCO₂), higher concentrations of nutrients, and lower levels of oxygen saturation were observed in the bottom water near the eroded coasts, implying that coastal erosion and subsequent oxidation of eroded organic matter (OM) rather than the Lena River serves as the predominant source of nutrients to the BKB. Atmospheric CO₂ fluxes from the sea surface in the BKB vary from 1 to 95 mmol m⁻² day⁻¹ and are determined by specific features of hydrology and wind conditions, which change spatially, seasonally, and inter-annually. Mean values of CO₂ emission from the shallow Laptev Sea were similar in September 1999 and 2005 (7.2 and 7.8 mmol m⁻² day⁻¹, respectively), while the CO₂ efflux can be one order lower after a strong storm such as in September 2011. Atmospheric CO₂ emissions from a thawed coastal ice complex in the BKB area varied from 9 to 439 mmol m⁻² day⁻¹, with the mean value ranged from 75.7 to 101 mmol m⁻² day⁻¹ in two years (September 2006 and 2009), suggesting that at the time of observations the eroded coastal area served as a more significant source of CO₂ to the atmosphere than the tundra (mean value: 22.7 mmol m⁻² day⁻¹) on the neighboring Primorsky coastal plain (September 2006). The observed increase in the Lena River discharge since the 1990s suggests that increased levels of "satellite-derived" annual primary production could be explained by an increasing load of humic acids delivered to shelf water; in this water the color resulting from the presence of CDOM (colored dissolved organic matter) mimics the color resulting from the presence of Chl a when seen from space. Because the BKB area can be employed as an integrator of ongoing changes in the surrounding environment, we suggest that under ongoing changes, more nutrients, products of eroded OC transformation and river transport, will be delivered to the Arctic Ocean with its shrinking ice cover, potentially increasing primary production outside of the shallow East Siberian Arctic Shelf (ESAS). At the same time, because the ESAS is characterized by very low transparency which limits euphotic layer thickness, excessive pCO₂ will not be utilized by photosynthesis but will rather be emitted to the atmosphere at increasing rates, affecting regional CO₂ balance.