• DOC was measured in the Bransfield Strait during the end-summer of 2015 and 2016.

• Physicochemical parameters related to the variability of DOC were investigated.

• Significant interannual differences were observed between DOC concentrations.

• ENSO and SAM influenced the interannual structure of the water column.

Contrasting dissolved organic carbon concentrations in the Bransfield Strait, Northern Antarctic Peninsula: insights into ENSO and SAM effects

Abstract - https://doi.org/10.1016/j.jmarsys.2020.103457

The marine reservoir of dissolved organic carbon (DOC) is equivalent to the atmospheric carbon reservoir and represents the largest reduced carbon stock in the ocean. However, the role of DOC in the global carbon cycle is not fully understood. In this study, we analyzed the distribution of DOC in the water column of the Bransfield Strait, Northern Antarctic Peninsula. Data were collected during two summer cruises of the NAUTILUS project conducted by the Brazilian High Latitude Oceanography Group in February 2015 and 2016. We investigated the possible effects of the El Niño - Southern Oscillation (ENSO) and Southern Annular Mode (SAM) climate modes on DOC variability based on correlations between DOC and temperature, salinity, and dissolved oxygen. Concentrations of DOC ranged from 40.70 to 122.82 μmol kg⁻¹ in 2015 and 33.12 to 112.20 μmol kg⁻¹ in 2016. The analysis shows that the vertical distribution of DOC in the Bransfield Strait may be controlled by a combination of vertical stratification, vertical export of organic matter and biological activity in the upper mixed layer during summertime. The significant interannual differences observed between DOC concentration and most physicochemical parameters may be linked to ENSO and SAM climate modes. The highest DOC concentration was observed in February 2015, associated with a layer of warm surface water, due to the higher stratification of the water column. The ENSO and SAM climate modes were low and positive in that year, favoring intrusions of waters from the Antarctic Circumpolar Current. In contrast, February 2016 was characterized by record positive ENSO and SAM modes. The intensification of the Weddell Gyre induced the advection of cold, more oxygenated and recently formed shelf waters into the Bransfield Strait, which may have contributed to the lower DOC concentrations. This study contributes to a better understanding of the role of DOC in the global carbon cycle in a region of complex ocean circulation and mixing, and sensitive to global climate change.