Articles | Volume 8, issue 3
Earth Syst. Dynam., 8, 817–826, 2017

Special issue: Multiple drivers for Earth system changes in the Baltic Sea...

Earth Syst. Dynam., 8, 817–826, 2017

Research article 14 Sep 2017

Research article | 14 Sep 2017

Effects of the 2014 major Baltic inflow on methane and nitrous oxide dynamics in the water column of the central Baltic Sea

Jukka-Pekka Myllykangas1, Tom Jilbert1, Gunnar Jakobs1,2, Gregor Rehder3, Jan Werner3, and Susanna Hietanen1 Jukka-Pekka Myllykangas et al.
  • 1Department of Environmental Sciences, University of Helsinki, P.O. Box 65, 00014 University of Helsinki, Helsinki, Finland
  • 2Technical University of Denmark, Frederiksborgvej 399, 4000 Roskilde, Denmark
  • 3Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Seestraße 15, 18119 Rostock, Germany

Abstract. In late 2014, a large, oxygen-rich salt water inflow entered the Baltic Sea and caused considerable changes in deep water oxygen concentrations. We studied the effects of the inflow on the concentration patterns of two greenhouse gases, methane and nitrous oxide, during the following year (2015) in the water column of the Gotland Basin. In the eastern basin, methane which had previously accumulated in the deep waters was largely removed during the year. Here, volume-weighted mean concentration below 70 m decreased from 108 nM in March to 16.3 nM over a period of 141 days (0.65 nM d−1), predominantly due to oxidation (up to 79 %) following turbulent mixing with the oxygen-rich inflow. In contrast nitrous oxide, which was previously absent from deep waters, accumulated in deep waters due to enhanced nitrification following the inflow. Volume-weighted mean concentration of nitrous oxide below 70 m increased from 11.8 nM in March to 24.4 nM in 141 days (0.09 nM d−1). A transient extreme accumulation of nitrous oxide (877 nM) was observed in the deep waters of the Eastern Gotland Basin towards the end of 2015, when deep waters turned anoxic again, sedimentary denitrification was induced and methane was reintroduced to the bottom waters. The Western Gotland Basin gas biogeochemistry was not affected by the inflow.

Short summary
The deep waters of the Baltic Sea host an expanding dead zone, where low-oxygen conditions favour the natural production of two strong greenhouse gases, methane and nitrous oxide. Oxygen is introduced into the deeps only during rare salt pulses. We studied the effects of a recent salt pulse on Baltic greenhouse gas production. We found that where oxygen was introduced, methane was largely removed, while nitrous oxide production increased, indicating strong effects on greenhouse gas dynamics.
Final-revised paper