High Emissions of Carbon Dioxide and Methane From the Coastal Baltic Sea at the End of a Summer Heat Wave

Abstract

The summer heat wave in 2018 led to the highest recorded water temperatures since 1926 -up to 21 • C -in bottom coastal waters of the Baltic Sea, with implications for the respiration patterns in these shallow coastal systems. We applied cavity ringdown spectrometer measurements to continuously monitor carbon dioxide (CO 2 ) and methane (CH 4 ) surface-water concentrations, covering the coastal archipelagos of Sweden and Finland and the open and deeper parts of the Northern Baltic Proper. This allowed us to (i) follow an upwelling event near the Swedish coast leading to elevated CO 2 and moderate CH 4 outgassing, and (ii) to estimate CH 4 sources and fluxes along the coast by investigating water column inventories and air-sea fluxes during a storm and an associated downwelling event. At the end of the heat wave, before the storm event, we found elevated CO 2 (1583 µatm) and CH 4 (70 nmol/L) concentrations. During the storm, a massive CO 2 sea-air flux of up to 274 mmol m −2 d −1 was observed. While water-column CO 2 concentrations were depleted during several hours of the storm, CH 4 concentrations remained elevated. Overall, we found a positive relationship between CO 2 and CH 4 wind-driven sea-air fluxes, however, the highest CH 4 fluxes were observed at low winds whereas highest CO 2 fluxes were during peak winds, suggesting different sources and processes controlling their fluxes besides wind. We applied a box-model approach to estimate the CH 4 supply needed to sustain these elevated CH 4 concentrations and the results suggest a large source flux of CH 4 to the water column of 2.5 mmol m −2 d −1 . These results are qualitatively supported by acoustic observations of vigorous and widespread outgassing from the sediments, with flares that could be traced throughout the water column penetrating the pycnocline and reaching the sea surface. The results suggest that the heat wave triggered CO 2 and CH 4 fluxes in the coastal zones that are comparable with maximum emission rates found in other hot spots, such as boreal and arctic lakes and wetlands. Further, the results suggest that heat waves are as important for CO 2 and CH 4 sea-air fluxes as the ice break up in spring.

Low Abundance of Methanotrophs in Sediments of Shallow Boreal Coastal Zones With High Water Methane Concentrations

“…At the time of sampling there was just minor changes in salinity in the studied stations (range: 6.5-7.0), compared to previous studies having a factor from freshwater-to-marine salinity (Bange, 2006;Osudar et al, 2015;Borges et al, 2016). Furthermore, a separate study conducted at the same time in the study area found no significant link between CH 4 concentrations in the water and salinity [full details in Humborg et al (2019)] indicating that salinity does not explain the patterns of relative abundance of methanotrophs and pMMO RNA transcript data in our study. The bottom water was oxygenated at all studied stations and was therefore unlikely to be a limiting factor for methanotrophs in the study area.…”

“…Seeps were defined as either trains of bubbles or bubble plumes (many bubbles overlapping in vertical structures) and were identified through visual inspection of the processed acoustic data. Ebullition from sediments has been observed in the study area and reported in Humborg et al (2019). Here we present in addition high resolution acoustic data on: (1) the number of seeps and (2) the relation of seeps to water depth in the study area.…”

“…For example, CH 4 emissions from inland waters are known to significantly contribute to the atmospheric budget (estimated to 77 Tg C yr −1 ) (Bastviken et al, 2011). Humborg et al (2019) calculated a daily sediment flux-water column CH 4 reservoir of 2.5 mmol m −2 (or 30 mg C m −2 ) in the coastal waters of Tvärminne during September 2018 (same sampling campaign as reported in this study). This is within the range of CH 4 emissions reported from subarctic lakes (Matveev et al, 2016), and it is suggested that shallow coastal waters, similarly to inland waters, are hotspots for CH 4 emission.…”

“…In September 2018 CH 4 concentrations in the surface water at a 0.5-1.0 m water depth were measured in situ using a Water Equilibration Gas Analyzer System (WEGAS). A full method description along with the results are presented in Humborg et al (2019). In brief, circulation pumps equipped to a seawater inlet transfer seawater into an equilibrator with showerhead.…”

“…This system also tracked temperature and salinity as long as R/V Electra was cruising. Salinity, temperature, and CH 4 for September 2018 have been measured and is available in Humborg et al (2019). However, data from the specific stations presented here have not been reported.…”