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Several methanogenic pathways in oxic surface waters were recently discovered, but their relevance in the natural environment is still unknown. Our study examines distinct methane (CH4) enrichments that repeatedly occur below the thermocline during the summer months in the central Baltic Sea. In agreement with previous studies in this region, we discovered differences in the methane distributions between the western and eastern Gotland Basin, pointing to in situ methane production below the thermocline in the latter (concentration of CH414.1±6.1 nM, δ13C CH4 -62.9 ‰). Through the use of a high-resolution hydrographic model of the Baltic Sea, we showed that methane below the thermocline can be transported by upwelling events towards the sea surface, thus contributing to the methane flux at the sea–air interface. To quantify zooplankton-associated methane production rates, we developed a sea-going methane stripping-oxidation line to determine methane release rates from copepods grazing on 14C -labelled phytoplankton. We found that (1) methane production increased with the number of copepods, (2) higher methane production rates were measured in incubations with Temora longicornis (125±49 fmol methane copepod -1 d -1) than in incubations with Acartia spp. (84±19 fmol CH4 copepod -1 d -1) dominated zooplankton communities, and (3) methane was only produced on a Rhodomonas sp. diet, and not on a cyanobacteria diet. Furthermore, copepod-specific methane production rates increased with incubation time. The latter finding suggests that methanogenic substrates for water-dwelling microbes are released by cell disruption during feeding, defecation, or diffusion from fecal pellets. In the field, particularly high methane concentrations coincided with stations showing a high abundance of DMSP/DMSO-rich Dinophyceae. Lipid biomarkers extracted from phytoplankton- and copepod-rich samples revealed that Dinophyceae are a major food source of the T. longicornis dominated zooplankton community, supporting the proposed link between copepod grazing, DMSP/DMSO release, and the build-up of subthermocline methane enrichments in the central Baltic Sea. [ABSTRACT FROM AUTHOR]

Several methanogenic pathways in oxic surface waters were recently discovered, but their relevance in the natural environment is still unknown. Our study examines distinct methane enrichments that repeatedly occur below the thermocline during the summer months in the central Baltic Sea. In agreement with previous studies in this region, we discovered differences in the methane distributions between the Western and Eastern Gotland Basin, pointing to in situ methane production below the thermocline in the latter (conc. CH4 14.1 ± 6.1 nM, δ13C CH4 -62.9 ‰). Through the use of a high resolution hydrographic model of the Baltic Sea, we showed that methane below the thermocline can be transported by upwelling events towards the sea surface thus contributing to the methane flux at the sea/air interface. To quantify zooplankton-associated methane production rates, we developed a sea-going methane stripping-oxidation line to determine methane release rates from copepods grazing on 14C-labelled phytoplankton. We found that: (1) methane production increased with the number of copepods, (2) higher methane production rates were measured in incubations with Temora longicornis (125 ± 49 fmol methane copepod-1 d-1) than incubations with Acartia spp. (84 ± 19 fmol CH4 copepod-1 d-1) dominated zooplankton communities, and (3) methane was only produced on a Rhodomonas sp. diet, but not on a cyanobacteria diet. Furthermore, copepod-specific methane production rates increased with incubation time. The latter finding suggests that methanogenic substrates for water-dwelling microbes are released by cell disruption during feeding, defecation, or diffusion from fecal pellets. In the field, particularly high methane concentrations coincided with stations showing a high abundance of DMSP-rich Dinophyceae. Lipid biomarkers extracted from phytoplankton- and copepod-rich samples revealed that Dinophyceae are a major food source of the T. longicornis dominated zooplankton community, supporting the proposed link between copepod grazing, DMSP release, and the buildup of subthermocline methane enrichments in the central Baltic Sea. [ABSTRACT FROM AUTHOR]

Abstract: We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (δ13C‐CH4 of −67.6‰) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet‐consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L−1 d−1 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d−1 per adult T. longicornis (about 1 pmol L−1 d−1) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper‐ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L−1 d−1 are needed to maintain the observed methane air–sea flux. [ABSTRACT FROM AUTHOR]

Anthropogenic CO2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity ( AT) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such AT dynamics, due to its direct link to terrestrial processes, short water residence time and long history of AT measurements dating back to the early 20th century. We compiled an extensive AT data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water AT levels increased throughout the Baltic Sea. The rates of change were highest in the low-saline, northern areas and decreased gradually toward constant levels in the North Sea. The AT increase observed in the Central Baltic Sea (+3.4 µmol kg−1 yr−1) and the Gulf of Bothnia (+7 µmol kg−1 yr−1) has compensated CO2-induced acidification by almost 50% and 100%, respectively. Further, the AT trends enhanced the CO2 storage capacity and stabilized the CaCO3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the AT trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO2, agricultural liming and internal AT sources. [ABSTRACT FROM AUTHOR]

A fully automated measurement system for recording of the surface water CO 2 partial pressure, pCO 2 , was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH 4 . The ship commutes regularly at 2–3 day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea. The pCO 2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO 2 consumption in spring/summer caused pCO 2 that were by up to 300 μatm below the atmospheric level. In contrast, the pCO 2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO 2 -enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO 2 , an air–sea CO 2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO 2 with uptake rates ranging between 0.60 and 0.89 mol m − 2 yr − 1 . The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO 2 uptake. The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events. Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130 nmol L − 1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air–sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage. [ABSTRACT FROM AUTHOR]