In:
Biogeosciences, Copernicus GmbH, Vol. 16, No. 1 ( 2019-01-07), p. 1-16
Abstract:
Abstract. 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 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 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.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-16-1-2019
DOI:
10.5194/bg-16-1-2019-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2019
detail.hit.zdb_id:
2158181-2
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