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Abstract

Over the last decades, the Arctic Ocean has suffered a substantial
decline in sea ice cover due to global warming. The impacts of these
variations on primary productivity, fluxes of dissolved and particulate
organic matter (OM) and turnover at the seafloor are still poorly
understood. Here we focus on the characteristics and dynamics of the
pool of marine dissolved OM (DOM) in surface sediments of the Arctic
Ocean. To investigate spatial and temporal variations of DOM in relation
to particulate OM input and benthic microbial community parameters,
sediment porewater and overlying bottom water were collected from the
long-term observatory HAUSGARTEN in June 2013 and 2014. The study area
in the Fram Strait, which is partially covered by sea ice, was sampled
along a bathymetric transect (1050-5500 m water depth), from east to
west (7 degrees 0.2' E to 5 degrees 17' W), and from south to north (78
degrees 37' to 79 degrees 43' N). Molecular data on solid phase
extracted DOM obtained via Fourier Transform Ion Cyclotron Resonance
Mass Spectrometric analysis and a suite of bulk chemical parameters were
related to benthic biogeochemical data. Our results demonstrate a close
coupling between the production and input of OM from the surface ocean
to the seafloor, and the concentration and composition of DOC/DOM in the
deep sea. Surface porewaters collected in 2013 from shallower stations
(<= 1500 m water depth) in the eastern Fram Strait, had a signal of a
larger and more recent input of OM (higher concentrations of
phytodetritus). This was associated with higher numbers of molecular
formulas, abundances of unsaturated aliphatic and N-containing formulas,
in concert with higher enzymatic activity, phospholipids, total organic
carbon and protein content. In contrast, porewaters collected in 2014
from deeper stations and from the West, were associated with lower OM
input, and showed higher abundances of aromatic and oxygen-poor
compounds. Higher OM input was also reflected in higher DOC
concentrations and fluxes from the sediment into the water column. Our
study demonstrates that regional and temporal variations in OM input can
quickly translate into changes in the quantity and quality of surface
porewater DOM, the latter substantially altered by deep-sea sediment
bacteria.