Composition and vertical flux of particulate organic matter to the oxygen minimum zone of the central Baltic Sea: impact of a sporadic North Sea inflow
Authors: Cisternas-Novoa Carolina , Le Moigne Frédéric A. C. , Engel Anja 
Published: February 2019

Abstract. Particle sinking is a major form of transport for photosynthetically fixed
carbon to below the euphotic zone via the biological carbon pump (BCP).
Oxygen (O2) depletion may improve the efficiency of the BCP.
However, the mechanisms by which O2 deficiency can enhance
particulate organic matter (POM) vertical fluxes are not well understood.
Here, we investigate the composition and vertical fluxes of POM in two deep
basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two
basins showed different O2 regimes resulting from the intrusion of
oxygen-rich water from the North Sea that ventilated the water column below
140 m in GB, but not in LD, during the time of sampling. In June 2015, we
deployed surface-tethered drifting sediment traps in oxic surface waters (GB:
40 and 60 m; LD: 40 and 55 m), within the oxygen minimum zone (OMZ; GB:
110 m and LD: 110 and 180 m) and at recently oxygenated waters by the North
Sea inflow in GB (180 m). The primary objective of this study was to test
the hypothesis that the different O2 conditions in the water column
of GB and LD affected the composition and vertical flux of sinking particles
and caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and
LD. In GB, particulate organic carbon (POC) flux was 18 % lower in the
shallowest trap (40 m) than in the deepest sediment trap (at 180 m).
Particulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes
decreased with depth, while particulate organic phosphorus (POP), biogenic
silicate (BSi), chlorophyll a (Chl a) and transparent exopolymeric particle
(TEP) fluxes peaked within the core of the OMZ (110 m); this coincided with
the presence of manganese oxide-like (MnOx-like) particles aggregated with
organic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28 %,
42 % and 56 %, respectively, from the surface to deep waters. POP, BSi and
TEP fluxes did not decrease continuously with depth, but they were higher at
110 m. Although we observe a higher vertical flux of POP, BSi and TEPs
coinciding with abundant MnOx-like particles at 110 m in both basins, the
peak in the vertical flux of POM and MnOx-like particles was much higher in
GB than in LD. Sinking particles were remarkably enriched in BSi, indicating
that diatoms were preferentially included in sinking aggregates and/or there
was an inclusion of lithogenic Si (scavenged into sinking particles) in our
analysis. During this study, the POC transfer efficiency (POC flux at 180 m
over 40 m) was higher in GB (115 %) than in LD (69 %), suggesting that
under anoxic conditions a smaller portion of the POC exported below the
euphotic zone was transferred to 180 m than under reoxygenated conditions
present in GB. In addition, the vertical fluxes of MnOx-like particles were
2 orders of magnitude higher in GB than LD. Our results suggest that
POM aggregates with MnOx-like particles formed after the inflow of
oxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may
alter the composition and vertical flux of POM, potentially contributing to
a higher transfer efficiency of POC in GB. This idea is consistent with
observations of fresher and less degraded organic matter in deep waters of
GB than LD.