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

Abstract

Abstract. Particle sinking is a major form of transport for photosynthetically fixed\ncarbon to below the euphotic zone via the biological carbon pump (BCP).\nOxygen ( O2 ) depletion may improve the efficiency of the BCP.\nHowever, the mechanisms by which O2 deficiency can enhance\nparticulate organic matter (POM) vertical fluxes are not well understood.\nHere, we investigate the composition and vertical fluxes of POM in two deep\nbasins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two\nbasins showed different O2 regimes resulting from the intrusion of\noxygen-rich water from the North Sea that ventilated the water column below\n140\u2009m in GB, but not in LD, during the time of sampling. In June\xa02015, we\ndeployed surface-tethered drifting sediment traps in oxic surface waters (GB:\n40 and 60\u2009m; LD: 40 and 55\u2009m), within the oxygen minimum zone (OMZ; GB:\n110\u2009m and LD: 110 and 180\u2009m) and at recently oxygenated waters by the North\nSea inflow in GB (180\u2009m). The primary objective of this study was to test\nthe hypothesis that the different O2 conditions in the water column\nof GB and LD affected the composition and vertical flux of sinking particles\nand caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and\nLD. In GB, particulate organic carbon (POC) flux was 18\u2009% lower in the\nshallowest trap (40\u2009m) than in the deepest sediment trap (at 180\u2009m).\nParticulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes\ndecreased with depth, while particulate organic phosphorus (POP), biogenic\nsilicate (BSi), chlorophyll\xa0 a (Chl\xa0 a ) and transparent exopolymeric particle\n(TEP) fluxes peaked within the core of the OMZ (110\u2009m); this coincided with\nthe presence of manganese oxide-like (MnOx-like) particles aggregated with\norganic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28\u2009%,\n42\u2009% and 56\u2009%, respectively, from the surface to deep waters. POP, BSi and\nTEP fluxes did not decrease continuously with depth, but they were higher at\n110\u2009m. Although we observe a higher vertical flux of POP, BSi and TEPs\ncoinciding with abundant MnOx-like particles at 110\u2009m in both basins, the\npeak in the vertical flux of POM and MnOx-like particles was much higher in\nGB than in LD. Sinking particles were remarkably enriched in BSi, indicating\nthat diatoms were preferentially included in sinking aggregates and/or there\nwas an inclusion of lithogenic Si (scavenged into sinking particles) in our\nanalysis. During this study, the POC transfer efficiency (POC flux at 180\u2009m\nover 40\u2009m) was higher in GB (115\u2009%) than in LD (69\u2009%), suggesting that\nunder anoxic conditions a smaller portion of the POC exported below the\neuphotic zone was transferred to 180\u2009m than under reoxygenated conditions\npresent in GB. In addition, the vertical fluxes of MnOx-like particles were\n2\xa0orders of magnitude higher in GB than LD. Our results suggest that\nPOM aggregates with MnOx-like particles formed after the inflow of\noxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may\nalter the composition and vertical flux of POM, potentially contributing to\na higher transfer efficiency of POC in GB. This idea is consistent with\nobservations of fresher and less degraded organic matter in deep waters of\nGB than LD.