| Summary: | We report measurements of pore-water dissolved organic carbon (DOC), dissolved organic nitrogen, total dissolved carbohydrates, dissolved free mono saccharides, and ammonium in recovered deep-sea sediments from the Porcupine Abyssal Plain (PAP), Northeast Atlantic. There were distinct maxima close to the sediment-water interface of these constituents at all times of the year. The very high diffusive effluxes calculated from these pore-water distributions were not compatible with simultaneous sediment trap measurements of particulate organic carbon, nitrogen, and carbohydrate fluxes toward the seafloor. Effluxes calculated from pore-water DOC distributions in recovered cores from another Atlantic deep-sea site, showing almost identical maxima as those at PAP, were more than an order of magnitude greater than simultaneous in situ chamber DOC flux measurements. We suggest that the dissolved organic matter maxima are predominantly artifacts induced by lysis of, or leakage from, mainly bacterial biomass resulting from decompression and/or warming during recovery of the sediment cores from the abyssal seafloor. Temperature elevation during core recovery from the abyss gives a N-2 saturation of about 150%, and the combined effect of warming and decompression results in a CO2 saturation of about 135%, which together plausibly are associated with bubble formation creating cell bursting. Previous estimates of microbial biomass in abyssal sediments may be underestimates because of the difficulty of counting lysed bacterial cells. Since exoenzymes are inducible, previous measurements of their activities in recovered abyssal sediments may be overestimates.
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