Rapid abiotic transformation of marine dissolved organic material to particulate organic material in surface and deep waters

Abstract. Marine particulate organic matter (POM) supports the vertical transport of organics in the oceans, and the ecology of microbes and filter feeders. POM is collected on GFF filters and quantified as particulate organic carbon (POC) and nitrogen (PON). The filtrate contains dissolved organic matter (DOM) that partly is abiotically converted into POM and can be collected on GFF filters. We filtered seawater from cultures and the pelagic ocean, after the initial sample filtration yielding the conventional POC/PON sample (POM₁), we refiltered the filtrate yielding POM₂, refiltering its filtrate we obtained POM₃ and so on till POM_i. Refiltered POM tended to the same concentration independent of the sample depth and even after a few refiltrations, independent of the original particulate organic load. POM₂/POM_i for surface water (< 100 m) was about 0.1 and for deep water (> 1000 m) was > 1.0. We considered adsorption of DOM and bacteria that had passed through the first filter but concluded that these could not explain the high POM concentration in the filtrates. We suggest that POM in filtrates represent gels formed due to hydraulic stress during filtration. The ratio of POM₂/POM_i will partly depend on environmental conditions like turbulent energy. We suggest that the increase of POM₂/POM_i with depth is related to the lower in situ turbulent energy at greater depth. We discuss aspects of POM methodology, including problems with acidification of samples, and the wider ecological implications of our results.