Effects of physical fragmentation on remineralization of marine snow

Abstract

The vast majority of carbon lost from ocean surface waters sinks as large, relatively rare, marine snow and fecal pellets. Fragmentation of these particles into more slowly sinking daughter particles has been proposed previously to partly explain the rapid loss of sinking carbon below the mixed layer. In this study we investigated 2 other less obvious potential consequences of aggregate disruption, namely the release of dissolved interstitial compounds upon fragmentation and accelerated degradation due to increased particle surface area. We found that upon fragmentation natural marine aggregates, ranging in size from 3 to 6 mm diameter, released dissolved organic carbon (DOC) (mean 0.12 μmol aggregate-1) and nitrate (mean of 0.013 μmol aggregate-1) into surrounding seawater, making these nutrients available to free-living biota. Filtration of whole aggregates failed to result in an equivalent release, suggesting that marine snow may not be as leaky as expected based on high aggregate porosity. Decomposition of aggregate particulate organic carbon (POC) to DOC was similar for whole and fragmented aggregates ranging from 0.6 to 0.9 μmol POC aggregate-1 d -1, resulting in calculated aggregate POC turnover times of 2 to 11 d. Remineralization of DOC was also similar for both aggregate treatments and suggested a tight coupling between solubilization and uptake by attached bacteria. Our results indicate that the longer residence times predicted for smaller aggregates in the mixed layer, rather than changes in decomposition rate, may be the most influential impact of aggregate fragmentation on reduction of particle flux to depth. © Inter-Research 2005.