Role of large particles in the transport of elements and organic compounds through the oceanic water column
During the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare, large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea. This finding has important implications for understanding the transfer, distribution and fate of elements and organic compounds in marine waters. The “large” (> 100 μm) detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small, discrete fecal pellets and plankton hard parts to large, amorphous aggregates or “snow” which contain both organic and inorganic constituents. Depending on size, shape and density, these particles sink at rates ranging from <1 to >1000 m day -1. Several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport. Flux studies have highlighted the importance of marine heterotrophs in packaging fine, suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth. Large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column. Chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements, radionuclides and organic compounds (natural and anthropogenic) in various oceanic regimes. Pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested.