Normal and accelerated sinking of phytoplankton in the sea

Abstract

The sinking and deposition of phytoplankton remains lead to sediment varving, the formation of sedimentary oozes, and the transport of partially decomposed and viable, autotrophic cells to great depths. Some specific examples are discussed, and the associated settling rates, depositional characteristics and individual species responses are examined in relationship to experimentally determined sinking rates of both living and dead, intact phytoplankton. The data include field and experimental observations on the sinking behavior for the same species. The in vitro rates are too low, and a mechanism or mechanisms causing accelerated rates of phytoplankton sinking in situ seems necessary to account adequately for some of the observed transport of phytoplankton remains to great depths and to the sea floor. Various potential mechanisms of heightened diatom, dinoflagellate and coccolithophorid sinking rates are then outlined and examined quantitatively. These include density inversion plumes, downwelling, and fecal pellet sinking. Sinking rates of naturally occurring fecal pellets and those produced by cultured zooplankton are presented. The importance of fecal pellet settling as a means of accelerated sinking of phytoplankton remains to depth, and their subsequent contribution to the fossil record and organic content of ocean floor sediments are then discussed.