275
generates from the photic zone: that is ( ... ) the amount of organic matter which will
accumulate in a trap set below this zone" (Berger et al., 1989). Material passed on to the food
web also is, in the ecological sense, export from the primary production system. It may end
up being exported from the production area either horizontally, through passive transport
associated with circulation patterns or active migration of large animals, or vertically, again
passively, through sedimentation of living or detrital particles, or actively, through vertical
migrations. The production of long-lived dissolved organic matter should also be considered
(see above). Not all export pathways result in the sequestration of carbon; commercial
harvesting of fish, for instance, will eventually lead to carbon recycling through respiration,
as will some other types of export to the food webs. The fraction of the original carbon
production that will eventually be sequestered is largely controlled by ecosystem processes,
which may result in a variable extent of uncoupling, in time and in space, between production
and export in the geochemical sense. Legendre and Le Fevre (1989) have reviewed the various
export pathways from the primary producers, their hydrodynamic control, and their
significance in terms of particulate organic matter (POM) fallout from the surface layer
(Fig. 2). The possibilities include the sinking of large intact cells, the grazing of large cells
by herbivores and production of faecal pellets as well as active transport by vertically
migrating organisms, the grazing of detrital biogenic material (accumulated in hydrodynamic
traps) by microphagous feeders, the incorporation of small cells into sinking marine snow, and
the direct grazing of ultraplankton by metazoans. As defined in Legendre and Le Fevre
(1989), large phytoplankton cells (> 5 J.'m, including chains of small cells) generally belong
to diatoms, dinoflagellates and coccolithophores, while small cells « 5 J.'m) comprise both
eukaryotic and prokaryotic photoautotrophs.
Massive sedimentation of large intact cells, at rates that may exceed 100 m ct 1 (refs. in
Goldman, 1988), mainly occurs under bloom conditions. Legendre's (1990) review shows that
microalgal blooms largely depend on the balance between phytoplankton production (resulting
from hydrodynamic forcing) and grazing by herbivorous zooplankton (see also Parsons and
Lalli, 1988). Under low grazing pressure, most of the large cells may sediment. Many cases
are reported in the literature of large fallout of often intact diatom cells following a bloom,
either in upwelling regions, or at certain phases of the annual plankton cycle. Smith et al.
generates from the photic zone: that is ( ... ) the amount of organic matter which will
accumulate in a trap set below this zone" (Berger et al., 1989). Material passed on to the food
web also is, in the ecological sense, export from the primary production system. It may end
up being exported from the production area either horizontally, through passive transport
associated with circulation patterns or active migration of large animals, or vertically, again
passively, through sedimentation of living or detrital particles, or actively, through vertical
migrations. The production of long-lived dissolved organic matter should also be considered
(see above). Not all export pathways result in the sequestration of carbon; commercial
harvesting of fish, for instance, will eventually lead to carbon recycling through respiration,
as will some other types of export to the food webs. The fraction of the original carbon
production that will eventually be sequestered is largely controlled by ecosystem processes,
which may result in a variable extent of uncoupling, in time and in space, between production
and export in the geochemical sense. Legendre and Le Fevre (1989) have reviewed the various
export pathways from the primary producers, their hydrodynamic control, and their
significance in terms of particulate organic matter (POM) fallout from the surface layer
(Fig. 2). The possibilities include the sinking of large intact cells, the grazing of large cells
by herbivores and production of faecal pellets as well as active transport by vertically
migrating organisms, the grazing of detrital biogenic material (accumulated in hydrodynamic
traps) by microphagous feeders, the incorporation of small cells into sinking marine snow, and
the direct grazing of ultraplankton by metazoans. As defined in Legendre and Le Fevre
(1989), large phytoplankton cells (> 5 J.'m, including chains of small cells) generally belong
to diatoms, dinoflagellates and coccolithophores, while small cells « 5 J.'m) comprise both
eukaryotic and prokaryotic photoautotrophs.
Massive sedimentation of large intact cells, at rates that may exceed 100 m ct 1 (refs. in
Goldman, 1988), mainly occurs under bloom conditions. Legendre's (1990) review shows that
microalgal blooms largely depend on the balance between phytoplankton production (resulting
from hydrodynamic forcing) and grazing by herbivorous zooplankton (see also Parsons and
Lalli, 1988). Under low grazing pressure, most of the large cells may sediment. Many cases
are reported in the literature of large fallout of often intact diatom cells following a bloom,
either in upwelling regions, or at certain phases of the annual plankton cycle. Smith et al.