277
in the water column only for a short time; the authors emphasized that such production events
(which may have gone unnoticed in past studies) can be instrumental in the deposition of the
large amounts of diatomaceous ooze found in the Southern Ocean. Episodes of phytoplankton
sedimentation following blooms are also well known on temperate continental shelves. They
can be associated with blooms occurring regularly in the annual cycle, as exemplified by the
Kiel Bight of the Baltic Sea, which is particularly well documented (e.g. Graf et al., 1982,
1984; Smetacek et al., 1984), or with episodic blooms resulting from summer mixing events,
as reported a number of times, for instance, from coastal areas in the northeastern Pacific
Ocean (e.g. Iverson et al., 1974; Takahashi et al., 1977). In the shallower areas, such as the
Kiel Bight, the fallout from the blooms is likely to be utilized by the benthos rather than
sequestered in the sediments.
Active grazing by herbivores, on the other hand, leads to sedimentation of faecal pellets.
Copepod faecal pellets are wrapped in a protective membrane that prevents degradation of the
content during at least part of the downwards transit and they package fine material (including
coccoliths) into larger, and therefore faster-sinking, particles. The sinking velocity of copepod
faecal pellets is of the order of 100 m d- I and shows variations with water temperature; they
will sink faster at higher temperature which, on the other hand, will tend to accelerate the
microbial degradation of their contents (e.g. Honjo and Roman, 1978), so that there are limits
to the effectiveness of copepod faecal pellet contribution to the export and eventual
sequestration of biogenic carbon. Microheterotrophs (bacteria and their protozoan predators)
will develop, for instance, by taking advantage of ageing biomass and detrital material not
being directly consumed by metazoans, especially if the material is being accumulated in
hydrodynamic traps, and, more generally, in situations that do not favour the buildup of a
full-fledged herbivore food chain (see Legendre and Le Fevre, 1989). As pointed out above,
the specialized consumers of microzooplankton are gelatinous organisms (e.g. salps, doliolids,
appendicularians) and pteropods. All of these produce faecal pellets that sink faster than those
of copepods or euphausiids (Alldredge, 1984). In addition, gelatinous organisms are
instrumental in the production of marine snow, where ultraplankton cells (which would
normally not sink at all) are aggregated together with other material and thus made available
for export to depth (e.g. Silver et al., 1986). As regards pteropods, a major aspect of their
contribution to carbon sequestration is sedimentation of their calcareous shells (see above).
in the water column only for a short time; the authors emphasized that such production events
(which may have gone unnoticed in past studies) can be instrumental in the deposition of the
large amounts of diatomaceous ooze found in the Southern Ocean. Episodes of phytoplankton
sedimentation following blooms are also well known on temperate continental shelves. They
can be associated with blooms occurring regularly in the annual cycle, as exemplified by the
Kiel Bight of the Baltic Sea, which is particularly well documented (e.g. Graf et al., 1982,
1984; Smetacek et al., 1984), or with episodic blooms resulting from summer mixing events,
as reported a number of times, for instance, from coastal areas in the northeastern Pacific
Ocean (e.g. Iverson et al., 1974; Takahashi et al., 1977). In the shallower areas, such as the
Kiel Bight, the fallout from the blooms is likely to be utilized by the benthos rather than
sequestered in the sediments.
Active grazing by herbivores, on the other hand, leads to sedimentation of faecal pellets.
Copepod faecal pellets are wrapped in a protective membrane that prevents degradation of the
content during at least part of the downwards transit and they package fine material (including
coccoliths) into larger, and therefore faster-sinking, particles. The sinking velocity of copepod
faecal pellets is of the order of 100 m d- I and shows variations with water temperature; they
will sink faster at higher temperature which, on the other hand, will tend to accelerate the
microbial degradation of their contents (e.g. Honjo and Roman, 1978), so that there are limits
to the effectiveness of copepod faecal pellet contribution to the export and eventual
sequestration of biogenic carbon. Microheterotrophs (bacteria and their protozoan predators)
will develop, for instance, by taking advantage of ageing biomass and detrital material not
being directly consumed by metazoans, especially if the material is being accumulated in
hydrodynamic traps, and, more generally, in situations that do not favour the buildup of a
full-fledged herbivore food chain (see Legendre and Le Fevre, 1989). As pointed out above,
the specialized consumers of microzooplankton are gelatinous organisms (e.g. salps, doliolids,
appendicularians) and pteropods. All of these produce faecal pellets that sink faster than those
of copepods or euphausiids (Alldredge, 1984). In addition, gelatinous organisms are
instrumental in the production of marine snow, where ultraplankton cells (which would
normally not sink at all) are aggregated together with other material and thus made available
for export to depth (e.g. Silver et al., 1986). As regards pteropods, a major aspect of their
contribution to carbon sequestration is sedimentation of their calcareous shells (see above).