32
Chapter 2: Biogeographic Partition of the Ocean
diatoms nevertheless remain very important constituents in some regions and at some
seasons. If a species is transported unintentionally in ballast water and implants itself as
an exotic in a new region, consequences may ensue that are unpredictable in their details,
but are perhaps not insignificant. Take the case of Coscinodiscus wailesii.
Coscinodiscus is, of course, a well-known genus of discoid centric diatoms, represented
in most regions where phytoplankton blooms occur by several species having a range
of cell diameters from 50 to 350 m. Relative taxonomic abundance changes seasonally,
and the small to moderate-sized species, at least, are important in the diet of herbivorous
copepods. Larger species exist, principally oceanic in distribution. One such is C. wailesii of
the northwest Pacific, having rather wide salinity and temperature tolerances (Nishikawa
et al., 2000); its cells of 160–450 m are dominant in the spring bloom in the Seto Inland
Sea, reaching cell concentrations sufficiently high as to interfere with the raft culture of
the macroalga Porphyrea by competing for light and nutrients. Mass sedimentation of
dead cells after a bloom may create oxygen deficiency in bottom water.
In the mid-1970s, C. wailesi was observed in the English Channel for the first time,
whence it spread rapidly into the North Sea, the German Bight, and more widely in
the northeast Atlantic (Edwards et al., 2001). It became firmly established as an invasive
species and continues to be dominant in the spring diatom bloom of these regions. More
recently, it has been observed on the continental shelf off southern Brazil, where it now
forms seasonal blooms in the Gulf of Parana, and also off the coast of Nova Scotia in
the NW Atlantic. All that might not matter very much if the cells were not so large
and could be consumed by the indigenous copepods and other herbivores. One author
has described this as an inedible diatom in the seas that it has invaded, although that is
not strictly the case for “sloppy” feeding by copepods has been recorded. Perhaps only
the largest cell sizes of C. wailesi are not readily available to copepods—recall that cell
volume progressively diminishes as diatoms reproduce during the season of growth. In
warm oceans, of course, centric diatoms as large as C. wailesi are the central item in the
diet of some filter-feeding fish (i.e., Ethmalosa, Brevoortia), rather than of copepods.
We may expect that such invasions will have measurable impacts on the structure and
flow of material through pelagic food chains and it has been suggested (Sommer, 1998)
that the effects may be comparable to those expected from eutrophication of semienclosed
seas by waste discharge. This is deficient in Si, compared to N and P, and so leads not
to diatom blooms, but to a more complex phytoplankton community and less efficient
transfer of energy up the food chain. Tunicates and coelenterates may be favored at
the expense of diatoms and fish. In this case, the fear is that the diatoms will not be
consumed.
The second consequence of the role of “intelligent” biota that I want to discuss
briefly is far more urgent and is perhaps—just perhaps—controllable. A century or
more of industrial fishing in large stretches of the coastal ocean has created havoc
with their pristine ecosystems. Indeed, as Jackson et al. (2001) comment “Ecological
extinction caused by overfishing precedes all other pervasive human disturbance of coastal
ecosystems, including pollution, degradation of water quality and anthropogenic climate
change.” One must agree with their assessment.
Unlike the introduction of exotic species of plankton, the impacts of fishing on the
marine food chain are (in the ecological sense) top-down, rather than bottom-up, and
are more immediate. Some regional food webs have already been entirely transformed, to
the common detriment of fisherfolk and fish. Recent studies of the destruction of benthic
invertebrates caused by the passage of a commercial trawl over the seabed have given
alarming results: individual scours left by otter boards may be visible for more than a
year. The mortality of gastropods, echinoderms, crustaceans, and annelids caused by the
passage of a single beam trawl is 5–40%, and for bivalve mollusks it is 20–65%. Taken
as a whole, population mortality of macrobenthos due to fishing in the eastern North
Chapter 2: Biogeographic Partition of the Ocean
diatoms nevertheless remain very important constituents in some regions and at some
seasons. If a species is transported unintentionally in ballast water and implants itself as
an exotic in a new region, consequences may ensue that are unpredictable in their details,
but are perhaps not insignificant. Take the case of Coscinodiscus wailesii.
Coscinodiscus is, of course, a well-known genus of discoid centric diatoms, represented
in most regions where phytoplankton blooms occur by several species having a range
of cell diameters from 50 to 350 m. Relative taxonomic abundance changes seasonally,
and the small to moderate-sized species, at least, are important in the diet of herbivorous
copepods. Larger species exist, principally oceanic in distribution. One such is C. wailesii of
the northwest Pacific, having rather wide salinity and temperature tolerances (Nishikawa
et al., 2000); its cells of 160–450 m are dominant in the spring bloom in the Seto Inland
Sea, reaching cell concentrations sufficiently high as to interfere with the raft culture of
the macroalga Porphyrea by competing for light and nutrients. Mass sedimentation of
dead cells after a bloom may create oxygen deficiency in bottom water.
In the mid-1970s, C. wailesi was observed in the English Channel for the first time,
whence it spread rapidly into the North Sea, the German Bight, and more widely in
the northeast Atlantic (Edwards et al., 2001). It became firmly established as an invasive
species and continues to be dominant in the spring diatom bloom of these regions. More
recently, it has been observed on the continental shelf off southern Brazil, where it now
forms seasonal blooms in the Gulf of Parana, and also off the coast of Nova Scotia in
the NW Atlantic. All that might not matter very much if the cells were not so large
and could be consumed by the indigenous copepods and other herbivores. One author
has described this as an inedible diatom in the seas that it has invaded, although that is
not strictly the case for “sloppy” feeding by copepods has been recorded. Perhaps only
the largest cell sizes of C. wailesi are not readily available to copepods—recall that cell
volume progressively diminishes as diatoms reproduce during the season of growth. In
warm oceans, of course, centric diatoms as large as C. wailesi are the central item in the
diet of some filter-feeding fish (i.e., Ethmalosa, Brevoortia), rather than of copepods.
We may expect that such invasions will have measurable impacts on the structure and
flow of material through pelagic food chains and it has been suggested (Sommer, 1998)
that the effects may be comparable to those expected from eutrophication of semienclosed
seas by waste discharge. This is deficient in Si, compared to N and P, and so leads not
to diatom blooms, but to a more complex phytoplankton community and less efficient
transfer of energy up the food chain. Tunicates and coelenterates may be favored at
the expense of diatoms and fish. In this case, the fear is that the diatoms will not be
consumed.
The second consequence of the role of “intelligent” biota that I want to discuss
briefly is far more urgent and is perhaps—just perhaps—controllable. A century or
more of industrial fishing in large stretches of the coastal ocean has created havoc
with their pristine ecosystems. Indeed, as Jackson et al. (2001) comment “Ecological
extinction caused by overfishing precedes all other pervasive human disturbance of coastal
ecosystems, including pollution, degradation of water quality and anthropogenic climate
change.” One must agree with their assessment.
Unlike the introduction of exotic species of plankton, the impacts of fishing on the
marine food chain are (in the ecological sense) top-down, rather than bottom-up, and
are more immediate. Some regional food webs have already been entirely transformed, to
the common detriment of fisherfolk and fish. Recent studies of the destruction of benthic
invertebrates caused by the passage of a commercial trawl over the seabed have given
alarming results: individual scours left by otter boards may be visible for more than a
year. The mortality of gastropods, echinoderms, crustaceans, and annelids caused by the
passage of a single beam trawl is 5–40%, and for bivalve mollusks it is 20–65%. Taken
as a whole, population mortality of macrobenthos due to fishing in the eastern North