The Four Primary Biomes of the Upper Ocean
101
Plankton, and especially the zooplankton, of the coastal biome at all latitudes differs
consistently from the plankton of the open ocean because it comprises a higher proportion
of meroplankton—the planktonic larvae of benthic and littoral invertebrates. But because
benthic invertebrates in higher latitudes tend toward direct development, so the relative
contribution of meroplankton to the coastal plankton is another variable that is modified
by latitude. For the rest, species comprising the coastal holoplankton are most frequently
congeneric with species of the oceanic plankton; only a few higher taxonomic groups of
holoplankton, such as the cladocerans, are coastal specialists. These crustacea, most species
of which occur in freshwater, occur as seasonal swarms, especially after phytoplankton
blooms, in coastal regions at all latitudes (Longhurst, 1985b).
It is characteristic of both major and minor upwelling sites that the specialized zooplankton herbivore should be a seasonally migrant calanoid copepod: in low latitudes
this is frequently Calanoides carinatus or a closely related taxon. The life history strategy
of these organisms is a modification of that of oceanic calanoids of high latitudes: a
descent to depths of 500–1000 m during the non-upwelling season and a rapid ascent
when upwelling commences. Unlike their boreal analogues, however, their generation
time is rapid and several generations may be achieved during a single upwelling season.
Nevertheless, in all sites investigated, the resting population comprises only copepodites
of the final instar before sexual maturity. In this way, reproduction occurs rapidly after
the rise to the surface. In the coastal biome at higher latitudes, adjacent to oceanic regions
in which some of the large copepods perform seasonal vertical migrations, parts of these
populations are advected over the shelf during summer and are unable to descend to
their full overwintering depths at the end of the season. These individuals may gather in
deep basins on the shelf in some regions, where they may form apparently permanent
expatriate populations.
Although the existence of coral formations in only the tropical seas is a very important
source of nonuniformity within the coastal biome, we should not make too much of it
in this analysis. Since the littoral zone itself is to be excluded from this study, we shall be
concerned only with the consequences of offshore barrier and fringing reefs lying deeper
than the atoll and platform reefs of shoal water. I have already discussed in Chapter 3 the
coherence of the benthic invertebrate fauna of continental shelves at all latitudes, so that
ecologically similar organisms, forming associations of similar diversity, occupy similar
kinds of deposits wherever they occur.
We shall have to be aware when discussing biological production and consumption
of organic material in the pelagic ecosystem over continental shelves that the trophic
coupling of pelagic and benthic ecosystems introduces a factor with which we do not
need to concern ourselves when considering the ecology of the oceanic biomes. We shall
encounter numerous examples of shelf regions where only quite a small fraction of the
phytoplankton biomass is consumed within the plankton ecosystem, and we shall have to
follow the fate of these cells as they sink unconsumed to the shallow sea floor. There, they
are metabolized within the benthic ecosystem and sustain a bewildering array of pathways
through the macro, the meio-, and the micro-benthic communities. It is no more than
a statement of the obvious, but remineralized nutrient molecules in shallow water are
of course much more directly accessible to the photoautotrophic plankton than in the
open seas, even if (as a generality) 80% of the available nutrients on shelves originates in
the deep sea. Not only do such molecules “leak” continuously from sediment porewater
into the overlying water mass, but any transient mixing process will tend to resuspend
superficial organic matter from the surface of the sediments and so accelerate the process
of renewing mixed-layer nutrients.
We must also be aware that even in the absence of macroalgal beds as occur
along temperate-zone upwelling coasts, there is significant autotrophic production on
many continental shelves by benthic diatoms on the surface of sediments down to as
101
Plankton, and especially the zooplankton, of the coastal biome at all latitudes differs
consistently from the plankton of the open ocean because it comprises a higher proportion
of meroplankton—the planktonic larvae of benthic and littoral invertebrates. But because
benthic invertebrates in higher latitudes tend toward direct development, so the relative
contribution of meroplankton to the coastal plankton is another variable that is modified
by latitude. For the rest, species comprising the coastal holoplankton are most frequently
congeneric with species of the oceanic plankton; only a few higher taxonomic groups of
holoplankton, such as the cladocerans, are coastal specialists. These crustacea, most species
of which occur in freshwater, occur as seasonal swarms, especially after phytoplankton
blooms, in coastal regions at all latitudes (Longhurst, 1985b).
It is characteristic of both major and minor upwelling sites that the specialized zooplankton herbivore should be a seasonally migrant calanoid copepod: in low latitudes
this is frequently Calanoides carinatus or a closely related taxon. The life history strategy
of these organisms is a modification of that of oceanic calanoids of high latitudes: a
descent to depths of 500–1000 m during the non-upwelling season and a rapid ascent
when upwelling commences. Unlike their boreal analogues, however, their generation
time is rapid and several generations may be achieved during a single upwelling season.
Nevertheless, in all sites investigated, the resting population comprises only copepodites
of the final instar before sexual maturity. In this way, reproduction occurs rapidly after
the rise to the surface. In the coastal biome at higher latitudes, adjacent to oceanic regions
in which some of the large copepods perform seasonal vertical migrations, parts of these
populations are advected over the shelf during summer and are unable to descend to
their full overwintering depths at the end of the season. These individuals may gather in
deep basins on the shelf in some regions, where they may form apparently permanent
expatriate populations.
Although the existence of coral formations in only the tropical seas is a very important
source of nonuniformity within the coastal biome, we should not make too much of it
in this analysis. Since the littoral zone itself is to be excluded from this study, we shall be
concerned only with the consequences of offshore barrier and fringing reefs lying deeper
than the atoll and platform reefs of shoal water. I have already discussed in Chapter 3 the
coherence of the benthic invertebrate fauna of continental shelves at all latitudes, so that
ecologically similar organisms, forming associations of similar diversity, occupy similar
kinds of deposits wherever they occur.
We shall have to be aware when discussing biological production and consumption
of organic material in the pelagic ecosystem over continental shelves that the trophic
coupling of pelagic and benthic ecosystems introduces a factor with which we do not
need to concern ourselves when considering the ecology of the oceanic biomes. We shall
encounter numerous examples of shelf regions where only quite a small fraction of the
phytoplankton biomass is consumed within the plankton ecosystem, and we shall have to
follow the fate of these cells as they sink unconsumed to the shallow sea floor. There, they
are metabolized within the benthic ecosystem and sustain a bewildering array of pathways
through the macro, the meio-, and the micro-benthic communities. It is no more than
a statement of the obvious, but remineralized nutrient molecules in shallow water are
of course much more directly accessible to the photoautotrophic plankton than in the
open seas, even if (as a generality) 80% of the available nutrients on shelves originates in
the deep sea. Not only do such molecules “leak” continuously from sediment porewater
into the overlying water mass, but any transient mixing process will tend to resuspend
superficial organic matter from the surface of the sediments and so accelerate the process
of renewing mixed-layer nutrients.
We must also be aware that even in the absence of macroalgal beds as occur
along temperate-zone upwelling coasts, there is significant autotrophic production on
many continental shelves by benthic diatoms on the surface of sediments down to as