Coastal Asymmetry, Geomorphology, and Tidal Forcing
67
of stability. The coastward upwelling of deep water therefore brings not only nutrient-rich
water into the photic zone, but also seed populations of large calanoid copepods. In all
eastern boundary currents there is a seasonal, latitudinal march of upwelling, poleward
in summer, equatorward in winter, with a central region where upwelling is essentially
nonseasonal. The mean seasonal changes in, for instance, chlorophyll values for each
region may obscure these facts and present a pattern that is not as helpful as it might be.
You may have noticed that I have not identified the so-called HNLC regions as an
individual case, as was done by Cullen et al. (op. cit.). This is because it is the stratification
and irradiance cycles that determine which kind of pelagic ecosystem shall develop, not
whether the Liebigian limiting element is a trace metal, such as Fe, or one of the major
nutrients, such as NO 3 . As I shall discuss in the next chapter, only very recently has
anybody commented on the simple fact that, in all so-called HNLC situations, the nutrient
that limits phytoplankton growth is delivered to the phytoplankton upward from deeper
layers by Ekman upwelling or by entrainment, but not from the skies. We can therefore
include equatorial iron-limited regions in Case 4 and those at high latitudes in Cases 1
or 2. This argument is expanded in Chapter 5.
It is more difficult to extend the foregoing analysis into the coastal seas, so as to fit
Margalef’s “low turbulence and high nutrients” quadrant, appropriate to uncontrolled
blooms in the littoral zone. Next, I shall review some of the physical processes to which
phytoplankton must respond in shallow seas over the continental shelves; if such processes
did not exist, then the six Cases discussed above would be appropriate over the shelves, right
in to shoal water. The role of the physical forcing that actually does occur in shelf regions
strongly modifies the pelagic regime that is characteristic of the adjacent open ocean. Unfortunately, we can place much less confidence in the seasonal cycle of chlorophyll biomass in
shallow water derived from satellite imagery because of the interference of pigmented dissolved organic material (CDOM) in river water: some components of CDOM absorb light
preferentially at wavelengths very similar to those of chlorophyll absorption. A routine filter
for these has yet to be devised and applied to the available images.
Even as we argue that ecosystem types are determined primarily by the action of the
external environment on plant growth, we must also recognize the moderating effects of
consumer organisms. In the jargon of general ecology, we must evaluate both top-down
and bottom-up forcing of the ecosystem. For terrestrial ecologists it is evident that dry forest, savanna, and grassland country is maintained in a certain state by the action of large
herbivores and the observed vegetation represents a dynamic balance between browsing
or grazing and the response of plants. In the pelagic ecosystem, the same dynamic balance
exists between production of algal cells and their consumption by appropriate herbivores—
diatoms by copepods, nanoplankton by tunicates. But it is neither copepods nor tunicates,
but rather their physical-chemical environment, that determines whether nanoplankton or
diatoms shall dominate and whether a Case 2 or a Case 4 model shall characterize a region.
Similarly, it is neither kelp-eating sea urchins nor sea-grass-eating manatees that determine
whether macroalgae or corals shall dominate the littoral fringe.
Coastal Asymmetry, Geomorphology,
and Tidal Forcing
So far, we have been concerned only with the consequences of ocean physics for the
development of pelagic ecosystems characteristic of the open ocean. We should also note,
very briefly, the fact that in the shallow seas, whether enclosed or over continental shelves
fronting the open ocean, ideal physical processes are dominated by the consequences
of land form. This brief excursion into the asymmetries of coastal oceans, and their
67
of stability. The coastward upwelling of deep water therefore brings not only nutrient-rich
water into the photic zone, but also seed populations of large calanoid copepods. In all
eastern boundary currents there is a seasonal, latitudinal march of upwelling, poleward
in summer, equatorward in winter, with a central region where upwelling is essentially
nonseasonal. The mean seasonal changes in, for instance, chlorophyll values for each
region may obscure these facts and present a pattern that is not as helpful as it might be.
You may have noticed that I have not identified the so-called HNLC regions as an
individual case, as was done by Cullen et al. (op. cit.). This is because it is the stratification
and irradiance cycles that determine which kind of pelagic ecosystem shall develop, not
whether the Liebigian limiting element is a trace metal, such as Fe, or one of the major
nutrients, such as NO 3 . As I shall discuss in the next chapter, only very recently has
anybody commented on the simple fact that, in all so-called HNLC situations, the nutrient
that limits phytoplankton growth is delivered to the phytoplankton upward from deeper
layers by Ekman upwelling or by entrainment, but not from the skies. We can therefore
include equatorial iron-limited regions in Case 4 and those at high latitudes in Cases 1
or 2. This argument is expanded in Chapter 5.
It is more difficult to extend the foregoing analysis into the coastal seas, so as to fit
Margalef’s “low turbulence and high nutrients” quadrant, appropriate to uncontrolled
blooms in the littoral zone. Next, I shall review some of the physical processes to which
phytoplankton must respond in shallow seas over the continental shelves; if such processes
did not exist, then the six Cases discussed above would be appropriate over the shelves, right
in to shoal water. The role of the physical forcing that actually does occur in shelf regions
strongly modifies the pelagic regime that is characteristic of the adjacent open ocean. Unfortunately, we can place much less confidence in the seasonal cycle of chlorophyll biomass in
shallow water derived from satellite imagery because of the interference of pigmented dissolved organic material (CDOM) in river water: some components of CDOM absorb light
preferentially at wavelengths very similar to those of chlorophyll absorption. A routine filter
for these has yet to be devised and applied to the available images.
Even as we argue that ecosystem types are determined primarily by the action of the
external environment on plant growth, we must also recognize the moderating effects of
consumer organisms. In the jargon of general ecology, we must evaluate both top-down
and bottom-up forcing of the ecosystem. For terrestrial ecologists it is evident that dry forest, savanna, and grassland country is maintained in a certain state by the action of large
herbivores and the observed vegetation represents a dynamic balance between browsing
or grazing and the response of plants. In the pelagic ecosystem, the same dynamic balance
exists between production of algal cells and their consumption by appropriate herbivores—
diatoms by copepods, nanoplankton by tunicates. But it is neither copepods nor tunicates,
but rather their physical-chemical environment, that determines whether nanoplankton or
diatoms shall dominate and whether a Case 2 or a Case 4 model shall characterize a region.
Similarly, it is neither kelp-eating sea urchins nor sea-grass-eating manatees that determine
whether macroalgae or corals shall dominate the littoral fringe.
Coastal Asymmetry, Geomorphology,
and Tidal Forcing
So far, we have been concerned only with the consequences of ocean physics for the
development of pelagic ecosystems characteristic of the open ocean. We should also note,
very briefly, the fact that in the shallow seas, whether enclosed or over continental shelves
fronting the open ocean, ideal physical processes are dominated by the consequences
of land form. This brief excursion into the asymmetries of coastal oceans, and their