Chapter 3
Fronts and Pycnoclines:
Ecological Discontinuities
B
oundaries of biogeographical or ecological regions will be least ambiguous where
discontinuities in the physical environment are strongest and these, in the open
ocean, will be located along major fronts and frontal systems; for our purposes
as ecologists, however, the treatment given these features in most oceanographic texts
does not really tell us what we need to know. Fortunately, what we can now observe
in high-resolution satellite images of sea-surface chlorophyll and sea-surface elevation
brings us closer to reality, for where the texts may lead us to expect featureless central
gyres, we see instead much spatial nonuniformity; we also see the real complexity of
fronts and frontal systems that cannot be observed at sea level. So TOPEX-POSEIDON
and MODIS have given us new confidence in extrapolating from studies of ecological
dynamics done at sea along necessarily very short sections of a frontal system.
Although, as discussed in the previous chapter, fronts are the locations of greatest
ecological discontinuity, they can really represent no more than a leaky boundary between
different regimes. This is because the physical dynamics of fronts require that parcels
of water should pass from one side to the other, a mechanism that may be observed
even in those cases where tidally mixed shelf water meets stratified water at a very sharp
discontinuity. This exchange of water has an important consequence that again is clearly
observed in satellite imagery: frontal zones are frequently areas of biological enhancement.
This occurs at all scales, from tidal fronts in the North Sea to the globe-encircling,
convergent frontal zone in the Southern Ocean at which subantarctic water passes below
the subtropical surface water mass. As Margalef (1997) has emphasized, fronts in the
ocean are not only boundaries but also habitats having the attributes of ecotones, in the
sense of Shelford (1963) or Odum (1971).
The ecotone, or transition zone between two ecological communities, became an
established concept in terrestrial ecology but has been very little discussed in biological
oceanography. Ecotones are, by definition, linear and less extensive than the communities
they separate; they are associated with a gradient either in the physical environment or
in an external stress, such as might be imposed if herbivore biomass differs on either
side. Ecotones may exhibit special ecological characteristics that differ from either of the
separated communities, and they may be the habitat of specialized “edge-effect” species.
For all these reasons, Odum notes, “we would not be surprised to find the variety and
density of life greater in the ecotone.” Because the biota in convergent oceanic fronts may
have access to resources supplied from each of the adjacent water masses, and because
of physical aggregation there, a greater biomass may indeed build up within the frontal
zone than on either side. Generally, we may expect that ecotones at sea shall be associated
(i) with conjunctions, principally convergent but also divergent, between two surface
35
Fronts and Pycnoclines:
Ecological Discontinuities
B
oundaries of biogeographical or ecological regions will be least ambiguous where
discontinuities in the physical environment are strongest and these, in the open
ocean, will be located along major fronts and frontal systems; for our purposes
as ecologists, however, the treatment given these features in most oceanographic texts
does not really tell us what we need to know. Fortunately, what we can now observe
in high-resolution satellite images of sea-surface chlorophyll and sea-surface elevation
brings us closer to reality, for where the texts may lead us to expect featureless central
gyres, we see instead much spatial nonuniformity; we also see the real complexity of
fronts and frontal systems that cannot be observed at sea level. So TOPEX-POSEIDON
and MODIS have given us new confidence in extrapolating from studies of ecological
dynamics done at sea along necessarily very short sections of a frontal system.
Although, as discussed in the previous chapter, fronts are the locations of greatest
ecological discontinuity, they can really represent no more than a leaky boundary between
different regimes. This is because the physical dynamics of fronts require that parcels
of water should pass from one side to the other, a mechanism that may be observed
even in those cases where tidally mixed shelf water meets stratified water at a very sharp
discontinuity. This exchange of water has an important consequence that again is clearly
observed in satellite imagery: frontal zones are frequently areas of biological enhancement.
This occurs at all scales, from tidal fronts in the North Sea to the globe-encircling,
convergent frontal zone in the Southern Ocean at which subantarctic water passes below
the subtropical surface water mass. As Margalef (1997) has emphasized, fronts in the
ocean are not only boundaries but also habitats having the attributes of ecotones, in the
sense of Shelford (1963) or Odum (1971).
The ecotone, or transition zone between two ecological communities, became an
established concept in terrestrial ecology but has been very little discussed in biological
oceanography. Ecotones are, by definition, linear and less extensive than the communities
they separate; they are associated with a gradient either in the physical environment or
in an external stress, such as might be imposed if herbivore biomass differs on either
side. Ecotones may exhibit special ecological characteristics that differ from either of the
separated communities, and they may be the habitat of specialized “edge-effect” species.
For all these reasons, Odum notes, “we would not be surprised to find the variety and
density of life greater in the ecotone.” Because the biota in convergent oceanic fronts may
have access to resources supplied from each of the adjacent water masses, and because
of physical aggregation there, a greater biomass may indeed build up within the frontal
zone than on either side. Generally, we may expect that ecotones at sea shall be associated
(i) with conjunctions, principally convergent but also divergent, between two surface
35