44
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
to deeper ecosystems, so it is the most significant feature in the three-dimensional
ecological geography of the oceans. It may be useful to remind ourselves of some of its
main characteristics: most importantly, what determines its depth and the strength of its
density gradient and whether or not it occurs within the lighted zone.
The epipelagic zone, whose ecological geography is the subject of this book, is a thin
layer of light, lighted, and wind-mixed water lying atop the cooler mass of the interior of
the ocean; the change in physical and chemical properties between the two depth zones
is more frequently abrupt than gradual. When abrupt, the ecological changes that occur
across a few tens of meters are greater than across most vertical fronts that intersect the sea
surface. This phenomenon is most striking in the eastern parts of the tropical oceans: here,
at around 35–40 m below the surface, the temperature drops from 28
C to about 16
C.
Although the organisms of the epipelagic zone differ fundamentally between the tropical
and temperate regions, the deeper-living biota are much more similar. Consequently, as
has been noted by many authors, a diel migrant copepod or euphausiid moving across
this boundary in the tropics at dawn and dusk makes an environmental adjustment as
great as traveling several thousand kilometers equatorward or poleward.
Though the depth at which the pycnocline occurs is determined principally by baroclinicity associated with the ocean circulation, local processes also intervene, at least
seasonally: turbulence induced by wind stress at the sea surface, shear-stress turbulence
due to inertial oscillations of the mixed layer water mass that are induced by impulsive
changes in wind strength, local heating at the surface by short-wave solar radiation, and
the local supply of fresh or brackish water. All these factors will be discussed in the next
chapter, so here it is sufficient to note that the depth of the surface wind-mixed layer
varies from 25 m in the eastern tropical oceans to 250 m in the center of the subtropical
gyres. It may also be noted, parenthetically, that the baroclinic upsloping of nitrate isopleths toward the edges of the anticyclonic subtropical gyres is a necessary consequence
of the bowl-shaped mixed layer of the gyres and is reflected in the surface chlorophyll
field. For this reason, chlorophyll takes consistently higher values around the margins,
and lowest values in the center, of the subtropical gyres of each ocean.
At higher latitudes, where seasonality of wind stress and solar heating is stronger,
there is a clear discontinuity between the mixed layer of one year and that of the next.
Winter mixing and surface cooling progressively deepen the surface mixed layer down
to the depth of the deep permanent pycnocline until, with the return of surface heating
and reduced wind stress in spring, a new shallow seasonal mixed layer develops. This
progressively deepens and strengthens during the summer, only to be eroded again by
wind mixing during the subsequent winter. Peak development of the summer mixed layer
is typically several months after midsummer, and greatest penetration of deep convection
occurs several months after midwinter. In some regions, a halocline may somewhat
obscure the simple model.
You should be aware that seasonal graphs of mixed layer depth, which are based on
archived time series of monthly mean values (such as those used in this book), may
not capture the establishment of near-surface stratification in spring. This occurs de
novo, far above the winter pycnocline, by solar warming and the consequent induction
of buoyancy in the near-surface layer. Instead, seasonal graphs may seem to show a
progressive shoaling from a very deep to a very shallow mixed layer depth—a process that
simply cannot happen, or at least only as a consequence of geostrophic adjustment. This,
of course, would violate Dodimead’s first rule for thermoclines: that they can deepen by
mixing warmer surface with cold deeper waters but they cannot shoal by unmixing the
same! Confusion appears to have arisen between the diel depth of wind mixing, which
does of course follow the pattern often used by modelers, and the depth to the seasonal
pycnocline. It is this latter, of course, that has the greater biological significance because
it carries the seasonally variable nutricline with it. The confusion may also arise by the
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
to deeper ecosystems, so it is the most significant feature in the three-dimensional
ecological geography of the oceans. It may be useful to remind ourselves of some of its
main characteristics: most importantly, what determines its depth and the strength of its
density gradient and whether or not it occurs within the lighted zone.
The epipelagic zone, whose ecological geography is the subject of this book, is a thin
layer of light, lighted, and wind-mixed water lying atop the cooler mass of the interior of
the ocean; the change in physical and chemical properties between the two depth zones
is more frequently abrupt than gradual. When abrupt, the ecological changes that occur
across a few tens of meters are greater than across most vertical fronts that intersect the sea
surface. This phenomenon is most striking in the eastern parts of the tropical oceans: here,
at around 35–40 m below the surface, the temperature drops from 28
C to about 16
C.
Although the organisms of the epipelagic zone differ fundamentally between the tropical
and temperate regions, the deeper-living biota are much more similar. Consequently, as
has been noted by many authors, a diel migrant copepod or euphausiid moving across
this boundary in the tropics at dawn and dusk makes an environmental adjustment as
great as traveling several thousand kilometers equatorward or poleward.
Though the depth at which the pycnocline occurs is determined principally by baroclinicity associated with the ocean circulation, local processes also intervene, at least
seasonally: turbulence induced by wind stress at the sea surface, shear-stress turbulence
due to inertial oscillations of the mixed layer water mass that are induced by impulsive
changes in wind strength, local heating at the surface by short-wave solar radiation, and
the local supply of fresh or brackish water. All these factors will be discussed in the next
chapter, so here it is sufficient to note that the depth of the surface wind-mixed layer
varies from 25 m in the eastern tropical oceans to 250 m in the center of the subtropical
gyres. It may also be noted, parenthetically, that the baroclinic upsloping of nitrate isopleths toward the edges of the anticyclonic subtropical gyres is a necessary consequence
of the bowl-shaped mixed layer of the gyres and is reflected in the surface chlorophyll
field. For this reason, chlorophyll takes consistently higher values around the margins,
and lowest values in the center, of the subtropical gyres of each ocean.
At higher latitudes, where seasonality of wind stress and solar heating is stronger,
there is a clear discontinuity between the mixed layer of one year and that of the next.
Winter mixing and surface cooling progressively deepen the surface mixed layer down
to the depth of the deep permanent pycnocline until, with the return of surface heating
and reduced wind stress in spring, a new shallow seasonal mixed layer develops. This
progressively deepens and strengthens during the summer, only to be eroded again by
wind mixing during the subsequent winter. Peak development of the summer mixed layer
is typically several months after midsummer, and greatest penetration of deep convection
occurs several months after midwinter. In some regions, a halocline may somewhat
obscure the simple model.
You should be aware that seasonal graphs of mixed layer depth, which are based on
archived time series of monthly mean values (such as those used in this book), may
not capture the establishment of near-surface stratification in spring. This occurs de
novo, far above the winter pycnocline, by solar warming and the consequent induction
of buoyancy in the near-surface layer. Instead, seasonal graphs may seem to show a
progressive shoaling from a very deep to a very shallow mixed layer depth—a process that
simply cannot happen, or at least only as a consequence of geostrophic adjustment. This,
of course, would violate Dodimead’s first rule for thermoclines: that they can deepen by
mixing warmer surface with cold deeper waters but they cannot shoal by unmixing the
same! Confusion appears to have arisen between the diel depth of wind mixing, which
does of course follow the pattern often used by modelers, and the depth to the seasonal
pycnocline. It is this latter, of course, that has the greater biological significance because
it carries the seasonally variable nutricline with it. The confusion may also arise by the