25
populations. At first it may seem rather counter-intuitive that fronts are where
waters mix, but several studies have demonstrated the extent to which parcels of
water, containing biota, pass from one side to other by a variety of mechanisms
(Ashjian 1993; Sournia 1994; Longhurst 1998). For example, despite the importance of the Gulf Stream as a biogeographical boundary, it appears to function as
a leaky interface permitting some cross-stream exchange of water and therefore,
plankton populations. The meanders of the Gulf Stream may be sites of cross-front
exchange of plankton populations between the Sargasso Sea and the Slope Water
(Ashjian 1993). Another proposed mechanism is that instead of a slow continuous
exchange, many fronts accumulate material for a while and then through an evolving instability lead to a single large pulse of cross-front exchange. This pulsed
exchange may be as effective in bringing about exchange across the front as if
there were no front (Largier 1993). Though it is commonly observed that different
groups of species and different ecological conditions dominate on either side of
fronts, it is also commonplace to observe that individuals of many, perhaps most,
of the relevant species can also be found in small numbers on the opposite side of
the front. Given the dynamic exchange of water across fronts at all scales, it could
hardly be otherwise (Longhurst 1998).
Though there are numerous studies about fronts and the spatial patterns at different scales of planktonic, benthic and nektonic organisms, such information has
not been yet systematized nor analyzed in a comparative way. This could be in part
the reason why mechanistic explanations on how fronts can create or influence those
patterns are limited. It is commonly assumed that gradients in environmental conditions are the primary determinant of the boundaries of the ranges of species, particularly when species’ boundaries cluster at a given location. In setting such boundaries
there are two classes of causes: one based on mortality outside the specie’s range,
either due to physical, chemical or biological processes, the other based on barriers
to larval dispersal. Unfortunately, the underlying oceanographic mechanisms potentially responsible for these two causes of range limits—steep physical gradients
versus hydrographic barriers to dispersal—are typically confounded in space. Steep
gradients in ocean temperature or other physical parameter cannot be generated and
maintained without anomalous circulation patterns (e.g. convergent currents) which
tend to restrict larval dispersal (Gaines et al. 2009). Another point of view is that
distributional patterns of populations of marine species with complex life histories
(i.e. those with planktonic egg and larval stages) are controlled by oceanographic
processes that facilitate birth site fidelity to reproductive grounds (Sinclair 1988).
Many fishes and invertebrates having planktonic larvae, which represent most of
the cases, choose fronts as spawning grounds (see the Section on Larvae retention).
Thus, marine fronts may play a role in setting populations’ spatial structures but not
necessarily being or defining the borders of the species’ geographical distributions.
Although the current systems capture the major elements of the biogeographic
patterns (i.e. provinces), considerable further “texture” does exist in the oceans
at smaller scales, including fronts (Spalding et al. 2012). It seems that the role of
fronts in setting biogeographic boundaries depends on their spatial scale, physical
contrast, and persistence. As those properties increase, so does the frontal influence.
3.3 Biogeography
populations. At first it may seem rather counter-intuitive that fronts are where
waters mix, but several studies have demonstrated the extent to which parcels of
water, containing biota, pass from one side to other by a variety of mechanisms
(Ashjian 1993; Sournia 1994; Longhurst 1998). For example, despite the importance of the Gulf Stream as a biogeographical boundary, it appears to function as
a leaky interface permitting some cross-stream exchange of water and therefore,
plankton populations. The meanders of the Gulf Stream may be sites of cross-front
exchange of plankton populations between the Sargasso Sea and the Slope Water
(Ashjian 1993). Another proposed mechanism is that instead of a slow continuous
exchange, many fronts accumulate material for a while and then through an evolving instability lead to a single large pulse of cross-front exchange. This pulsed
exchange may be as effective in bringing about exchange across the front as if
there were no front (Largier 1993). Though it is commonly observed that different
groups of species and different ecological conditions dominate on either side of
fronts, it is also commonplace to observe that individuals of many, perhaps most,
of the relevant species can also be found in small numbers on the opposite side of
the front. Given the dynamic exchange of water across fronts at all scales, it could
hardly be otherwise (Longhurst 1998).
Though there are numerous studies about fronts and the spatial patterns at different scales of planktonic, benthic and nektonic organisms, such information has
not been yet systematized nor analyzed in a comparative way. This could be in part
the reason why mechanistic explanations on how fronts can create or influence those
patterns are limited. It is commonly assumed that gradients in environmental conditions are the primary determinant of the boundaries of the ranges of species, particularly when species’ boundaries cluster at a given location. In setting such boundaries
there are two classes of causes: one based on mortality outside the specie’s range,
either due to physical, chemical or biological processes, the other based on barriers
to larval dispersal. Unfortunately, the underlying oceanographic mechanisms potentially responsible for these two causes of range limits—steep physical gradients
versus hydrographic barriers to dispersal—are typically confounded in space. Steep
gradients in ocean temperature or other physical parameter cannot be generated and
maintained without anomalous circulation patterns (e.g. convergent currents) which
tend to restrict larval dispersal (Gaines et al. 2009). Another point of view is that
distributional patterns of populations of marine species with complex life histories
(i.e. those with planktonic egg and larval stages) are controlled by oceanographic
processes that facilitate birth site fidelity to reproductive grounds (Sinclair 1988).
Many fishes and invertebrates having planktonic larvae, which represent most of
the cases, choose fronts as spawning grounds (see the Section on Larvae retention).
Thus, marine fronts may play a role in setting populations’ spatial structures but not
necessarily being or defining the borders of the species’ geographical distributions.
Although the current systems capture the major elements of the biogeographic
patterns (i.e. provinces), considerable further “texture” does exist in the oceans
at smaller scales, including fronts (Spalding et al. 2012). It seems that the role of
fronts in setting biogeographic boundaries depends on their spatial scale, physical
contrast, and persistence. As those properties increase, so does the frontal influence.
3.3 Biogeography