31
can guide nektonic organism in their far ranging migrations across the open ocean.
Westward movement of loggerhead sea turtles (Caretta caretta) across the central
North Pacific occurs along fronts, moving north and south to stay within a specific frontal zone. Horizontal gradients in temperature, current, chlorophyll, and
possibly prey abundance levels around the fronts may provide cues that loggerheads would use to maintain their association with fronts (Polovina et al. 2000).
However, other species such as the leatherback turtle (Dermochelys coriacea)
possess a truly remarkable compass sense, allowing them to follow precise tracks
even in the presence of strong currents; in these cases the orientation of their
tracks is independent of fronts (Gaspar et al. 2006).
Many of the large pelagics (e.g. swordfish, tunas, whales) also seem to use
fronts as pathways. For example, swordfish could navigate in a coordinates system defined along fronts by isotherms and isolumes. Frontal pathways provide
energy savings in migration, enhanced foraging sites, and with the ability of adults
to place their young in particular locations, an important factor in reproductive
success (Olson 2002). Geographic predictability appears as a needed condition
for front functioning as migratory routes. It is hard to differentiate if fronts provide navigational clues or feeding opportunities; or both. In any case, perception
of frontal features such as patch contrast or abruptness may be fundamental. The
disciplines of animal behavior and landscape ecology thus become tightly interwoven in interpreting boundary function and the response of moving organisms
(Cadenasso et al. 2003b).
At smaller spatial scales, it has been shown that under certain circumstances,
mobile fronts may offer a transport mechanism for some components of the
plankton. Though most of planktonic organisms perform daily vertical migrations
in the water column, they are not able to accomplish migratory movements in
the most energetic horizontal flow fields. In Eastern Boundary Current ecosystems, seasonal wind-driven upwelling brings nutrient-rich water to the surface
along the coast. Fronts develop between the cold waters near the coast and the
warmer offshore waters. As the wind forcing relaxes following coastal upwelling
events, the upwelling fronts move onshore. The low-density surface water moves
shoreward over the upwelled water, forming a convergence zone at the front.
This shoreward-moving front concentrates and transports larvae. This may be an
important mechanism promoting the shoreward migration of larval invertebrates
and fish. The relaxation of winds can bring upwelling fronts to shore periodically,
a process that has been linked to intertidal invertebrate recruitment. In this way
front probability is an important predictor of recruitment of multiple taxa across
the California Current Large Marine Ecosystem. Moreover it appears that owing
to variations in bottom topography and/or coast direction bearing, fronts moving
towards the shore do not impinge everywhere along a coast. This alongshore difference in the contact of upwelling fronts might cause the observed alongshore
differences in recruitment of intertidal invertebrates (Roughgarden et al. 1991;
Shanks et al. 2000; Woodson et al. 2012). These mechanisms may also account
for larvae to settle jointly, increasing population cohesiveness (see the Larvae
retention section).
3.6 Migrations and Transport
can guide nektonic organism in their far ranging migrations across the open ocean.
Westward movement of loggerhead sea turtles (Caretta caretta) across the central
North Pacific occurs along fronts, moving north and south to stay within a specific frontal zone. Horizontal gradients in temperature, current, chlorophyll, and
possibly prey abundance levels around the fronts may provide cues that loggerheads would use to maintain their association with fronts (Polovina et al. 2000).
However, other species such as the leatherback turtle (Dermochelys coriacea)
possess a truly remarkable compass sense, allowing them to follow precise tracks
even in the presence of strong currents; in these cases the orientation of their
tracks is independent of fronts (Gaspar et al. 2006).
Many of the large pelagics (e.g. swordfish, tunas, whales) also seem to use
fronts as pathways. For example, swordfish could navigate in a coordinates system defined along fronts by isotherms and isolumes. Frontal pathways provide
energy savings in migration, enhanced foraging sites, and with the ability of adults
to place their young in particular locations, an important factor in reproductive
success (Olson 2002). Geographic predictability appears as a needed condition
for front functioning as migratory routes. It is hard to differentiate if fronts provide navigational clues or feeding opportunities; or both. In any case, perception
of frontal features such as patch contrast or abruptness may be fundamental. The
disciplines of animal behavior and landscape ecology thus become tightly interwoven in interpreting boundary function and the response of moving organisms
(Cadenasso et al. 2003b).
At smaller spatial scales, it has been shown that under certain circumstances,
mobile fronts may offer a transport mechanism for some components of the
plankton. Though most of planktonic organisms perform daily vertical migrations
in the water column, they are not able to accomplish migratory movements in
the most energetic horizontal flow fields. In Eastern Boundary Current ecosystems, seasonal wind-driven upwelling brings nutrient-rich water to the surface
along the coast. Fronts develop between the cold waters near the coast and the
warmer offshore waters. As the wind forcing relaxes following coastal upwelling
events, the upwelling fronts move onshore. The low-density surface water moves
shoreward over the upwelled water, forming a convergence zone at the front.
This shoreward-moving front concentrates and transports larvae. This may be an
important mechanism promoting the shoreward migration of larval invertebrates
and fish. The relaxation of winds can bring upwelling fronts to shore periodically,
a process that has been linked to intertidal invertebrate recruitment. In this way
front probability is an important predictor of recruitment of multiple taxa across
the California Current Large Marine Ecosystem. Moreover it appears that owing
to variations in bottom topography and/or coast direction bearing, fronts moving
towards the shore do not impinge everywhere along a coast. This alongshore difference in the contact of upwelling fronts might cause the observed alongshore
differences in recruitment of intertidal invertebrates (Roughgarden et al. 1991;
Shanks et al. 2000; Woodson et al. 2012). These mechanisms may also account
for larvae to settle jointly, increasing population cohesiveness (see the Larvae
retention section).
3.6 Migrations and Transport