29
of most ciliates, flagellates, coccolithophorids, and diatoms (sinking). Most crustacean (e.g. copepods, euphausiids, amphipods) as well as many types of fecal pellets (sinking) have vertical velocities 1 or 2 orders of magnitude higher than the
vertical velocities at fronts (Franks 1992). Such organisms will thus accumulate
in the slowly sinking waters of the convergent frontal interfaces (Fig. 3.2) (Bakun
1996; Olson 2002; McManus and Woodson 2012). Larger organisms can break
through vertical density gradients (pycnoclines) between different water masses;
thus vertical migrations between water masses moving in different directions at
tidal, diel or longer timescales permit retention of planktonic larvae and adults in
favorable ecological locations (Fig. 3.6). Exogenous factors serving as cues for, or
directly controlling, vertical migrations rhythms include light, hydrostatic pressure,
salinity, temperature and water movements. The interaction of vertical migrating
behavior with vertically structured ocean transport processes offers a mean by
which living organisms are potentially able to follow drift trajectories that may in
no way resemble those followed by passive particles (Sinclair 1988; Bakun 1996;
Naylor 2006). Flexibility of behavior in response to hydrographic conditions
gives larvae unexpected freedom from normal restraints in controlling their movements. Models that incorporate vertical migration often show that vertical movements have a significant effect on plankton transport, and can lead to retention
which would not otherwise occur (Levin 2006). Frontal zones are characterized
by complex internal structure and may incorporate features across a wide range of
spatial scales. For example, the Mississippi River plume front consists of a largescale (2–20 km width) frontal zone within which small-scale (10–50 m wide) and
ephemeral convergence zones (Govoni and Grimes 1992) are embedded. Densities
of larval fishes within the large-scale frontal zone are probably the result of their
accumulation along ephemeral convergence zones and subsequent dispersal and
mixing during relaxation of convergence, so the spatial distribution of larvae in the
vicinity of the front is the aggregate result of the repeated formation and relaxation
of small-scale convergence zones (Govoni and Grimes 1992). Olson et al. (1994)
also distinguished between large-scale frontal zones and fronts, arguing that the
Fig. 3.6 In a counter-current
system, simple behavioral
traits (e.g. vertical migrations
associated to the day/night
passage) can generate
plankton retention, modified
from Weinstein et al. (1980)
3.5 Life Histories Traits in Relation to Fronts
of most ciliates, flagellates, coccolithophorids, and diatoms (sinking). Most crustacean (e.g. copepods, euphausiids, amphipods) as well as many types of fecal pellets (sinking) have vertical velocities 1 or 2 orders of magnitude higher than the
vertical velocities at fronts (Franks 1992). Such organisms will thus accumulate
in the slowly sinking waters of the convergent frontal interfaces (Fig. 3.2) (Bakun
1996; Olson 2002; McManus and Woodson 2012). Larger organisms can break
through vertical density gradients (pycnoclines) between different water masses;
thus vertical migrations between water masses moving in different directions at
tidal, diel or longer timescales permit retention of planktonic larvae and adults in
favorable ecological locations (Fig. 3.6). Exogenous factors serving as cues for, or
directly controlling, vertical migrations rhythms include light, hydrostatic pressure,
salinity, temperature and water movements. The interaction of vertical migrating
behavior with vertically structured ocean transport processes offers a mean by
which living organisms are potentially able to follow drift trajectories that may in
no way resemble those followed by passive particles (Sinclair 1988; Bakun 1996;
Naylor 2006). Flexibility of behavior in response to hydrographic conditions
gives larvae unexpected freedom from normal restraints in controlling their movements. Models that incorporate vertical migration often show that vertical movements have a significant effect on plankton transport, and can lead to retention
which would not otherwise occur (Levin 2006). Frontal zones are characterized
by complex internal structure and may incorporate features across a wide range of
spatial scales. For example, the Mississippi River plume front consists of a largescale (2–20 km width) frontal zone within which small-scale (10–50 m wide) and
ephemeral convergence zones (Govoni and Grimes 1992) are embedded. Densities
of larval fishes within the large-scale frontal zone are probably the result of their
accumulation along ephemeral convergence zones and subsequent dispersal and
mixing during relaxation of convergence, so the spatial distribution of larvae in the
vicinity of the front is the aggregate result of the repeated formation and relaxation
of small-scale convergence zones (Govoni and Grimes 1992). Olson et al. (1994)
also distinguished between large-scale frontal zones and fronts, arguing that the
Fig. 3.6 In a counter-current
system, simple behavioral
traits (e.g. vertical migrations
associated to the day/night
passage) can generate
plankton retention, modified
from Weinstein et al. (1980)
3.5 Life Histories Traits in Relation to Fronts