24
3 Biology of Fronts
from historical events such as glacial intrusion or land bridge submergence. In
marine systems, however, it becomes more difficult to envision how persistent range
boundaries can become locally concentrated. Although substrate types vary spatially
and rivers locally alter salinity, a single, continuous, dispersal medium (the ocean)
connects all available habitats, and environmental gradients within this medium are
neither as contrasting nor as immutable as on land (Gaylord and Gaines 2000).
The idea that fronts separate different water masses, hence different pelagic
populations, is an old one and it is sustained by a number of observations at various scales (Sournia 1994). However, the role of fronts in marine biogeography is
not yet fully understood. Conceptually, oceanographic mechanisms acting across a
range of spatial and temporal scales can be viewed as a base level of environmental structure that influence a suite of biological processes that in turn influence the
formation of macroecological patterns, both directly and indirectly (Leichter and
Witman 2009). Though the tight linkages between biogeography and a small number of ocean drivers is recognized (Spalding et al. 2012), there is a lack of comprehensive interpretation of the function of fronts in setting biogeographic boundaries.
Longhurst (1998) pointed out that subdivisions based on oceanographic criteria (primarily the positions of fronts) may be appropriate in certain circumstances. Some
fronts seem to constitute biogeographic boundaries such as the Antarctic Polar Front
(Sournia 1994; Boltovskoy et al. 2005; Bost et al. 2009; Spalding et al. 2012), or the
shelf-break fronts that usually mark the neritic-oceanic transition (Longhurst 1998;
Spalding et al. 2007) however fronts having lesser spatial scale or persistence do not
constitute the boundaries between the geographic units in the most recent biogeographic marine systems (Longhurst 1998; Spalding et al. 2007, 2012). This means
that in some way organisms are able to cross fronts. Notwithstanding, fronts have
been recognized in finer resolution regional classification systems such as ecoregions (Spalding et al. 2007) or faunal assemblages’ boundaries. For example, zooplankton assemblages including chaetognaths, salps, krill larvae and copepods may
be strongly influenced by frontal structures (McGinty et al. 2011). Frontal discontinuities may also manifest in terms of the developmental stages, as seen in the distributions of eggs, larvae and juveniles of fishes across the Ushant front or by different
copepodite stages in the Ligurian Sea (Sournia 1994).
Most efforts have been devoted to study the role of fronts on spatial patterns of
plankton (Hunt and Hosie 2003; Boltovskoy et al. 2005), but frontal effects have
been also reported for pelagic fishes (Moteki et al. 2011), demersal fishes (Gaertner
et al. 2005), benthic invertebrates (Boltovskoy et al. 2005) and marine birds (Piatt and
Springer 2003; Bost et al. 2009). Physical contrast across a front may determine its
influence on the community structure. Minor physical differences that cannot generate
changes in the specific composition of the community may produce major changes
in the dominance pattern across fronts (Angel 1986). The degree to which different
groups of organisms are affected by fronts is also variable. Antartic endemism for
benthic invertebrates is higher than for zooplankton, showing that the effects of the
Antartic Polar Front are different for such groups (Boltovskoy et al. 2005).
In most cases fronts represent a leaky boundary between different ecological
regimes and several data indicate that fronts exert mixing effects on the adjacent
3 Biology of Fronts
from historical events such as glacial intrusion or land bridge submergence. In
marine systems, however, it becomes more difficult to envision how persistent range
boundaries can become locally concentrated. Although substrate types vary spatially
and rivers locally alter salinity, a single, continuous, dispersal medium (the ocean)
connects all available habitats, and environmental gradients within this medium are
neither as contrasting nor as immutable as on land (Gaylord and Gaines 2000).
The idea that fronts separate different water masses, hence different pelagic
populations, is an old one and it is sustained by a number of observations at various scales (Sournia 1994). However, the role of fronts in marine biogeography is
not yet fully understood. Conceptually, oceanographic mechanisms acting across a
range of spatial and temporal scales can be viewed as a base level of environmental structure that influence a suite of biological processes that in turn influence the
formation of macroecological patterns, both directly and indirectly (Leichter and
Witman 2009). Though the tight linkages between biogeography and a small number of ocean drivers is recognized (Spalding et al. 2012), there is a lack of comprehensive interpretation of the function of fronts in setting biogeographic boundaries.
Longhurst (1998) pointed out that subdivisions based on oceanographic criteria (primarily the positions of fronts) may be appropriate in certain circumstances. Some
fronts seem to constitute biogeographic boundaries such as the Antarctic Polar Front
(Sournia 1994; Boltovskoy et al. 2005; Bost et al. 2009; Spalding et al. 2012), or the
shelf-break fronts that usually mark the neritic-oceanic transition (Longhurst 1998;
Spalding et al. 2007) however fronts having lesser spatial scale or persistence do not
constitute the boundaries between the geographic units in the most recent biogeographic marine systems (Longhurst 1998; Spalding et al. 2007, 2012). This means
that in some way organisms are able to cross fronts. Notwithstanding, fronts have
been recognized in finer resolution regional classification systems such as ecoregions (Spalding et al. 2007) or faunal assemblages’ boundaries. For example, zooplankton assemblages including chaetognaths, salps, krill larvae and copepods may
be strongly influenced by frontal structures (McGinty et al. 2011). Frontal discontinuities may also manifest in terms of the developmental stages, as seen in the distributions of eggs, larvae and juveniles of fishes across the Ushant front or by different
copepodite stages in the Ligurian Sea (Sournia 1994).
Most efforts have been devoted to study the role of fronts on spatial patterns of
plankton (Hunt and Hosie 2003; Boltovskoy et al. 2005), but frontal effects have
been also reported for pelagic fishes (Moteki et al. 2011), demersal fishes (Gaertner
et al. 2005), benthic invertebrates (Boltovskoy et al. 2005) and marine birds (Piatt and
Springer 2003; Bost et al. 2009). Physical contrast across a front may determine its
influence on the community structure. Minor physical differences that cannot generate
changes in the specific composition of the community may produce major changes
in the dominance pattern across fronts (Angel 1986). The degree to which different
groups of organisms are affected by fronts is also variable. Antartic endemism for
benthic invertebrates is higher than for zooplankton, showing that the effects of the
Antartic Polar Front are different for such groups (Boltovskoy et al. 2005).
In most cases fronts represent a leaky boundary between different ecological
regimes and several data indicate that fronts exert mixing effects on the adjacent