23
exceed a minimum density before they can forage successfully. In the neighborhood of fronts predators may feed upon small prey present in high density because
frontal dynamics aggregate food particles near the surface; thus, fronts may create
new foraging opportunities by concentrating prey items that are usually too small
to be profitably fed upon if concentrations are low (Vlietstra et al. 2005). Although
predators may prefer relatively large prey items over small ones, the amount of
energy per unit volume of water of small prey may equal or exceed that of large
prey when marine fronts cause small prey to concentrate in dense patches. Fronts
may therefore represent profitable foraging sites for some predators (Vlietstra
et al. 2005), turning otherwise unprofitable sections of the marine landscape into
profitable foraging grounds.
Different fronts are able to concentrate different prey types and this determines their use by predators of diverse body size and diet. In the Southern Ocean,
large marine birds such as albatrosses and gadfly petrels, which partially depend
on squid, dominate seabird biomass near the Subtropical Front; while small species that feed primarily on macrozooplankton, such as prions, dominate near the
Polar Front. The highest concentrations of blue whales (Balaenoptera musculus),
humpback whales (Megaptera novaeangliae), Fin whales (B. physalus) and Minke
whales (B. bonarensis) have been found in strong association with the Antarctic
Divergence, reflecting the distribution patterns of their main prey, the Antarctic
krill. Sei whales (B. borealis) on the other hand, having a diet dominated by copepods, are concentrated close to the Subtropical Front (Bost et al. 2009).
The food web is a composite not only of trophic levels but also of organisms
with differing time scales of life cycles; thus predator/prey interactions imply
interactions across scales (Steele 1989). Variations in the timing of events play an
important role in marine life, including the rhythms of nutrient enrichment processes in fronts that can affect trophic webs. Le Févre and Frontier (1988) considered a tidal front in which the fertilizing mixing process occurs in a 14 days
cycle, matching the neap-spring tides, and a shelf-break front, where the fertilization process is of high frequency, 12 h periodicity matching the semi-diurnal tides.
Based on distributions of zooplankton biomass they concluded that in the latter
case enhanced productivity was in the form of a classical herbivore food chain,
while in the former case primary production was consumed by microorganisms,
because herbivorous copepods cannot adapt to short-lived, fortnightly phytoplankton blooms.
3.3 Biogeography
There are several differences in the distribution patterns of terrestrial and marine
organisms. These differences are not surprising since distinct mechanisms are at
work in these two realms. On land, critical habitat characteristics can change drastically over short distances because of explicit barriers such as mountains, deserts,
and watergaps. In other cases, the clustering of terrestrial range limits may derive
3.2 Trophic Webs
exceed a minimum density before they can forage successfully. In the neighborhood of fronts predators may feed upon small prey present in high density because
frontal dynamics aggregate food particles near the surface; thus, fronts may create
new foraging opportunities by concentrating prey items that are usually too small
to be profitably fed upon if concentrations are low (Vlietstra et al. 2005). Although
predators may prefer relatively large prey items over small ones, the amount of
energy per unit volume of water of small prey may equal or exceed that of large
prey when marine fronts cause small prey to concentrate in dense patches. Fronts
may therefore represent profitable foraging sites for some predators (Vlietstra
et al. 2005), turning otherwise unprofitable sections of the marine landscape into
profitable foraging grounds.
Different fronts are able to concentrate different prey types and this determines their use by predators of diverse body size and diet. In the Southern Ocean,
large marine birds such as albatrosses and gadfly petrels, which partially depend
on squid, dominate seabird biomass near the Subtropical Front; while small species that feed primarily on macrozooplankton, such as prions, dominate near the
Polar Front. The highest concentrations of blue whales (Balaenoptera musculus),
humpback whales (Megaptera novaeangliae), Fin whales (B. physalus) and Minke
whales (B. bonarensis) have been found in strong association with the Antarctic
Divergence, reflecting the distribution patterns of their main prey, the Antarctic
krill. Sei whales (B. borealis) on the other hand, having a diet dominated by copepods, are concentrated close to the Subtropical Front (Bost et al. 2009).
The food web is a composite not only of trophic levels but also of organisms
with differing time scales of life cycles; thus predator/prey interactions imply
interactions across scales (Steele 1989). Variations in the timing of events play an
important role in marine life, including the rhythms of nutrient enrichment processes in fronts that can affect trophic webs. Le Févre and Frontier (1988) considered a tidal front in which the fertilizing mixing process occurs in a 14 days
cycle, matching the neap-spring tides, and a shelf-break front, where the fertilization process is of high frequency, 12 h periodicity matching the semi-diurnal tides.
Based on distributions of zooplankton biomass they concluded that in the latter
case enhanced productivity was in the form of a classical herbivore food chain,
while in the former case primary production was consumed by microorganisms,
because herbivorous copepods cannot adapt to short-lived, fortnightly phytoplankton blooms.
3.3 Biogeography
There are several differences in the distribution patterns of terrestrial and marine
organisms. These differences are not surprising since distinct mechanisms are at
work in these two realms. On land, critical habitat characteristics can change drastically over short distances because of explicit barriers such as mountains, deserts,
and watergaps. In other cases, the clustering of terrestrial range limits may derive
3.2 Trophic Webs