Environmental Engineering Reference
In-Depth Information
in salps, which occupy less productive and warmer regions (Atkinson et
al. 2004). These changes are likely the consequence of global warming.
Summer phytoplankton blooms and the extent of winter sea ice are the key
factors triggering the high krill densities observed in the southwest Atlantic
Ocean (Atkinson et al. 2004). In fact, krill larvae as well as the recruitment
to adult stocks depend on phytoplankton blooms at the margins of sea ice
cover (Atkinson et al. 2004, Richardson 2008). As waters have warmed, the
extent of winter sea ice and its persistence have declined, leading to lower
larval survival and explaining the observed decline in krill density. As krill
densities decreased, salps appear to have synchronously increased in the
southern part of their range distribution. These changes have had profound
effects within the Southern Ocean food web, especially the populations of
baleen whales, fi shes, penguins, seabirds, and seals that depend upon krill
as their primary food source (Richardson 2008).
Population outbreaks of gelatinous zooplankton have been increasingly
detected in recent years in many marine ecosystems (Mills 2001, Attrill et
al. 2007, Brotz et al. 2012). Jellyfi sh and ctenophore blooms are part of the
natural seasonal cycle of these species (Boero et al. 2008). Nevertheless,
climate warming has been suggested as one of the main driving forces
for changes in the abundance of gelatinous plankton, given that warmer
temperatures can trigger greater and more rapid production of many
species (Purcell 2005). As gelatinous organisms are key predators of other
zooplankton species, including fi sh eggs and larvae (Purcell and Arai 2001),
an increase in their populations could implicate the disruption of pelagic
ecosystems (Mills 2001, Oguz et al. 2008).
The effects of physical/chemical changes due to climate change are
transmitted through networks of interacting organisms to shape the
structure of communities and the dynamics of ecosystems (Shurin et al.
2012). Biological systems are generally controlled by their top predators
through
top-down control
, by their producers through
bottom-up
control,
or by a number of key species in the middle through
wasp-waist control
(Cury et al. 2000). Strong
bottom-up control
results in a positive correlation
between predator and prey whereas strong
top-down control
, results in
a negative correlation (Richardson and Schoeman 2004). A variety of
evidences suggest that increased temperatures may affect the sensitivity
of food webs to
top-down
and
bottom-up
forcing (Shurin et al. 2012). For
example, organisms at different positions within aquatic food webs have
a specifi c sensitivity to temperature, leading to imbalanced responses to
temperature change among trophic levels (Shurin et al. 2012). More active
primary consumers may exert stronger
top-down
effects on producers;
however, their greater metabolic demands may intensify resource limitation
and reduce their abundances, leading to weaker effects at the long-term
population level (Shurin et al. 2012). The close coupling between trophic
