Environmental Engineering Reference
In-Depth Information
a)
b)
Fig. 2.
Contrasting scenarios of vertical mixing, light and nutrient regime. Solar radiation is
exponentially attenuated through the water column. The
euphotic zone
is defi ned as the depth
at which the underwater irradiance reaches the 1% of the incident irradiance at the surface,
and mainly depends on turbidity. The
pycnocline
is the vertical gradient in water density
caused by differences in temperature (thermocline) or salinity (halocline) and defi nes the limit
of the
mixing depth
. The
critical depth
in a) is smaller compared to the one observed in b), but
a weaker and deeper pycnocline allows the upward transport of bottom nutrients through
strong vertical mixing. In b), a shallow and strong pycnocline acts as a barrier against mixing
causing nutrient depletion in the upper layers. Furthermore, the phytoplankton cells in b) could
be negatively affected by prolonged exposures to high irradiance, causing photoinhibition or
photodamage. Further consequences of strong vertical stratifi cation are higher sinking rates
and cell loses beyond the mixing depth.
The documented contrasting results highlight complex non-linear
responses of phytoplankton to global ocean climate and cascading
consequences linked to predator-prey interactions, resource limitation
and community species composition. Moreover, predicting future
distributional shifts in accordance to warming—i.e., temperature
gradients—(Rühland et al. 2008, Morán et al. 2009) requires additional
knowledge of species' range boundaries (population plasticity) and their
driving factors. Particular attention should be paid to alongshore where
warming-associated weakening of advection could break down certain
marine biogeographical barriers that currently prevent range expansions
(Harley et al. 2006). We describe hereinafter the common ecophysiological
responses of phytoplankton to modifi cations in the pelagic environment
driven by warming, including: size structure, bloom phenology, elemental
stoichiometry and food quality.
Cell size:
Increasing evidence of changes in plankton size structure has been
reported worldwide in relation to global warming (Forster et al. 2012 and
references therein). Temperature effects on the size structure (i.e., a reduction
in the mean size) have been detected in microzooplankton (Molinero et
