Environmental Engineering Reference
In-Depth Information
the world's oceans will modify water chemistry through carbon enrichment
and nutrient depletion from the upper layers due to thermal stratifi cation.
Because taxonomic groups (i.e., diatoms and non-diatoms) have different
nutrient concentration/ratios requirements and different sensitivity to light
levels, shifts in species dominance and size-structure are expected under
modifi cations of the surrounding environment (Tilman 1982, Litchman et
al. 2007). The environmental stoichiometry affects the metabolic rate of
photosynthetic organisms, because the rate at which they acquire energy
and materials for maintenance, growth and reproduction depends on both,
their specifi c cellular requirements, as well as external bio-availability of
elements (Sterner and Elser 2002, Finkel et al. 2010). Accordingly, element
imbalance in the seawater has signifi cant implications in the phytoplankton
stoichiometry and therefore in the quality of food available for higher
trophic levels (Fig. 3)
(van de Waal et al. 2010, Klauschies et al. 2012). It is
worth noting that the food quality encompasses all features of the food that
make it suitable for ingestion and for fulfilling the consumer's nutritional
requirements (Sommer et al. 2012). Therefore, quality properties not only
include stoichiometric composition and biochemical make-up but also
morphological characteristics such as presence of setae or cell projections
and life-styles concerning solitary or chain forming and motile (e.g.,
fl agellates) or non-motile cells.
Fig. 3.
Changes in the plankton elemental stoichiometry driven by the interactive effects
between the rise in the atmospheric carbon dioxide (CO
2
) and sea-surface warming. Higher CO
2
levels are available for primary producers and vertical thermal stratifi cation causes nutrient
depletion in surface layers. Under these conditions, the phytoplankton stoichiometry shifts
towards high C-to-nutrient ratios and the community structure towards dominance of smaller
species. These changes in phytoplankton affect the zooplankton composition promoting shifts
towards species with low nutrient requirements and high recycling effi ciency of nutrients.
To compensate the low food quality, zooplankton excrete high carbon levels acting as a
feedback mechanism for the imbalanced C-to-nutrient ratios in the environment. The high
loads of dissolved organic carbon (DOC) are in turn transformed into CO
2
and liberated into
the atmosphere through microbial decomposition. Storms and eutrophication may partially
compensate the water column stratifi cation and nutrient depletion.
