Environmental Engineering Reference
In-Depth Information
The elemental composition of phytoplankton affects the cellular
concentrations of proteins, fatty acids and other important constituents
for zooplankton growth. The most widely used stoichiometric relationship
in marine systems is the Redfi eld ratio of around 106:16:1 for the molar
C:N:P ratio (Redfield 1934). In general, individual species in natural
assemblages display signifi cant variability in this proportion depending
upon the concentration of bio-available N and P and changes in irradiance,
temperature and carbon dioxide (e.g., Hessen et al. 2002, Finkel et al. 2006,
Fu et al. 2007). In particular, the predicted excess of C in relation to N and
P, and the increase in underwater irradiance, associated to projections of
CO
2
and warming (i.e., thermal stratifi cation), will shift the phytoplankton
cellular stoichiometry towards higher C-to-nutrient ratios (Fig. 3), with
negative cascading effects on herbivores performance (e.g., high C:P
ratios, Hessen et al. 2002) and eventually on upper trophic levels through
trophic linkages amplifi cation (Hessen and Anderson 2008). In agreement
to this, negative changes in algal fatty acid composition (i.e., decrease of
polyunsaturated and increase of saturated fatty acids) have been shown
under acidifi ed conditions with detrimental effects on the reproduction of
copepods (Rossoll et al. 2012). Changes in algae stoichiometry further drive
shifts in zooplankton community composition (i.e., microzooplancton
vs.
mesozooplankton dominance) and in the whole pelagic food web in relation
to phytoplankton community size-structure and food quality and quantity
(Fig. 3) (Sterner and Elser 2002). The zooplankton community shifts towards
organisms with high effi ciency for nutrient recycling regarding the elevated
C-to-nutrient stoichiometry of their food and consequently high amounts of
POC are excreted (van de Waal et al. 2010). The offset between food quality
and quantity depends strongly upon facilitation via grazing and recycling
by grazers, and this effect is more important in systems with low renewal
rates (Hessen and Anderson 2008).
Overall, both CO
2
water enrichment and the strengthening of thermal
stratification will enhance elemental imbalance between phyto- and
zooplankton generating a feedback mechanism of increasing atmospheric
carbon dioxide and climate warming driven by plankton activity (Fig. 3)
(van de Waal et al. 2010). It is important to consider that the increase in
storms frequency and eutrophication of coastal environment may in part
prevent the projected shift to more stratifi ed and oligotrophic sea-surface
waters.
Harmful Algal Blooms (HABs) and Parasitism of Phytoplankton
Coastal ecosystems are highly dynamic in terms of hydro-climatic
variability, biogeochemical processes, occurrence of phytoplankton blooms
and food webs structure (Cloern and Jassby 2008, Winder and Cloern 2010).
