Geoscience Reference
In-Depth Information
shown a greater consumption of dissolved inorganic carbon when the
pH decreases, indicating a stimulation of the net community primary
production. In some of these experiments, it has been observed that
the consumption of nutrients remained the same and the C:N ratio
increased, exceeding the Redfield ratio (106:16; [RED 34]). This
change of the stoichiometry of organic matter could reduce its
nutritional value and affect the efficiency of bacterial degradation and
secondary production [RIE 11].
5.4.1.1.2. Nitrogen fixation
Nitrogen is a limiting nutrient for photosynthesis in many oceanic
provinces. A larger supply of nitrogen via an increase in fixation of
nitrogen gas could have major consequences on primary production.
Growth and nitrogen fixation of
Trichodesmium sp
. are stimulated at
elevated pCO
2
(e.g. [HUT 07]). However this species is not the only
cyanobacteria performing nitrogen fixation. Only three studies focus
on other species, including one on unicellular cyanobacteria
Crocosphaera
[FU 08], for which N
2
fixation is also stimulated at a
high level of elevated pCO
2
, and the heterocystous cyanobacteria
Nodularia spumigena
for which the two studies carried out provided
conflicting results [CZE 09, WAN 12].
5.4.1.1.3. Calcification
Calcification, the formation of calcareous skeletons and shells by
organisms such as calcareous algae, corals, mollusks, foraminifera,
echinoderms, crustaceans and bryozoans, can be negatively affected by
the decreased availability of carbonate ions associated with the increase
in acidity (Table 5.1). While the primary production of
coccolithophores can, in certain cases, be slightly stimulated by ocean
acidification (see section 5.4.1.1.1), their calcification can be altered.
Coccolithophores are the first planktonic organisms for which the
impact of the elevation in CO
2
on calcification has been studied
[RIE 00]. Their response varies greatly depending on the species
studied, or even for different strains of a same species. In many cases,
while calcification decreases with the increase of pCO
2
(for example,
[RIE 00]), certain species have a bell-shaped response curve with a
maximum value at the current levels of pCO
2
and a decrease on either
