Page 20 - climate-change2013

Another type of bloom, mucilaginous aggregates, is
caused by the proliferation of several phytoplankton Eutrophication: Process by which
species developing seasonally and at different depths. a body of water (usually shallow)
Marine mucilage floating on the surface or in the water acquires a high concentration of
column can have a long life span (up to 2–3 months) nutrients, especially phosphates and
and when thick, dense mats of it settle on the sea bot- nitrates and decrease in dissolved
tom they can sometimes completely cover entire ben- oxygen, caused by either natural
thic communities, such as seagrass (Posidonia oceani- processes or pollution. This typically
ca) meadows and gorgonian (e.g. Paramuricea clavata) promotes excessive growth of algae.
forests, causing hypoxic and/or anoxic conditions over
several square kilometres of sediments (Danovaro et al.,
2009). At this stage they can be especially harmful to
gorgonian populations, and severe mortality events as- Effects of acidification
sociated with these outbreaks have been reported from
Italy (Sicily) and Spain (Columbretes Marine Reserve)
(Mistri and Ceccherelli, 1996). In other cases, however, The absorption by seawater of atmospheric CO lead-
2
the mucilage disappeared after several weeks leaving ing to a decrease in pH (acidification) can have a severe
no apparent signs of impact on the communities. A impact on the performance and survival of many or-
considerable increase in the frequency of these muci- ganisms with calcium carbonate structures, and con-
lage events has been observed in different parts of the sequently affect the composition and productivity of
north-western Mediterranean, around Sicily and par- marine communities.
ticularly in the northern Adriatic Sea, the last being the
area most severely affected by these outbreaks. The Little is understood at present, however, about the im-
timing of these events and of the climate anomalies ob- pact that will have on marine biodiversity. Ocean acidi-
served in parallel with them (e.g. the rise in sea-surface fication has the potential to affect individuals’ growth,
temperatures) indicates a clear relationship with climate reproduction and activity rates. Some animals will toler-
change (Danovaro et al., 2009). ate higher acidity; some may even thrive on it, but the
overall community changes will be different at each
Warmer coastal waters combined with eutrophication locality. Shifts in species composition along pH gradi-
can also increase the intensity, duration and extent of ents suggest that calcified species might not survive
harmful algal blooms, which can damage marine com- the increased metabolic costs of coping with low-pH
munities and coastal industries such as aquaculture. environments while competing for resources with other,
Many of these events are thought to be a consequence uncalcified organisms.
of climate change. Warming may also raise the possibil-
ity that new parasites and diseases will arrive in Medi- Understanding how Mediterranean coastal ecosystems
terranean waters, and some recent studies suggest an will react to increased seawater acidity is one of the prior-
increased frequency of such outbreaks in invertebrates, ities for many national and international research groups
making large mortality events more likely (Lejeusne et as very few field studies have been carried out. Besides
al., 2009; Calvo et al., 2011). However, not enough is laboratory experiments, shallow marine habitats with
yet known to predict the consequences of these patho- volcanic CO vents have offered researchers a good
2
gens and their connection with the changing climate. environment to learn about the effects of acidification

Photo: J. Hall-Spencer

Posidonia oceanica meadows in volcanic

Discharges of waste through underwater drainage system. CO 2 vents of Italy.
Photo: V. Tasso, OCEANSNELL
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