Climate Change and the European Water Dimension - Agri ...

looms are often associated with toxic or harmful species (see box #2), which
have proliferated during the past two decades due to a combination of mild
winters, appropriate nutrient ratios, higher stratification, and possibly exposure
to UV radiation. In contrast, neither N nor Si seems to be limiting the
phytoplankton growth rate in the North Adriatic, which may be more sensitive
to changes in light and temperature (Bernardi-Aubry et al. 2004). Nitrogen
fixers such as Trichodesmium are mainly distributed in subtropical and
tropical areas where the combination of warm temperature and stratification is
favorable to their growth. This community has also a strong iron requirement
and could therefore benefit from global warming, subsequent land
desertification and an intensification of dust events. Lenes et al. (2001)
observed a 100-fold increase in Trichodesmium biomass at the West Florida
shelf after a major Saharian dust event.
Box #2: Harmful Algal Blooms (HABs)
Over the last several decades, the world coastal environment has encountered an
increasing and problematic situation of anomalous phytoplankton blooms (AABs).
The term ‘anomalous’ is used here to differentiate these blooms with those occurring
regularly following well-known seasonal processes (e.g. spring blooms, seasonal
upwellings). In addition, AABs are very often associated with harmful or negative
consequences (i.e., HABs) on the surrounding ecosystem, or even toxic material
causing mass mortalities of marine organisms, as well as affecting human health
through contaminated shellfish and fish populations.
The increasing frequency of these blooms and their socio-economical impacts have
lead to the development of important regional and national programmes such as
EUROHAB (Granéli et al. 1999) in Europe and ECOHAB in the United States to
investigate on possible causes that have triggered such an upsurge of HABs events
threatening today most coastal systems, and to better understand the dynamics and
trophic interactions of these blooming species. In response to the globalization of the
phenomena, international efforts have been initiated (GEOHAB 2001; EU-US HAB
2003) to coordinate scientific progress made locally and develop common
approaches to evaluate the impact of HABs.
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HABs have been classified within two groups on the basis of their
noxiousness and geographical distribution: the toxin producers mainly
occurring in open coastal systems which may lead to harmful impact even at
low density; and high-biomass producers which tend to develop in enclosed or
semi-enclosed seas obstructing light penetration and depleting oxygen in
subjacent layers. The initiation, growth and maintenance of these blooms are still
under studies with possible explanations around man-induced pollution effects and/or
climate shifts. The hemispheric differences in the distribution of some toxic species
(Figure IV.C.9) would support the role of anthropogenic pollution and increase of
nutrient loadings in the northern coastal zone through rivers and atmospheric
deposition.
Cyanobacterial blooms have been present in the Baltic Sea during the summer
periods for thousand years as documented from fossil records of cyanobacterial
pigments (Bianchi et al. 2000). Since the early 1960’s, the occurrence of these
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