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reach high abundances after storms (Kingsford 1992 ), while during El Ni˜o events,
high water temperature and nutrient limitation can cause the disappearance of
benthic seaweeds (Dayton et al. 1999 ), thus also leading to the elimination of the
floating populations.
17.3 Ecophysiology of Floating Seaweeds
Growth and reproduction of floating seaweeds depend (as for their benthic
counterparts) on a variety of abiotic and biotic conditions. At the sea surface,
factors such as grazing activity, epibiont overgrowth, high water temperature, and
solar radiation have been repeatedly inferred to have a negative effect on growth
and health status of floating seaweeds. Also, it has been discussed that nutrient
limitation might affect physiological functioning and growth of floating algae
(Edgar 1987 ). While nutrient-limited open ocean waters suppress the physiological
functioning of floating S. natans (Lapointe 1995 ), algae that accumulate in frontal
systems where organic matter is efficiently recycled are thought to have sufficient
nutrients to sustain algal growth (Thiel and Gutow 2005b ).
Abrupt changes in environmental factors, as experienced especially by floating
algae, can impact their photosynthetic apparatus, which is most susceptible to
damage under stressful conditions. Stressed algae invest energy to adjust and
maintain photosynthetic activity. However, this energy investment comes at the
expense of algal growth. Consequently, algae respond with variable growth to
changing environmental conditions because growth integrates all physiological
costs and gains. Growth can be directly related to the overall health status of the
algae and thus to their persistence at the sea surface (Fig. 17.5 ).
17.3.1 Light
At the sea surface floating algae are often exposed to intense visible (400-700 nm)
and ultraviolet radiation (280-400 nm), which may induce photoprotective
processes. In large outdoor mesocosm studies conducted along the Chilean coast,
M. pyrifera reacted to high solar irradiance by lowering their pigment contents and
by energy dissipation via heat (Roth
ausler et al. 2011a , b ). Similar physiological
responses were observed for the holopelagic S. natans, floating in tropical waters of
the Gulf of Mexico (Schofield et al. 1998 ). These processes are relevant because
they permit algae to tolerate the stressful conditions of extreme irradiance and even
to continue growing at the sea surface.
While the positively buoyant M. pyrifera , of which the attached sporophytes
grow throughout the entire water column (up to 40 m in length), can efficiently
acclimate to a broad range of irradiance prevailing along the Chilean Pacific coast
(Roth
ausler et al. 2011a , b ), negatively buoyant seaweeds showed strong
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