Biology Reference
In-Depth Information
Centuries after Columbus' voyage, contemporary marine scientists have
reported floating seaweeds from all major oceans (Ohno 1984 ; Kingsford 1992 ;
Helmuth et al. 1994 ; Ing ยด lfsson 1998 ; Hobday 2000a ; Hirata et al. 2001 ; Macaya
et al. 2005 ; Vandendriessche et al. 2006 ; Komatsu et al. 2008 ; Hinojosa et al. 2010 ).
Recent studies confirm that floating algae can cross large ocean basins, facilitating
the colonization of new habitats on remote shores by the algae themselves but also
by organisms living associated with floating algae (Fraser et al. 2009 , 2011 ; Macaya
and Zuccarello 2010 ; Nikula et al. 2010 ). Accordingly, these authors emphasized
the importance of dispersal of organisms via these floating islands.
There is a wide variety of positively buoyant seaweeds in the oceans that become
detached by breakage of stipes, thallus fragmentation, erosion of holdfasts, storms,
or grazing activities. Once detached, floating seaweeds continue to grow if envi-
ronmental conditions are optimal, and they form habitat for many associated
organisms. Also these algae are known to travel considerable distances at the
mercy of winds and currents, having the potential to release spores or gametes at
a new site and thereby connecting distant populations (Fraser et al. 2009 ). In order
to understand the fate of floating seaweeds it is crucial to address a set of questions.
Where can we regularly find floating algae and where do they come from? How do
abiotic and biotic factors affect the persistence of floating algae at the sea surface?
How can floating algae acclimate or withstand sea surface conditions? Which
organisms are traveling with them and how can they affect raft viability? Answering
these questions will help to understand the role of floating algae as dispersal agents in
the world's oceans.
17.2 Spatial and Seasonal Distribution of Floating Seaweeds
A wide diversity of seaweeds can be found floating in the world's oceans, where
they persist at the sea surface thanks to the presence of gas-filled tissues or
pneumatocysts (Fig. 17.1 ). The dominant floating seaweeds in the world's oceans
are brown algae belonging to the genera Macrocystis , Fucus , Sargassum ,
Ascophyllum, Durvillaea, Carpophyllum, Phyllospora, and Cystophora . Some red
and green algae have also been reported floating, albeit with a more limited floating
potential than brown algae (Thiel and Gutow 2005a ; Gagnon et al. 2011 ). Besides a
few entirely pelagic species such as Sargassum natans and S. fluitans , most floating
algae grow in benthic habitats during their earlier life stages and can vary widely in
size after detachment, ranging from a few centimeters to tens of meters. Floating
seaweeds can aggregate at the sea surface and form large rafts of many individuals
with biomasses of up to 300 kg wet weight per seaweed patch (e.g., Senta 1962 for
the Japanese coast) or more (Mitchell and Hunter 1970 for the southern Californian
coast and Baja California).
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