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results were shown for Cystoseira amentacea var. stricta from the Mediterranean
Sea (Susini et al. 2007 ). While A. nodosum is highly buoyant, individuals of
C. amentacea var. stricta lack vesicles but thallus fragments mix with other floating
algae at the sea surface (Susini et al. 2007 ). Overall, the dispersal of gametes and
spores via floating reproductive fragments or adults may also facilitate the reestab-
lishment of algal populations after large-scale disturbances (such as El Ni ˜ o events)
(e.g., Dayton et al. 1999 ).
17.6 Outlook
Floating seaweeds can be found from the tropics to the Arctic and the sub-Antarctic
islands. Since most studies on the physiology and ecology of floating algae were
conducted at mid and high latitudes (e.g., Hobday 2000a , b ; Macaya et al. 2005 ;
Roth
ausler et al. 2009 ), our knowledge about floating algae from tropical and
subtropical regions is scarce. We propose that future research should examine the
floating persistence of algae traveling in warm waters and under high radiation
conditions, which will help to understand the role of low latitude algae as potential
dispersal agents. In the face of global climate change, which predicts to drive
species ranges towards the poles (see also Chap. 18 by Bartsch et al.; Parmesan
and Yohe 2003 ), it can be expected that tropical and subtropical algae with positive
buoyancy might expand farther towards mid latitudes, while the dispersal potential
of temperate floating algae will be suppressed at mid latitudes (see above).
While afloat, algae can be concentrated in frontal systems (convergence zones)
and eddies, which can result in the formation of huge biomass accumulations (e.g.,
Gower et al. 2006 ). Particulate organic matter such as algal detritus can also
aggregate within these zones, contributing to biogeochemical cycles (Thiel and
Gutow 2005b ). Decomposers may use the organic matter as a substrate and
nutrients released can be assimilated by the algae. Efforts should be made to better
understand these processes within zones where floating algae and other floating
items accumulate (Hinojosa et al. 2011 ).
So far, molecular studies have helped to reveal population connectivity of
seaweeds and associated organisms over a broad range of spatial scales. However,
they should also include the temporal scale. Rafting frequencies mainly depend
on the availability and the persistence of the floating algae at the sea surface.
Depending on latitude and local conditions, supply of floating algae can be
spatially and temporally variable. Furthermore, the benthic distribution of floating
algae might have changed substantially over evolutionary and ecological
timescales. Future studies should therefore not only examine the relevance of
rafting for recent population connectivity, but also how episodic rafting in the past
has facilitated the establishment of isolated populations and subsequently led to
allopatric speciation.
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