Biology Reference
In-Depth Information
of high biological activity, where decomposers that are associated with the floating
algae recycle organic remains into nutrients and other compounds (Thiel and
Gutow
2005b
).
Floating seaweeds are often more abundant in coastal waters than in the open
ocean (Segawa et al.
1961
; Kingsford
1992
; Ing
´
lfsson
1995
; Hobday
2000b
). High
nearshore densities of floating algae are primarily due to the proximity to coastal
source populations. Additionally, floating algae are known to accumulate in coastal
areas with complex current conditions and frontal zones (Segawa et al.
1961
).
Harrold and Lisin (
1989
) tracked floating
M. pyrifera
equipped with radio
transmitters in coastal waters of California. They could show that the majority of
algal rafts are deposited on beaches soon after detachment while others become
entangled with benthic sporophytes in coastal kelp forests (see also Dayton et al.
1984
; Graham et al.
1997
). The onshore transport of floating
M. pyrifera
in coastal
waters of California was confirmed by Hern´ndez-Carmona et al. (
2006
)
(Fig.
17.4
). Results from experiments with artificial drifters revealed that direc-
tional net transport of floating algae in coastal systems is mainly the result of
prevailing winds that interact with surface currents (Tapia et al.
2004
). During
periods of low wind, no substantial transport of the drifters by currents alone could
be detected (Fig.
17.4
). Apparently, only a relatively small fraction of detached
seaweeds escapes to offshore waters where they might enter larger oceanic currents.
In offshore regions conspicuous amounts of seaweeds have repeatedly been
observed in surface eddies that develop above seamounts in the NE Pacific (Parker
and Tunnycliffe
1994
). Additionally, floating algae often accumulate in the con-
vergence zones of wind-induced Langmuir circulations (Faller and Woodcock
1964
). In the N Atlantic, pelagic
Sargassum
has been observed in such windrows
extending over tens to hundreds of miles (Carr
1986
). Drift lines of floating
Sargassum
are transient structures that disintegrate when wind exceeds a certain
threshold speed (Marmorino et al.
2011
). Johnson and Richardson (
1977
) suggested
that the downwelling of surface waters in the zones of convergence might carry
Sargassum
below a critical depth where the thalli lose buoyancy and finally sink to
the seafloor, thereby exporting biomass from the surface to the deep sea benthos
(Schoener and Rowe
1970
).
17.2.3 Seasonal Patterns of Floating Populations
While large quantities of seaweeds appear during some periods of the year, floating
algae can virtually disappear at other times. For instance, along the coast of Japan
high densities of floating individuals of the annual
Sargassum horneri
can be found
throughout spring and summer when the algae shed off most of their thalli during
their reproductive season (Yoshida
1963
; Ohno
1984
; Hirata et al.
2001
). Similar
observations have been made by Kingsford (
1992
) for floating
S. sinclairii
from
coastal waters of New Zealand, indicating that the appearance of large quantities of
Sargassum
is driven by their seasonal growth cycles. Additionally, the invasive
