Floating Seaweeds and Their Communities - Seaweed Biology - page 360

17.3.4 Epibiosis

Floating seaweeds are commonly colonized by larvae of sessile epibionts such as
bryozoans and lepadid barnacles (Thiel and Gutow 2005b ), which can influence
algal growth by covering photosynthetic tissues. At mid latitudes in the Humboldt
Current System, it was observed that bryozoan cover of M. pyrifera increased with
distance from potential source populations and that physiological parameters (e.g.,
maximal quantum yield and overall photosynthetic efficiency) declined with
increasing bryozoan size (Roth

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ausler et al. 2011d ). Small benthic algae can main-
tain high photosynthetic activity under bryozoan cover by increasing the pigment
content (Mu˜oz et al. 1998 ). However, such shade adaptation was not observed in
benthic and floating M. pyrifera (Hurd et al. 2000 ; Hepburn et al. 2006 ; Rothausler
et al. 2011d ). While dense epibiont cover can increase the specific density of
floating algae and even cause sinking, uncalcified young bryozoan colonies might
have a positive effect on algal tissue by shielding them against high solar radiation.
Algae may also benefit from encrusting bryozoans through the provision of carbon
dioxide and ammonium released directly from epibionts onto the algal tissue
(Mu˜oz et al. 1998 ; Hurd et al. 1994 , 2000 ), but these advantageous effects
probably depend on the degree of colonization. In general, over long floating
times, continuous epibiont growth and grazing pressure negatively affect algal
persistence at the sea surface. However, depending on the presence of alternative
food sources and the colonization progress, not all organisms associated with
floating algae (e.g., detritus feeders, scavengers, and predators) may have the
same destructive effects on their rafts.

17.4 Rafting Communities

Floating seaweeds carry with them a wide diversity of organisms (Thiel and Gutow
2005b and references therein). The composition of the rafting community and the
abundance of individual species can be highly variable in time and space, and
successional changes are probably related to travel time of the floating algae, their
distance from the shore, competition, and predation.
Not all organisms are equally adapted to the rafting life style because not all of
them can efficiently hold onto floating algae. Large invertebrates such as sea
urchins, crustaceans, and gastropods immediately drop off when detached algae
float to the sea surface. Also small organisms such as the boring peracarid isopods
Limnoria spp. quickly left their burrows after kelp holdfasts were detached from
benthic substrata (Miranda and Thiel 2008 ). Similarly, Gutow et al. ( 2009 ) showed
that the densities of some peracarid crustaceans dropped immediately after algae
became dislodged. Some species, such as ampithoid amphipods that build nest-like
domiciles on algal blades (e.g., Peramphithoe femorata Cerda et al. 2010 ), may
persist and even reproduce after detachment, resulting in a population increase
during rafting journeys (see also Thiel 2003 ). Even though these motile organisms