Macroalgae in Tropical Marine Coastal Systems - Seaweed Biology - page 328

further investigate the role of herbivory on regulation of macroalgae community
structure, numerous herbivory exclusion experiments have been conducted in
tropical coral reefs (Lirman 2001 ; Hughes et al . 2007 ; Smith et al . 2010a ; Hoey
and Bellwood 2010 ). These studies provide evidence of how exclusion of large
herbivores is followed by undesirable blooms of upright macroalgal species, which
are considered less susceptible to herbivory and capable of causing phase shifts
from coral to macroalgae dominated systems. Mork et al . ( 2009 ) conducted a
6-week herbivore exclusion study on a moderately disturbed coral reef on the
Kenyan coast. Their results show a 77% increase of algal biomass, largely
attributed to a 1,000% increase in corticated forms of algae, dominated by Dictyota
spp. Sotka and Hay ( 2009 ) reported similar results from an experiment conducted in
the Florida Keys. After 142 days of monitoring, they observed that coral slabs
exposed to natural densities of large herbivorous fishes were dominated by crustose
coralline algae, short ( 0.5 cm) filamentous turf algae, and upright macrophytes,

which never exceeded 15% cover. In contrast, herbivore exclusion treatments were
subject to 80-100% upright macroalgae cover.
The role of herbivory in regulating macroalgal communities can be severely
compromised in overfished and degraded reefs and may result in phase shifts from
coral to algae dominated systems (Mumby et al . 2006 ). In areas suffering from
moderate levels of overfishing, a lack of top predators can stimulate an increase of
herbivore biomass. However, in severely overfished areas, herbivorous fish are not
able to control macroalgae because they too become subject to overfishing and are
unable to become sufficiently large in size to regulate algal production, even if they
are numerically abundant (Kopp et al. 2010 ). A significant reduction of fishing
pressure of herbivorous fish (Mumby and Harborne 2010 ) as well as a reduction of
confounding disturbances such as nutrient pollution (Smith et al . 2010a ) has been
suggested as a means of reversing algal phase shifts.

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16.4.1.2 Chemical Defenses

Macroalgae defend against herbivory using a number of strategies, including
morphological, structural, and chemical defenses as well as associations with
other algae or benthic organisms (Hay 1997 ; Paul and Puglisi 2004 ; Amsler
2008 ; Smith et al . 2010b ; Fong and Paul 2011 ; see Chap. 9 by Amsler). Morpho-
logical and structural defenses include calcification and toughness, which are
characteristics of the Halimeda, Dictyota, and Lobophora genera (Paul et al .
2001 ). These species are coarsely branched, leathery, or rubbery and have hard
calcium carbonate tissues which are of little interest or nutritional value for
herbivores (Schupp and Paul 1994 ; Hay 1997 ). Some macroalgae utilize secondary
metabolites to help protect against pathogens, fouling organisms, and herbivores
(Fong and Paul 2011 ). A significant quantity of natural products has been extracted
and isolated from tropical marine green, red, and brown macroalgae, of which
halides like bromine and chloride are very common as well as compounds like
terpenoids (Fong and Paul 2011 ). Chemical deterrents in calcified green algae from