Biology Reference
In-Depth Information
tidal flushing helps to remove anoxic waters and nutrients from the system,
minimizing hypoxia-related stress on grazers and negative cascading effects up
the food web (Martinetto et al.
2010
,
2011
). Therefore, the high biomass of
nutrient-rich macroalgae can provide a large quantity of food with higher
nutritional quality to grazers and support higher grazer abundances (Martinetto
et al.
2010
,
2011
).
21.5.2.2 Coral reefs
In the last four to five decades, there has been an increasing trend of macroalgal
blooms in coral reefs coinciding with a decrease in coral cover (Hughes
1994
;
McManus and Polsenberg
2004
; Nugues and Bak
2008
). This shift from coral to
macroalgal-dominated reefs, known as coral-algal phase shifts, has stimulated
much of the debate in the relative role of top-down and bottom-up controls in
marine habitats. Some studies have shown that the removal of the dominant
herbivores, through overfishing or natural causes, has been more important as a
control on macroalgal growth than nutrient enrichment (Hughes
1994
; Hughes et al.
1999
; Burkepile and Hay
2006
; Sotka and Hay
2009
). Others state that recent
increases in nutrient inputs from land to coastal reefs have triggered macroalgal
blooms (Lapointe
1997
; Lapointe et al.
2005a
,
b
). And then there are those studies
that support both top-down and bottom-up controls as important influencing factors
(McClanahan et al.
2003
; Littler et al.
2006
; Smith et al.
2001
). In this chapter, we
do not attempt to provide a comprehensive review of the current extensive literature
on coral-algal phase shifts in coral reefs, but rather focus on a few specific studies
that examine the effects of top-down and bottom-up controls on macroalgal growth.
In Hughes (
1994
), one of the first long-term studies of coral reef decline in the
Caribbean reported large-scale natural and human disturbances were linked to
coral-algal phase shifts. In most of the reefs around Jamaica, the driving factors
of these phase shifts were thought to be the increase in human population, which
drove an increase in overfishing, followed by the mass mortality of the sea urchin
Diadema antillarum
. Without herbivorous fish and
D. antillarum
, the dominant
grazers of macroalgae on the reefs, blooms of filamentous macroalgae began to
overgrow corals, which were then replaced by late successional stage species, such
as
Dictyota
,
Lobophora
,
Halimeda
, and
Sargassum
spp. These macroalgae
inhibited coral reef recovery by effectively competing for open space needed for
recruitment of coral larvae (Hughes
1994
). Lapointe (
1997
) introduced another
potential trigger of macroalgal blooms on these Jamaican reefs by showing that
bottom-up control also played a role. It was demonstrated that dissolved inorganic
nitrogen from wastewater entered the reefs through groundwater discharge and
elevated reef nutrient water concentrations sufficiently to sustain macroalgal
blooms in these habitats (Lapointe
1997
). These studies spurred a debate as to the
relative role of bottom-up and top-down controls on coral reefs (Hughes et al.
1999
;
Lapointe
1999
).
