Biology Reference
In-Depth Information
number of intense storms and decrease those of intermediate intensity (Webster
et al
.
2005
). The consequences of these changing weather patterns on the distribu-
tion of drift algae and their persistence within the coastal environment require close
monitoring.
16.4.3.1 Calcareous Algae Within the Seagrass Habitat
High densities of seagrasses have the capacity to increase the pH of entire water
bodies due to their uptake of CO
2
and release of OH
(Beer et al
.
2006
). For
example,
Cymodocea rotunda, Halophila ovalis,
and
Thalassia hemprichii
have the
capacity to raise water pH from 8.1 to 8.6, 8.8, and 9.2, respectively (Beer et al
.
2006
). The pH compensation point of each species as well as the inability of
seagrasses like
H. ovalis
to tolerate the high pH values and low inorganic carbon
concentrations generated by the other two species forces these seagrasses into
monospecific tide pools. However, Semesi et al
.
(
2009
) determined that calcifying
macroalgae, such as
Hydrolithon
spp.,
Mesophyllum
spp., and
Halimeda renschii
,
can buffer against this increase in pH via intensified calcification. In fact calcifica-
tion rates of
Hydrolithon
spp. can be five to six times higher in the presence of
seagrasses. Similarly, photosynthetic activity of
Mesophyllum
spp
.
increased by
15% in the presence of seagrasses. Thus, when coexisting these two groups are
capable of maintaining high levels of productivity while encouraging higher levels
of biodiversity. Monitoring of the balance between these two groups is important as
the dominance of one over the other could lead to dramatic shifts in water chemistry
and a regime shift. Particularly important are the consequences that ocean acidifi-
cation may have on this balance.
16.4.3.2 Regime Shifts and Controls Within the Seagrass Habitat
Human development of the coastal zone has often been identified as the source of
nutrient pollution that disrupts the balance between seagrasses and macroalgae. In
2008
, Houk and van Woesik collected evidence suggesting that seagrass species
respond differently to environmental pressures. In a Saipan lagoon they found that
increased watershed development stimulated macroalgal proliferation within
Halodule uninervis
seagrass meadows, severely impacting the integrity of the
meadows. However, the same study revealed that the growth of a different seagrass,
Enhalus acoroides,
was positively correlated with human development and water-
shed size. The authors suggest that the physical and/or morphological
characteristics of seagrass species determine their susceptibility to macroalgal
overgrowth. Similarly, McGlathery (
2001
) suggests that
Thalassia testudinum
may be more resistant to the pressures of algal blooms because of a proportionately
greater allocation of resources to below-ground biomass, allowing them to persist
during temporary algal blooms.
