Biology Reference
In-Depth Information
strong top-down control of epiphytes on seagrasses (Moksnes et al
.
2008
). In fact,
Spivak et al
.
(
2009
) found that the strongest effect during their nutrient-enrichment
experiments was an increase in grazer biomass, not epiphytes, indicating an
efficient transfer of nutrients through trophic levels. This evidence suggests that
removal of large predators (tertiary consumers) is likely to cause trophic cascades,
resulting in the increased severity and frequency of algal blooms in the seagrass
environment.
16.4.3 Drift Algae Within Seagrass Meadows
In the tropics, drift algae can have a significant impact on the well-being of seagrass
communities. Drift algae originate as epiphytes growing within seagrass meadows
and neighboring habitats, having been liberated from the substrate during height-
ened hydrological disturbances. Accumulation of drift macroalgae within seagrass
meadows is a function of water circulation characteristics determined by coastal
physiography (Kopecky and Dunton
2006
) and rates of entanglement in the
seagrass canopy (Biber
2007
). Transport of drift algae across the seabed is depen-
dent upon substrate complexity, and in the case of seagrasses, it is determined by a
meadow's patchiness, blade length, and shoot density. In shallow waters where
water velocity is reduced drift algae can become entrained in bare patches. Algae
also have a tendency to accumulate where wind or tidal generated water currents
are sufficiently reduced to allow deposition out of the water column, be it on the
leeward side of a bank or within a protected depression (Kopecky and Dunton
2006
). Once entangled, drift algae can rapidly proliferate under favorable
conditions generated by freshwater inputs, ideal temperatures, and nutrient loading
(Biber
2007
). In the Gulf of Mexico, drift macroalgae can reach a biomass exceed-
ing that of the dominant seagrass,
Thalassia testudinum
(Kopecky and Dunton
2006
). The adverse weather conditions typical of the tropical hurricane season
generate current velocities sufficient for the removal of accumulated algal biomass
(10 cm s
1
) (Biber
2007
). Drift algae biomass varies by season as well as from
1 year to the next (Houk and Camacho
2010
).
It has long been recognized that excessive quantities of drift macroalgae can
have serious impacts on underlying seagrasses via competition for nutrients and
PAR (Hauxwell et al
.
2003
). Excessive macroalgal loads can become so dense that
they restrict water flow, thereby depriving seagrasses of oxygen and driving up
concentrations of sulfides in the subsurface sediments (Holmer and Nielsen
2007
).
In addition to its impact on seagrasses, light attenuation by blooms of drift algae off
the coast of Texas has caused significant reductions in seagrass epiphyte biomass
(Kopecky and Dunton
2006
). Given that the integrity of seagrasses can be severely
compromised during these short-term seasonal blooms, the long-term repercussions
of persistent blooms that may arise as a result of climate change demand serious
consideration. Climate experts suspect that a changing climate will increase the
