Biology Reference
In-Depth Information
The consequences of nutrient enrichment are not always easily predicted.
Nutrient addition experiments in Florida resulted in increased seagrass biomass
and decreased macroalgal epiphyte coverage offshore. Conversely, the inshore
treatment had no effect on two species of seagrasses while macroalgae biomass
increased significantly (Ferdie and Fourqurean
2004
). In a similar Florida-based
study, Armitage et al
.
(
2005
) found that enrichment of sediments stimulated highly
variable group and site-specific responses by macroalgae. The resulting macroalgal
blooms (calcareous green and filamentous red) were ephemeral and had no long-
lasting impact on the system. Their results also showed that areas containing a high
abundance of macroalgae were exposed to an array of additional chronic pressures
including nutrient enrichment, current and erosion pressures, and heavy boat traffic.
During nutrient treatments, phosphate (not nitrate) stimulated growth, prompting
the researchers to conclude that Florida Bay is phosphate limited, a consequence of
the high carbonate content of the sediments, perhaps owing to the calcareous algae
contained therein. Results from a similar study in Puerto Rico show that several
species of nutrient-depleted nuisance bloom (
Hypnea musciformis
) and mat-
forming (
Acanthophora spicifera, Dictyota cervicornis
) macroalgae increased
growth rates significantly when exposed to nutrient enrichment. However, enriched
algae of the same species had no response to increased nutrient concentrations.
Calcareous green algae (CGA) (e.g.,
Halimeda incrassata
) did not respond to
treatments within the 3-day experiment, suggesting limitation by some other meta-
bolic process, such as calcification (Fong et al
.
2003
). Therefore, in order to predict
the consequences of nutrient enrichment, coastal managers must consider the
existing concentration of each relevant nutrient within the system as well as the
primary producers contained therein.
Evidence suggests that storms arriving in the late summer cyclone/hurricane
season play an important role in maintaining a balance between seagrasses and
algae. Frequent disturbances such as large swells and intense currents prevent
epiphyte growth and thus maintain seagrasses as the dominant flora (Houk and
Camacho
2010
). However, the impacts of massive natural disturbances on coastal
systems (erosion, mechanical thinning, and burial) can completely transform ben-
thic communities, often irreversibly. Following Hurricane Georges in 1998,
Fourqurean and Rutten (
2004
) observed that while only 3% of
T. testudinum
was
removed, 24% of CGAs were lost. CGAs are much more susceptible to hurricane
damage because they lack the extensive rhizomes of seagrasses and are thus easily
liberated from the substrate during sediment displacement. As the system recov-
ered, CGAs were the first primary producers to recolonize bare patches, although its
density did not return to prehurricane levels for another 3 years. This storm-
dependent balance within the seagrass-algae complex and the biodiversity that it
maintains can be considered within the context of the Intermediate Disturbance
Hypothesis (Connell
1978
).
