Oyster Physiology and Ecological Distribution as an Indication of Environmental Health
and as a Performance Measure of Restoration Effectiveness of Southwest Florida’s
Estuaries
Michael Savarese, Aswani Volety, and Greg Tolley
Florida Gulf Coast University, Ft. Myers, FL
Oyster reefs make excellent sentinels for estuarine health in Southwest Florida. Oyster
reef development is critical to estuarine ecology and to the geomorphologic structure of
the region. The development of mangrove islands that differentiate the inner and outer
bays and passages along the coast is founded upon oyster reefs. Additionally, because
the eastern oyster, Crassostrea virginica, is the principal suspension feeder within these
estuarine waters, oyster reefs are critical to pelagic – benthic coupling. Reefs remove the
carbon fixed by these waters’ principal primary producers, the phytoplankton, and
transfer that carbon to the benthos. This filter feeding ability also serves to improve
water quality and clarity for benthic primary production. Lastly, oyster reefs are
“essential fish habitat” and provide substrate for many other fin- and shellfish species.
Because many of Southwest Florida’s estuaries lack prolific sea grass beds, oyster reefs
are therefore the most “valued ecosystem component” of our estuarine system.
Consequently, oysters and their reefs are ideal measures of estuarine health and can be
used to monitor management and restoration practices.
Our research investigates the response of oyster growth, standing stock, mortality, larval
recruitment, disease prevalence (to DERMO, caused by the protozoan parasite Perkinsus
marinus), and living density on reefs, and upon oyster reef distribution and aerial extent
to water management within altered and pristine estuaries in Southwest Florida. Studies
have been completed or are presently underway in the Ten Thousand Islands, Henderson
Creek (within waters of Rookery Bay NERR), Estero Bay, and the Caloosahatchee River.
The results presented herein concern three estuaries within the Ten Thousand Islands that
are germane to a pending CERP restoration project: South Golden Gate Estates. South
Golden Gate Estates (SGGE) is a 94 square mile tract of drained wetland that was
originally altered for a large housing development in the 1960s. The developer, Gulf
America Corporation, had planned and subdivided the area, but went out of business
before homes were built. Development, however, was not halted before networks of
drainage canals and roads were constructed. These canals and roadways have disrupted
sheet flow into the estuaries of the central Ten Thousand Islands and have generated a
freshwater pollution point source, through the main Faka Union trunk canal, to Faka
Union Bay. Estuarine ecology has been drastically altered as a result. SGGE has been
targeted for restoration since the 1970s as the impacts of excessive drainage became
apparent. Since that time, SGGE restoration was designated a South Florida Critical
Ecosystems Restoration Project under the Water Resources Development Act of 1996
and is presently a proposed project of CERP. The expected engineering improvements to
SGGE include the disassembly of the canals feeding the Faka Union canal, the possible
use of spreader canals, and the removal of roads.
Three estuaries were considered: (1) Faka Union, the estuary most impacted by presentday water management and the one to be monitored most carefully during and after
SGGE restoration; (2) Henderson Creek, located south of the city of Naples, outside the
influence of SGGE management but also an estuary under restoration; and (3) Blackwater
River, a relatively pristine estuary within the Ten Thousand Islands west of Faka Union.
Some of the measures concerning oyster physiology and ecological distribution outlined
above were made seasonally (wet versus dry season); others were made monthly; and
oyster reef distribution was mapped once at the study’s initiation. Measurements were
compared using a “spatial homologue” approach. Because the three estuarine systems
have a comparable geomorphology (i.e., similar bay distributions and sizes, watershed
areas, and drainage lengths), this suggests that prior to human alteration they had similar
hydrologies. Consequently, it is appropriate to assume that spatially homologous points
along the onshore-offshore estuarine axis (i.e., those located at equal distances upstream
from the outermost marine coastline) had similar water quality characteristics (e.g.,
salinity) and that oyster ecology and physiology were comparable when the estuaries
were in their natural state. Aspects of water quality and oyster physiology and ecology
were compared among 5 spatially homologous points within the 3 estuaries. Any
deviation in conditions from those measured in Blackwater suggests a human-effected
change in Faka Union.
Results clearly demonstrate that water management practices, specifically the impacts of
freshwater inundation from the uncontrolled draining and channeling of the wetlands
within SGGE, have adversely affected oysters and the development of reefs. In Faka
Union, an estuary that receives excessive freshwater during the rainy season because its
watershed has been enlarged, reef distribution, regions of maximum living density,
recruitment, and standing stock are displaced seaward along the estuarine axis relative to
the pristine estuary. Henderson Creek, an estuary receiving pulses of nutrient-rich
freshwater because of a crude weir design, has populations with higher standing stocks
and densities. Two demographic patterns in standing stock are consistently discernable:
one of greater variance, indicating a wider age distribution and a second skewed toward
smaller individuals, indicating greater juvenile mortality. The first pattern persists where
pulsed release or protracted inundation of freshwater occurs. The alternative pattern
dominates in natural settings. The persistence of small oysters with few older
individuals, seen in the latter, is related to the net effect of recruitment, growth rate, and
susceptibility to the disease-causing parasite Perkinsus marinus. Higher salinity and
temperature increase P. marinus prevalence in adult oysters within all estuaries.
Ironically, excessive freshwater input or freshwater pulsing favors disease resistance.
However, despite the lower disease prevalence among adult oysters at upstream
locations, juveniles in these water-managed estuaries experience heavy mortality due to
freshwater releases. Similar patterns are seen in recruitment. Although spat recruit
effectively to upstream locations within the managed estuaries, few of those individuals
survive to reach adulthood.
The location and aerial coverage of reefs best exemplify the effect of water management.
Of the 3 estuaries, Faka Union has the lowest area of reef occurrence (24,300 m2; 1.0 %
coverage) compared to 35,400 m2 (1.7%) in Blackwater and 47,700 m2 (1.6%) in
Henderson. Additionally, the area of densest reef coverage in Faka Union is positioned
downstream relative to Blackwater and Henderson and is now located in a region of the
estuary where the accommodation space for reef growth is most limited. There is greater
intertidal wetted area for reef development further upstream in and around the inner bays
in the region that historically had the best water quality for reef development.
These results have great implications for hydrologic restoration. Because of the above
effects, oyster physiology and distribution is proposed as a performance measure of
restoration effectiveness. Target salinities to generate normal oyster distribution and
physiological state are obtained by comparing altered to pristine estuaries using the
spatial homologue approach or by comparison against hypothetically modeled conditions.
Such approaches have been applied to restoration of SGGE and the Ten Thousand
Islands. Performance measure targets have been established for the western Ten
Thousand Islands by comparing these estuaries with spatial homologues in Fakahatchee
Bay, a pristine estuary just east of the effected region. We predict that the restoration of
sheet flow should improve oyster health, physiology, and ecologic distribution. The
decrease in freshwater point-source discharge into Faka Union Bay that would
accompany improvements in sheet flow should shift the foci of oyster growth,
recruitment, standing stock, and reef distribution back to the middle regions of the
estuary. Consequently, any restoration strategy that augments sheet flow and decreases
freshwater point-source discharge should improve oyster physiology and reef
development. The closer a strategy restores the natural pre-alteration sheet flow
conditions, the better the predicted improvement in estuarine health.
Michael Savarese, Florida Gulf Coast University, Marine Science Program, 10501 FGCU
Blvd South, Ft. Myers, FL 39965, Phone: 239-590-7165, Fax: 239-590-7200,
msavares@fgcu.edu