Our case study has revealed that the shallow patch reef community on the
carbonate platform of the northern Florida reef tract is net calcifying and net
heterotrophic. Indeed, taken as a whole the platform segment studied, covered by
seagrass meadows, bare sand, and scattered patch reefs, is also net heterotrophic, but
exhibits net carbonate dissolution. Spatial extrapolation of community metabolism
based on mapping with airborne hyperspectral and lidar scanning allows us to infer that
within this reef zone, much of the carbonate sediment produced on the patch reef crests
is deposited on the adjacent platform and consumed by dissolution. Therefore,
although the live substrate classes exhibit positive rates for G, the likelihood of future
reef accretion is limited. This finding, based on biogeochemical function, is consistent
with the observed dominance of octocorals over stony corals in the present-day patch
reef community.
Although near the perimeter of the reef system, the carbonate platform in the
northern Florida reef tract was determined to be net heterotrophic, as are all it’s
component biotopes. In the classical paradigm (Hatcher, 1990), this platform
immediately landward of the northern Florida bank-barrier reef zone might be expected
to be a net autotrophic shallow reef system perimeter region. The expected high rates
of P g at this outer reef system location would result in a P/R ratio in excess of 1, with
the advection of excess production towards more closed backreef sinks that are in close
mixotrophic balance (Hatcher, 1990). Based on our determination of a P/R ratio less
than 1, we infer that metabolism on the carbonate platform of the Florida reef tract is
dominated by external inputs.
Although this location is near a large urban population, the net autotrophy that
might be expected to result from associated eutrophication does not exist. Accordingly,
we infer that at the study site the inward advection of inorganic nutrients is not the
dominant forcing mechanism for benthic biogeochemical function. Indeed, the
observed net heterotrophy suggests that influxes of organic detritus, most likely of land
origin, followed by in situ remineralization, is driving overall benthic metabolic
function. This result implies that nutrient loading is occurring at the substrate/water
column interface. Therefore, the monitoring of water column nutrient concentrations
should be focused at the benthic boundary layer to be of maximal value in the
management of this reef system zone.
6. Conclusions
Coral reefs are extremely high in habitat complexity, with a diverse assemblage of
plants and animals embedded across a substrate that is convoluted topographically
(Hatcher, 1990). They function as highly efficient carbon uptake and recycling systems
based on intimate interactions between resident organisms. Although this web of
interlocking physical, chemical, and biological processes is quite dynamic (Sargent and
Austin, 1949), significant amounts of organic material are normally not accumulated
over time periods longer than a single diurnal cycle (Buddemeier and Keinzie, 1976;
Hatcher, 1990). A rigorous, holistic approach to the assessment of entire reef systems
is needed for the timely appraisal of pollution effects or generalized endemic stress
(Grigg and Dollar, 1990; Wilkinson et al., 1999). Excess organic-carbon production
(E), the integration of net community production over time, and net calcification (G),
are all key rate variables that describe the status of coral reef ecosystems (Hatcher,
1997a,b). The integration of metabolic process measurements and remote sensing
within a spatially distributed model can enable estimation of E and G scaled up over
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Brock, Yates and Halley
carbonate platform of the northern Florida reef tract is net calcifying and net
heterotrophic. Indeed, taken as a whole the platform segment studied, covered by
seagrass meadows, bare sand, and scattered patch reefs, is also net heterotrophic, but
exhibits net carbonate dissolution. Spatial extrapolation of community metabolism
based on mapping with airborne hyperspectral and lidar scanning allows us to infer that
within this reef zone, much of the carbonate sediment produced on the patch reef crests
is deposited on the adjacent platform and consumed by dissolution. Therefore,
although the live substrate classes exhibit positive rates for G, the likelihood of future
reef accretion is limited. This finding, based on biogeochemical function, is consistent
with the observed dominance of octocorals over stony corals in the present-day patch
reef community.
Although near the perimeter of the reef system, the carbonate platform in the
northern Florida reef tract was determined to be net heterotrophic, as are all it’s
component biotopes. In the classical paradigm (Hatcher, 1990), this platform
immediately landward of the northern Florida bank-barrier reef zone might be expected
to be a net autotrophic shallow reef system perimeter region. The expected high rates
of P g at this outer reef system location would result in a P/R ratio in excess of 1, with
the advection of excess production towards more closed backreef sinks that are in close
mixotrophic balance (Hatcher, 1990). Based on our determination of a P/R ratio less
than 1, we infer that metabolism on the carbonate platform of the Florida reef tract is
dominated by external inputs.
Although this location is near a large urban population, the net autotrophy that
might be expected to result from associated eutrophication does not exist. Accordingly,
we infer that at the study site the inward advection of inorganic nutrients is not the
dominant forcing mechanism for benthic biogeochemical function. Indeed, the
observed net heterotrophy suggests that influxes of organic detritus, most likely of land
origin, followed by in situ remineralization, is driving overall benthic metabolic
function. This result implies that nutrient loading is occurring at the substrate/water
column interface. Therefore, the monitoring of water column nutrient concentrations
should be focused at the benthic boundary layer to be of maximal value in the
management of this reef system zone.
6. Conclusions
Coral reefs are extremely high in habitat complexity, with a diverse assemblage of
plants and animals embedded across a substrate that is convoluted topographically
(Hatcher, 1990). They function as highly efficient carbon uptake and recycling systems
based on intimate interactions between resident organisms. Although this web of
interlocking physical, chemical, and biological processes is quite dynamic (Sargent and
Austin, 1949), significant amounts of organic material are normally not accumulated
over time periods longer than a single diurnal cycle (Buddemeier and Keinzie, 1976;
Hatcher, 1990). A rigorous, holistic approach to the assessment of entire reef systems
is needed for the timely appraisal of pollution effects or generalized endemic stress
(Grigg and Dollar, 1990; Wilkinson et al., 1999). Excess organic-carbon production
(E), the integration of net community production over time, and net calcification (G),
are all key rate variables that describe the status of coral reef ecosystems (Hatcher,
1997a,b). The integration of metabolic process measurements and remote sensing
within a spatially distributed model can enable estimation of E and G scaled up over
126
Brock, Yates and Halley