Florida Keys Reef Tract Research Articles

Geomorphology of the Southeast Florida continental reef tract (Miami-Dade, Broward, and Palm Beach Counties, USA)

This paper describes the geomorphology of the northern, relict, portion of the Florida reef tract that extends along the southeast Florida (USA) continental coast northward to latitude N26° 43.1′. The descriptions are based on high-resolution Laser Airborne Depth Sounding bathymetric data along Miami-Dade, Broward, and Palm Beach Counties as well as chirp subbottom-profiles and reef cores. The SE Florida continental reef tract consists of relict Holocene reefs and lithified sand ridges that apparently formed during backstepping of the reefs in response to sea-level rise. The outer reef is a linear structure that is more or less continuous from Miami-Dade to Palm Beach County and as such is one of the longest continuous reef structures in the western Atlantic. Its relation with the modern Florida Keys reef tract cannot be clearly resolved without further study. The outer reef may be equivalent to the shelf-edge reefs of the Florida Keys. It is broken by reef gaps that likely represent paleo-river channels cross-cutting the underlying Pleistocene substrate. The upward growth of the outer reef ceased approximately 8,000 cal BP (calibrated 14C age before present). A middle reef may represent a remnant shoreface with continuous reef framework. Its growth ceased approximately 3,700 cal BP. An inner reef of Acropora palmata framework is perched on shoreline deposits. It ceased growth approximately 6,000 cal BP. Several nearshore ridges consist of coquina and carbonate/quartz sandstone.

Nitrogen and oxygen isotopic signatures of subsurface nitrate seaward of the Florida Keys reef tract

Hydrographic and nutrient structure and the nitrogen and oxygen isotopic compositions (d15N and d18O) of the nitrate pool seaward of the Florida Keys reef tract were measured in Summer 2003. Nitrate and soluble reactive phosphate concentrations were tightly associated with depth and temperature and were close to 0 in surface waters, increased to 2–10 µmol L−1 and 0.1–1.0 µmol L−1, respectively, at intermediate depths associated with the thermocline and 15–25 µmol L−1 and 1.0–1.5 µmol L−1, respectively, in deeper water. Ammonium concentrations were low (0–0.6 µmol L−1) at all depths. Dissolved organic carbon concentrations ranged from 70–120 µmol L−1 near the surface to 40–60 µmol L−1 below the thermocline. Mean nitrate δ15N was 4.78% (±0.05) overall and varied from 4.24%(±0.07) within the chlorophyll maximum layer at 50–70 m depth to 5.26% (±0.05) below the thermocline at 150–242 m depth. Mean δ18O mean was 3.26% (±0.08) and varied from 4.89% (±0.14) within the chlorophyll maximum layer to 3.02% (±0.07) below the thermocline. These observations suggest an oceanic, deep‐water nitrate source below the thermocline, a mix of sources from upwelling and N2 fixation, and the effects of fractionation associated with uptake by primary producers within the thermocline. The water column nitrate isotopic data support the general hypothesis that primary producers on the Florida Keys reef tract directly utilize the transiently available subsurface nitrate pool.

The short-term influence of herbivory near patch reefs varies between seagrass species

The coexistence of multiple species within a trophic level can be regulated by consumer preferences and nutrient supply, but the influence of these factors on the co-occurrence of seagrass species is not well understood. We examined the biomass and density responses of two seagrass species in the Florida Keys Reef Tract to grazing pressure near patch reefs, and evaluated how nutrient enrichment impacted herbivory dynamics. We transplanted Halodule wrightii (shoalgrass) sprigs into caged and uncaged plots in a Thalassia testudinum (turtlegrass) bed near a patch reef. Nutrients (N and P) were added to half of the experimental plots. We recorded changes in seagrass shoot density, and after three months, we measured above- and belowground biomass and tissue nutrient content of both species. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and belowground biomass of nearly an order of magnitude. T. testudinum shoot density and belowground biomass were not affected by herbivory, but aboveground biomass and leaf surface area were higher within cages, suggesting that although herbivory influenced both seagrass species, T. testudinum was more resistant to herbivory pressure than H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and N:P was near the 30:1 threshold that suggested a balance between N and P. The different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.

Fragment generation, survival, and attachment of Dictyota spp. at Conch Reef in the Florida Keys, USA

During the past decade, the relative abundance of the brown macroalgae Dictyota spp. has been high in the Florida Keys. Recent studies have shown that members of this genus successfully reproduce via vegetative fragmentation. To investigate the importance of fragmentation on the reef community, this study examined: (1) the degree of epiphytism on benthic organisms, (2) the rate of fragment production through fish foraging activities, (3) the likelihood of fragment entanglement, and (4) the fragment attachment and success rate. It was found that reef fish contributed substantially to the fragment pool; furthermore, most fish-produced fragments produced rhizoids and attached to sand grains within 24 h in the field. Fragments of Dictyota spp. most commonly became entangled around and then attached themselves to the green alga Halimeda tuna, and other Dictyota spp. These results suggest that vegetative fragmentation of Dictyota spp. plays an important role in the changing community structure on the Florida Keys reef tract.

Synoptic water clarity assessment in the Florida Keys using diffuse attenuation coefficient estimated from Landsat imagery

The diffuse attenuation coefficient, K (m− 1), is a measure of the effective attenuation of light in the water column. It characterizes water clarity and is used as a proxy for water quality. Mapping of shallow water benthic habitats using optical means, including daytime visible satellite imagery, requires knowledge of K to correct for water column effects such as light absorption and scattering. Traditionally, K is derived from imagery using a priori knowledge of bottom types at different depths and specific locations, and assuming that light attenuates exponentially with depth. This technique is applied to three Landsat 7 satellite images (February, May, and July 2000) from the Florida Keys Reef Tract between Key Largo and Key West. Interpolated depth data, initially from NOAA vector chart data, with uncertainties of ±0.5 m, were draped over 30 m spatial resolution Landsat satellite imagery. K was derived for 27 sites where bright sand could be observed at depths between 3 and 20 m. The blue and green bands (Landsat bands 1 and 2 at 450–520 nm and 520–600 nm, respectively) provided K values consistent with time and location. Average K values for bands 1 and 2, respectively, were 0.029 and 0.043 m− 1 (Lower Keys), 0.050 and 0.072 m− 1 (Middle Keys), and 0.063 and 0.082 m− 1 (Upper Keys). The red band (band 3, 630–690 nm) provided more ambiguous and erroneous results with several negative K values, attributed primarily to three factors use of a simple atmospheric correction, the high absorption and rapid attenuation of red light in water, and low radiometric sensitivity of the Landsat ETM + sensor. Despite the fact that these observations were a snapshot in time, trends were observed for the Upper, Middle, and Lower Keys, possibly due to the influence of more turbid Florida Bay waters.

Analysis of Coral Mucus as an Improved Medium for Detection of Enteric Microbes and for Determining Patterns of Sewage Contamination in Reef Environments

Traditional fecal indicator bacteria are often subject to a high degree of die-off and dilution in tropical marine waters, particularly in offshore areas such as coral reefs. Furthermore, these microbes are often not associated with human waste, and their presence may not be indicative of health risk. To address the offshore extent of wastewater contamination in the Florida Keys reef tract, we assayed coral surfaces for the presence of human-specific enteric viruses. The overlying water column and surface mucopolysaccharide (mucus) layers from scleractinian corals were sampled from three stations along a nearshore-to-offshore transect beginning at Long Key in the middle Florida Keys, USA. Samples were assayed for standard bacterial water quality indicators (fecal coliform bacteria and enterococci) and for human enteroviruses by direct reverse transcriptase-polymerase chain reaction (RT-PCR). The concentration of the bacterial indicators was greatest at the nearshore station in both the water column and corals, and decreased with distance from shore; no indicator bacteria were detected at the offshore station. Whereas human enteroviruses were not detected in any of the water column samples, they were detected in 50–80% of coral mucus samples at each station. These data provide evidence that human sewage is impacting the reef tract up to ~6.5 km from shore in the middle Florida Keys and that coral mucus is an efficient trap for viral markers associated with anthropogenic pollution.

Taphonomic Alteration of Reef Corals: Effects of Reef Environment and Coral Growth Form II: The Florida Keys

Abstract In a companion study to earlier work in the Indo-Pacific, taphonomic alteration in reef-coral death assemblages was assessed in four distinct reef habitats ranging from 2–30 m water depth in the Florida Keys reef tract. Physical and biological taphonomic attributes measured from coral specimens showed great variability with respect to reef environment. Physico-chemical degradation (abrasion and dissolution) was greatest in reef-crest and patch-reef environments. With the exception of encrusting foraminifera, coverage by epi- and endobionts was higher in deep-reef environments (20 m and 30 m). Variability in dissolution and abrasion is likely the result of the different energy regimes present in the reef habitats examined. Variability in biological attributes results from a combination of increased residence time of coral skeletons on substrates in deep-reef environments, higher overall coral skeletal densities of corals inhabiting deep reef environments, and increased nutrient availability in the...

Episodic nutrient transport to Florida coral reefs

This study documents the changes in nutrient fluxes associated with internal tidal bores arriving on Florida Keys coral reefs and points to biological use of subthermocline nitrate brought onshore by this mechanism. Internal bores on Conch Reef, Florida Keys, are associated with concentrations of 1.0–4.0 µmol L−1 nitrate (NO3− ) and 0.1–0.3 µmol L−1 soluble reactive phosphate (SRP) and onshore flow velocities of 0.1–0.3 m s−1. The arrival of internal bores causes 10–40 fold increases in nutrient concentrations and 1–2 orders of magnitude increases in nutrient flux relative to ambient, nonbore conditions. The magnitude and duration of cool‐water nutrient transport events increases significantly with increasing depth on reef slopes. In June 2001, the gradient of increased exposure to subsurface water with depth corresponded to increased percentage of N and δ15N and decreased C:N ratio in a common benthic macroalga, Codium isthmocladum. Internal tidal bores are widespread throughout the Florida Keys reef tract, with cool‐water episodes influencing reefs up to 10%–25% of the time during summer months and with significant variability among years. Estimated inputs of nitrogen and phosphorus by internal tidal bores to Florida Keys reef slopes are as much as 40‐fold larger than published estimates of inputs to near‐shore waters from waste water and storm water runoff. Internal tidal upwelling represents an important, previously underestimated, episodic source of nutrients on the Florida Keys reef tract. In order to assess nutrient availability in this system accurately it is essential to understand natural sources of high‐frequency variability.

Depth-dependent photoacclimatization to solar ultraviolet radiation in the Caribbean coral Montastraea faveolata

The importance of solar ultraviolet radiation (UVR, 290 to 400 nm), and UVB (290 to 320 nm) in particular, as an environmental factor affecting the biology and ecology of coral reefs has taken on renewed interest since the demonstration of global stratospheric ozone loss through human activities. The hermatypic coral Montastraea faveolata occurs over a wide bathymetric range in the Florida Keys reef tract. The bathymetric range of M. faveolata and its role in reef community structure make it an important species for which to assess the effects of present day UVR irradiances. Both UVR irradiances and UVR-absorbing mycosporine-like amino acids (MAAs) decreased significantly with increasing depth in M. faveolata. UVB irradiances were measured to a depth of 30 m during this study and maximum rates of productivity in M. faveolata were significantly affected by the presence of UVR. Action spectra (= biological weighting function) for the photoinhibition of photosynthesis by UVR were measured for samples of M. faveolata at 3, 10, 18, 23, and 30 m. Using these action spectra, radiation amplification factors (RAFs) were calculated for corals at the same depths. RAFs for M. faveolata suggest that corals at depths greater than 3 m will be more sensitive to increases in UVB irradiance, such as might be caused by ozone depletion, than their shallow-water counterparts. Despite this increase in sensitivity to UVB, calculations incorporating continued ozone depletion suggest that at the present rate of stratospheric ozone depletion corals deeper than 5 m in the Florida Keys will be affected very little, if at all, by increases in UVR. These results, however, do not incorporate the interacting and synergistic effects of UVR with other physical parameters (e.g. temperature) that will be essential to understanding and predicting the fate of coral reefs under conditions of global change.

Sediment retention in encrusting Palythoa spp.--a biological twist to a geological process

Carpet sea anemones of the genus Palythoa are common inhabitants of reef crest environments in the Florida Keys reef tract. Through a unique assimilation mechanism, Palythoa spp. entomb carbonate sediment within their tissues. The amount of sediment assimilated is significant, averaging almost 45% of wet tissue weight. Palythoa spp. assimilate all available minerals on the reef. Aragonite, magnesium calcite, calcite and minor quantities of siliciclastic components are all assimilated in proportions comparable to their content in adjacent sediment sinks. There is also no preference in terms of skeletal composition; coral grit, coralline red algae, Halimeda and other allochems are all equally assimilated into Palythoa spp. tissue. The only preference is particle size. Sediment extracted from tissue samples is generally ?125 μm in size, far finer than ambient sediment found adjacent to Palythoa spp. colonies (predominantly >500 μm). Much of the finest sediment extracted from Palythoa spp. tissue is composed of elongated crystal aggregates of aragonite. These particles appear to have been produced in situ through biologically influenced mineralization. Aggregates nucleated on exogenous sediment and attained their elongated form as assimilation proceeded. When Palythoa spp. colonies die, the assimilated sediment and the crystal aggregates are released back into the reef environment. The eventual fate of this material has yet to be determined.

Sea Level and Geostrophic Current Control on Carbonate Shelf-Slope Depositional Sequences and Erosional Patterns, South Florida Platform Margin

High-resolution seismic reflection profiles across the shelf-slope margin between the Dry Tortugas and Key West, Florida, indicate that sea-level fluctuations and the eastward flowing Florida Current are major controls on late Quaternary sequence stratigraphy. The study area, a transition zone between the open south Florida shelf and the lower Florida Keys island/reef system, is typified by a shallow shelf with reef margin adjacent to a deeper lower-shelf/slope. The lower-shelf/slope is composed of stacked or prograding sequences that downlap and pinchout on the Pourtales Terrace. Strike oriented stratigraphic sections exhibit many sea-level controlled features such as lowstand erosion, transgressive unconformities, and highstand system tracts. Lowstand reefs, notches, or barriers are observed as deep as 150 m below present sea level. Depositional styles change along-slope from west to east. The western portion of the study area is characterized by thick, low-amplitude, prograding sequences related to abundant supply of sediment through off-shelf transport during high sea-levels as well as along-slope reworking by Florida current. Part of this section has been severely eroded by along-slope currents producing localized cut-and-fill structures and widespread erosional unconformities. To the east, a thinner section of high-amplitude reflections is common seaward of the lower Florida Keys reef tract system.more » This study provides a new evidence of how strong geostrophic boundary currents along with fluctuating sea levels have interacted to control depositional sequences on a carbonate slope in the Florida/Bahamas platform complex.« less

A New Holocene Sea Level Curve for Upper Florida Keys and Florida Reef Tract: ABSTRACT

A new Holocene sea level curve for the upper Florida Keys and Florida reef tract has been constructed by integrating existing and new data from 14C age analyses. New data are derived from 21 mangrove peat samples from 5 locations and 3 laminated CaCO3 soilstone crust (caliche) samples from 3 locations. The new sea level curve is based on 14C ages ranging from 360 ± 60 y.B.P. to 14,000 ± 160 y.B.P., and indicates a fluctuating sea level rise of approximately 0.3 mm/yr (from 14,000 to 7,000 y.B.P., sea level rose from 9.2 to 7.0 m, 30.2 to 23 ft, below MSL), approximately 1.2 mm/yr (from 7,000 to 2,000 y.B.P., sea level rose from 7.0 to 0.75 m, 23 to 2.5 ft, below MSL), and approximately 0.3 mm/yr (from 2,000 y.B.P. to present, sea level rose from 0.75 m, 2.5 ft, below MSL to present MSL). No evidence was found in this area that, during the last 14,000 yr, any highstand was greater than the present sea level. The rate of rising sea level, however, has varied. Sea level stand in this area at 14,000 y.B.P. is much shallower than indicated on other published curves for the east coast of the United States. End_of_Article - Last_Page 522------------