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An Investigation of Near-Bottom Flow Patterns Along and Across Hawk Channel, Florida Keys
Abstract
Current and wind data collected during a 124-day study period between August and December of 1991 as part of the SEAKEYS program are used to characterize near-bottom circulation at two study sites widely separated along the longitudinal axis of Hawk Channel, Florida Keys. Currents at both study sites are influenced primarily by local winds and secondarily by the tides. Spectral analysis indicates statistically significant coherence between along-channel wind stress and currents at periodicities of 3-4 days. Tidal co-oscillations are responsible for 17-19% and 5-25% of the total variance of the along-channel and across-channel currents, respectively. Net flows appear to reflect seasonal changes in local winds. During the first half of the study net flow is towards the northeast at a resultant speed of 0.4 cm·s−1 at the Upper Keys site and towards the south, southwest at 2.3 cm·s−1 at the Lower Keys site. During the second half of the study net flow is towards the southwest at both locations, with resultant speeds of 1.0 cm·s−1 at the Upper Keys site and 6.1 cm·s−1 at the Lower Keys site. Evidence suggests that divergence may be occurring throughout the study period. Net inflow to Hawk Channel through tidal channels supports the idea of a divergence in the along-channel flow and probably explains a distinct across-channel motion at the Lower Keys site.
... ...:.:~:::::::::::::::: .. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::~I U~~¢ . .................................................................................. ida Bay with the human populations in the keys and the coral reef tract (Pitts, 1994;Smith, 1994). REGIONAL CIRCULATION PATTERNS. ...
... Circulation in the Lower Keys near Key West has not yet been studied in detail. Note also the water movement into western Florida Bay from the Shark River region west of Cape Sable (Pitts, 1994;Smith, 1994). ...
... Most of the anthropogenically introduced nutrients entering the coastal waters appear to be taken up by nearshore algal and seagrass communities before they reach patch reef areas. Smith (1994) and Pitts (1994) report a seaward deflection in the near-bottom flow in Hawk Channel which may represent a mechanism to move nearshore water offshore. Further work needs to be done to determine whether nutrient-enriched groundwater flow may be reaching the reefs. ...
The SEAKEYS (Sustained Ecological Research Related to Management of the Florida Keys Seascape) program is a research framework which encompasses the large geographic scale and long time scale of natural marine processes and ecosystem variation upon which human impact is superimposed. The need for interdisciplinary long-term research in coastal ecosystems is critical as we anticipate extraordinary resource management obligations and scientific opportunities in the next decade. The core of the program is six instrumented, satellite-linked monitoring stations which span the 220 mile-long coral reef tract and Florida Bay and which, since 1991, have documented the potential impact of summer heating, winter cold fronts, storms, and distant floods. Meso-scale physical oceanographic studies have documented the net flow of water from Florida Bay to Hawk Channel which provides a potential mechanism to link water quality in Florida Bay with the waters of Sanctuary. Water column and sediment nutrient studies have shown elevated nutrient levels in nearshore waters decreasing sharply to low levels near the offshore coral reef tract. There is a potential link of nearshore and offshore via a seaward deflection in the near-bottom flow. Regional nutrient dynamics are complicated by periodic upwelling driven by the Florida Current. A series of long-term photomosaic stations have tracked coral community dynamics for more than 5 years and have indicated a loss of over 40% in coral cover at some sites. This loss may be linked to declining water quality in Florida Bay. As a large marine ecosystem, the new Florida Keys National Marine Sanctuary and adjoining parks and reserves must be studied and managed holistically if human use of the region is to be sustained.
... Previous studies of current variability in the Florida Keys coastal waters indicate that in the cross-shelf direction there are two distinct flow regimes, associated with responses to different forcing mechanisms. The first flow regime consists of the nearshore and Hawk Channel regions, where flow variability is controlled by tidal and local wind forcing (Lee, 1986;Pitts, 1994). Tidal currents are primarily in the alongshore direction (except for areas immediately adjacent to tidal passages between the Keys) and can account for 20 to 50% of the total current variance, but exhibit no significant tidal residual flow (Lee, 1986;Pitts, 1994). ...
... The first flow regime consists of the nearshore and Hawk Channel regions, where flow variability is controlled by tidal and local wind forcing (Lee, 1986;Pitts, 1994). Tidal currents are primarily in the alongshore direction (except for areas immediately adjacent to tidal passages between the Keys) and can account for 20 to 50% of the total current variance, but exhibit no significant tidal residual flow (Lee, 1986;Pitts, 1994). Low-frequency variability, primarily from local alongshore wind forcing, accounts for the remaining 50 to 80% of the total variance. ...
... Low-frequency variability, primarily from local alongshore wind forcing, accounts for the remaining 50 to 80% of the total variance. The wind response appears typical of other shallow coastal zones with long, straight coastlines (Lee and Mayer, 1977;Csanady, 1982) and consists of significant alongshore flow events directly forced by local alongshore winds and are primarily responsible for seasonal mean flows (Lee, 1986;Pitts, 1994). However, the Florida Keys coastline curves from a east-west to northsouth orientation and this can produce an interesting pattern of diverging alongshore flows forced by persistent easterly (westward) winds, with westward currents prevailing in the lower Keys and northward currents more common in the upper Keys (Pitts, 1994). ...
Long-term moored observations of current and temperature variability have been made along the offshore fringes of the Florida Keys reef tract from Carysfort Reef to the Dry Tortugas as part of multidisciplinary studies of larval recruitment processes (SEFCAR: South East Florida and Caribbean Recruitment) and studies of surface transport processes (SFOSRC: South Florida Oil Spill Research Center). These data are used to make robust estimates of magnitudes and patterns of mean flow and temperature fields in coastal waters of the outer shelf, and are compared to spatial patterns of the mean wind field measured at offshore Coastal Marine Automated Network (CMAN) weather stations and to the change in coastline orientation of the Florida Keys. Five years of moored current meter data from a station seaward of Looe Reef are analyzed using CMAN wind records and Acoustic Doppler Current Profile (ADCP) time series made offshore near the Florida Current front, revealing significant annual cycles in coastal current and temperature fields related to atmospheric and Florida Current forcing. These mean flow patterns and annual cycles are discussed in terms of their impact on larval recruitment pathways. The combined influences of downstream flow of the Florida Current, onshore Ekman transports in the upper layer, coastal countercurrents and cyclonic circulation in the Tortugas gyre tends to aid retention and ultimately recruitment of both locally- and foreign-spawned larvae into the Florida Keys. Seasonal cycles of currents and winds favors enhanced larval recruitment in the fall season of persistent northeast winds that can cause a coastal countercurrent over the entire length of the Keys from Key Largo to the Dry Tortugas, combined with seasonal maximum onshore surface Ekman transports and minimum downstream flow in the Florida Current.
... Wo selcctccl a l0 tl window ol-postlarval inllux lirllowins l.low ntoon bccausc the tirlescries analyses on wcekly data showecl that peak irrflux occurs clLrring this pcriocl (see "Results Transport clue to wind-lbrcing"). The 4 ancl 7 cl wind-lirrcing windows are bascd on eviclence litr coupling ol' wincl and surlircc ct-lrrents nearsl.rolc in the Flolida Kcys at liecFrencics ol-3 ro 4 cl ancl 5 to 12 cl (Pitts 1994). We ret.uovccl autocorrelations and perioclicities fl'ont thc pclstlarval inllux clata by fitting an 'autol'egrcssive integratcd moving irvefagc' (ARIMA) rnodel to the abundance clata, and checked the rcsicluals using a X-test for white noise. ...
... H owever, Ward ( | 9it9) recorded peak settlement of P. urgus postlarvae from full moon through thc new-moon period. The ecological signilicance of the timing of influx may be linked to the stronger flooding tides that occur during the new-moon phase (Pitts 1994), although this does not explain why postlarvae do not capitalize on the strong full-moon tides. It is generally assurned that mortillity attribr,rtablc to visual predators is lower dr,rring the darkest lunar phase, and we have conlirmed this in field and mesocosm experiments (Acosta 1997). ...
... Thus, wind movement of surface waters is likely to have a great impact on postlarval supply to the nearshore nursery. Strong atmospheric fronts and their associated winds are most common from late fall through early spring, when postlarval supply is maximum in the lower Florida Keys, and nearshore flow during this period is typically alongshore toward the southwest (Pitts 1994). We therefore suspected that the strongest association between postlarval supply and wind-forcing would be during this period when wind velocity was greatest and from a direction which would facilitate onshore movement of surface waters. ...
We used time-series analysis to identify weekly and annual patterns in the supply of spiny lobster, Panulirus argus (Latreille, 1804), postlarvae to the Florida Keys, USA, over an 8 yr period. We also investigated the relationship between
postlarval influx and wind forcing as a transport mechanism using the complex vector-scalar correlation analysis. Postlarval
supply had a lunar phase periodicity at 4.5 wk intervals, with postlarval abundance peaking between the new moon and first-quarter
lunar phases. A distinct annual cycle of postlarval supply with two peak periods was also apparent. Cross-correlation analysis
between relative postlarval abundance and a 12 mo cycle showed that the annual peak occurs in spring, centered around March.
With the 12 mo periodicity removed, a smaller peak at 5 mo intervals was also well defined. Wind-forcing for 7 d prior to
the time of postlarval collection was marginally correlated with postlarval abundance through the entire time-series; the
association was strongest during the late fall to early spring months. The analysis indicated that postlarval supply was correlated
with winds from the northeast (ca. 45°), which are associated with winter atmospheric fronts. In contrast to results reported
for other spiny lobster populations, these patterns suggest that recruitment of lobster postlarvae to south Florida is predictable
only at a gross level and is presumably affected by the temporally inconsistent structure of regional oceanic gyres and variability
in the timing of lobster spawning in the Caribbean.
... In the case of the Southeast Distal Florida Physiographic Realm, which consists of low-lying limestone uplands and minimally disturbed wetlands along the southeastern flanks of the Florida Peninsula, the morphodynamic zones of Tree Island Biohydrologic Systems, Marsh Prairie Regimes, Mangrove Forest Biomes, and Everglades Swampland Systems are found (Anon, 1990;Davis, 1943;Davis and Ogden, 1994;Douglas, 2007;Finkl, 1994Finkl, , 1995Finkl and Restrepo-Coupe, 2007;Gleason, 1984;Lodge, 2010;Myers and Ewel, 1990;Restrepo et al., 2006;White 1970). In the example of the Biscayne Bay and Card Sound Physiographic Realm, which is a sheltered, shoreparallel estuarine basin, three separate morphodynamic zones were interpreted: Nearshore Banks, Offshore Banks, and Structural Banks (Bader and Roessler, 1971;Hine, 2013;Lirman et al., 2003;McNulty, Work, and Moore, 1962;O'Gower and Wacasey, 1967;Wanless and Tagett, 1989 , 1968;Lidz, 2006;Lidz, Reich, and Shinn, 2003;Lidz et al., 2006;Lighty, 1977;Precht and Miller, 2007 (Ball, 1967;Jaap, 1984;Lidz, 2004;Lidz, Reich, and Shinn, 2003;Pitts, 1994Pitts, , 1997. Further offshore, the FRT Physiographic Realm, a broad, discontinuous shelf-edge Holocene coral reef system, includes Inner Reef, Outer Reef, Intra-Reefal, Backreef, and Forereef morphodynamic zones (Banks et al., 2007(Banks et al., , 2008Finkl, 2004;Gischler, 2010;Hoffmeister, 1974;Jaap, 1984;Lidz, 2004Lidz, , 2006Lidz et al., 1997;Precht and Miller, 2007). ...
... This seaward extension of the gently sloping limestone ledge drops less than 30 cm toward the parabathic clastic shelves of Hawk Channel;. the rock ledge can be uncolonized or contain macroalgae (e.g., Lidz, 2006;Lidz, Reich, and Shinn, 2003;Lidz et al., 2006;Zieman, Fourqurean, and Iverson, 1989) Hawk Channel Shallow water body separating Biscayne Bay, Card Sound, and the Florida Keys from the Florida Reef Tract (FRT) An elongated, shallow basin (approximately 2300 km 2 ) containing exposed bedrock shelves and sandy sediments in planar bed forms and ridges that overlie Pleistocene Limestone; backreef deposits, patch reefs, and aggregated reefs merge into the seaward margins of Hawk Channel and generally define the shoreward flanks of the Florida Reef Tract (FRT), which include coral reefs, intrareefal sand flats, and reef gaps (e.g., Jaap, 1984;Lidz, 2004;Lidz, Reich, and Shinn, 2003;Pitts, 1994Pitts, , 1997 Bay, Card Sound, and parabathic lithic shelves, and seaward by the FRT Morphosedimentary transitional basin containing sandy sediments and other unconsolidated materials that are uncolonized or covered by seagrass meadows (e.g., Lidz, 2004) Florida Reef Tract (FRT) ...
The continental shelf off southeast Florida contains a range of benthic environments that are discernible via remote sensing platforms because of low turbidity in the water column. Using the Geospatially Integrated Seafloor Classification Scheme (G-ISCS), physiographic realms and associated morphodynamic zones were cognitively interpreted and classified at a nominal scale of 1:6000 across four remote sensing platforms (i.e. GeoEye IKONOS-2, Landsat-5 Thematic Mapper UK, Landsat-7 Enhanced Thematic Mapper [ETM], and National Agriculture Imagery Program [NAIP] high-resolution aerial orthoimagery). Attribute tables were created in conjunction with interpretations to quantify and compare spatial relationships between classificatory units and the different remote sensing platforms. Resultant maps exported from ESRI ArcGIS (R) ArcMap software showed that while IKONOS-2 satellite imagery and NAIP aerial orthoimagery provided the greatest detail, classification of physiographic realms and morphodynamic zones was still possible using TM and ETM satellite images. Overall, it was determined that IKONOS-2 provided the most beneficial imagery when applying such a classification of coastal and seafloor features. It is postulated that accurate delineation of physiographic realms and morphodynamic zones can provide a foundation for more in-depth biogeomorpholocial classification (e.g., further subdivision into benthic geoforms, coastal landforms, biological cover) along continental shelves.
... This natural feature is enclosed on the seaward side (~6 km offshore) by numerous patch reefs and eventually the bank reef (~10 km offshore), which breaks the surface at low tide in many locations. Beyond the bank reef the continental shelf drops to a break at approximately 30 m, where the shelf edge plummets hundreds of meters into the Straits of Florida (SOF; Pitts 1994, Lee & Williams 1999. ...
... The finding that larval sphyraenids, gerreids, and lutjanids Lutjanus griseus, which settle to nearshore seagrass and mangrove habitats, also follow this pattern suggests that larvae passing over the bank reef may be lost not only to settlement, but possibly also to predation. Additionally, currents inside Hawk Channel are variable, mostly influenced by tide and wind, and are much weaker than those on the outer shelf (Lee 1986, Pitts 1994. Weaker, more variable currents may have also contributed to the observed difference in larval supply between inshore (WB) and offshore (FR) reefs, as traps over WB presumably sampled less water. ...
To identify temporal and spatial patterns in the supply of late-stage fish larvae to the coral reefs of the upper Florida Keys, 3 replicate light traps that collect larvae in the process of settling were deployed at each of 2 reefs from May 2002 to February 2004. Traps were deployed every other night from May through October and twice monthly from November through April to examine cross-shelf (2002 and 2003) and alongshore (2002 and 2004) patterns. The nightly abundance of settling larvae was compared to concurrently collected environmental data to assess the relationships between these variables and temporal patterns of larval supply. In total, 26 185 fish larvae from 55 families were collected during 174 nights of sampling. Seasonally, the supply of larvae peaked in the late winter to early spring and reached a minimum in late fall. Within-season supply of late-stage larvae was cyclic and strongly related to the lunar cycle, and to a lesser extent, the maximum tidal amplitude cycle. Strong negative correlations between larval supply and both moon illumination and maximum tidal amplitude each night resulted in peak supply of fish larvae between the third quarter moon/minimum amplitude tides and the new moon/maximum amplitude tides. While these cyclic environmental cues provided a high degree of temporal predictability of larval pulses, the magnitude of pulses was stochastic, with some variability related to the passage of mesoscale frontal eddies by the upper Florida Keys. However, the effects of these physical features on larval supply were not consistent over time. Spatial variability (kilometer-scale) was greater in the cross-shelf direction than in the alongshore direction: the supply of larval fishes was significantly greater to the fringing reef site (French Reef) than to the inshore patch reef (White Banks), due to either active larval avoidance of inshore environments or depletion of larvae originating offshore as they pass over and settle on the fringing reef.
... Upwelling enhances phytoplankton biomass and increases primary production as nutrient-rich waters of the upper thermocline shoal into the euphotic zone along the shoreward edge of the Gulf Stream (Yentsch, 1974;Hitchcock et al., 1993). The nearshore region south of the lower Florida Keys and north of the Florida Current is characterized by frontal mixing zones and coastal eddies (Lee et al., 1992;1994;Shay et al., 1998). We hypothesized that elevated abundances of copepod larvae would be present in convergent frontal zones resulting from aggregation of adult and larval copepods in the presence of relatively higher food (chlorophyll a) concentrations. ...
... Objectives of the present study were to measure the surface distribution and abundance of zooplankton larvae in relation to nearshore frontal zones and submesoscale regional circulation patterns in the southern Straits of Florida (SSF). The general regional circulation of the study site includes a nearshore wind-and tide-forced coastal current running westward, parallel to the lower Keys along Hawk Channel at velocities ranging from -2 to 6 cm s ', depending primarily on seasonal changes in wind speed and direction (Pitts, 1994;Lee and Williams, 1999). Offshore, over the shelf break, is the strong, eastward-flowing Florida Current, with velocities of 150 cm s-I (Shay et al., 1998). ...
Near-surface zooplankton samples were collected during an interdisciplinary study of biological responses to oceanographic circulation south of Looe Reef in the southern Straits of Florida (SSF) in late May 1994. Surface currents in the sampling region were monitored during the study with the University of Miami's high frequency (HF) Ocean Surface Current Radar (OSCR). HF-radar was highly effective in resolving submesoscale (<30 km) circulation patterns that would have been difficult to discern using conventional current meters and shipboard surveys, particularly in this coastal region where local circulation patterns undergo rapid temporal transitions. The present study focused on the surface abundance and distribution of copepod nauplii and copepodites. These groups generally remain in the near-surface layer, tend to behave as passive drifting particles and in the case of nauplii, are good indicators of recent egg production. Copepod nauplii and juveniles were found to be most abundant in cyclonic submesoscale eddies (SMEs; 10-12 km) that traversed the frontal-mixing zone between the Florida Current and nearshore coastal water during the field study. The region of greatest zooplankton abundance was also associated with elevated pigment fluorescence. These data suggest that the SMEs in the SSF may enhance biological productivity, probably through a combination of accumulation and retention of zooplankton and upwelling of nutrients. General distribution patterns of numerically dominant copepod taxa were determined and linked with water of Florida Current origin or nearshore coastal water.
... Smith and Pitts (2002), using current meter measurements in the major tidal passes in the Middle and Lower Florida Keys, reported a net long-term Gulf of Mexico to Atlantic flow of seawater. Pitts (1994) showed that these ''outwelled'' net cumulative flows entering Hawk Channel have an offshore deflection, which combined with elevated DIN concentrations from land-based sources, can result in considerable advection of DIN to the offshore reef tract (Pitts, 2002). Shark River Slough and Taylor Slough in Everglades National Park, watersheds that drain upland agricultural areas, can deliver significant loads of nitrogen (up to 4545 mton N/year for Shark River Slough in 1995) during periods of high flows (Rudnick et al., 1999; and this enrichment has been linked to NH 4 + enrichment and the generation of phytoplankton blooms and turbidity in both central and western Florida Bay as well as downstream bank reefs at Looe Key Brand, 2002). ...
... This pattern resulted in a seasonal decrease in the f-ratio, indicating a reduced importance of NO 3 À during summer when the potential for upwelling is maximum (Green, 1944;Lapointe and Smith, 1987;Leichter et al., 2003). Our data support the widely reported pattern of long-term seawater flow in an offshore-not onshore-trajectory in the Lower Keys (Pitts, 1994;Smith, 1994;Smith and Pitts, 2002). A long-term across-shelf flow of f 2.0 cm/s during a 13-month study in the Lower Keys in 1992 -1993 yielded an average DIN net transport rate of 55 AM m À 2 s À 1 toward the outer reef tract (Pitts, 2002). ...
Land-based nutrient pollution represents a significant human threat to coral reefs globally. We examined this phenomenon in shallow seagrass and coral reef communities between the Content Keys (southern Florida Bay) and Looe Key (south of Big Pine Key) in the Lower Florida Keys by quantifying the role of physical forcing (rainfall, wind, tides) and water management on mainland South Florida to nutrient enrichment and blooms of phytoplankton, macroalgae, and seagrass epiphytes. Initial studies (Phase I) in 1996 involved daily water quality sampling (prior to, during, and following physical forcing events) at three stations (AJ, an inshore area directly impacted by sewage discharges; PR, a nearshore patch reef located inshore of Hawk Channel; and LK, an offshore bank reef at Looe Key) to assess the spatial and temporal patterns in advection of land-based nutrients to the offshore reefs. Concentrations of dissolved inorganic nitrogen (DIN=NH4++NO3−+NO2−), soluble reactive phosphorus (SRP), and chlorophyll a increased at PR and LK following a wind event (∼15 knots, northeast) in mid-February. The highest DIN (mostly NH4+) and SRP concentrations of the entire study occurred at the inshore AJ during an extreme low tide in March. Following the onset of the wet season in May, mean NH4+ and chlorophyll a concentrations increased significantly to maximum seasonal values at PR and LK during summer; relatively low concentrations of NO3− and a low f-ratio (NO3−/NH4++NO3−) at all stations during summer do not support the hypothesis that the seasonal phytoplankton blooms resulted from upwelling of NO3−. A bloom of the seagrass epiphyte Cladosiphon occidentalis (phaeophyta) followed the onset of the rainy season and increased NH4+ concentrations at LK, resulting in very high epiphyte:blade ratios (∼3:1) on Thalassia testudinum. Biomass of macroalgae increased at all three stations from relatively low values (<50 g dry wt m−2) in winter and early spring to higher values (∼100–300 g dry wt m−2) typical of eutrophic seagrass meadows and coral reefs following the onset of the rainy season. The mean δ15N value of Laurencia intricata (rhodophyta) during 1996 at AJ (+4.7‰) was within the range reported for macroalgae growing on sewage nitrogen; lower values at the more offshore PR (+3.1‰) and LK (+2.9‰) were at the low end of the sewage range, indicating an offshore dilution of the sewage signal during the 1996 study. However, transient increases in δ15N of Cladophora catanata (chlorophtyta) from ~+2% to +5% at LK concurrent with elevated NH4+ concentrations following rain and/or wind events in May and July suggest episodic advection of sewage nitrogen to the offshore LK station. The Phase II study involved sampling of macroalgae for δ15N along a gradient from the Content Keys through Big Pine Key and offshore to LK in the summer wet season of 2000 and again in the drought of spring 2001. During the July 2000 sampling, macroalgae in nearshore waters around Big Pine Key had elevated δ15N values (~+4‰) characteristic of sewage enrichment; lower values (~+2‰) at LK were similar to values reported for macroalgae in upstream waters of western Florida Bay influenced by nitrogen-rich Everglades runoff. That pattern contrasted with the drought sampling in March 2001, when δ15N values of macroalgae were elevated (+6‰) to levels characteristic of sewage enrichment over a broad spatial scale from the Content Keys to LK. These results suggest that regional-scale agricultural runoff from the mainland Everglades watersheds as well as local sewage discharges from the Florida Keys are both significant nitrogen sources supporting eutrophication and algal blooms in seagrass and coral reef communities in the Lower Florida Keys. Hydrological and physical forcing mechanisms, including rainfall, water management on the South Florida mainland, wind, and tides, regulate the relative importance and variability of these anthropogenic nitrogen inputs over gradients extending to the offshore waters of the Florida Reef Tract.
... It is possible that these populations are highly mixed and lack fine-scale structure. Connectivity between different regions may be aided by the northward flow of the Florida Current or local eddies and wind-driven counter currents in some cases (Drury et al., 2018;Lee et al., 1994;Pitts, 1994). Although increasing the number of loci would not necessarily change our overall findings (Björklund & Bergek, 2009), additional loci and/or larger sample sizes could possibly better distinguish symbionts between these groups and provide a better understanding of the population structure of symbionts in these areas ( Figure A3 in Appendix 3). ...
The symbiotic relationship between dinoflagellate algae in the family Symbiodiniaceae and scleractinian corals forms the base of the tropical reef ecosystem. In scleractinian corals, recruits acquire symbionts either “vertically” from the maternal colony or initially lack symbionts and acquire them “horizontally” from the environment. Regardless of the mode of acquisition, coral species and individual colonies harbor only a subset of the highly diverse complex of species/taxa within the Symbiodiniaceae. This suggests a genetic basis for specificity, but local environmental conditions and/or symbiont availability may also play a role in determining which symbionts within the Symbiodiniaceae are initially taken up by the host. To address the relative importance of genetic and environmental drivers of symbiont uptake/establishment, we examined the acquisition of these dinoflagellate symbionts in one to three‐month‐old recruits of Orbicella faveolata to compare symbiont types present in recruits to those of parental populations versus co‐occurring adults in their destination reef. Variation in chloroplast 23S ribosomal DNA and in three polymorphic microsatellite loci was examined. We found that, in general, symbiont communities within adult colonies differed between reefs, suggesting that endemism is common among symbiont populations of O. faveolata on a local scale. Among recruits, initial symbiont acquisition was selective. O. faveolata recruits only acquired a subset of locally available symbionts, and these generally did not reflect symbiont populations in adults at either the parental or the outplant reef. Instead, symbiont communities within new recruits at a given outplant site and region tended to be similar to each other, regardless of parental source population. These results suggest temporal variation in the local symbiont source pool, although other possible drivers behind the distinct difference between symbionts within O. faveolata adults and new generations of recruits may include different ontogenetic requirements and/or reduced host selectivity in early ontogeny. Although showing fidelity at genus/species level, at the level of genotype/population, symbiont populations within recruits did not reflect the symbiont populations in adults at either the parental or the outplant reef. Instead, symbiont communities within new recruits at a given outplant site tended to be similar to each other, regardless of parental source population. While drivers behind the distinct difference between symbionts within O. faveolata adults and new generations of recruits may include different ontogenetic requirements and/or reduced host selectivity in early ontogeny, our results indicate that variation in the local symbiont source pool likely explains our observations. This change in the local symbiont pools may reflect adaptations in symbionts in response a changing climate, thus suggesting the possibility for a new generation of more resilient corals.
... Although some species may be able to delay metamorphosis and avoid settlement for extended periods (McCormick, 1999), finding a settlement site is ultimately essential to their survival. Because currents in the ocean and around reefs can be quite strong (Hamner and Hauri, 1981;Andrews, 1983;Wolanski and Pickard, 1983;Frith et al., 1986;Cowen and Castro, 1994;Pitts, 1994), larvae may be carried large distances in the several days to months before they are ready to leave the pelagic environment and transition to the adult/ juvenile habitat . Consequently, larvae may need to cover considerable distances in order to locate suitable settlement habitat, requiring substantial sustained swimming capabilities. ...
... Channel generally flows in a southwest direction (Pitts 1994), i.e. toward the lower keys. The water exiting FL Bay is very hot with high salinity in the summer, and very cold in the winter (Manzello et al. 2012(Manzello et al. , 2015b. ...
The goal of this study was to gain an understanding of intra-and inter-specific variation in calcification rate, lipid content, symbiont density, and chlorophyll aa of corals in the Florida Reef Tract to improve our insight of in-situ variation and resilience capacity in coral physiology. The Florida Keys are an excellent place to assess this question regarding resilience because coral cover has declined dramatically since the late 1970s, yet has remained relatively high on some inshore patch reefs. Coral lipid content has been shown to be an accurate predictor of resilience under stress, however much of the current lipid data in the literature comes from laboratory-based studies, and previous in-situ lipid work has been highly variable. The calcification rates of three species were monitored over a seven-month period at three sites and lipid content was quantified at two seasonal time points at each of the three sites. Montastraea cavernosa had the highest mean calcification rate (4.7 mg cm−2−2 day−1−1) and lowest mean lipid content (1.6 mg cm−2−2) across sites and seasons. In contrast, Orbicella faveolata and Porites astreoides had lower mean calcification rates (2.8 mg cm−2−2 day−1−1 and 2.4 mg cm−2−2 day−1−1, respectively) and higher mean lipid contents (3.5 mg cm−2−2 and 2.3 mg cm−2−2, respectively) across sites and seasons. Given the recent Endangered Species Act (ESA) listing of O. faveolata and the relative persistence of M. cavernosa and P. astreoides on a population-scale, this study suggests that the hypothesis that coral lipids are good indicators of resilience may be species-specific, or more complex and interrelated with other environmental factors than previously understood. Additionally, coral lipid storage under benign thermal conditions may differ from lipid storage before, during, and after thermal stress events.
... In their compilation of historical fire events, they found evidence that a wildfire during 1961 was spotted over a short distance (1 km) between two areas on BPK. The characteristics of fire synchrony between keys (widespread on NNK vs. localized on BPK) and the geography of the two islands support an explanation that widespread NNK fires spotted over to BPK, especially given that a westward (easterly) wind prevails during the year (Pitts 1994; Lee and Williams 1999). However, the closest distance between any of the four BPK study sites and NNK is about 5 km, making spotting between islands unlikely over this distance (e.g., Albini 1983; Albini, Alexander, and Cruz 2012). ...
We investigated relationships between climate variability and wildfires in endangered pine rockland communities in the Florida Keys, United States, using fire-scarred samples from the canopy dominant Pinus elliottii var. densa. To test broad-scale, spatiotemporal relationships between wildfires and climate, we compared cross-dated fire-scar chronologies from two islands in the lower Florida Keys, Big Pine Key (BPK) and No Name Key (NNK), to measured values of the Atlantic Multidecadal Oscillation (AMO), North Atlantic Oscillation (NAO), El Niño-Southern Oscillation (ENSO; NIÑO3.4), Pacific Decadal Oscillation (PDO), Interdecadal Pacific Oscillation (IPO), and divisional temperature and precipitation over the period from 1856 to 1956. Large-scale climate anomalies captured by ENSO (NIÑO3.4) and IPO indexes had combined effects on widespread fires. Superposed epoch analysis revealed that widespread fires on BPK occurred during years that were drier than average and when constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the IPO occurred three years and one year prior to fires. Positive phases of the PDO were also significantly associated with widespread fires three years prior to events, but the PDO was not influential one year prior to fires. Although fire years were temporally synchronous between the two islands during the period between 1818 and 1924 (n = 10), we did not find significant relationships between climate and fire on the smaller NNK, which suggests that island size influences the ability to detect broad-scale climate forcing of wildfires in the Florida Keys.
... The gyre does not persist into the upper Keys, and the absence o f such circulation may mean that many spawned larvae in the upper Keys are transported away from reefs before they reach competency to settle. In addition, there is evidence that net flow of near-bottom circulation in Hawk Channel, the inshore channel that runs the length of the Florida Keys, is toward the south, southwest in the lower Keys and toward the northeast in the upper Keys during the summer and early autumn months (Pitts 1994). Thus, if coral larvae move up and down in the water column, they may be able to influence their dispersal rather than drifting like passive particles. ...
Coral recruitment is an essential process that influences community structure and can determine a reef's resiliency and ability to recover from decline. Understanding coral recruitment is particularly important in the face of continuing decline of the world's coral reefs from a variety of both natural and anthropogenic stresses. The goal of this study was to examine coral recruitment in the Florida Keys and to investigate some factors that may influence patterns of coral community structure observed on the reefs. Nine patch reefs from the upper to the lower Florida Keys were surveyed for differences in juvenile coral distribution. In addition, coral recruitment, growth and mortality rates in the upper Keys were measured and utilized to construct a model of juvenile coral population dynamics. Both lower densities and dissimilar taxonomic distribution of recruits were found in the upper Keys region versus the middle and lower Keys. The presence of macroalgae and crustose coralline algae and the availability of topographically complex habitat, all of which have the potential to affect coral recruitment, did not correlate with observed juvenile coral distribution patterns. The only factor examined that correlated with juvenile coral abundance was the presence of adult colonies. The density of Siderastrea spp. recruits increased with increasing benthic cover of Siderastrea spp. Similarly, density and surface area of adult Porites astreoides colonies were positively correlated with juvenile density. Differences in coral community structure could not be attributed to differences in recruitment, growth, or mortality. However, the occurrence of partial mortality varied between sites and years and may be a process that produces reduced coral cover and smaller size class distributions over time. Lastly, coral recruitment to reef restoration sites designed to enhance and speed recovery from vessel groundings was assessed to determine the efficacy of these restoration projects. While all the methods employed were deemed successful based on the equal or increased density and taxonomic richness of recruits compared to reference sites, some techniques attracted higher densities of recruits. Both the material used in the restoration and the orientation of the restoration material were factors that contributed to the effectiveness of the restoration efforts.
... Florida Bay water intrusions on the coral reef-Due to the confluence of easterly currents, Florida Bay is 7 cm higher than the Atlantic Ocean; therefore on average, bay water flows between the keys and out to the Atlantic Ocean (Smith 1994). Historically, this water was primarily hyposaline, and therefore floated over offshore coral reefs in its path; however, in the recent past, this water has been hypersaline and sinks under Hawk Channel (Pitts 1994) as it moves onto coral reefs offshore (Fig. 3). Very little is known regarding the quality, frequency of occurrence, distribution, and transport of Florida Bay water masses into and within the Florida Keys National Marine Sanctuary. ...
Changes in land use and water management practices in south Florida have altered the quality and quantity of freshwater flowing into Florida Bay. By the 1980s, reduced inflow and drought led to an extensive hypersaline phase in the bay. This phase had a drastic effect on benthic communities within the bay and possibly also on coral communities within the bay and the Florida Keys National Marine Sanctuary. Physical oceanographic measurements demonstrate the presence of warm, hypersaline, and turbid water on coral reefs offshore from the Florida Keys, especially near passes which conduct water from Florida Bay to the Atlantic Ocean. To examine the effect of Florida Bay water intrusions on coral reefs, we tested for significant effects of two stressors, elevated temperature and salinity, on coral production, respiration, and survival. Elevated temperatures produce significant reductions in photosynthesis, respiration, and net P:R ratios afte r6ho fexposure, and elevated salinities produce similar results after 30 h. Exposure to both elevated temperature and salinity produces a highly significant (P. 0.01), but short-term, mitigative interactive effect. The combination of the two stressors was less stressful (for the response variables measured) than the sum of the stressors acting independently. After 36 h of exposure, however, the mitigating effect disappeared and corals exposed to the combined stresses did not survive. A three-dimensional response surface, which predicts P:R ratios as a function of varying salinity and temperature, is used to construct a testable hypothesis to explain recent declines in coral cover on some reefs within Florida Bay and the Florida Keys. We chose salinity and temperature to test a multiple stressor model because they are relatively easy to manipulate. However, any enviromentally realistic model must include other potential stressors, such as turbidity, elevated nutrients, and environmental contaminants.
... Thus, these larvae should be able maintain their position for extended times as they near settlement, even against relatively fast currents. Average current speeds on reefs range from around 5 cm s À1 (Pitts, 1994) up to 25-30 cm s À1 (Wolanski and Pickard, 1983). Maximum current speeds may be considerably greater (100 cm s À1 ; Cowen and Castro, 1994); however, if larvae use behavioural mechanisms such as vertical migration to ride favourable currents (Armsworth, 2001), they could significantly alter their dispersal patterns even in areas of high current flow. ...
We examined the maximum sustainable swimming speed of late-stage larvae of nine species of tropical reef fishes from around Lizard Island, Great Barrier Reef, Australia. Larvae were captured in light traps and were swum in flumes at different experimental swimming speeds (of 5 cm s−1 intervals) continuously for 24 h. Logistic regression was used to determine the speed at which 90% of larvae were able to maintain swimming, and this was used to indicate the maximum sustainable swimming speed for each species. Maximum sustainable swimming speeds varied among the species examined, with the lethrinid maintaining the fastest sustainable swimming speed (24 cm s−1), followed by the Pomacentridae (10–20 cm s−1) and the Apogonidae (8–12 cm s−1). U-crit (maximum speed) explained 64% of the variation in sustainable speed among species, whereas total length only explained 33% of the variation in sustained swimming. A regression fitted across species suggests that 50% U-crit is a good approximation of the speed able to be maintained by these larvae for 24 h. A model based on a cubic relationship between sustained swimming time and speed was found to be more successful than either length or U-crit as a method of estimating sustainable swimming speed for most of the species examined. Overall, we found that swimming speed is an important factor when considering the potential for active swimming behaviour to influence dispersal patterns, recruitment success and levels of self-recruitment in reef fish larvae and needs to be carefully considered in models of larval dispersal.
... Because of boundary layer effects, net speeds may be much slower near the substratum. For example, in the Florida Keys, Pitts (1994) recorded speeds between 2.3 and 6.1 cm s -1 measured 4 m above the substratum vs 0.4 to 2.3 cm s -1 2 m above the substratum. In comparison, the average undisturbed swimming speeds of Amphiprion melanopus larvae ranged from 1.7 to 2.9 cm s -1 from hatching to settlement (3.8 to 6.6 cm s -1 maximum) and larval Pomacentrus amboinensis swam between 1.5 and 7.6 cm s -1 (2.9 to 14.9 cm s -1 maximum). ...
Larval dispersal is shaped by the interaction between oceanographic processes and larval behaviour. To evaluate the potential impact of larval behaviour on this process, we quantified the undisturbed swimming speeds and nocturnal swimming activity of 5 reef fish species throughout their larval phase. We used video techniques to obtain undisturbed observations of swimming behaviour in captive bred larvae. The results conclusively demonstrate that larvae maintain relatively high swimming speeds throughout development. Speeds were consistent among 3 anemonefish species (Amphiprioninae; Amphiprion melanopus, A. percula and Premnas biaculeatus), which swam an average of 3.9 and a maximum of 8.4 body lengths (bl) s(-1). However, differences may exist among taxa in the undisturbed swimming speeds of larvae. Highest speeds were recorded in the damselfish Pomacentrus amboinensis (Pomacentridae) and the slowest speeds in the cardinalfish Sphaeramia nematoptera (Apogonidae). The results support short-duration experimental and in situ evidence of high sustained swimming speeds. However, it is striking that larvae routinely swim at such speeds without external stimuli. The proportion of time larvae spent swimming at night increased rapidly towards the end of the larval phase in all 5 species examined. In addition, the undisturbed swimming speeds of larvae were significantly greater at night than during the day. Patterns of nocturnal activity appear to relate to the active nocturnal settlement behaviour of larvae. The pattern of swimming, and speeds achieved, suggest that an active behavioural mechanism for self-recruitment is well within the capabilities of the reef fish larvae examined.
... The marine environment surrounding the lower FK is a dynamic area affected by tides, seasonal winds, and the Florida Current (FC) and its associated meanders and eddies. The net westward water movement inshore of the fringing reef is driven by the prevailing westward winds that persist for most of the year (Pitts, 1994). The combination of these winds and the east-west orientation of the coastline in the lower FK also causes downwelling along the coast, with onshore flow in the upper layers (Lee et al., 1992). ...
... Lapointe et al. (1993) have reported on similar studies in the waters off Big Pine Key. Pitts (1994) has compared current patterns recorded simultaneously off Key Largo and Bahia Honda Key and found high coherence in along-channel transport. ...
Current meter time series collected between 1987 and 1993 from tidal channels in the Middle and Lower Keys quantify long-term net displacement and volume transport between the Gulf of Mexico and Hawk Channel on the Atlantic Ocean side of the Florida Keys. NOAA-supported field studies conducted near Looe Key Marine Sanctuary during autumn, winter and spring months of 1987 and 1988 reveal a quasi-steady nontidal flow into Hawk Channel. Resultant speeds in Newfound Harbor Channel, Bahia Honda Channel and Moser Channel are 0.05, 0.11 and 0.04 m·s−1, respectively. More recent studies of Long Key Channel and Channel No. Five show temporal variability over time scales on the order of 1-2 weeks; seasonal variations are not well defined. The long-term net flow is consistently out of the Gulf of Mexico. Investigations conducted as part of the SEAKEYS program included translating cumulative net displacement into cumulative volume transport. Current profiles at anchor stations under both flood and ebb conditions are used to calibrate two tidal channels. For a 34-day time period in October and November 1990, the resultant volume transport through Bahia Honda Channel is 620 m3·s−l. The tidal contribution to the total is isolated and found to provide a resultant volume transport of 78 m3·s−l in the opposite direction— into the Gulf of Mexico. A similar analysis of a one-year record from Long Key Channel indicates a resultant volume transport of 262 m3·s−l with strongest Gulf-to-Atlantic transport in winter and spring months.
... The tidal passes of the Keys island chain serve as a point of exchange for waters from Florida Bay, the Southwest Florida shelf and the Gulf of Mexico with the waters of the Atlantic coastal environment. One scenario is that bay waters may affect the reef by delivering relatively nutrient rich, hypersaline, turbid, hot, or cool waters to the offshore reef environment (Pitts 1994). The purpose of this study was to quantify water volume transport and total nutrient flux between Florida Bay and Hawk Channel through Long Key Channel in the Middle Keys. ...
There is a net discharge of water and nutrients through Long Key Channel from Florida Bay to the Florida Keys National Marine
Sanctuary (FKNMS). There has been speculation that this water and its constituents may be contributing to the loss of coral
cover on the Florida Keys Reef tract over the past few decades, as well as speculation that changes in freshwater flow in
the upstream Everglades ecosystem associated with the Comprehensive Everglades Restoration Plan may exacerbate this phenomenon.
The results of this study indicate that although there is a net export of approximately 3,850 (±404) ton N year−1 and 63 (±7) ton P year−1, the concentrations of these nutrients flowing out of Florida Bay are the same as those flowing in. This implies that no
significant nutrient enrichment is occurring in the waters of the FKNMS in the vicinity of Long Key Channel. Because of the
effect of restricted southwestward water flow through Florida Bay by shallow banks and small islands, the volume of relatively
high-nutrient water from central and eastern portions of the bay exiting through the channel is small compared to the average
tidal exchange. Nutrient loading of relatively enriched bay waters is mediated by tidal exchange and mixing with more ambient
concentrations of the western Florida Bay and Hawk Channel. System-wide budgets indicate that the contribution of Florida
Bay waters to the inorganic nitrogen pool of the Keys coral reef is small relative to offshore inputs.
... This current flows northeast along the Gulf Stream and is a major influence on water flow along the Florida Keys reef tract, especially in the Upper Keys (Lee et al. 1992; Lee and Williams 1999). Specifically, seasonal winds largely dictate the current direction in the Upper Keys along Hawks Channel and in the vicinity of Molasses Reef, with northern flow in the summer and southward flow the rest of the year (Pitts 1994; Lee and Smith 2002; Willemsen 2005). Thus, the strong downstream effects of the Florida Current, along with wind and eddy driven counter-currents, could result in mixing of Symbiodinium populations in the Upper Keys. ...
Numerous marine invertebrates form endosymbiotic relationships with dinoflagellates in the genus Symbiodinium. However, few studies have examined the fine-scale population structure of these symbionts. Here, we describe the genetic
structure of Symbiodinium type “B1/B184” inhabiting the gorgonian Gorgonia
ventalina along the Florida Keys. Six polymorphic microsatellite loci were utilized to examine 16 populations along the Upper, Middle,
and Lower Keys spanning a range of ~200km. Multiple statistical tests detected significant differentiation in 54–92% of the
120 possible pairwise comparisons between localities, suggesting low levels of gene flow in these dinoflagellates. In general,
populations clustered by geographic region and/or reefs in close proximity. Some of the sharpest population differentiation
was detected between Symbiodinium from deep and shallow sites on the same reef. In spite of the high degree of population structure, alleles and genotypes
were shared among localities, indicating some connectivity between Symbiodinium populations associated with G. ventalina.
... Our results confirmed the existence of two distinct dynamic regimes in the southeast Florida Shelf flow (Lee and Mayer, 1977;Lee, 1986;Pitts, 1994;Lee and Williams, 1999). The flows over the shelf break and in the offshore region were dominated by the FC and its meanders and eddies, while most of Hawk Channel and the inner shelf responded to tides and primarily local winds. ...
Tides, wind and the Florida Current affect the dynamics of the South Florida continental shelf over a range of scales. Two field experiments in the winter (LARGO1) and summer (LARGO2) employed a high-frequency (HF) Ocean Surface Current Radar (OSCR), moored current meters and a boat-mounted Acoustic Doppler Current Profiler (ADCP) to study the circulation over the southeast Florida Shelf. The OSCR measured near-surface currents over an approximately 30 km by 30 km grid, with 1-km horizontal resolution, covering the shelf and extending offshore of the shelf break. By using observed 20-min surface vector field time series to backward track patches with defined volumes of shelf water, their recent origin was identified. The proportion of surface volume flux past various points on the shelf that originated offshore, along the shelf edge and inshore was then estimated. Onshore winds were linked to periods of transport of offshore water all the way to the inner shelf during both the summer and winter seasons. In LARGO2 the exchange of offshore water with water over the middle and inner shelf was closely linked to onshore and downcoast winds. Simulated particle trajectories revealed that Florida Current spin-off eddies caused off-shelf water to be advected onto the middle (LARGO1) or outer (LARGO2) shelf. However, because eddies had short residence times and small spatial scales their contribution to the overall exchange was much smaller than the wind.
... Currents in the mid shelf region of the Keys are mostly tide and wind forced. Subtidal currents in the lower and western Keys are persistent toward the west, whereas in the upper Keys there are seasonal reversals due to seasonal shifts in local winds (Lee, 1986; Pitts, 1994; Lee et al., 2000). Mean currents in the upper Keys are toward the north in summer when the winds are from a southeasterly direction, and shift to southward in the fall, winter and spring as the winds are mostly from the northeast and east. ...
Shipboard measurements of volume transports through the passages of the middle Florida Keys are used together with time series of moored transports, cross-Key sea level slopes and local wind records to investigate the mechanisms controlling transport variability. Predicted tidal transport amplitudes ranged from ±6000 m3/s in Long Key Channel to ±1500 m3/s in Channel 2. Subtidal transport variations are primarily due to local wind driven cross-Key sea level slopes. Subtidal transports through Long Key Channel ranged from +1000 m3/s inflow to Florida Bay to −2500 m3/s outflow to the reef tract. Wind directions oriented toward 190–315° will cause a positive cross-Key sea level slope and inflow to Florida Bay. All other wind directions will tend to cause a negative cross-Key slope of sea level and Gulf to Atlantic outflow toward the reef tract. Seasonal variation in local wind forcing results in maximum outflows from Florida Bay in the winter when increased winds toward the SE and S occur following cold front passages. Minimum outflow occurs in fall when winds toward the SW and W are more frequent and inflows to Florida Bay can persist for several days. The long-term mean flow is toward the southeast and the reef tract and is estimated at –740 m3/s for the combined flows of the major channels in the middle Keys, with 7-Mile Bridge Channel accounting for about 50% of this flow, Long Key Channel carries about 35% of the flow and Channels 5 and 2 account for about 7% each. The mean Gulf to Atlantic flow is supported by sea level standing higher in western Florida Bay and the eastern Gulf than in the Keys Atlantic coastal zone.
... At the southern boundary, the Florida Keys is composed of a vulnerable and delicate archipelago of islands jutting southwest into the Gulf of Mexico. With a wide entrance from the west and a leaky boundary to the Straits of Florida through the Florida Keys, the flow of water between Florida Bay and the Atlantic Ocean is characterized by a net flux of water out of the bay (Smith, 1994;Pitts, 1994). The working hypothesis defining circulation is that wind and wave-driven flux of water enters the estuary from the west, whereas lower water level along the path of the Florida Current produces a pressure gradient conducive to outflow into the Florida Straits (Wang et al., 1994). ...
The carbonate system has been studied in the Florida Bay from 1997 to 2000. Measurements of pH, total alkalinity (TA) and total inorganic carbon dioxide (TCO2) were made from 20 stations in the bay and used to calculate the partial pressure of carbon dioxide (pCO2) and the saturation states of aragonite (ΩArg) and calcite (ΩCal). The results were found to correlate with the salinity. The pH was low and the pCO2 was high for the freshwater input from the mangrove fringe due to the photochemical and biological oxidation of organic material. The TA and TCO2 for the freshwater input are higher than seawater due to the low values of pH and Ω. The pH was high and the pCO2 was low in November in regions where the chlorophyll is high due to biological production. During the summer when the salinity is the highest the normalized values of TA and TCO2 were lower than average seawater, due to the inorganic precipitation of CaCO3 caused by the resuspension of sediments or the biological loss by macroalgae. A transect across the mangrove fringe near the outflow of Taylor Slough shows that PO4 and TA increases as the freshwater enters the Bay. This is thought to be due to the dissolution of CaCO3 in the low pH waters from the bacterial and photo oxidation of plant material.
... Florida reefs are separated from inshore waters by Hawk Channel, a deeper channel that runs the length of the Florida reef tract 1-3 km from shore. It serves as a partial hydrographic barrier between inshore and offshore waters of the Florida reef tract lagoon (Pitts 1994;, and Szmant and Forrester (1996) found that offshore waters had significantly lower nutrient concentrations than inshore waters. The question is whether anthropogenic nutrients are reaching Florida reefs at rates that can alter reef community structure. ...
Coral reefs are degrading worldwide at an alarming rate. Nutrient over-enrichment is considered a major cause of this decline
because degraded coral reefs generally exhibit a shift from high coral cover (low algal cover) to low coral cover with an
accompanying high cover and biomass of fleshy algae. Support for such claims is equivocal at best. Critical examination of
both experimental laboratory and field studies of nutrient effects on corals and coral reefs, including the Elevated Nutrient
on Coral Reefs Experiment (ENCORE) enrichment experiment conducted on the Great Barrier Reef, does not support the idea that
the levels of nutrient enrichment documented at anthropogenically-enriched sites can affect the physiology of corals in a
harmful way, or for most cases, be the sole or major cause of shifts in coralalgal abundance. Factors other than nutrient
enrichment can be significant causes of coral death and affect algal cover, and include decreased abundance of grazing fishes
by fishing, and of grazing sea urchins to disease; grazing preferences of remaining grazers; temperature stress that kills
coral (i.e., coral bleaching) and creates more open substrate for algal colonization; sedimentation stress that can weaken
adult corals and prevent coral recruitment; coral diseases that may be secondary to coral bleaching; and outbreaks of coral
predators and sea urchins that may be secondary effects of overfishing. Any factor that leads to coral death or reduces levels
of herbivory will leave more substrate open for algal colonization or make the effects of even low-level enrichment more severe.
Factors that contribute to an imbalance between production and consumption will result in community structure changes similar
to those expected from over-enrichment. Over-enrichment can be and has been the cause of localized coral reef degradation,
but the case for widespread effects is not substantiated.
Current and wind data collected during the summer of 1992 and the fall and winter of 1993 1994 are used to characterize near-bottom flow patterns at a study site in Hawk Channel southwest of Key West. Currents are dominated by along-channel motions with low-frequency reversals occurring over time scales of 2 3 weeks. A seaward deflection of along-channel flow is apparent in both records. Harmonic analysis indicates that tidal co-oscillations contribute 6 10% and 48 70% of the total variance of the along-channel and across-channel currents, respectively. Spectral analysis indicates a close coupling between along-channel wind stress and along-channel currents at periodicities of 2 3 days. A comparison of across-channel winds and across-channel currents suggests an inverse relationship indicating an upwind return flow of near-bottom currents.
The dispersal of larvae during their time in the pelagic environment is critically important to our understanding of marine populations. Recent publications have highlighted the potential importance of larval behaviour in influencing dispersal patterns of larval reef fishes. However, it has not been clearly established if their abilities are of a magnitude comparable to the potential effects of oceanic processes and whether larval behaviour is sufficient to facilitate self-recruitment, This study presents new data on the swimming speed of late-stage larvae to determine how they can swim relative to oceanic currents. The families examined comprised the Acanthuridae, Siganidae, Lutjanidae, Lethrinidae, Pomacentridae, Chaetodontidae, Nemipteridae, Monacanthidae, Psuedochromidae, Pomacanthidae and Apogonidae. The late-stage larvae of all reef fish families examined were able to swim at speeds greater than the mean transport speeds reported around reefs in most locations. However, even the best-swimming reef fish families could not swim faster than the maximum current speeds reported. Based on new and previously published data it appears that the development of swimming ability can be described adequately (80% of variation explained) as a linear increase from zero at hatching to a species-specific maximum at settlement. Calculations based on this developmental pattern suggest that most reef fish families could substantially influence their dispersal patterns relative to ocean currents for over 50% of their larval phase. For all families examined, swimming behaviour could potentially affect dispersal patterns on a magnitude similar to the dispersing effect of oceanic currents. In addition, the swimming capabilities of several reef fish families have the potential to facilitate active self-recruitment in a range of reef systems.
Like many marine species with meroplanktonic larvae, the Caribbean spiny lobster (Panulirus argus) has a postlarval stage that moves from the oceanic plankton to inshore nurseries only under specific environmental conditions (i.e., at night, in the surface water layer, on the flood tide, and during new moon), presumably to avoid predation or to enhance onshore transport. Using field and mesocosm experiments, we compared predation on planlConic postlarvae swimming at night near the surface and bottom over coastal habitats along typical offshore-inshore transport paths and determined whether predation rates differed between lunar periods (new moon vs. full moon) and with prey density (i.e., predator encounter rates). We also measured predation on transparent (newly settled) and pigmented (nearing metamorphosis) postlarvae sheltering in coral reef, seagrass, and macroalgal habitats during the day. We measured predation on postlarvae swimming near the surface and bottom along typical offshore-inshore transport paths (i.e., coral reefs, coastal lagoon, and bay) by tethering postlarvae to floats that drifted on the nightly flood tide during new moon. To test the hypothesis that new-moon transport of postlarvae may have evolved as a means to avoid higher predation under the bright full moon, we repeated the pelagic tethering experiments at the reef and in the bay during full moon. Mortality was highest over coral reefs regardless of lunar phase, but it was lower nearshore, especially in the bay near the surface and during new moon. Predation on benthic, recently settled transparent postlarvae and pigmented postlarvae (nearing metamorphosis) was also higher when tethered on the reef as opposed to vegetated habitats in the lagoon and bay. In experimental mesocosms, planktivorous fish were equally efficient at consuming postlarvae under new- and full-moon conditions when postlarval density was high, as it is in the constricted water column over the reefs. However, when postlarvae were less dense, mortality was significantly lower during new moon. Collectively, these results indicate that several behavioral traits exhibited by postlarval spiny lobsters, including inshore migration during the darkest lunar phase, use of surface waters, and settlement in vegetated habitats, reduce their risk of predation, particularly in the shallow bay. No single behavioral strategy is universally advantageous across all coastal habitats, but combined, they are an effective means to reduce predation across heterogeneous environments that postlarvae must transit during recruitment.
Current meter data from a 411-day study are used to characterize the movement of water through Tavernier Creek, a tidal channel in the Upper Florida Keys that connects northeastern Florida Bay with the narrow continental shelf on the Atlantic Ocean side of the Keys. The record reveals active tidal and nontidal exchanges. Strongest flood and ebb current speeds commonly reach 50 cm s−1. Low-frequency exchanges are highly coherent with the across-shelf component of local wind stress over time scales in excess of 2.5 days. Bay-shelf exchanges are investigated in four ways. Current measurements made while a drogue was tracked from one end of the creek to the other provide a relationship that can be applied to the time series of current meter data. Results suggest that ocean water reaches the bay end of the creek on 92% of the floods, and bay water reaches the ocean end on 94% of the ebbs. The Eulerian tidal excursion calculated from the amplitude of the M
2 tidal constituent is 1.42 times the length of the channel, and half-tidal cycle Eulerian displacements are commonly 1.5 times the length of the channel. Salinity measurements over a 165-day period document the arrival of bay and ocean water at a study site at about the midpoint of Tavernier Creek. Results suggest that the creek becomes completely flushed after about 450,000 m3 of water have entered from either end. Histograms of ebb and flood volume transports indicate that half-tidal cycle transports are commonly between 800,000 and 1,100,000 m3. The long-term movement of water through Tavernier Creek is a net outflow from Florida Bay. Results support the idea that Tavernier Creek serves as an effective conduit for exchanging bay and ocean water, and especially for draining the northeast corner of Florida Bay.
Measurements of the distribution patterns of nutrients (ammonium, nitrate, orthophosphate, total N and total P) and chlorophyll concentrations were conducted under an interdisciplinary program known as SEAKEYS, initiated because of concern that anthropogenic nutrients may be impacting Florida coral reefs. Samples were collected along transects that extended from passes or canals to 0.5 km offshore of the outermost reefs. Seven of the transects were either in the Biscayne National Park (BNP) and Key Largo (upper keys) or Seven Mile Bridge/Looe Key (upper part of lower keys) areas, which have the best present-day reef development; the two in the middle keys off Long Key were in an area of minimal reef development where passes allow estuarine Florida Bay water to flow onto the Florida reef platform. Off the upper keys, water column concentrations of N and chl a were elevated near marinas and canals (1 M NO3, 1 g/l chl a), but returned to oligotrophic levels (e.g., chl a 0.25 g/l; NO3 0.25 M; NH4 0.10 M) within 0.5 km of shore. Phosphorus concentrations, however, were often higher offshore 0.2 M PO4). Sediment interstitial nutrient concentrations decreased from inshore to the offshore reef areas (e.g., 100 M NH4 inshore to 50 M NH4 offshore) and were comparable to those of some presumably pristine coastal and reef carbonate sediments. Sediment bulk N was higher nearshore and decreased steeply offshore ( 60 g-at N/gm sediment to 20 g-at N/gm sediment, respectively); bulk P concentrations ( 6 g- at P/gm sediment) varied little or exhibited the reverse pattern. Sediment N:P ratios were consistently lower offshore (1–10 vs. 20–40 nearshore). Higher offshore P concentrations are attributed to periodic upwelling along the shelf edge. In the middle keys water column nutrients and chl a concentrations were both higher than those in the upper keys, and there was less of an inshore-offshore decrease than that noted in the upper keys. Sediment nutrients were higher also, and nearshore and offshore areas did not differ. Water column and sediment nutrient concentrations and distribution patterns in the upper part of the lower keys were most similar to those measured in the upper keys. Overall, the present data do not support the contention that reef areas in the upper keys are accumulating elevated loads of land-derived nutrients via surface water flow, but does document moderately elevated nutrient and chl a levels in many developed nearshore areas. Most of the anthropogenic and natural nutrients entering the coastal waters from shore appear to be taken up by near shore algal and seagrass communities before they reach patch reef areas. Further work is needed to determine whether nutrient-enriched ground waters reach the reefs, however these would be expected to cause an enrichment of reef sediments, which was not observed.
Wind, bottom pressure and current meter data are used to describe water level differences and exchanges between Florida Bay and Atlantic shelf waters during the passage of Tropical Storm Gordon on 15 November 1994. The data indicate that winds associated with the approaching storm forced a 30 cm set-up of water levels over the inner shelf creating a shelf-to-bay pressure gradient. During that time, current meters in three tidal channels connecting eastern Florida Bay with the Atlantic recorded a nontidal inflow to the bay that averaged between 17 and 32 cm s−1for 2 days. This represents a 4–6-fold increase in the long-term flow through these channels. A maximum storm-induced inflow of 71 cm s−1was recorded in Long Key Channel. For comparison, the root-mean-square values for the predicted tidal currents in the channels range between 26 and 41 cm s−1. About a day after the storm had passed water levels peaked in Cotton Key Basin, a sub-basin of eastern Florida Bay, at almost 50 cm above the previous 2-month mean and the Atlantic-to-bay pressure gradient reversed. As a result. water exited by the bay through the channels at a rate ranging between −8 and −34 cm s−1for 2 days. A maximum nontidal outflow of −78 cm s−1was observed in Indian Key Channel. Results indicate that Gordon caused significant perturbations on the long-term flow that occurs between Florida Bay and adjacent shelf waters, and storm effects were apparent for about 5 days.
The possibility that the eggs and larvae of southeast Florida fishes can be retained by local hydrodynamic processes is examined using simple idealizations of the local currents, oceanic turbulence, Ekman drift, horizontal larval swimming and larval mortality. The simulated settling rates were greatly affected by the interaction between the hydrodynamics and the degree of spatial heterogeneity in the mortality fields. In the presence of an onshore Ekman drift, for example, settling rates tended to increase unless the mortality rate over the reef was much higher than elsewhere. Overall, the simulations suggest that the meanders, eddies and gyres associated with the Florida current front can retain between 0.07 and 41% of the larvae with planktonic life spans on the order of 1 month. However, only a recirculating gyre enabled large numbers of larvae to settle within a few kilometers of their birth place. Assuming the mortality rate near the reef is 20–90% per day, but lower offshore, between 0.1 and 7% of the larvae retained are likely to settle. This implies that southeast Florida reef fish populations should be able to replenish themselves without relying on upstream sources of recruitment, provided they exhibit normal fecundities. The simulations also suggest that natural levels of variation in the mortality field or local currents can both cause large fluctuations in recruitment.
Several polar continental air masses intruding into the south Florida/northern Bahama Bank region during January 1981 caused record low air temperatures and rapid chilling of extensive shallow-water carbonate systems. Numerous coral kills along the Florida reef tract and massive fish mortalities in Florida Bay were attributable to unusually cold waters generated at this time. Thermal evolution of Florida Bay/Florida reef tract and northern Bahama Bank waters from 8 to 21 January was assessed from thermal infrared data acquired by the NOAA-6 environmental satellite, in situ water temperatures, local meteorological data, and a computerized heat flux model. Field observations and laboratory experiments identify 16C as a thermal stress threshold for most reef corals (Mayor 1915; Davis 1981). Temperaturecorrected digital satellite data indicated that water temperatures below 16C were generated in Florida Bay and on Little and Great Bahama Banks during a 10-day period in January. Lowest temperatures on the Florida reef tract resulted from offshelf transport of Florida Bay water through major tidal channels. Offshelf movement of bay water is driven primarily by strong northerly winds, density gradients, and tidal pumping. Absence of reef development opposite major tidal passes along the Florida reef tract (Ginsburg and Shinn 1964) and aperiodic coral kills along bank margins can be attributed to this process, which has probably had a limiting influence on Holocene reef development in these areas.
Analysis of current meter data obtained in the Key Largo Coral Reef Marine Sanctuary over a 3 year period from 1981 to 1983 indicates that the Sanctuary can be separated into two flow regimes with greatly different flow properties: the deep water outershelf where current and temperature validity is dominated by the Florida Current; and the shallower parts of the Sanctuary including Hawk Channel and the reef tracks, where tidal and atmospheric forcing controls current and temperature variability. (from author's abstract)
During January 1977 three consecutive cold fronts crossed S Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. Coral mortality at Dry Tortugas was up to 91% during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow- water areas and have residence times of several days.-from Authors.
Tidal contributions to a sea‐level record can be reduced by a large factor by taking special weighted averages of hourly values. In the residual record it is possible to recognize changes of as little as a few millimeters taking place in a few days. The method might be useful in other geophysical problems.
The Charnock relation uses roughness length to describe the wind disturbed water surface, which is proportional to the ratio between the square of wind friction velocity and gravitational acceleration. Intrinsic errors and limitations of the Charnock relation are illustrated. A refinement is suggested which proposes a probable nondimensional expression between the roughness length and the wind friction velocity involving not only gravity but surface tension and viscosity. (from author's abstract)
Current meter time series collected between 1987 and 1993 from tidal channels in the Middle and Lower Keys quantify long-term net displacement and volume transport between the Gulf of Mexico and Hawk Channel on the Atlantic Ocean side of the Florida Keys. NOAA-supported field studies conducted near Looe Key Marine Sanctuary during autumn, winter and spring months of 1987 and 1988 reveal a quasi-steady nontidal flow into Hawk Channel. Resultant speeds in Newfound Harbor Channel, Bahia Honda Channel and Moser Channel are 0.05, 0.11 and 0.04 m·s−1, respectively. More recent studies of Long Key Channel and Channel No. Five show temporal variability over time scales on the order of 1-2 weeks; seasonal variations are not well defined. The long-term net flow is consistently out of the Gulf of Mexico. Investigations conducted as part of the SEAKEYS program included translating cumulative net displacement into cumulative volume transport. Current profiles at anchor stations under both flood and ebb conditions are used to calibrate two tidal channels. For a 34-day time period in October and November 1990, the resultant volume transport through Bahia Honda Channel is 620 m3·s−l. The tidal contribution to the total is isolated and found to provide a resultant volume transport of 78 m3·s−l in the opposite direction— into the Gulf of Mexico. A similar analysis of a one-year record from Long Key Channel indicates a resultant volume transport of 262 m3·s−l with strongest Gulf-to-Atlantic transport in winter and spring months.
Lunar barometric tidal determinations (L
2) have been made by the Chapman-Miller method for Zrich (493 m) and Sntis (2000 m) based on 49 years' data. The seasonal variations ofL
2 are much larger at Zrich than on the Sntis summit. However, a comparison with the five other pairs of stations for which tidal determinations at different altitudes, but short horizontal distances, are available do not indicate that this result is as an altitude effect, nor do they show any other systematic differences betweenL
2 at the lower and the higher station. The solar tidal determinations agree well with those made byEggenberger (1944) and fit in with those obtained for other stations. From the difference between the pressure oscillations at Zrich and on Sntis the corresponding meantemperature oscillations of the air column between the two stations has been computed.
Cold-air outbreak induced chilling of shallow bay, bank, and shelf waters of southern Florida and the northern Bahamas was examined using satellite thermal infra-red measurements, in situ measurements, and a shallow-water heat flux model. Vast expanses of shallow waters are rapidly modified by the cold, dry continental air and high wind speeds characteristic of cold-front passages. Although water mass modifications are more rapid in shallow areas, prolonged effects are experienced in deeper shelf regions. Northerly winds accompanying the cold-air outbreak induce a net offshelf circulation, subjecting deeper regions to an inflow of chilled waters generated in shallower areas. Absence of coral reef development along preferential routes for offshelf water movement suggests that these winter processes are a limiting influence to southern Florida and northern Bahamas reef distribution.
Baseline characterizations of chemical and hydrographic processes in the water column of Looe Key National Marine Sanctuary. Final Report to
- B E Lapointe
- D O'connell
LaPointe, B. E. and 1ulie D. O'Connell. 1990. Nutrient couplings between on-site sewage disposal
systems, groundwaters and nearshore surface waters of the Florida Keys. 1. Biogeochem. 10: 289-307.
---and Mark Clarke. ]992. Nutrient inputs from the watershed and coastal eutrophication in the
Florida Keys, U.S.A. Estuaries 15: 465-476.
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Ned P. Smith, Patrick A. Pitts and Mark Clark. 1992. Baseline characterizations of chemical
and hydrographic processes in the water column of Looe Key National Marine Sanctuary. Final
Report to U.S. Dept. of Commerce, NOAA, Office of Ocean and Coastal Resource Management,
Contract No.NA86AA-H-CZ071, Washington, D.C. 59 pp.
Signal processing toolbox for PC-Mallab
- L Little
- Shure
Little, 1. and L. Shure. 1988. Signal processing toolbox for PC-Mallab. The MathWorks, Inc., Cochituate Place 24 Prime Park Way, Natick, MA 01760. 167 pp.
Evolution of cold-water stress conditions in high-latitude reef systems: Florida reef tract and the Bahama Banks
- H H Roberts
- L J Rouse
- N D Walker
Roberts, H. H., L. J. Rouse, 1r. and N. D. Walker. 1983. Evolution of cold-water stress conditions in
high-latitude reef systems: Florida reef tract and the Bahama Banks. Carib. 1. Sci. 19(1-2):
55-60.
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