BULLETIN OF MARINE SCIENCE, 54(3): 602-ti09, 1994

LONG-TERM GULF-TO-ATLANTIC TRANSPORT THROUGH TIDAL CHANNELS IN THE

Ned P. Smith

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-I, respectively. More recent studies of 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 trans- lating cumulative net displacement into cumulative volume transport. Current profiles at an- chor 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-1 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-1 with strongest Gulf-to-Atlantic transport in winter and spring months,

An understanding of advective and diffusive transport processes can be applied to a wide variety of issues of importance in marine science. Transport mechanisms act as a least common denominator relating studies that deal more specifically with larval transport, nutrient transport, suspended sediment transport and pollut- ant transport among other things. Transport processes in the Florida Keys include the wind-driven circulation in shallow-water areas on both the Atlantic and Gulf sides of the keys; the ebb and flood of the tide, especially through the major tidal passes separating the keys; and the poorly understood density currents, arising from the enhanced response to sensible and latent heat fluxes in the shallower waters on the Gulf side of the keys. Lee (1986) has described wind-driven cir- culation between the keys and the reef tract off the Upper Keys near . 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 and found high coherence in along-channel transport. The emerging pattern includes a wind-forced seasonally-varying circulation in the shallow waters on the Atlantic side of the keys. In the Upper Keys, the long- term net flow is northerly in summer months and southerly in winter months. In the Lower Keys along-channel flow toward the northeast is rare. Southwesterly flow is fastest in winter months, when winds are out of the north-easterly quadrant. In summer months, winds are more directly shoreward. Neither the circulation of the shallow waters on the Gulf side of the keys nor the flow through the major tidal channels has been systematically studied and reported. The purpose of my paper is to extend previous work on the regional circulation of the keys in a geographical sense by summarizing results that have come from a series of studies of major tidal passes (Fig. 1). Results of field studies conducted in 1987 and 1988 in the northern part of the Lower Keys are combined with

602 SMITH: CHANNEL TRANSPORT IN FLORIDA KEYS 603

I I ,I ,I , ,, I I I ,I , "," " ,.... '" -. .",; ..,....:. .- ~.'","" ••••• ~., ><'\-" ~ •• . •• }: ---:N1''1-..•.•.B..MT" -_ ..... ,- .

..,------..,-----, o 50 100Km

Figure 1. Map showing locations of tidal channels investigated between 1987 and 1993: Newfound Harbor Channel (N), Bahia Honda Channel (B), Moser Channel (M), Long Key Channel (L) and Channel No. Five (F). The approximate position of the reef tract is indicated by the hatched line that parallels the keys on the Atlantic side. Hawk channel lies between the keys and the reef tract. results from more recent studies conducted as part of the SEAKEYS program (Ogden et aI., 1994) to characterize long-term net flow patterns. Results show a quasi-steady nontidal flow from the Gulf side of the keys to the Atlantic side, however the data base assembled at this time is not sufficient to establish either the long-term variability or its underlying causes. An estimation of the associated volume transport suggests that the flow through tidal channels may represent a flushing mechanism for waters landward of the reef tract on the Atlantic side of the keys.

DATA

The data base assembled for describing long-term net flow through tidal channels includes current meter time series, time series of bottom pressure used to represent water level and flow meter profiles needed to quantify bottom roughness. General Oceanics Mark I and Mark II recording current meters were moored approximately 2 m above the bottom in depths ranging from 4 to 5 m. Sampling stations were generally in mid channel at a point where the channel cross-section was at a minimum between the Gulf and Atlantic sides of the keys. Currents were recorded hourly, following a burst sampling of four readings. A vector-average of the burst reduced high-frequency wave effects. The accuracy of the inclination and direction is ± 0.5° and ± 2°, respectively, according to manufacturer's specifications. Within the range of incli- 604 BULLETIN OF MARINE SCIENCE, VOL. 54, NO.3, 1994 nation angles characteristic of the tidal channels studied, the ± 0.50 accuracy translates into an accuracy of approximately ±0.9 cm·s-I• Current vectors were decomposed into along-channel and across- channel components. The along-channel axis was defined as the heading for which the average of the across-channel components was zero. Only the along-channel components were used in the study. Bottom pressures were recorded using a Sea Data TDR-3 pressure recorder. The resolution is 0.004 decibar (db), according to the manufacturer's specifications. A 1 db change in bottom pressure cor- responds approximately to aIm change in water level. Current profiles were obtained using a General Oceanics Model 2035 Mark III direct readout flow meter. Flow meter readings were calibrated at the surface using a tethered drogue. Profiles were constructed by taking readings at approximately I-m intervals. At each level, current speeds were integrated over a one-minute sampling interval to reduce effects of wave noise and boat motion.

METHODOLOGY

Cumulative net displacement plots have been constructed to show the long-term net flow through the tidal channels. Displacement, DO'is defined to be the product of the hourly along-channel com- ponent current speed and the time interval it represents. Thus, for the m'h hour of the time series

In Dc = L u, d.t, (1) 1=1 where u, is the along-channel component of the ith observation recorded by the current meter. Net displacement is obtained by defining the flow leaving the Gulf to be negative, and values are accu- mulated over the entire study period. Plots show both the flood and ebb of the tide (high-frequency positive and negative displacements, respectively), lower-frequency displacement associated with me- teorological forcing in a variety of forms, and any quasi-steady flow due to regional scale slopes in sea level. The local transport at each channel study site is obtained by extrapolating the measured current to the surface and bottom. Current profiles indicated that the current speed, u, as a function of height above the bottom, z, can be represented by the logarithmic expression

u* Z Uz = -k In-, (2) Zo where u* is the friction velocity, k is the Karman constant (0.41) and Zo is the roughness length. When current speeds were regressed against the natural log of the height above the bottom to determine zo' the correlation coefficient averaged 0.92. The friction velocity was then obtained using (2), and the vertically-integrated transport at the jth anchor station was determined from Z Tj = luzdz = u* [z In(~) - (Z - Zo)j, (3) " k(Z - zo) Zo where Z is the sum of the time-averaged water depth and the time-varying deviation about the mean. Z and u* are functions of time and space; Zo varies in space only. The transport for a given segment was obtained by assuming that the measured water depth and current profile at each anchor station represented the entire segment. The segment transport was then obtained by multiplying the vertically integrated transport by the segment width, Ay, and the total transport was obtained by summing the contributions from each segment:

5 TT = L Tj Ay. (4) j=!

By sub-dividing the channel and occupying anchor stations during both flood and ebb conditions, ebb- dominant and flood-dominant parts of the channel were identified. Temporal variations in tidal current speed occurring during the approximately 2-h time period needed for channel calibration were removed by interpolation. The current speed at each anchor station was adjusted to a common time using measurements made during two visits. The interpolation procedure assumed that the tidal current was in phase through the entire cross-section of the channel, and that the nontidal flow was constant during the calibration. Given the relationship between the surface current speed at the reference station and the surface current speed at each anchor station, and given the water depth and roughness length at each anchor station, current meter time series from the reference station were converted to volume transport for the channel as a whole. SMITH: CHANNEL TRANSPORT IN FLORIDA KEYS 605

E ~ ~E I-- Z -200 W ::> -400 W () I« « .J -400 .J en!l- g; is is -800 l- I- -600 W !!! Z W W > > >= >= -1200 « -BOO « .J :J 5 ::> ::> :J :J () -1000 () -1600 5 3 31 28 26 23 20 5 3 31 28 26 23 20 SE? OCT NOV DEC JAN FEB SE? OCT NOV OEC JAN FEB

E E i!i i!i l- I-- Z W ~ -200 -100 W () « ~ .J .J !l; g; 15 -400 is -200 l- I-- ~. W W Z Z W W > 2: -600 I-- -300 «>= « .J .J ~, ::>:J ~ :J :J () -800 () -400 5 3 31 28 26 23 20 19 3 17 31 14 28 12 26 9 23 7 21 SE? OCT NOV DEC JAN FEB MAR AUG SEP OCT NOV DEC

100 ~ I- Z -100 W::;: W -200 () « ...J -300 en!l- 15 -400 I- W -600 Z w > -600 >= « -700 ...J :J ::;: -BOO :J () -900 1 26 21 16 10 7 2 JUL AUG OCT DEC FEB APR JUN 1992-93

Figures 2-6. Cumulative net displacement diagrams (CNDD) for currents recorded in channels of the Florida Keys; negative values indicate a Gulf-to-Atlantic flow. 2 (upper left) CNDD for Newfound Harbor Channel,S September ]987 to ]2 March ]988; 3 (upper right), CNDD for Bahia Honda Channel,S September ]987 to 21 February 1988; 4 (center left), CNDD for Moser Channel,S Sep- tember 1987 to 3 April 1988; 5 (center right), CNDD for Channel No. Five, 3 August ]990 to 3 January 1991; 6 (lower center) CNDD for Long Key Channel, 1 July 1992 to 21 July 1993.

RESULTS Cumulative Net Displacement.-Three cumulative net displacement diagrams rep- resenting six current meter deployments during the 1987-88 field studies suggest a quasi-steady nontidal outflow of water past the current meters and into shallow waters on the Atlantic side of the keys. The plot for Newfound Harbor Channel is shown in Figure 2 for a 188-day time period from 5 September 1987, to 12 March 1988. The ebb and flood of the tide appear as relatively minor and transient 606 BULLETIN OF MARINE SCIENCE. VOL. 54. NO.3, 1994

Table ]. Harmonic constants of the principal semi-diurnal and diurnal tidal constituents for Bahia Honda ChanneL Computed from near-bottom pressures and mid-depth along-channel current compo- nents measured 17 October to 20 November 1990. Amplitudes (Tj) in decibars or m·s-1; local phase angles (

Tidal constituent

M, S, N, K, 0, P,

Bottom pressure Tj 0.18 0.06 0.04 0.08 0.08 0.03 248 274 226 250 273 250

Along-channel current Tj 0.59 0.14 0.10 0.11 0.10 0.04 234 256 216 199 190 199 features on a curve that decreases steadily with time, The end points of the curve indicate a mean outflow of -0.05 m·s-I• A 68-day continuation of this time series (not shown) indicates a similar pattern, after a break in the record of nearly 5 months. The long-term resultant speed was reduced to -0.03 m·s-I, however. Current meter measurements made over the 161-day time period from 5 Sep- tember 1987 to 21 February 1988 in Bahia Honda Channel are shown in Figure 3. For this time series, the long-term resultant flow past the current meter was -0.11 m·s-I. A second study at the same location during the 160-day time period from 7 May to 15 October 1988 (not shown) produced a resultant current speed of -0.09 m·s-I. A third study, conducted between 17 October and 20 November 1990 (not shown) included bottom pressure measurements (Table 1) but produced similar results. The resultant current was a Gulf-to-Atlantic flow of -0.11 m·s-I. Figure 4 shows the long-term net displacement through Moser Channel for the 21O-day time period from 5 September 1987 to 3 April 1988. The cumulative net displacement diagram indicates a mean flow past the current meter of -0.04 m·s-I for this time series. Results from the follow-up study in Moser Channel (not shown) during the 146-day time period from 23 May to 15 October 1988 indicate a resultant current speed of -0.03 m·s-I. Results obtained from the SEAKEYS program during 1990-93 extend the sur- vey in a geographical sense to include two tidal channels in the Middle Keys. The cumulative net displacement of water past a current meter moored in Channel No. Five during the 154-day time period from 3 August 1990 to 3 January 1991 is shown in Figure 5. The pattern differs somewhat from those described above in the sense that the nontidal outflow of water slows at about the midpoint of the record. From 3 August to 25 September the resultant flow was -0.06 m·s-I; from late September through the end of the record, the resultant flow was only -0.02 m·s-I. A more recent SEAKEYS time series from Long Key Channel (Fig. 6) shows a long-term net outflow past the current meter during the 385-day time period between 1 July 1992 and 21 July 1993. The resultant speed is -0.026 m·s-1, but considerable variability is apparent, including a possible seasonal cycle with max- imum outflow in late winter and spring months, and low-frequency reversals over time scales on the order of 1-2 weeks. The general pattern of a long-term net flow from the Gulf to the Atlantic is maintained, however. Cumulative Volume Transport.-The logical follow-up to characterizing and quantifying flow past a current meter involves the conversion of cumulative net displacement into cumulative net transport. Time series from Bahia Honda Chan- nel and Long Key Channel have been investigated in this way. Results from the 34-day study of Bahia Honda Channel are shown in Figure 7. The cumulative SMITH: CHANNEL TRANSPORT IN FLORIDA KEYS 607

..,...... a E III .•...o c:: ;=:; -400 -o'- a. U) c:: as -800 I-'- CD E :::::l -1200 '0 > CD ;:::> -1600 as "3 E :::::l o -2000 17 24 31 7 14 OCT NOV

Figure 7. Cumulative net volume transport through Bahia Honda Channel, 17 October to 20 Novem- ber 1990. Negative values indicate a Gulf-to-Atlantic volume transport. net value of -1.87·1 09·m3 at the end of the study period is equivalent to a quasi- steady nontidal Gulf-to-Atlantic volume transport of -620 m3·s-l• The similarity of the resultant flow found in Bahia Honda Channel during the 1987-88 studies with the resultant flow found during the 1990 study suggests that the volume transport calculated for the latter study may be more generally representative in time. The role that tidal exchanges alone play in the long-term net transport was quantified using tidal harmonic constants computed from time series of along- channel current components and bottom pressure (Table 1). For tidal pumping alone, the residual volume transport was +78 m3·s-1 into the Gulf of Mexico. Results for Long Key Channel indicate a substantially lower long-term net volume transport, although the Gulf-to-Atlantic net direction is consistent with the direction found for Bahia Honda Channel. For the 385-day study conducted between midsummer months of 1992 and 1993, the resultant outflow from the Gulf side of the keys averaged 262 m3·s-l• This is about 42% of the flow through Bahia Honda Channel, even though the cross-sectional area of Long Key Channel is just over six and a half times greater.

DISCUSSION The significance of the resultant Gulf-to-Atlantic flow stems from the hydro- graphic properties of the water itself, plus whatever might be dissolved or sus- pended in the water column. Ginsburg and Shinn (1964), Shinn (1988), Shinn et al. (1989) and Shinn et al. (1994) called attention to the distribution of coral reefs on the Atlantic side of the keys relative to the locations of major tidal passes. Where tidal channels permit an influx of cold waters in winter months, coral growth is inhibited. Studies by Roberts et al. (1982), Roberts et al. 608 BULLETIN OF MARINE SCIENCE, VOL 54, NO.3, 1994

(1983), Walker et al. (1982), and Walker et al. (1987) documented the formation, transport and effect on coral reefs of cold water forced out of Florida Bay during cold-air outbreaks. Salt transport can also be significant. Studies by Zieman et al. (1994) demonstrated that portions of Florida Bay had average salinities in excess of 50%0 over the past few years. Individual measurements were as high as 70%0. Gulf-to-Atlantic density differences create baroclinic pressure gradients that re- inforce the long-term net transport into Hawk Channel. Lapointe and Clark (1992) reported elevated nutrient concentrations in shallow waters near keys as a result of ground water and surface runoff. The net Gulf-to- Atlantic flow through tidal channels encourages the accumulation and availability of nutrients in Hawk Channel. A major cause for concern stems from the potential impact that elevated nutrient concentrations could have on the coral reef tract, lying 10 km from the keys on the Atlantic side, Results from Bahia Honda Channel and Long Key Channel presented here do not necessarily indicate that the long-term volume transport is from the Gulf side of the keys into Hawk Channel in all other channels as well. However, the sim- ilarity between cumulative net displacement and cumulative volume transport for these two channels suggests that this may be the case generally in the Middle and Lower Keys. Follow-up field studies will be needed to calibrate other major tidal channels and quantify both the total and tidal volume transport. The cause of the quasi-steady net outflow through tidal channels cannot be confirmed with the available data base, but it is likely that spatial gradients in mean sea level play a role. Chew (1982) has reported an 8-9 em drop in mean sea level between and Miami. While a quasi-steady outflow through tidal channels would require a slope perpendicular, rather than parallel to the keys, it is probable that the observed outflow arises for the same reason that the Florida Current leaves, rather than enters the Gulf of Mexico. Namely, water levels in the Gulf of Mexico are higher than water levels in adjacent parts of the Atlantic Ocean. Although datum planes connecting widely separated sites can vary as a result of inaccurate leveling, available data for Cedar Key and Daytona Beach, Florida (Permanent Service for Mean Sea Level, 1977) suggest that mean sea level along Florida's Gulf coast is approximately 30 cm higher than mean sea level along the Atlantic coast. While differences would undoubtedly be less fur- ther south, closer to the tidal channels investigated for this study, it is likely that a Gulf-to-Atlantic slope in sea level is at least partially responsible for the ob- served resultant flow. van de Kreeke et al. (1994) have reported an alternate mechanism that helps explain the net outflow through tidal passes in the Middle Keys. A numerical model involving tidal forcing alone predicts a set-up of water level as damped progressive waves move into Florida Bay. One might expect a fortnightly peri- odicity, according to spring tide and neap tide (or tropic and equatorial tide) conditions, superimposed onto a mean transport. Low-frequency fluctuations do appear in the Long Key Channel data (Fig. 6), but neither time scales nor wind conditions have been explored for this record to estimate the relative importance of tidal pumping as a forcing mechanism.

ACKNOWLEDGMENTS

Support for the 1987-88 studies in the Lower Keys was provided through the National Marine Sanctuaries Program of NOAA's Office of Ocean and Coastal Resource Management. Field studies conducted in the Middle Keys since 1990 have been part of the SEA KEYS program with support from the Florida Institute of Oceanography through a grant from the John D. and Catherine T. Mac- Arthur Foundation. Harbor Branch Oceanographic Institution, Contribution No, 969, SMrfH:CHANNELTRANSPORTIN FLORIDAKEYS 609

LITERATURE CITED

Chew, E 1982. The slope of the mean sea level along the Florida Straits and its dynamical impli- cations. Oceanolog. Acta 5: 21-30. Ginsburg, R. N. and E. A. Shinn. 1964. Distribution of reef-building community in South Florida and the Bahamas. (Abstract) Am. Assoc. of Petrol. Geolog. Bull. 48: 527. Lapointe, B. E. and M. W. Clark. 1992. Nutrient inputs from the watershed and coastal eutrophication in the Florida Keys. Estuaries 15: 465-476. ---, N. P. Smith, P. A. Pitts and M. Clark. 1993. Baseline characterizations of chemical and hydrographic processes in the water column of Looe Key National Marine Sanctuary. Final Rept. to U.S. Dept. of Comm., NOAA Office of Ocean and Coastal Res. Mgmnt., Contract NA86AA- H-CZ071, Wash., D.C., 103 pp. Lee, T. N. 1986. Coastal circulation in the Key Largo Coral Reef Marine Sanctuary. Pages 178-198 in J. van de Kreeke, ed. Physics of shallow estuaries and bays. Springer-Verlag, New York, 280 pp. Ogden, J. C., J. W. Porter, N. P. Smith, A. M. Szmant, W. C. Jaap and D. Forcucci. 1994. A long- term interdisciplinary study of the Florida Keys seascape. Bull. Mar. Sci. 54: 1059-1071. Permanent Service for Mean Sea Level. 1977. Monthly and annual mean heights of sea level. Vol. 2, North, Central and South America. Inst. Oceanogr. Sci., Bidston Observatory, Birkenhead, Merseyside, United Kingdom (unpaginated). Pitts, P. A. 1994. An investigation of near-bottom flow patterns along and across in Hawk Channel, Florida Keys. Bull. Mar. Sci. 54: 610-620. Roberts, H. H., L. J. Rouse, Jr. and N. D. Walker. 1983. Evolution of cold-water stress conditions in high-latitude reef systems: tract and the Bahama Banks. Carib. J. Sci. 19: 55-60. ---, ---, Jr., N. D. Walker and J. H. Hudson. 1982. Cold-water stress in Florida Bay and Northern Bahamas: a product of winter cold-air outbreaks. J. Sed. Petrology 52: 145-155. Shinn, E. A. 1988. The geology of the Florida Keys. Oceanus 31: 46-53. ---, B. H. Lidz and M. W. Harris. 1994. Factors controlling distribution of Florida Keys reefs. Bull. Mar. Sci. 54: 1084. ---, ---, R. B. Halley, J. H. Hudson and J. L. Kindinger. 1989. Reefs of Florida and the . Field Trip Guidebook T 176, 28th International Geological Congress, Am. Geophys. Union, ISBN: 0-87590-648-6, 53 pp. van de Kreeke, J., J. D. Wang, N. Krishnan and D. Smith. 1994. Wind and tide response in Florida Bay. Bull. Mar. Sci. 54: 579-601. Walker, N. D., L. J. Rouse, Jr. and O. K. Huh. 1987. Response of subtropical shallow-water environ- ments to cold-air outbreak events: Satellite radiometry and heat flux modeling. Continental Shelf Res. 7: 735-757. ---, H. H. Roberts, L. J. Rouse, Jr. and O. K. Huh. 1982. Thermal history of reef-associated environments during a record cold-air outbreak event. Coral Reefs I: 83-87. Zieman, J. C., R. Davis, J. W. Fourqurean and M. B. Robblee. 1994. The role of climate in the Florida Bay seagrass dieoff. Bull. Mar. Sci. 54: 1088.

DATE ACCEPTED: October 5, 1993.

ADDRESS: Harbor Branch Oceanographic Institution, 5600 North U.S. Highway I, Fort Pierce, Flor- ida 34946.