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Pulsed Delivery of Subthermocline Water to Conch (Florida Keys) by Internal Tidal Bores Author(s): James J. Leichter, Stephen R. Wing, Steven L. Miller, Mark W. Denny Reviewed work(s): Source: Limnology and , Vol. 41, No. 7 (Nov., 1996), pp. 1490-1501 Published by: American Society of Limnology and Oceanography Stable URL: http://www.jstor.org/stable/2838529 . Accessed: 29/03/2012 21:28

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http://www.jstor.org Limnol.Oceanogr., 41(7), 1996, 1490-1501 ? 1996, by the American Society of Limnologyand Oceanography,Inc. Pulseddelivery of subthermoclinewater to ConchReef (FloridaKeys) by internaltidal bores

JamesJ. Leichter StanfordUniversity, Hopkins Marine Station,Pacific Grove, California93950

StephenR. Wing Wildlife,Fish and ConservationBiology, University of Californiaat Davis 95616

StevenL. Miller National Undersea Research Center,University of North Carolina at Wilmington28403

Mark W. Denny StanfordUniversity, Hopkins Marine Station

Abstract Internaltidal boresgenerated by breakinginternal waves cause dramatic,high-frequency variation in ,salinity, water velocities, and concentrationof chlorophylla on ConchReef, Florida Keys. The arrivalof bores on thereef slope is linkedto a semidiurnalinternal and is markedby temperature drops of up to 5.4?Cand salinityincreases of up to 0.60ooin 1-20 min.These changesare accompaniedby the suddenonset of upslopeflow 1-15 m above thebottom with speeds of 10-30 cm s-'. Cool, high-salinity wateris transportedfrom below the seaward of the reef and is residenton thereef slope for up to 4 h beforeit mixeswith surface waters and recedesdownslope. Compared with ambient surface water, thisdeep watercan containsignificantly elevated of dissolvednitrate. Physical variability producedby this mechanism increases significantly with depth on thereef slope. Analysis of 3-yr temperature recordsindicates the arrival of internal bores is a consistentfeature at thissite from May through November, withpeak activity in July-September.Pulsed delivery of subthermocline water appears to significantlyaffect thetemperature, nutrient, and particleflux regimes on thiscoral reef.

The sources, dynamics,and consequences of physical cause reefsflourish in environmentswith limited variabilityin coral reefecosystems have interestedbiol- seasonal variabilityand low nutrientconcentrations, in- ogistfor at least 150 years.For example, Darwin's (1962 termittentdelivery of deep, nutrient-richwater may sig- [ 1842]) observationthat reefs grow fastest near shelfedges, nificantlyaffect temperature and nutrientregimes (Wo- Goreau's (1959) descriptionof the effect of wave exposure lanskiand Pickard 1983; Wolanski 1994). Similarly,tran- on coral species distributions,Connell's (1978) formu- sientmovement of plankton-richwater onto a reefcould lation ofthe relationship between disturbance and species be an importantsource of suspendedparticles and larvae. diversity,and manyrecent studies of physical disturbance This studyexamines internaltidal bores as a mechanism in reef ecosystems(see Hughes 1993) all recognize the of persistent,high-frequency physical variabilityon the fundamentalimportance of temporal and spatial envi- slope of (Florida Keys). ronmentalvariability. Single factorsrarely explain the Internalbores are generatedby breakinginternal waves. patternsand dynamicsof coral reefs.However, the study In any stablystratified body of water,where densityin- of environmentalvariability provides an underlyingcon- creases either continuouslyor discretelywith depth, a text for understandingthese complex ecosystems. Be- varietyof mechanisms can produceinternal waves (Baines 1986). Tidal forcingof a stratifiedwater column over the margins of a continentalshelf can generate packets of Acknowledgments internalwaves that tend to propagate inshore along the We thankC. Cooper,S. Genovese,D. Hanisak,T. Hopkins, thermocline(Baines 1986; Holloway 1991). As these L. Morgan,M. Samples,G. Shellenbarger,J. Styren,G. Villa, waves run into shallow water over gradually sloping D. Ward,K. Watkins,J. Witting,and thestaff of the National shelves,they steepen, and increasingshear at the density UnderseaResearch Center for assistance in thefield. J. Bascom, discontinuityleads to instabilityand breaking.The re- R. Bourgeirie,M. Connolly,and W. Wilmotof the National Ocean Servicesprovided instrumentation. We thankR. Jones sultingturbulent bores of subthermoclinewater -internal fornutrient analysis. Two anonymousreviewers provided crit- surf-continue to travel inshore, mixing with ambient icismof the manuscript. surfacewater (Wallace and Wilkinson 1988; Holloway This researchwas supportedby the NationalOceanic and 1991; Pineda 1994). In wave tanks, breaking internal AtmosphericAdministration through National Undersea Re- waves cause vertical mixing (Wallace and Wilkinson searchProgram grants 9306 and 9422. 1988), and it has been suggested that internal waves 1490 Internalbores on ConchReef 1491 breakingon continentalshelves may cause transientup- wellingin coastal marineenvironments (e.g., Cooper 1947; Wolanskiand Pickard 1983; Sandstromand Elliott1984). The existenceof internalwaves was well documented by the 1950s (Ewing 1950), and theirpresence in all ocean basins has been recognized since the 1970s. Only since the 1970s, however, has their potential ecological im- portance been studied in detail. Haury et al. (1979) re- ported high frequencyinternal waves in Massachusetts and proposed that they could produce significant redistributionof plankton. Shea and Broenkow (1982) detected large amplitude internaltides and a potential effecton thenutrient regime of MontereyBay, California. Surface slicks travelingwith internalwaves have been implicatedin the shorewardtransport of neustonic larvae along the of southernCalifornia (Shanks 1983) and New Zealand (Kingsfordand Choat 1986). Pineda (1991, Fig. 1. Map of southernFlorida with schematic represen- 1994) showed shoreward transportof cool, subsurface tationof the studysite at ConchReef (inset) seaward of Key water in turbulentbores, followed by onshore transport Largo.An arrayof instruments deployed across the reef on five of surfacewarm frontsand larvae in southernCalifornia. 7-15-d cruises consisted of vertical strings of thermistors based Semidiurnalvertical oscillations of the thermoclinehave at 35 and 21 m,electromagnetic and conductivity-tem- been described as a mechanism of nutrientdelivery to perature-depthmeters at 21 and3 5 m,moored acoustic Doppler southernCalifornia kelp forests(Zimmerman and Kre- currentprofiler at 35 m (November1993 only), and individual mer 1984). Frederiksenet al. (1992) suggestedsuspended thermistorsat 3-mdepth increments from 30 to 12 m. particledelivery by internalwaves could explain patchy distributionsof a deep-waterscleractinian coral in the that run fromthe reefcrest at a depth of 12 to -30 m, northeasternAtlantic. Witman et al. (1993) documented where the formationsbreak up into a series of isolated transportof warm, phytoplankton-richwater to dense patches surroundedby coral . A mix of primarily aggregationsof sessile suspensionfeeders on shallow pin- mounding and plating as well as benthic , nacles in theGulf of Maine. Andrewsand Gentien(1982), sponges,and softcorals covers most available space on Wolanski and Pickard (1983), and Wolanski (1994) have the reef. At -35 m, the reef ends on a gentlysloping, pointed to tidal oscillations of the thermoclinein the continuoussand plain thatextends, uninterrupted, for 8- Coral Sea as a potentialsource of nutrientsfor the outer 10 km to the deep of the Florida .The reef shelf of the , and Novozhilov et al. lies withinthe Florida Keys National Marine Sanctuary, (1992) reportedevidence of tidal upwellingnear reefsin and all measurementswere made at the site of the Na- the SeychellesIslands. Internalwaves and bores seem to tional Undersea Research CenterAquarius . The be a widespreadphenomenon (Pineda 1995), but reports site was accessed from small boats and measurements fromcoral reefsare few,and other studies to date have were made with both (36%0 2) and saturation not documented the dramatic and repeated forcingof diving. subthermoclinewater onto a coral reefthat we observed at Conch Reef. Long-term measurements-Long-term temperature Our observationsof largetemperature and salinityfluc- data were collectedby continuousdeployment of instru- tuations accompanied by rapid increases in flow speeds mentson the reefslope at 7, 21, and 3 5 m fromlate 1991 on a tidal cycle led to a workinghypothesis that internal to 1994. Ryan Tempmentorswere moored 1 m above waves weredriving the shorewardtransport of water from the bottom and sampled at 20-min intervalsfor deploy- belowthe thermocline. Physical conditions associated with ments lastingup to 3 months. Daily temperaturemean, the runup turbulentbores on the reefslope were subse- variance,and range(max - min) werecalculated for each quentlyexamined in detail. Althoughall measurements depth. The daily data for 1992 were divided into two were conducted on a single reef,the longshorescale of 6-monthseasons, "winter"(January-April, plus Novem- the phenomenon seems large enough (1-10s of km) that ber-December)and "summer" (May-October),and these otherreefs in the Florida Keys reeftrack may be similarly data were analyzed for differencesamong depths with affected. respectto daily mean and variance. Because the temper- ature data were not normallydistributed within depths and were not serially independent between successive Methods dates,Friedman's nonparametric test was used withdepth as the treatment(N = 3) and the observationsblocked by Site description-Conch Reef (24?59'N, 80?25'W) is a timeinterval (days) (Sokal and Rohlf 1981). Separatetests fringingreef 8 km southeastof Key Largo, Florida (Fig. were run forthe daily mean and daily variance data for 1). The reef lines the submergedmargin of Key Largo both seasons. Where significanttreatment effects were and is characterizedby coral spurand groove formations detected,nonparametric post hoc multiplecomparisons 1492 Leichteret al. betweendepth pairs were conducted by STP (Sokal and waterwith temperatureresolution of 0. 1?C) spaced 2 m Rohlf 1981). apart were deployed verticallyat 35- and 24-m depthsto Time-seriesanalysis was used to examine the frequen- measure temperaturevariability through the water col- cies of temperaturevariability in the longestcontinuous umn above the reef.Time series of temperature,salinity, data set- 18 monthsfrom November 1991 throughApril and tidal heightwere recordedby moored SeaBird SB-9 1993. Data were averaged in 2-h intervalsto remove the CTDs and Interocean S4 electromagneticcurrent/CTD highestfrequency variations, and long seasonal trends metersfixed 1 m above the bottom at 35 and at 21 m, were removed by subtractinga 35-d moving average. A samplingat 0.5-s intervalsand recording1 -min averages. fast Fourier transform(FFT) was calculated on the de- Watervelocities 1 m above the reefwere recordedby the trendedresiduals, with spectralpower per frequencycal- two-axis S4 currentmeters at 35 and 21 m. During No- culated by dividing the power in each frequencyby the vember 1993, velocities above the reefwere recordedby totalpower of the entireFourier transform. The resulting an acoustic Doppler currentprofiler (ADCP) moored at powerspectrum was smoothedwith a fixed-intervalmov- 30 m and profilingthree current axes in 1-m depth bins ingaverage, and 95% confidenceintervals for the spectral from 30 m to the surface.This instrumentsampled at peaks werecalculated following Jenkins and Watts(1968). 0.5-s intervalsand stored2-min averages. Currentmeter data were analyzed along 3300/1500 and 240?/60?axes Short-termmeasurements-Intensive short-term data (vectorcomponents), where 330?/1 500 correspondsto flow were collected duringfive 7-15-d cruises in September onto (positive) and off(negative) the reefslope and 2400/ 1992, August 1993, November 1993 (including 4 d of 600 contains the major component of tidal-drivenflows saturationdiving), July 1994, and November 1994. Phys- at the site. Correlationsbetween temperature at the bot- ical sampling consisted of concurrentmeasurement of tom and at each positionin the verticalthermistor strings temperature,salinity, chlorophyll a, and nutrientcon- werecalculated to determinethe spatial coherence of vari- centrationsthrough the watercolumn seaward of and on ability across the water column. Similarly,correlations the reefslope coupled withwater velocity measurements werecalculated between bottom temperature and the two above the reef.A SeaBird SB- 19 conductivity-tempera- componentsof flow in each 1-m bin ofthe vertical current ture-depth(CTD) meterwith attached Turner fluorom- profilerrecord as well as between flowspeed 1 m above eterand General Oceanics 1.5-literNiskin bottlesat sur- the bottom and flow speed in each 1-m bin throughthe face, midwater,and near-bottomdepths were used to watercolumn. sample the water column. Immediatelyafter collection, Water-columnconditions during the arrival of a packet watersamples were filter-sterilized(Whatman GF/F 0.7- of turbulentbores were measuredby rapid CTD profiling ,umfilter paper in Gelman filters)into clean 60-ml Nal- (yo-yo sampling) above the 35-m site every 10 min for gene bottles.Samples were kept dark on ice on the boat 8-h periods on 25 and 26 July1994. The run up of tur- (1-4 h max) and frozenupon returnto until anal- bulent bores onto the reefwas measured with 14 Onset ysis. Nutrient determinationsfor nitrite,nitrate, am- Computersrecording thermistors (calibrated in waterwith monium,and phosphatewere performed(Alpkem RFA- temperatureresolution of 0.20C) moored 1 m above the 300 nutrientanalyzer), with nutrient determinations mea- bottom at 3-m depth incrementsfrom 12 to 30 m along sured against internalstandards according to the RFA two transectsparallel to the reefspur and groove. These methodologyhandbook (Alpkem). instrumentsrecorded average values at 4.8-minintervals. CTD profileswere used to determinedensity structure All instrumentsin the sampling array contained syn- and positionof the pycnocline,if present, and to calculate chronized internalclocks, and positions and distances Brunt-Vaisalii(B-V) buoyancyfrequencies as a function between instrumentswere measured underwaterto the of the local densitygradient (Pond and Pickard 1983). nearest 1 m. Lag times in the arrival of cool water at B-V frequencyis directlyrelated to water-columnstabil- thermistorslocated successivelyupslope were used to es- ityand indicates the highestfrequency pycnocline oscil- timatepropagation speeds of cool-waterfronts. lations that can be attributedto internalwaves (Baines 1986). During each cruise,daily profilingwas conducted Results at six evenly spaced stations fromthe reef crest (1 0-m depth) to a station located 2 km seaward (50-m depth). Long-termrecord- The largestcomponent of long-term In November 1994, profilingwas conducted at 1.5-km variationwas due to seasonal warmingfrom May to Sep- stations out to 8 km. A CTD cast was made at each tember and cooling from October to March. Superim- station,and data were averaged in 1-m depth bins. On posed on this long-termtrend were high-frequencyfluc- several occasions when three or more CTD casts were tuationsthat were greatestin summerat the deepest sta- made in quick succession at the same station,very little tion. Figure 2 shows these patternsin a 16-monthtime instrumentvariation was detectedbetween casts. seriesof temperatureat 35 m (top panel) fromDecember High-frequencyvariations in temperature,salinity, and 1991 throughMarch 1993. Over thisperiod, temperature flow velocities were measured with a spatial array of rangedfrom a low of 19.70C to a high of 30.0?C. Figure moored recordingthermistors, CTD meters,and current 2 also shows an expanded view of 4 weeks of the same metersdeployed across thereef during each cruise.Figure recordfrom 26 Augustto 23 September1992 plottedwith 1 shows a schematic representationof the instrument lunar phase. Precipitousand repeatedtemperature drops array.Recording strings of 12 thermistors(calibrated in (some > 50C) at - 12-h periods were associated withdays Internal bores on Conch Reef 1493

30 28 oc26 24 22 20 Jan Mar May Jul Sep Nov Jan Mar 30 coc 29 28 27 26 25 24 23 29 Aug 5 Sep 12 Sep 19 Sep Fig. 2. Upper panel shows 16-monthrecord of temperature1 m above the bottom at 35 m (December 199 1-April 1993) averaged to 2-h intervals.Lower panel shows 20-min sample intervaldata forperiod correspondingto bar in upper panel (26 August-23 September1992), withlunar phase (0-full moon). surroundingboth the new and fullmoon. These fluctu- probablynot significantlyaliased by variabilityabove the ationswere also detectedat 21 m, butnot at 7 m. Table samplingfrequency (Jenkins and Watts 1968). 1 summarizesthe 1992 daily temperaturedata across depths.In bothwinter and summer,mean temperature Short-termmeasurements-Table 3 summarizesCTD decreasedwith depth, while standard deviation, coeffi- profilingdata for two sample periods, November 1993 cientof variance,and rangeall increasedwith depth. Daily temperaturerange and variance(plotted as SD) at 6- 7, 21, and 35 m for1992 are shownin Fig. 3. At 21 and 6 0 0 0 0 0 0 0 0 0 0 0 0 35 m, the high-frequencytemperature variation was greatestin July,August, and September.This seasonal 4 - patternwas consistentover 3 yr of measurements.Al- 3- thoughthe occurrenceof the fullmoon was generally 2 - accompaniedby increased temperature variability, other timeswithin the lunar cycle also showedhighly variable temperature.Statistical tests indicate a significanteffect 5 21 m ofdepth on dailymean temperature and variancein both 4 1992 seasons(Table 2). Multiplecomparisons show that 3 all thepairwise groupings of depths differed significantly 3C 2- in summer. Figure4 showslow-frequency and high-frequencypor- tionsof the powerspectrum for the 35-m temperature record.At highfrequencies, a single,well-defined peak 4- correspondingto a periodof 12.4 h is evident.The 95% i~~~~~~~~. , . .. 4. C.I. indicatesthis peak stands out significantly from other 3- high-frequencypeaks. At the low-frequencyend of the 2- powerspectrum, variance is spreadover a groupof peaks thatcorrespond to periodsfrom 13.3 to 42.6 d. The 95% C.I. indicatesthat none of these peaks stands out signif- Jan Mar May Jul Sep Nov Jan icantlyfrom the others.The portionsof the spectrum Fig. 3. 1992 dailytemperature range (solid lines) and stan- shownin Fig. 4 contain96% of thepower of theentire darddeviation (broken lines) 1 m above thebottom at 7, 21, spectrum.Spectral power drops close to zerowell below and 35 m. Daily valuescalculated from data takenat 20-min theNyquist frequency, indicating that the time series was intervals.Circles indicate date of full moons. 1494 Leichteret al.

Table 1. Daily temperaturedata from7, 21, and 35 m for Table 2. Friedman's testand STP multiplecomparisons for winter (January-April,November-December) and summer 1992 daily mean temperatureand daily temperaturevariance. (May-October) 1992. Treatment= depth. Asterisks:* P < 0.05, ** P < 0.01; ns- not significant. Depth (m) Mean (?C) SD C.V. Range n Sig. x2 n df Winter Winter 7 24.5 1.36 0.076 2.4 179 Daily mean ** 129.0 178 2 21 24.2 1.42 0.083 2.8 182 7 vs. 21 m ns 35 24.0 1.53 0.098 3.4 182 7 vs. 35m ** Summer 21 vs. 35 m ns 7 28.2 1.48 0.078 3.1 173 Daily variance ** 22.3 178 2 21 27.7 1.51 0.082 4.4 173 7 vs. 21 m ns 35 27.1 1.60 0.094 5.4 173 7 vs. 35 m ns 21vs.35m * and July 1994, 1.5 km seaward of Conch Reef. In No- Summer vember 1993, thewater column was well mixed to a depth Daily mean ** 270.8 173 2 of 30 m and continuouslystratified below than to 50 m. 7 vs. 21m ** Total change in temperatureand salinity between the 7 vs. 35m ** surfaceand 50 m was -0.6?C and 0.40oo.B-V frequency 21 vs. 35m ** calculations predicteda minimum period of oscillation Daily variance ** 69.7 178 2 of 6.67 min at 48 m. In July1994, by contrast,the water 7vs.21m * column was mixed to depth of 20 m with a well-defined 7 vs. 35m ** pycnoclineat -50 m. Stratificationwas essentiallytwo- 21 vs. 35m ** layered. Maximum temperatureand salinitychange be- tweenthe surfaceand 50 m were -7.3?C (29.18?C max- imum at 2 m, 21.87?C minimum at 49 m) and 1.5%oo, Profilingin November 1994 showed a deeper pycnocline witha minimumB-V period of 2.12 min at 49 m. Figure at 80-95 m, 5-8 km seaward of the reef.Nutrient data 5 shows sectionplots of temperature,salinity, and Chl a fromNovember 1994 hydrocastsshowed concentrations forthe mixed watercolumn on 19 November 1993 (left of nitratebetween 3.98 and 4.67 AM below the 80-m panels), compared with the much strongerstratification pycnocline.Phosphate concentrationalso increasedfrom on 23 July1994 (rightpanels). On 23 July1994 the sur- 0.02 to 0.30 AM across thepycnocline in November 1994. face layer (29?C) was much warmerthan in November, while the top of the themocline(25.5?C) was at 44 m. A Internalbore arrival-Variation in temperature,salin- relativelyhigh of Chl a was associated with ity,two componentsof flow speed, and tidalheight (depth) the thermoclinein July1994. On 19 November 1993 the measured 1 m above the bottom at 33 m from 15 to 20 concentrationof inorganicnitrate increased significantly November 1993 are shown in Fig. 6. Approximatelyev- with depth from 0.1 ,uM at 24 m to 0.7 ,uM at 48 m. ery 12 h, on the floodingtide, rapid drops in temperature were accompanied by dramatic increases in salinityand spikes of high flow speeds at 3300. From 45 min to 2 h 1.000 42.6d aftereach event, positive flow at 3300 was followed by 27.3d negative (offslope) flow as temperaturegradually in- creased. The 600 d 12.4 component of flow showed consistent .100 ~~13.3 hI tidal forcingbut contained very few of the sudden in- creases associated with temperaturedrops. Temperature and salinitywere tightlynegatively correlated, indicating .010 a coupled change fromwarm, relativelylow salinityto cooler, more salty conditions as water was transported frombelow the thermoclineonto the reef.Figure 7 shows .001 temperaturevariation throughthe water column above .05 .10 .15 .20 .25 1 2 3 4 5 6 the 33-m stationfrom 17 to 20 November 1993. Rapid Frequency (d-1) drops in temperatureand more gradual warmingin the bottom 8-12 m of the water column coincide with tem- Fig. 4. Low frequency(left, 5-point smoothed) and highfre- quency (right,33-point smoothed) portionsof the power spec- perature1 m above the bottom (Fig. 6). trumfor 18-monthtemperature record from35 m. Errorbars Figure 8 shows the upslope component of the current represent95% C.I. Each portionis normalized to mean-square profilerdata duringcooling on 20 November 1993 shown amplitude for that frequencyrange. Mean-square amplitudes in Fig. 6 and Fig. 7. Within one 2-min span at the start (0C2) = 0.181 low-frequencyportion, 0.105 high-frequencypor- of cooling,upslope flowincreased fromnear 0 to 15 cm tion, 0.298 total spectrum. s-'. As water was flowingrapidly upslope near the bot- Internalbores on ConchReef 1495

Table 3. CTD data summary 1.5 km seaward of Conch Reef, November 1993 and July 1994. Maximum and minimum averaged values with standard deviation and depth of oc- currence.

Max Min 8-21 November 1993 (n = 13 casts) Temp. (?C) 27.02 (0.21) 1 m 26.39 (0.46) 48 m Salinity(%Oo) 36.08 (0.18) 49 m 35.73 (0.15) 1 m Chl a (,ug liter1) 0.619(0.258) 48 m 0.082(0.050) 4 m Density (?,) 23.74 (0.17) 48 m 23.26 (0.15) 1 m B-V frequency(min-') 0.15 48 m Stratification:continuous 23 July1994 (n= 3 casts) Temp. (?C) 29.18 (0.04) 2 m 21.87 (0.18) 49 m Salinity(%oo) 36.63 (0.17) 49 m 35.16 (0.23) 1 m Chl a (,ug liter1) 0.951(0.061) 49 m 0.038(0.009) 12 m Density (?,) 25.49 (0.18) 49 m 22.13 (0.17) 1 m B-V frequency(min-') 0.47 49 m Stratification:two-layered

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00 08 16 24 32 40 32~~~f00 08 16 24 40. 10 ...... ~~~~~~~~itacfo quru H btt kn 099(de )j. Daacnordfof)CDcs vrgdt lmdphbn tec 0.1-msainfonte efrs o40k Fi2Wtrclm0etoso 5 eprtr aiiyadCh ocnrto n1 oebr19 abc n 3Jl seaward along a headingof 1500. ~ ~ ~ ~ ~ phvl a(gglit 1496 Leichteret al.

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3 36.1 10 0 36.0 Cl)35.9 15 C ir 20.0 5 10.0 20 o 0.00i ? -10.0 20.0 25 10.0 %~0.00 i 0 30 0 'NO-10.0 0 20 40 60 80 100 32.4 Time(min) 32.8 33.0 18 Nov 19 Nov 20 Nov 21 Nov 22 Nov Fig. 6. Temperature,salinity, flow speeds at 3300 and 600, -15.0-10.0 -5.0 0.0 5.0 10.0 15.0 and tidalheight (depth) recorded 1 m above thebottom at 33 m from17 to 23 November1993. Flow speed (cm s-1) Fig. 8. AcousticDoppler currentprofiler record of330'com- tom, flowwas offslopein the surface 15 m of the water ponentof flow speeds in 1r-mdepth increments through the water column.These conditionslasted just under20 min. About column above the 33-m station before,during, and afterthe 45 min afterthe initial arrival of cool water,flow in the arrival of an internalbore on 20 November 1993. bottom 4 m of the watercolumn reversedto 4-6 cm s-I downslope. This "slosh back" coincidingwith the onset of warmingis a consistentfeature of each event in the currentprofiler record. The 3300 componentof flowand temperaturethrough the watercolumn 20 min before,at the onset of, and 50 min afterthe 20 November cooling are shown in Fig. 9. At the onset of the event, currents near the bottomand near the surfacewere in opposition, with a shear zone between 16 and 17 m. Figure 9 also Depth(i) 14.0 showscorrelation coefficients between 3300 flow1 m above thebottom and at each 1r-mbin throughthe water column 17-22 3300 22.0 for November. Below 16 m, flowin the water v~~~~~~~~~7. column was positivelycorrelated with flownear the bot- tom. Above this depth,the correlationwas negative,in- 26.5 dicating a persistentopposition of surface and bottom onshore-offshorecurrents. Correlation coefficientsbe- 26.0 tween bottom temperatureand temperatureat each po- (5 in 25.5 sition thewater column decreased roughly linearly from 323.5 324.0 324.5 325.0 325.5 326.0 326.5 1.0 at thebottom to 0.18 at 6 m withlittle apparent phase shift,indicating a decrease in the extentof repeatedcool- Day ofthe year ing withdistance above the bottom.Bottom temperature Fig. 7. Temperature recorded at 2-m depth increments was moderatelycorrelated with the 3300 component of throughthe water column above the 33-m station on 19-23 flowthrough the watercolumn but poorlycorrelated with November 1993. total flowspeed. Internal bores on Conch Reef 1497

A time series of the water-columnprofile above the reefon 25 July 1994 is shown in Fig. 10. In the 10-min intervalfrom 1720 to 1730 hours,temperature 1 m above 5 the bottom decreased from 29.3 to 27.8?C, salinityin- creased from35.50 to 36.15%oo,and Chl a concentration more than doubled, from 0.32 to 0.66 ,g liter-'. This ~15 sudden change in conditionswas consistentthrough the bottom 10 m of the water column. The arrival of cool ~20 I waternear the bottom was accompanied by a slightwarm- 25- ing near the surface.The sudden cooling was followedby a much more gradual warming as cool water receded downslope duringthe next 2-4 h. A sudden reversal of -15 0 15 25.7 26.2 26.7 -0.5 0 0.5 surfacecurrent that caused the sampling boat to rotate cm s ?C r 1800 on its mooring above the 33-m site was noted at Fig. 9. Water-columnprofiles of 3300 componentof flow exactly 1726 hours, and observersnoted the arrival of speed(left panel) and temperature(center panel) 20 minbefore the firstof six sharplydefined surface slicks at the same (- - -) at the onset of ( ) and 60 min after( - -) the arrival time that instrumentsrecorded the sudden incursionof ofan internalbore on 20 November1993. Coefficients of cor- cool, high-salinitywater near the bottom. Separated by relation(right panel) for 17-22 November 1993 between bottom 50-75 m, orientedparallel to the reefcrest, and slowly and water-columnmeasurements of 330?components of flow travelingonshore at - 10-20 cm s- 1,these slicks extended (O), temperatureand 330?flow (0), and temperatureand total to the horizon (several kilometers)and contained dense flowspeed (0). concentrationsof floatingseaweed (Sargasum spp.) jel- lyfish(Aurelia aurita), and flotsam at the surface and slick propagationspeeds cannot be used directlyto infer feedingfish (Atlantic spadefish, Chaetodipterussfaber, and wave lengthsor propagationspeeds. horse-eyedjacks, latus) just beneath the surface. Summer water-columnprofiles showed an essentially During this time,the surfacewas calm, with wind speed two-layeredstratification seaward of the reef,and B-V < 5 km h- 1. Divers on the reefduring the onset of several calculations predictminimum periods of buoyant oscil- such events noted the movement upslope of a visible, lations of 2.1 min. Under these conditions,tidal flooding shimmeringdiscontinuity within several meters of the over the shelfbreak of the Florida Straitis likelyto gen- bottomand observed schools of reeffish orienting to the erate packets of shoreward-travelinginternal waves moving front. (Baines 1986). Studies of stratificationprocesses in this Figure 11 shows temperaturevariation at 3-m depth region(e.g. Atkinsonand Blanton 1986) confirmour ob- incrementsacross thereef slope duringthe cooling shown servationsof a sharplydefined shallow pycnocline and in Fig. 10. The rapid decrease in temperatureon the bot- two-layerstratification in summergiving way to weaker tom at 27 m was followed by decreases at successively stratificationand a deeper pycnoclinein winter.The lo- shallowerdepths with a significanttime lag and decrease cation of internalwave breakingdepends on the depth of in the relativetemperature change with decreasing depth. thepycnocline and wave amplitudeand will varybetween The onset of cooling at 15 m occurred -20 min after packets of waves and between seasons. cooling at 27 m. The cross-slopedistance betweenthese The correlationbetween temperatureon the bottom two stationswas 161 m, givingan estimatedbore speed (30 m) and temperatureat elementsin the verticaltherm- of 13 cm s-'. istor stringsdecreased with distance from the bottom because near-surfacetemperatures did not consistently Discussion track near-bottomvariability (Fig. 9). The correlations between3 300 flownear thebottom and 3 300 flowin water The coupled changes in temperatureand salinityand column indicates that strongnear-bottom and surface the onset of rapid, upslope flownear the bottomindicate currentsco-occur in opposingonshore-offshore direction. themovement of well-defined masses ofwater onto Conch Near the bottom,temperature was negativelycorrelated Reef. These events are well explained as the periodic withthe 3300 componentof flowdue to the co-occurrence arrivalof internalbores linked to a semidiurnalinternal of strongupslope (positive) flowwith cool . tide. The recurrentpattern of gradual warmingaccom- The magnitude of this negative correlationwas small, panied by downslope flow45 min to 2 h afterthe initial however,because temperaturesremained low as flowwas temperaturedrops shows the water transportedin bores downslope (negative). Temperaturewas not well corre- receding downslope as a gravitycurrent (Wallace and lated withthe total flowspeed (Fig. 9), indicatingthat the Wilkinson1988; Pineda 1994). The observedsurface slicks tidal flow patternsthat dominate the total flow speeds can be explained as surfacemanifestations of packets of were not responsible for the rapid temperaturefluctua- internalwaves (Shanks 1983; Kingsfordand Choat 1986), tionsin thelower portion of the water column. Deviations and theseinternal waves can break in shallow water,gen- of 15-20? in the directionof water flow at the onset of eratingturbulent internal bores. The exact relationships some cooling events are presentin the data, but 3300 is betweensurface slicks and underlyingfields of linearand the predominant flow direction coinciding with rapid nonlinear internalwaves are not well understood,and cooling near the bottom. 1498 Leichteret al.

0.0 30.0 10.0 ~2.~ 29.0 20.0 28.5

30.0 28.0 27.5

0.0 _ 36.2 10.0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~3.

20.01 30.0 0635.4 4 30.0 - _ S135.2 b 35.0 0.020.7 * 0.6~ 10.0 s 0. 0.4 C 20.0 0.2 30.0 .0.1 0.0 1500 1600 1700 1800 1900 2000 2100 2200 U Time (min) Fig. 10. Time-seriesprofiles of temperature, salinity, and Chl a concentrationon 25 July 1994 above the33-m station. Data from1 CTD castevery 10 minfor 8 h.

The interpretationthat internal bores cause the ob- steepnessof theslope to steepnessof thewaves leading servedhigh-frequency physical variations is supportedby faceexceeds 1.0, waves are reflectedoffshore rather than similarobservations on slopingshelves in other geograph- breakingand flowingupslope (Holloway 1991; Wolanski ic regions.Smyth and Holloway(1988) characterized 1994).Slope steepness helps explain the limited transport shorewardpropagation of large-amplitude internal waves ofdeep wateronto a reeffront in theGreat Barrier Reef alongthe wide,gradually sloping northwest Australian despitesignificant internal wave activityat depths> 100 shelf.Crossing the shelf, internal waves steepen,leading m nearbyin theCoral Sea (Wolanskiand Pickard1983). to instabilityand formationof undular,turbulent bores An alternativeinterpretation of the Conch Reefdata withonshore components of flowas highas 40 cm s-I mightattribute temperature variability to spin-offeddies at 40 m and concurrentstrong offshore surface flows. andmeanders of the Florida Current. However, this would Verticalmixing that results from this onshore transport notexplain the high-frequency, rapid cross-shelf flow of causesrapid dissipation of internal tidal energy (Holloway cool wateronto the reef,the correlationbetween tem- 1991). Similarmechanisms have been describedfor in- peraturedrops and upslopeflow, or the observationof ternalwaves on theScotian shelf (Sandstrom and Elliott movingsurface slicks. Variability in theFlorida Current 1984),the coast of BritishColumbia (Drakopoulos and is wellestablished (Lee and Mayer1977; Lee et al. 1994) Marsden1993), the of Maine (Brickmanand Loder andvery likely influences the generation of internal waves 1993),and the narrow shelf at themargin of the San Diego throughchanges in water-columndensity structure and marinecanyon (Pineda 1991, 1994). Wherethe ratio of shearacross the pycnocline. Internal bores on Conch Reef 1499

The high-frequencyspectral peak at 12.4 h resultsfrom 30.0 the regulararrival of cool waterin bores linkedto a semi- diurnal (M2) internaltide. The lower frequencypeaks at 29.5-,;, 13.3 and 27.3 d correspondapproximately to the lunar ,fi; .--;-- fortnightly(Mf, 13.6 d) and lunar monthly(Mm, 27.5 d) tidal periods which may contributeto the amplitude of 29.0 I internaltides (Pond and Pickard 1983). Analysisof daily temperaturevariance vs. day of the lunar cycle forMay- 28.5I October 1992 showed no clear pattern.Although days - ...... 15 m shortlypreceding and followingthe summerfull moons 28.0 21m did exhibitthe greatesttemperature variability (Fig. 2), 27.527m temperaturevariations were also large on other days 27.5-...... I I .. . throughoutthe lunar cycle. Concurrenceof highlyvari- able temperatureswith the July,August, and September 17:00 19:00 21:00 fullmoons werealso observedin summer1994 and 1995, Fig. 11. Temperature1 m above thebottom at 15,21, and but again, temperaturevariability was not restrictedto 27 m on 25 July1994. these days. For sites between southern California and Washington,Pineda (1995) reportedgreatest internal bore activityon days between the new and full moons, with Jokieland Coles 1990), and both species-specificand col- seasonal variabilityin the extentto which internalbore ony-specificresponses to temperaturevariation have been activitytracked the lunar cycle. Although lunar phase implicatedas causes of this patchiness(Edmunds 1994). may influencethe amplitude of internaltides, the mech- By generatingspatial and temporal patternsof physical anisms of internalwave generationcan be complex and variationwithin a reefand by forcingcool wateronto the are not necessarilyconstrained to particulardays within reef during summer when ambient temperaturesare at thelunar cycle (Pineda 1995). This helpsexplain the spread theirhighest, internal bores may contributeto the patchy of spectralpower across a range of low frequenciesfrom distributionof .Spatial patchinessin tem- 13 to 28 d. The lowest frequencyspectral peak, corre- peraturevariability may also contributeto species-spe- spondingto a period of 42.6 d, is not well explained by cific and colony-specificpatterns of growthand survi- tidal harmonicsbut may representtemperature variabil- vorship. ityrelated to spin-offeddies of the Florida Current(Lee Concentrations of dissolved nutrientsgenerally in- and Mayer 1977; Lee et al. 1994). Longer term(seasonal crease with depth across the thermocline,especially in and annual) trendsin temperature,although presentin warm, oligotrophicwaters (Wolanski 1994). The water- the data, do not show up in the spectralanalysis because columnprofiles 2 km seawardof the reef on 19 November of detrending. 1993 show more than an order of magnitude increase Cross-shelftransport can cause exchangeof waterand (from 0.05 to 0.71 A,M)in the concentrationof nitrate materialsuch as nutrients,food particles,planktonic lar- from24 to 48 m. Sampling in November 1994 as well vae, and pollutantsbetween nearshore environments and as summers1994 and 1995 showed even greaterincreases coastal oceans (Pineda 1994). Both slope steepness and in nitrateand phosphateacross thepycnocline (J. Leichter small-scalereef topography, such as spur and groove for- unpubl. data). Lee et al. (1994) showed subthermocline mations,affect the runup of turbulentbores and contrib- concentrationsof nitrateas high as 3-15 AM seaward of ute to spatial patchinessof bore impacts. Physicalforcing the southernFlorida Keys. As pulses of subthermocline resultingfrom pulsed deliveryof subthermoclinewater water are forced onto the reef,dissolved nutrientcon- to Conch Reef may have ecosystem-wideconsequences. centrationswill trackthe changesin temperatureand sa- Several of these are consideredbelow. linity.Miller and Hanisak (unpubl. data) reportedhighly Water temperatureinfluences a wide range of physio- variable bottom concentrationsof nitrateat Conch Reef logical processessuch as energyutilization, photosynthe- and suggestedthat some variabilitymight be due to in- sis, growth,and calcificationin corals and otherreef or- ternalwaves. Symbioticscleractinian corals take up both ganisms. The long-termtemperature records at Conch dissolved ammonium and nitrate(Muscatine and D'Elia Reef show predictableseasonal patternsof cooling in win- 1978; Bythell 1990), and uptake of dissolved inorganic terand warmingin summer,with an annual temperature nitrogenmay supply up to 30% of the requirementsfor rangeof just over 10?C (Fig. 2). Internalbores add mark- growth,reproduction, and mucus secretionin edlyto the high-frequencyvariability in temperature,with palmata (Bythell1988). Rapid nutrientuptake may allow deeper sites experiencingsignificantly greater daily vari- corals efficientuse ofperiodic and patchynitrogen sources ance than shallowersites. At 35 m, the maximum daily (Wilkersonand Trench 1986), but responsesto high-fre- temperaturerange of 5.4?C in September1992 was more quency variation in ambient concentrationsare not well than halfof the annual temperaturerange. Both elevated understood. temperaturesand rapid fluctuationsnear extremehighs Nitrate uptake by A. palmata has been shown to be can cause corals to expel zooxanthellaeand bleach (Jokiel linearlydependent on ambientconcentrations with a rate and Coles 1990; Gates 1990). Coral bleaching can be of 0.896 Amol h-1 at a concentrationof 0.31 AM nitrate extremelypatchy both within and between depths (e.g. (Bythell 1990). When the impact of internalbores on 1500 Leichteret al. nitrateuptake is modeled as a linearresponse to changing sityand the forcesexerted by wave-drivenwater motion concentrationwith a correctionof Qlo = 2 forchanging (see Done 1983). Because the variance in lightand water temperature,uptake rate is predictedto increase nearly motion also tendto decrease withdepth, deeper zones on sixfold(from 0.29 to 1.64,mol h-1) followingthe arrival reefshave oftenbeen considered more constant,stable of an example internalbore thatcauses nitrateto increase environmentsthan shallower areas near reef crests. By from0.1 to 0.7 AM while temperaturedrops from28 to contrast,however, the physical variability associated with 25?C. Nutrientuptake can also be limited by transport the impact of internalbores increasesdramatically with across benthicboundary layers. Reduced boundarylayer increasingdepth at Conch Reef, especially duringsum- thicknessassociated with increased flowand turbulence mer. Species-specificresponses to the variance in tem- can resultin increaseduptake (Atkinson and Bilger1992), peraturestress, nutrient pulsing, or particledelivery pro- and increased flowspeeds coincide with increased nutri- duced by internalbores could significantlyaffect species ent concentrationsduring the arrival of internalbores. distributionsand abundance across the reefslope. Previous exposure, nutrientneeds, lag time in response to changingconcentration, or interactionbetween nitro- gen sourcesalso affectuptake. However, witha residence Conclusions time of 1-4 h and the semidiurnalfrequency of bores at the site, short-termpulsing may be an importantsource Internaltidal bores are a consistentfeature of the phys- of nitrate.Total uptake of nitratefrom two largeinternal ical environmentat Conch Reef. The runupof tidal bores bores per day with a residence on the reefof 2 h could exertsdirect influence on physicalfactors across the reef, equal or exceed uptake forthe restof the day. causing fluxto the reefof cool water and dissolved and Spatial variabilityin nutrientdynamics within and be- suspended materialassociated withthe thermocline.The tweenreefs can affectspecies abundance and competitive interactionof this runup with the reeftopography may interactionsbetween corals and benthicalgae (Tomascik generatephysically variable microhabitatsacross thereef. and Sander 1987). Spatial and temporal variabilityin The observationof surfaceslicks extending to the horizon onshoretransport of subthermocline water may also affect in both alongshoredirections suggests that this mecha- nutrientdynamics related to the movement of surface nism operates on a longshorescale of 1Os of kilometers and subsurfacepollution. Understandingthe spatial and and verylikely affects other reefs in the Florida Keys reef temporal dynamics of cross-shelftransport by internal track.Variation in reefmorphology and topographysep- bores could be of practicalimportance to pollution mit- aratingreefs from the shelfbreak may produce large be- igation effortsor managementdecisions, such as the lo- tween-reefvariation in the impact of internalbores. Un- cation of deep-watersewage outfalls. derstandingthe mechanism and spatial and temporal Internal bores also influencesuspended particle flux variation of internalbores may shed lighton basic eco- past Conch Reef. Figure 9 shows the changes in Chl a logical processes on coral reefs.Transient onshore trans- concentrationas a packet of bores arrived on the reef. port by internalbores may be particularlyimportant at Chl a, measured fromfluorescence, is a reliable estimate Conch Reefbecause analysisof long-termcurrent records of phytoplanktonconcentration (Townsend et al. 1984), suggestsnet offshoreflows near the bottom duringnon- and the formationof subsurfacechlorophyll maximum bore periods (S. Miller unpubl. data). The impact of in- layersassociated withthe thermoclineis well established ternal bores can be viewed as a dynamic process that forboth temperateand tropicalwaters (Townsend et al. redistributescool water and materialssuch as dissolved 1984; Wolanski 1994). As phytoplanktonconcentration nutrients,suspended particles,and plankton from sea- and flowspeeds increase,food supplyto passive suspen- ward distributionsin relationto the thermoclineto spa- sion feederssuch as corals and sponges should increase tially and temporallydynamic distributionsacross the rapidly.Observations of reeffish orienting to incoming reef. bores suggestthat mobile planktivoresmay be similarly affected.Because of the variabilityin internalbore im- pacts across depths, supply of suspended food particles References is likelyto differmarkedly across the reefslope. Floating ANDRPEws,J. C., AND P. GENTIEN. 1982. Upwellingas a source material and neustonic larvae may also be transported of nutrientsfor the Great BarrierReef ecosystem:A solu- onshore,entrained in surfaceslicks (Shanks 1983; Pineda tion to Darwin's question? Mar. Ecol. Prog. Ser. 8: 257- 1991, 1994). Pineda (1994) explained shorewardtrans- 269. port of surfacewarm frontsrich in neustonic larvae in ATKINSON, L. P., AND J. 0. BLANTON. 1986. Processes that southernCalifornia as an epiphenomenonof internal bores affectstratification in shelfwaters, p. 117-130. In N. K. drivenby thepostbore downslope gravity current. A large Moores [ed.], Baroclinic processes on continentalshelves. pool of warm surfacewater is presentover the reefflats AGU. shorewardof Conch Reef.As internalbores recededown- ATKINSON, M. J., AND R. W. BILGER. 1992. Effectsof water velocityon phosphate uptake in -flatcommuni- slope, warm reef-flatwater can flow seaward over the ties. Limnol. Oceanogr. 37: 273-279. forereef. BAINES,P. G. 1986. Internaltides, internalwaves, and near- Much of the zonation of species and growthmorphol- inertialmotions, p. 19-31. In N. K. Moores [ed.],Baroclinic ogies on coral reefs is related to gradientsin physical processes on continentalshelves. AGU. factors,most notablydecreases withdepth in lightinten- BRICKMAN, D., AND J. W. LODER. 1993. Energeticsof the Internal bores on Conch Reef 1501

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