Pacific Science (1973), Vol. 27, No.2, p. 197-206

Printed in Great Britain

Tides and Circulation in a Series of Saline Lakesat Christmas Island l

BRENT GALLAGHER 2

ABSTRACT: Hydrographic conditions are presented for a series of seven salinelakes which open off of the main lagoon at Christmas Island, and which are beingconsidered as a potential aquaculture site for brine shrimp. The first five pondshave typical tidal ranges of 1 foot and residence times of about 11 days. The twoponds farthest inland are penetrated only by spring tides and have residencetimes in the order of 15 months. The tides themselves, as well as mean water-leveland tidal phase differences, present several possibilities for producing a controlledflow through the ponds. Circulation and structure in one of the innermost pondsshow that no significant nutrient traps exist, and that flow patterns would serve tohelp collect floating brine shrimp eggs.

TIDES

economy of the Gilbert and Ellice Islands (andpresumably better the people's living standards,if population growth can be kept from wipingout the economic advance). The culture ofbrine shrimp (Artemia) might be especiallysuitable for the lakes, since it requires movingthe organisms through a series of enclosures ofincreasing salinity to cause egg production. Toinvestigate the feasibility of such a project, ateam from the University of Hawaii went toChristmas Island in November 1971 to surveythe physical and biological conditions in oneset of seven lakes which branch off from themain lagoon in a sequential series. This paperpresents the hydrographic findings. Waterlevels, winds, water temperatures, and currentswere measured to determine flushing ratesthrough the pond series, circulation patternswithin a single pond, and the potential uses ordifficulties these might present in an aquacul­ture operation.

Water levels in the ponds were studied forthe purpose of answering four questions:

1. What are the existing flushing rates orresidence times of water in each pond?

2. If the tides were used to produce a net cir­culation through the pond series, whatvolumes of flow could be expected?Are there naturally occurring tidal phase3.

197

AT CHRISTMAS ISLAND, much of the originalatoll lagoon has been rendered dry land­apparently by a sequence of erosion, reef­building, and sedimentation that accompaniedsea level changes and a possible, recent north­westward tilting of the atoll (Wiens, 1962). Pastreef growth within the lagoon followed asomewhat rectilinear, interconnecting patternthat probably established the geometry presenttoday in the form of the many ridges whichenclose dry basins, isolated ponds, and lakesintercommunicating through relatively narrowchannels. Other stages of a similar topographicdevelopment seem to be present elsewhere inthe Line Islands. Jarvis Atoll has a filled lagoon,and the lagoon at Fanning Island is honey­combed by interconnecting line reefs. ChristmasIsland may represent an intermediate stage.About half of the atoll is now above water, andanother fourth of its surface area (about 160km2) is covered by saline lakes and ponds.These extensive interior water bodies are aunique feature among the world's atolls.

The saline lakes and ponds are not currentlyused by man, but there is an idea that some ofthem might be suitable for aquaculture. Thiscould provide a new local industry to help the

1 Hawaii Institute of Marine Biology contributionno. 402. Hawaii Institute of Geophysics contributionno. 503. Manuscript received 30 August 1972.

2 Department of Oceanography, University ofHawaii, Honolulu, Hawaii 96822.

198 PACIFIC SCIENCE, Volume 27, April 1973

157° 27'W 157° 26'01 °54'N-r----!----------!-------r.,..----+------r-

S Automatic, contlnuous tide recorders

X Non-recording, water level gages

Ol051'N

FIG. 1. Map of F-pond series and adjacent areas, showing tide measurement locations.

differences between ponds that could be usedto promote net flows?

4. Are there mean-water-Ievel differences be­tween ponds that could be used to promotenet flows?

Water levels were measured in each lake atthe locations shown in Fig. 1. Readings weretaken every hour from 1100 (local time),8 November 1971 to 2000, 10 November 1971.During the same period, hourly readings were

taken of wind velocity and surface watertemperature in lake F1B. The relative elevationsofall gages shown in Fig. 1 were determined bysurvey so that differences in mean water levelbetween ponds could be computed.

The tide records will be discussed startingwith the outermost pond, which is the one mostdirectly connected with the main lagoon, andmoving landward through the series, passingtoward ponds that are sequentially fartherremoved from the lagoon.

Tides and Circulation at Christmas Island-GALLAGHER 199

2~wwILl

~

LL0

0z02ll.

Z

wIoi=

o

NOVEMBER 1971o Xonolulu ,6/",,- 7 lj I~ IU

I \ / A\ /\ /\ A /'\.J ~ ,\..'- ..... ,,\. ./ '-- ./ '\../ - "-./- .....,J ~ --I ............ ./ ........... f .......... ~

II 12 13 14 15 16

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O~i=:::>...J:::>

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Fig. 2. Recorded tide in pond FlA. Honolulu tide shown for comparison.

Tides in the Main Lagoon

The tide in the outermost pond, F1A, may betaken to represent the tide in the neighboringregion of the main lagoon. F1A does not openimmediately to the main lagoon, but we notethat wherever the lakes are interconnected byfairly wide passages there is negligible distor­tion of the tide wave over short distances. Thisshows clearly in the similarity of the recordsfrom F1A and 14, which are connected via themain lagoon. Thus we regard the record fromF1A as the tide in the local, inner portion of themain lagoon. This tide is shown in Fig. 2. Therange is about 1 foot, and the form of the waveis strongly distorted.

Tides measured in the inner end of the lagoondo not necessarily represent conditions through­out the whole htgoon. When a large lagoon haslimited communication with the surroundingocean, the tide in the inner parts of the lagoonwill lag that close to the atoll inlets. The delaycan be more than an hour. Such delays are foundat Fanning Island (Gallagher et aI., 1971) andare very likely to be present here. The tide isroutinely recorded in the lagoon at London(near Cook Inlet), and relatively simple em­pirical formulas could be derived to give thetide in the inner lagoon from the Londonmeasurements.

The distortion of the lagoon tide is interest­ing to note. The water rises quickly and re­cedes slowly, with the crest shifted forward intime-a pattern that characterizes nonlineardistortion of the wave. In general there are two

processes that can produce nonlinear distortionof a tide: passage of the wave through extensiveregions of shallow water, and bottom friction(Gallagher and Munk, 1971). Although bothprocesses are undoubtedlypresent in the lagoon,the shape of the wave indicates that the first ispredominant here. The lagoon is large enoughso that a nondistorted tide entering from theopen ocean will undergo a shoaling transfor­mation, becoming steeper in front somewhatlike an ordinary swell wave approaching abeach. (A good example of frictional distortionappears inside the pond series and will be dis­cussed later.) The nonlinear distortion of thelagoon tide at Christmas Island is very pro­nounced; the island would be an excellent placeto conduct field studies of this phenomenon.

There is some practical motivation in seeingwhether the Christmas Island lagoon tide bearsany predictable relation to the Honolulu tide.Both curves, therefore, are given in Fig. 2.There is no simple relation which would giveaccurate lagoon predictions based on Honolulu.However, rough estimates could be made: thelagoon tide has somewhat less than half theHonolulu amplitude, high water lagging byabout 2 hours and low by about 5 hours.

Tides in the F-Pond Series

A set of hourly water level readings throughthe pond series is presented in Fig. 3. Tides inthe outer lakes, through F4, follow the locallagoon tide with little if any discernible differ­ences. This is simply because these ponds are

200 PACIFIC SCIENCE, Volume 27, April 1973

•••••••••• •••••••••.•••••••••• 0.0 ••••••••••

F5

14

F6

F7

F4

F1B

F2

RECORDED TIDE,OUTER F1A

WATER LEVEL GAGES:

FIA

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10 NOVEMBER03 05 07 09 II 13 15 17 I~ 2[

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TEMPERATUREAT F1B GAGE

FIG. 3. Water levels, wind speed and direction, and surface water temperature measured in and near the F-pondseries.

connected by relatively large passes whichpermit appreciable volume flow.

The first noteworthy tidal difference occursbetween F4 and F5. These ponds are connectedby a shoal pass about 1 foot deep and in theorder of 25 feet wide which impedes the tidalflow. The impedance displays the character of anonlinear frictional process. High tides in F5are reduced in amplitude by as much as a factorof two and may lag those in F4 by as much as2 hours. The nonlinearity of the channel resis­tance enhances these effects (amplitude reduc­tion and phase delay) on the larger tides. Thetidal phase difference between F4 and F5 could

be used for limited practical purposes, and thiswill be discussed later.

The channels connecting F5 with F6, andF6 and F7 are less than 20 feet wide and lessthan 1 foot deep. In fact, the sills of thesechannels lie above the mean water level of F5,so that only the higher tides penetrate into F6and F7. These penetrations do not cause normaltides in F6 and F7; they are simply additions ofwater which tend to replace what has evapora­ted. Such additions did not occur during ourperiod of hourly monitoring, and the curves inFig. 3 show an evaporation rate of aboutO.5inchper day. From the evaporation rate, the height

Tides and Circulation at Christmas Island-GALLAGHER 201

TABLE 1

RESIDENCE TIMES IN PONDS Fl-F5

% OF POND

VOLUME

VOLUME EXCHANGED RESIDENCE

POND (FT3) PER DAY TIME (DAYS)

F1A 189 x 106 8.8 11.4FiB 154 x 106 8.8 11.4F2 274 x 106 8.8 11.4F3 105 x 106 8.8 11.4F4 42 x 106 8.8 11.4F5 30 x 106 4.5 22.2

TABLE 2

RESIDENCE TIMES IN PONDS F6 AND F7

% OF POND

VOLUME RESIDENCE

VOLUME EXCHANGED TIME

POND (FT3) PER MONTH (MONTHS)

F6 27 x 106 6.7 15F7 32 x 106 6.7 15

of the tide in F5, and the measured differencein mean water levels between F5 and F6, it canbe concluded that the occasional penetrations ofwater from F5 must occur at least as often asonce a month. It cannot be proved with thisset of measurements, but it seems highlyprobable that the penetrations occur diurnallyduring spring tides, happening 3 to 6 days in arow about every 2 weeks.

There are tidally driven changes in the heightof the water table at Christmas Island. Thepossibility arises that this has some effect on thetides observed in the ponds, but it seems quiteunlikely for several reasons. The water-tablefluctuations are damped out with increasingdistance from the coast; their amplitudes havealready decreased by about a factor of two at apoint 500 meters inland, where they are oforder 10 cm (Jenkin and Foale, 1968). Onewould expect the fluctuations to be even smallerbeneath the F-Ponds, about 5 km inland. Theactual records show no sign of any influencefrom water-table fluctuations. The amplitudesand phases of the observed tides are consistentwith their being driven only by water flowsthrough the channels interconnecting the ponds

and the main lagoon. And when the channelflows ceased in F6 and F7, no water levelfluctuations were discernible. It is concludedthat water-table tides are not influencing thetides seen in the F-Ponds.

Volume Exchanges and Residence Times

Residence times and daily percentage volumeexchanges are tabulated in Table 1 for the pondswhich have normal tidal fishing. For the com­putations, a mean depth of 15 feet was takenfor all ponds, and pond areas were planimeteredfrom a map based on aerial photos. Residencetime is computed as the volume of the ponddivided by the tidal volume-exchange rate. If apond were perfectly mixed at all times, then onthe average a water parcel would stay in thepond for the residence time. Since the pondsare probably not perfectly mixed in reality, thecomputed residence times should be treated asminimum values.

Ponds F6 and F7 do not have normal tidalexchange and must be treated differently.Volume exchange has been calculated for theobserved cycle of evaporation and occasionalrefilling. The numbers in Table 2 refer to waterbut not to dissolved substances.

Mean Water Level in the Pond Series

Mean water level for each pond is listed inTable 3, where the elevation in F1A has beenused as an arbitrary zero level. Errors in thelevel survey and in the tide readings couldamount to about ± 0.05 feet. Thus to withinthe limits of measurement accuracy, meanwater level is constant throughout the outerponds-as would be anticipated from theirfree communication with the lagoon tide. Notethat 14, which lies adjacent to the most land­ward F-ponds, is in good communication withthe main lagoon and shares the same water level.

Unlike the other table entries, the waterlevels listed for F6 and F7 do not representsteady, average values. As mentioned before,water is added to these ponds only duringspring tides; the rest of the time their levels arefalling due to evaporation at the rate of about0.50 inch per day. (In the rainy months ofJanuary-May, the average net rate of evapora-

202

TABLE 3

MEAN WATER LEVELS

PACIFIC SCIENCE, Volume 27, April 1973

during the midday hours when wind velocityis maximum, there is about 0.5 inch of set-up atthe leeward sides of the ponds. This effect canbe seen in Fig. 3.

tion over preCipitation would be reduced toabout 0.25 inch per day.) At the time of ourhourly monitoring, F6 and F7 stood about0.65 foot below the mean water level in theouter ponds. Their monthly or semimonthlyrange of water level is probably between 6 and12 inches. Most, or perhaps even all, of the time,F6 and F7 stand below the mean water level inF5 and I4--a fact which could be used in asystem of flow control.

TEMPERATURE AND WIND

Surface water temperature and wind velocitywere observed hourly during the period 8-10November, and the data are presented in Fig. 3.Surface temperature in pond F1B undergoes adiurnal cycle, peaking near midday with a rangeof about 50 C. Since no tidal effects are seen,this curve is probably typical of most of theouter ponds in the series. A similar range couldbe expected in F6 and F7, but the actual tem­peratures would probably be higher.

The Southeast Trades are present almostconstantly at Christmas Island. (The residentplantation manager could recall only 2 or 3 calmdays in the past 3 years.) During the measure­ment period the wind showed a clear diurnalpattern: strengthening to about 12 knots atmidday and dying off to 5 knots during thenight. Wind direction was also diurnal; thelighter wind blew toward about 2600 magneticand shifted to 3000 as it grew stronger. Thewinds produced no measured water level chan­ges in the ponds, except in F6 and F7 where,

POND

F1AFiBF2F3F4F5F6F714

MEAN WATER LEVEL (FT)

0.000.01

-0.04-0.05-0.04-0.05- 0.64 (on 9 November)- 0.65 (on 9 November)

0.00

POSSIBILITIES FOR ALTERING OR

CONTROLLING FLOW THROUGH THE PONDS

The phenomena discussed above will allowthree general types of flow-control utilizingenergy from the natural environment. (Avariety of additional things could be accom­plished by installing motors and pumps, butthese will not be discussed.)

It would be relatively simple to produce aone-way flow in either direction through theinner ponds in the F-series. This is due to thefortunate circumstances that lakes of the adja­cent I-series lie quite close to these ponds, andhave the tidal characteristics of the local mainlagoon. For example, a one-way-gated barrieracross the pass between F3 and F4, in combina­tion with a one-way-gated channel from F5 to13, would produce a unidirectional flushingthrough F4 and F5. The flow could be set ineither direction and would be entirely driven bythe tides. Variations on this theme could be usedto get many flushing and/or holding possibili­ties in ponds F3 through F7. Ponds F1 and F2would be more expensive to control becauseblocking their wide interconnecting passeswould involve much more extensive construc­tion work.

If F6 or F7 were to be included in the tidalflushing scheme, the channel entering eachwould have to be deepened. This could bedesirable, but it might also be useful to leaveF6 and F7 in their present state and take advan­tage of their unusual pattern of evaporationand occasional renewal. This natural flow pat­tern produces monthly or semimonthly salinitycycles in which salinity may vary by 5 to 10percent of its mean value. Other periodicitiescould be achieved by minor alterations of theinlet channels.

A third possibility exists for making use ofnaturally generated flow. Because of the non­linear impedance of the F4-F5 channel, thewater in F5 is lower than in F4 or 13 duringmost rising tides-by an amount on the order of

Tides and Circulation at Christmas Island-GALLAGHER

..... WINO

~';-;.';::O__::--~-':-':-=-=-=~2~6~.2==26."3=======-~"~ft~.=£:;;=======:::::::::::::::-­26.3

FIG. 4. Vertical temperature section across pond F6. (Section location shown in Fig. Sa.)

203

3 inches. This head difference could be used toproduce periodic flows through tanks or smallpools constructed adjacent to FS.

INTERNAL STRUCTURE AND CIRCULATION IN F6

Structure

The internal structure and circulation in asingle pond were studied to answer certainpractical questions:

1. How will nutrients be dispersed in the pond?In particular, will nutrients tend to betrapped in stagnant, lower waters?

2. How would passively floating, brine shrimpeggs be affected by circulation? Can thenatural circulation be used to collect themfor harvesting?

In general, questions such as these are verydifficult to answer in complete detail for naturalwater bodies. For a body the size of an F-pond,a carefully planned, fully instrumented studylasting several man-weeks would be necessary.In the present case, such time and man powerwere not available and advance knowledge ofthe ponds was inadequate for planning a com­prehensive study. However, one pond wasstudied for 3 days with crude techniques.Tentative answers can be offered and, ifnecessary, a more complete study could now beplanned.

F6 was selected for the preliminary survey.It is favorable because of its smaller size andsimple shape. By working during a period when

tidal effects were absent I was able to study theisolated influence of the wind rather than themore complex patterns that are present if cir­culation is driven by both wind and tides. Thissimplification not only increased the chances ofdiscerning sensible patterns during a shortsurvey, but may also have relevancy to actualconditions in an aquaculture operation. Quitepossibly certain stages of brine shrimp culture(especially egg production and harvesting)might be conducted in ponds such as F6, whichare periodically closed and purely wind-driven.

Fig. 4 is a vertical temperature section rough­ly aligned with the wind direction; the sectionlocation is indicated in Fig. Sa. The data weretaken in midmorning over a period of about1 hour. Subsurface readings were made withan instrument of questionable reliability. Thedevice showed serious zero drift, and it wasnearly impossible to get good absolute readings.The general nature of the vertical profile at eachstation was established by repetitive lowerings,and the values at each station were adjusted sothat surface temperatures agreed with mercurythermometer measurements. The section isprobably roughly correct in its main features,but it does not necessarily present an accuratepicture of details or exact absolute tempera­tures. Independent information about surfacecirculation was used in locating the inter­sections of isotherms with the pond surface.

There are four main features of the thermalstructure. The bulk of the pond is nearlyisothermal. Under the action of the wind, a poolof cooler surface water is accumulated along

204 PACIFIC SCIENCE, Volume 27, April 1973

FIG. 5. a, Tracks of drogues set at a depth of 1 footin pond F6; b, surface circulation in pond F6 as indicatedby drogue tracks; c, tracks of drogues set at a depth of5 feet in pond F6.

shoal shelf adjacent to the windward shore thereis also appreciable solar warming and evapora­tion. This appears to form water ofintermediatedensity which sinks along the bottom andspreads out with mixing at medium depths. Theexact extent of the horizontal spreading indi­cated by the isotherms in Fig. 4 is speculative.The bottom of the deep portion of the pond isoverlain by a layer of warmer (and thereforesaltier) water which appeared to be less than afoot thick. It is probably renewed (or partiallyrenewed) by the occasional formation of un­usually saline water-probably in the windwardshoals as discussed above. In connection withthis water, we note that most of the pond'sbottom, except along the leeward shore, iscovered with a thick, pink, gelatinous growth-probably a blue-green algae-which seemsto thrive in some of the more saline environ­ments at Christmas Island. The distribution ofthis algae in F6 tends to indicate the windwardshoals as the source of the bottom water.Samples of the algae dredged near the centerof the pond smelled strongly of H 2S. It couldbe that an anoxic condition is produced insidethe gelatinous layer (which is several inchesthick), or that the thin, bottom layer of water isanoxic. The existence of the warm bottom layerand its possible anoxic condition both indicatethat this water is relatively stagnant. Certainlythe lowest foot or so of water is not subject tovigorous mixing. However, it is unlikely that apotentially serious" nutrient trap" exists. Evenif molecular diffusion is the only mixing processpresent, a foot-thick bottom layer would have ahalf-life on the order of 1 week; the bottomlayer would lose over half of its excess concen­tration of any substance within 7 days. So,although the stagnant layer could tie up someof the pond's nutrients, it cannot act as a sinkor a permanent trap. (Of course, it would benecessary to minimize nutrient-use by benthicalgae, but this would be true whether thebottom layer were stagnant or not.)

The salinities in F6 were above the upperlimit of the field instrument (40%0) so distri­butions of this property were not determined.The temperature data permit some inferencesabout salinity, and these are presented above.

The temperature section and the structure itindicates are probably roughly typical of all

APPROXIM....TESHORElINE

,jj WIN'

FIG. 50

FIG. 5«:

FIG.5b

J/ WIN'

------

LEGENDV LOCATION OF VERTICAL

TEMPERATURE PROFilESX fiXED REFERENCE MARKo DROGUE fOLlOWED ON 16 NOV.• DROGUE FOllOWED ON 17 NOV.

the leeward shore. Its temperature and positionindicate that it must be the least saline water inthe pond; its origin is probably the monthly orsemimonthly inflow from FS. During the nightwhen the wind slackens, this water probablyextends farther upwind over the pond's surfaceand loses some of the heat it gains by dailywarming over the shoal, inshore flats. Theamount of this water and its physical propertiesare probably quite variable in time. Along the

Tides and Circulation at Christmas Island-GALLAGHER 205

downwind sections except those adjacent to theends of the pond. The gross features of thestructure shown are likely to be present nearlyall the time.

Circulation

Currents in F6 tended to be slow and com­plex in pattern. In this situation, relativelylittle would be learned from a fixed currentmeter, and so drogues were used. They wereset at i-foot and 5-foot depths and followedwith a skiff. Three fixed markers along thecrosswind axis of the pond and natural featureson the shoreline were used in estimating thepositions of the drogues. Any single positionmay be incorrect by 100 feet or so, but theoverall forms of the drogue tracks are fairlyaccurate. Figure Sa gives the tracks of the sur­face (i-foot) drogues, and 5b is a constructionshowing an average surface-flow pattern whichseems indicated. The wind drives the surfacewater in two large eddies which have narrow,return flows against the wind at the ends of thepond. This layer of surface circulation extendsdown less than 5 feet in the central portion ofthe pond, and to greater depths (probablyexceeding 5 feet) toward either end of the pond.In addition, the less dense water along thewindward shore seems to move in two elonga­ted eddies as shown. A typical surface currentspeed is 3 ft per minute.

The drogues set at 5 feet traced the patterns'shown in Fig. 5c. Generally the currents at thisdepth are as strong as those at the surface.Unfortunately, nothing more can be done withthe subsurface flow than to present the droguetracks. Whatever is going on is more compli­cated than the surface flow, and the data areinsufficient to allow interpretation.

Two additional points should be mentionedin connection with the pond's circulation.First, freely floating objects having even a smallexposure to the air are driven to the leewardshore by the wind. Second, near the end of theobservation period, the wind grew strongerthan at any time during the study; we estimateits speed at 12 knots. A definite pattern ofLangmuir circulation was established overmost of the surface of the pond. (In this form ofcirculation, water particles in the surface layer

are driven downwind in paths which areroughly helical. The helixes have axes in thedirection of the wind and lying parallel to thewater surface. The sense of rotation in the he­lixes alternates regularly from one to the next, sothat alternating bands of surface convergenceand divergence are created between them.Foam and floating organic material are collectedalong the convergences, producing characteris­tic, parallel" wind slicks.") From the spacing ofthe slicks observed in F6, we estimate that theLangmuir circulation was stirring the surfacelayer to a depth of about 3 feet. When thiscondition is established during strong winds, thehorizontal eddies pictured earlier probably ceaseto exist, and floating objects would be advectedtoward the leeward shore by the Langmuircells.

The natural circulation in a wind-drivenpond such as F6 could certainly be used incollecting and harvesting shrimp eggs. Theywould be conveyed to the leeward shore by thedirect action of the wind and/or by Langmuircirculation (assuming the eggs float at or nearthe surface). Under the action of the horizontaleddies that exist during light-to-medium winds,eggs that did not go aground would tend tocollect at the convergence near the midpointof the leeward shore.

ACKNOWLEDGMENTS

Each individual data point in Fig. 3 repre­sents a man walking a quarter-mile or so in thehot sun (or trying to follow a crude path with aflashlight at night), wading thigh-deep into apond, putting his eye down close to the water'soften choppy surface, and reading a gage. Some450 such trips were made by volunteer helpersfrom the expedition. The quality of the dataproves how well they cared. I am sincerelygrateful to the following devoted waders:John Ball, Andrew Berger, Fred Farrell, JohnHance, Philip Helfrich, Ed Heald, Mayo Ryder,and Mark Valencia.

I am also indebted to Fred Farrell and JohnHance for making drogues and piloting theskiff during the circulation study; they spentlong hours cruising search patterns in the hotsun.

206

Mayo Ryder and John Ball also spent longhours in the sun walking a 2-day level survey ofall the tide gages. They produced excellentresults, and I thank them.

Last, I want specially to thank two men whohelped me the most with getting equipmentready, installing it in the field, and removing itall afterward: Fred Farrell and John Hance.

LITERATURE CITED

GALLAGHER, B., and W. MUNK. 1971. Tides inshallow water: spectroscopy. Tellus 23:346-363.

PACIFIC SCIENCE, Volume 27, April 1973

GALLAGHER, B. S., K. M. SHIMADA, F. 1.GONZALEZ, JR., and E. D. STROUP. 1971.Tides and currents in Fanning atoll lagoon.Pacif. Sci. 25: 191-205.

JENKIN, R. N., and M. A. FOALE. 1968. Aninvestigation of the coconut-growing poten­tial of Christmas Island. Land ResourcesStudy 4. Vol. 1, The environment and theplantations. Land Resources Div., Director­ate Overseas Surveys, British MinistryOverseas Development, Tolworth, Surrey,England.

WIENS, H. J. 1962. Atoll environment andecology. Yale Univ. Press, New Haven,Conn. 532 p.