SCI. MAR., 65 (S@. 1): 155-166 SCIENTLAMARINA 2001 AN INTERDISCIPLINARY VIEW OF THE OCEAN. J.L PUEGR~;1. ALONSO and J. ARÍSTEGUI (d.) Tidal currents and mixing in the Lake Maracaibo estuarine system* 'Dcpanamemo & Físia Facultad & Cimcias del Mar, Univasidad de Las Pllmas & Gran Canaria, CampusUnivasitwo~Tafw.35017LasPalmas&GranCanana,Spain. 3NTEVEP S. A.. Centro de investigaci6n y Apoyo Tecnológico, Fide F'cm5leos & Vcnaue1a S. A., Los Tcqua, EscadoMirandavenezaela SUMMARY: The Lake Maracaibo cstuarinc systcm is a cooscilIating coupled ocean-lakc systern connected through a par- tialiy mixed estuay. In this paper we examine thc low frcsuency movcmam (o< 0.1 cyclcs/day) in uie differmt water bod- ies within thi system. The harmonic and rorary specaal analyscs of velocity data indicatcs the importante of tidai forcing within thc whole system and its intaauion with winds and fresh warcr discbarge. ln particular, the nsults indicate thc exis- te~of dominant semi-mdy osciUarions in iak Maracáibo. kely of non-iincar tidal origin, and suggest thai vertical smififaetion plays an important dein coí~troiiingthe three-dimcnsional strucnire of tidal cmu. Key words: estuarine sysrcm, tidal forcing, non-linear phenomena, stdfication. mixing, Caribbean Sea. and current distributions with limited data sets. The existing stratification, on the other hand, wili shape Any dynamicd study of an estuarine system has the instantaneous and mean cunrents. Stratification to include both current and density distributions, will enhance lateral advection but reduce vertical simply because rhey are deeply inte~co~e~ted.The exchange. Wíth strong vertical stratificatian, for relative forcing of river discharge and tidal range, exarnpie, the upper and lower layers may be rather togefher with the effect of winds, waves, and the uncoupled, the tidal wave having quite different geomorphology of the basin, will determine the amplitudes and phases. Ievel of vertical and horizontal stratification. In par- Furttiennore, tidal propagation is a highly non- ticular, stratif~cationwill be modified as the tidal linear phenomenon, which results in higher harmon- current changes its strength through an individual ics and residual currents with pronounced spatial tidal cycle or the neap-spnng cycle: either by differ- gradients. The effect of a non-linear mechanisrn on ential horizontal advection or through shear-induced tidal constituents is to cause a modulation and/or mixing. This results in a rapid response of he estu- distortion of these constituents, and to conmbute arine system and makes it difficult to assess the prin- towards a non-zero residual (zero-frequency) circu- cipal factors respnsihk fa &e nhmed de.nsity izhm -!m, 10531). The impftnt m!e ef SE& i residual cixculation in the net salt and sediment transport, as well as in pollutant flushing, within the TDAL CURRENTS AND MIXING 155 estuary, dernands an accurate description of the tidal tia1 variations in the phase and arnplitude of tidal circuiation in estuaries (LeBlond, 1991). currents as the estuary widens and the degree of The citculation-stratifícation feedback mechanism stratification changes. It appears that the tide is takes place in many different interading ways; rnak- gdy mudified throughout the esniary, mainly ing the overall pichire diacult to grasp and changing because of the way the different constituents res- this picture fnmi one estuary to another. The biologi- onate with the different basins composing the sys- cal and chemical consequences of this interaction are, tem, but also because of the cfeation of higher tidal however, important. A stable density gradient may harmonics. increase the horizontal dispersion and conmbute to the entrance of salt, nutrients and poliutants into the The Lake Maracaibo estuarine system estuary. Periodic and episodic vdcal homogeneity is responsible for the replenishment of nutrients in the This is a coupled ocean-lake system connected surface water, and oxygen in bottom waters, but may through a partially mixed estuary, located on rhe reduce the entrance of salt water. A subsequent peri- Caribbean coast of Venezuela. The dynamic charac- od of stratification in a shallow water column may teristics of rhis estuarine system result from the then lead ?o anoxic conditions, determhhg the vexti- motion and interaction of sea and fresh water in its cal transfer of heat and light. different water bodies: gulf, bay, síraii, lake, and In this wark we examine the propagation and nvers (Fig. 1). The Gdf of Venezuela cornmunicates modification of the tide through heLake Maracaibo at its southern boundaq with El Tablazo Bay estuarine system, in particular investigating he spa- through three narrow inlets. El Tablazo Bay is a - - CARIBBEANSEA - RG. 1. - Map of tbc Lake Maracaibo esniarine systcm with thc location of stations (black boxcs) and transcas (do& lim) used in this study. broad shallow embayment enclosed by banier bars and become domuiant in the southem comer of the and ishds and wmected with the Shait of Mara- Gulf of Venezuela, within El Tablazo Bay, and in the caibo. El Tablazo Bay and the Strait of Maracaibo Strait of Maracaibo. In Lake Maracati, because of form a typical paríiaüy mixed estuary. The Lake of its dimensions, the diumal constiments again pre- Maracaibo, a salt stratified water body, is located at vaiL Tlus, a majar characteristic of the Lake Mara- the southem end of &e Straít of Maracaibo. A navi- caibo eshiarine system is that the tidal ngirne gation channel, that reaches depths of 15 m, crosses changa hmmixed diumai to semidid in its the system from the Gulf to the Me. northern portion, to mainiy semi-diumal in its en- ?he circuiatíon in the esniarine system is con- trai region, and to diumai in its southem extreme tmiied by the conjunction of wind, river discharge, (RedfieId et al., 1955; Redfield, 1961; Molines e? and tidal forcing. In this work we have concentrated d,1989). on tidal forcing and its interacton wiCh other Redííeld's (1961) md Molines and Fornerino's processes. are important in this system, being (1985) analyses for the semidimal constituents responsible for cun-ent reversais and &-water show the existente of a nodai hein the northern intnisions. In particular, it is through tidal action that end of the Me,near La Salina, whüe an antínode is salt water enfers El Tablazo Bay and the Strait of present in the central portion of E1 Thlazo Bay. The Maracaibo, where it mixes with the outfiowing lake didconstiaieuts have a node con&nt with the water. The resuiting brackish mixture enters the lake semi-diurna1 antinode fod in El Tablam Bay. at its northem end, and flows down along the bottom Tidal amplitudes within Lake Maracaibo are las to fom a saline hypolimnion (iower layer). The two- than 3 cm at various frequency bands. Non-linear layer systern is stimd up, with salt being transport- interaction in the Bay and the Strait, however, gen- ed up into the epilimnion (upper layer). The water of erates smalI but measurable overtides in the third the epilimnion has a mean cyclonic circulation, with and fourth diurna1 bands, which have received littie surface water escaping from the lake along the east- a&on to date (Lynch er al., 1990). em shonr. The reiative importarice of the forces responsible for the generation of the cyclonic circu- lation is not yet well known. OBSERVATIONS AND METHODS The precipitation on the Lake Maracaibo basin p~senr;rmaximnm- vdue- in T)-~~~r,/Nov~-~rz~'nd_ Dwig he 1980 and 19Ws mVEP Apri1/May, and mullmurn values in February/March (Research and DeveIopment Center of Petróleos de and JulyIAugust. The October/Novernber maximum Venezuela, S.A.) made a number of measurements is greater than the ApnVMay, while the Febru- and deployed cmnt meter mysat stations situat- &-y., r--UU js iSíriucbl Iower 'uLiiili ,.A -l..-- *La -..k.-L-- --.-*-- 1- *L.:.. --u ...- U3 rUUiig U& WLUdiltLC 3Y5LClll. 111 ULW y-1 WG 4LId- July/August minimurn. The surface water flows out lyze wo different types of data set. FKst, the densi- along the Strait of Maracaibo and through El Tabla- ty data of two ~tansectsalong the estuary, talcm from zo Bay with velocities related to recent rainfail over El Tablazo Bay to the soiithern end of the Saait of tíle drainage basin, mking with sdt water &o- MaTacaiMaTacaiboduring flood and ebb tide condilions. duced by tidal action frorn &heGulf of Venezuela. Second, the current meter data recorded at several The wind variability in the estuarine system stations during both high and low lake-runoff condi- depends on the northeastern trade winds and sea- tions. Specificaily, we examine the current data from land breezcs. The bade winds blow dl year in thís two stations in the Gulf of Venezuel% tluee stations a being more kitense during the December-April along the main stem o£ the Strát, three stations period. The sea-land breezes present diurna1 oscilla- across the entrance to Lake Maramibo, and two sta- tions and predominate when &e üade winds are less tions in Lake Maxacaibo. 'ihe location of ihe stations uitense between May and November. is shown in Figue 1, and the instniment and water bottom &pths are &en in Table 1. Tidal forcing in the Lake Maracaibo estuarine Hourly water velocity were obtained frorn 10 rvctnm UJ Y.- rin.u*aizvtrzge Om&m mtm mm=.t~UI: &e. The data were hasmonicaüy analysed using a leas- Tides in the Caribbean Sea are of rnixeddiurnal squares technique. To investigate the structure of the type. The serni-diumai tides, however, resonate in tidai cmnsit is advantageous to use the clockwise ,e Guü oi Venezuela 'because oi its ciimensions, md anriciocicwise row compnents. Tis form oi TiDAL CURRENTS AND MZXING 157 TABLE1. - Station~in the Lake Mamcaibo Estuarine Systcm. polarized if one of the two components is zero, rec- tilinear if both circularly- -polarized components have Station Location Stationdepul lnstrummtdepth the -e magnitude.