A Water Movement Study in Lianzhou Bay, Guangxi Province,China

J. Ocean Univ. China (Oceanic and Coastal Sea Research)DOI 10.1007/s1 1802-014- 1963-4ISSN 1672-5182, 2014 13(1): 13-22http://www.ouc. edu.cn/xbywb/E-mail:xbywb@ouc.edu.cnA Water Movement Study in Lianzhou Bay, Guangxi Province,ChinaSUN Ting), *, Andreas Macrander), and David Kaiser3)1) College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, P. R. China2) Marine Research Institute, Shkilagata 4, 121 Reykjavik, Iceland3) Leibniz Centre for Tropical Marine Ecology, Fahrenheistr, 6, D-28359 Bremen, Germany(Received March 13, 2012; revised March 22, 2012; accepted July 26, 2013)@ Ocean University of China, Science Press and Springer-Verlag Berin Heidelberg 2014Abstract This study investigates the physical conditions (water depth, current speed, salinity, temperature) in Lianzhou Bay, ashallow coastal bay in southern China, during two expeditions in the dry and wet seasons of 2011. Based on these expedition data,basic hydrodynarmic parameters like Brunt-Vaisala Frequency, Richardson Number, Rossby radius, and Resonance Period are calcu-lated. The results show that Lianzhou Bay is characterized by comparatively small quantity of freshwater input and weak stratifica-tion. Strong tides, which are spatially uniform within the bay, cause turbulent mixing. Residence time of the water is shorter in winterdue to a stronger coastal current in that season. Consideration of the water movement may help to reduce the harmful ecologicalimpact of aquaculture waste water discharge.Key words water movement; water mixing process; Rossy radius; tidal resonance; residence time; Lianzhou Bay1 Introduction2 MethodsWater movement studies aim to characterize the physi- 2.1 Study Sitecal oceanography of a given area regarding tidal regime, .Lianzhou Bay is a shallow coastal bay located on thewater mixing processes, water exchange processes, etc.northern coast of the Gulf of Tonkin, which is part of theSuch studies have been carried out on different spatialSouth China Sea (Fig.1).scales, e.g, the Baltic Sea (Osinski et al, 2010), the GulfThe Nanliu River delta borders the Lianzhou Bay. In itsof Tonkin, which is also called the Beibu Gulf (Wu et al, .vicinity are Beihai city in the east and Beibu Gulf in the2008), the Tachibana Bay of Japan (Matsuno et al, 1999),south (Fig.1). Lianzhou Bay is among the five largest baysRio of Ferrol (deCastro et al, 2004), west coast of Indian Guangxi province, Southern China. The width of its(Unnikrishnan and Antony, 1990), and the estuary systemmouth and the length of its coastal line are 25 and 79 km,of the Columbia River (Chawla et al, 2008). In the man-respectively (Chen, 2003; Lv et al, 1995; Liang and Li, .agement of aquaculture, sea water movement studies may2002). The bay is rather shallow (average depth 5 m;be employed to minimize the ecological impact of waste-Chen, 1996), and has extensive tidal flats in its northernwater. Moreover, it has been suggested that, an appropri-half. There is intensive oyster farming in the shallowate understanding of the water movement dynamics couldbe applied for any kind of anthropogenic activity insubtidal area of the bay.coastal areas (Paez-Osuna, 2001; Ulses et al, 2005; Ma-Lianzhou Bay is typical for coastal bays with moderatetsuno et al, 1999).freshwater input and large tides. Nanliu River is the larg-The site of this study- Lianzhou Bay- is one of theest fresh water source to the sea in Guangxi Province. Itcoastal regions of China which accommodate intense has three main branches: Nandong River in the westaquaculture activities. The study assesses the physical(which is the largest in terms of size and runoff), Nanxioceanography of Lianzhou Bay and its driving factors, River, and Nangan River in the east (which is the small-using in situ data (temperature, salinity, pH, dissolvedest). According to the data provided by the Changle Hy-oxygen, water depth, current speed, etc.) from severaldrological Station. the average fresh water discharge ofexpeditions during August to October in 2010 and March Nanliu River f中国煤化工8m's'. In gen-to May in 2011.eral, the largesYHC N M H Gler during Juneto August (Gu and wu, 2U01; J1ang el al, 2008). This* Corresponding author. E-mail: entergoing_ 9@hotmail.comrunoff is small compared with estuaries of large rivers包Springer4SUN et al. 1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-22like, e.g, the Columbia River Systemwhich has a runoff mostly dominant over semi-diurnal tides. Previous studiesbetween 2000m's 1 and 10000m3s-' (Chawla et al, 2008). concluded that tidal currents dominate water movement inThe sediments carried by the Nanliu River are the main the bay, where the primary pattern are reversing tidalmaterial source for the Lianzhou Bay with the material currents (Lai and Wei, 2003; Jiang et al., 2008; Sun andtransport direction from NE to SW (Gu and Wu, 2001).Huang, 2001; Liang and Li, 2002). Sea water rushes to-The bay is affected by strong tides. The mean tidal dif- ward N and NE in flood, and returns sea-ward to S andference is 2.54 m, and the largest tidal difference reaches SW after reaching the head of the bay. The average tidal5.36m (Lai and Wei, 2003). Fig.2 shows a typical tidalcurrent velocity in Lianzhou Bay is larger on the ebb thantime series spanning the time of the dry season expedition, on the flood tide, being around 104 cms' and 88 cms"demonstrating that in Lianzhou Bay diurnal tides are respectively (Chen, 1996; Chen and Shi, 1996).Beihai CityBeibu20GulfOioiseheu SraiHainanNanliu River，Souh China Sea11129 HNanxi River21*40N+Nangan RiverNandong River官2103522130*9 21006400m0108955'109-0'10995'10910*10915'EFig.1 Map of Lianzhou Bay. The location is marked by the inset map. Local bathymetry data are obtainedfrom a local bathymetry map in 1987 based on theoretical depth datum. Depths are referenced to mean sealevel. Tidal flats are shaded.Tidal chart 2011/0460050400g300I哥200M10..33...9.......33.399中国煤化工TimeCNMHGFig.2 Predicted water level at Beihai city from 2011/4/1 to 201 1/4/30 during the dry season expedition.Mean sea level is 255 cm.自SpringerSUN et al.1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-225The ecology of Lianzhou Bay is affected by the eco- in this paper), and twice on sections across the bay. Dur-nomic activities of a population of about 320000 living ing the dry season, an expedition was done during Marchwithin the catchment area of Nanliu River (Qiu and Lai, and April in 2011 with samples taken once in each area.2004). The land is mostly used for agriculture further Sampling stations across the bay are shown in Fig.3.upstream and for aquaculture closer to the coast. Local Typical parameters, ie, temperature, salinity, pH, andfarmers discharge waste water from the aquaculture dissolved oxygen were measured using a Hach HQ40dponds into ditches, from where it flows into Nanliu River. portable sensor and compatible probes (Hach Company,The average annual runoff of nutrients (nitrogen, phos- 2012). All measurements, except for DO, were made atphorus, silicate) flushed from the river into Lianzhou Bay both surface and bottom at each station of the bay and theapproaches 5900t (Chen, 2001; Qiu and Lai, 2004). Con- three river branches. Water depths larger than 1 m weresequently, the large load of nutrients and organic matterdetermined using a handheld echo sounder, while shal-creates medium. polluted and eutrophicated conditions in lower depths were measured with a wooden stick andLianzhou Bay according to Sea water quality standard measuring tape. Current speed was measured with a sim-GB 3097-1997 and Environmental quality standard for ple and efctie method recording the time for a hand-surface water GB 3838-2002 (Chen, 2001). Algal (Mi- made drift bottle to drift over a distance of 10m along acrocystis Kuetz) blooms occurred in Lianzhou Bay in moored boat or a fixed structure.March 1995, and in February 2004 (Wei and He, 1998;In addition to sampling cruises, there were five fixedWei et al, 2004; Qiu and Lai, 2005).sampling sites to observe the 24-h tidal changes. Twowere at the mouth of Nangan River during dry season(Fig.3C), and the rest were in a‘Clam house' built by2.2 Data Collctionlocal fishermen around 1 km away from the other 24-hTwo expeditions have been crried out to cllete in-situ sampling station, more out to the sea (Figs.3A, C). Thedata of physical parameters. To cover seasonal variability,serial number instead of the date in Table 1 will be usedexpeditions have been done during wet scason and dry hereafter. Sampling was done every 2h during the wetseason. The wet season extended from late August to season (Station 1) and hourly during the dry season (Sta-early November in 2010. During this period, samples tions 2- -5). Procedures and parameters are similar to sam-were taken once along each river tributary (not discussed pling cruises but taken at the same location.配公7●8B●●102●1，6400 m中国煤化工Fig.3 Bay expeditions and 24 h sampling. In (A), (B), and (C), darkTHCNMH Guises of2010/08/30, 2010/10:05, and 201 1/04/08, respectively. Numbers acn....... J. MI .use. Lightsquares denote the 24-h sampling stations for No.1 wet season, and Nos.2 to 5 dry season.包Springer16SUN et al.1JL. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-22Table 1 Description of the 24-h samplingsExecuted timeStart timetEnd timetSeasonStation site2010/09/04, 13:002010/09/05.15:00Wet1 km south of the Nangan River mouth2011/03/19, 12:002011/03/20, 12:00ryNangan River mouth201 1/03/24.17:00201 1/03/25, 17:002011/03/28, 17:002011/03/29, 17:00)ry2011/04/05. 12:002011/04/06. 12:00bry. ! km south of the Nangan River mouthote: ' yyy/mm/dd, hh:mm.2.3 Data Processingthe stratification of a water body and is proportional toCalculation of parameters such as Brunt-Vaisala Fre-the stability of stratification (Houry et al, 1987; Osinskiquency (BVF), Richardson Number (Ri), and water vol-et al, 2010).ume differences related to tides are based on in situ data ofThe Brunt-Vaisili Frequency N in a given depth isLianzhou Bay. The potential density was calculated usinggiven by:the UNESCO equation of state (Fofonoff and Millard,|g dp(1)1983). Data were visualized using Ocean Data ViewVPo dz(Schlitzer, 2012) and overlain with a modified map pro-where g -9.81 msis the acceleration of gravity, ρo is thevided by colleagues from the Guangxi Mangrove ResearchCenter (GMRC) in Beihai, Guangxi, China. Numericalpotential density of the water, dp/dz is the vertical densitymodelling of the current dynamics in the bay is begradient. With just two data points at the surface and thethe scope of this study, as no high-resolution dataset of bottom. Pg(bottom)- Pg(surface)will be used as ap-the local bathymetry is available to set up a high-resolu-Depthtion occan circulation model for Lianzhou bay. Neverthe- proximation for dp/dz.less, it is possible to determine the general characteristicsIn Lianzhou Bay, N ranges between 91 cph (cycles peroftthewateT movement in the bay by hyrodynamic key hour) during wet season and 59 eph during dry season.parameters based on the data of the expeditions.These values are somewhat lower than in, e.g, the WeserEstuary (North Sea), which is under a stronger riverineinfluence (average runoff 323 m3 s', Grabemann and3 Results and DiscussionKrause, 2001). There, a mean N of 127 cph was found3.1 Physico-Chemical Profiles of Lianzhou Bay(Macrander, 2009), which indicates a stronger srtificae-Fig.4 shows the temperature and salinity distribution intion, especially during ebb tide when the fresh waterLianzhou Bay. Limited by the shallow water, the first wetflows out faster than the. (Geyer, 1993).season cruise on 2010/8/30 was conducted during a semi- In another highly stratifed water system, Baltic Sea, N isdiurmal tidal flood and the first half of the ebb; the dryfound between 126 and 198 cph within thermo- and halo-season cruise on 2011/4/8 was launched while a diurnal clines (Osinski et al, 2010). Compared to these examples,tidal flood started.stratification in Lianzhou Bay is weaker.Fig.4A gives a clear example of the seasonal variationFig.5 shows N at the surface of the bay during wetof water temperature. The average temperate was 30C season (2010/8/30) and dry season (2011/4/8). Towardsduring the wet season (AI) and 23C during the dry sea- the coast, salinity and density are lower, with the lowestson (A2). No significant temperature gradient was ob-salinities ocurring close to the river mouths. Stratifica-served between surface and bottom water. During wet tion in the rather shallow coastal areas is weak, too (smallseason, water temperature is rather uniform across the bay N in, e.g., Station 3 of Fig.5A). In contrast, a higher salinewith values up to 31.75C. During the cooler dry season, bottom layer is found in the seaward Station 8 (Fig.5A),temperatures decrease from the river mouth (approx.associated with a stronger stratification. Measurements at25' C) to the deep bay area (approx. 21"C).Stations 8 -10 were，; taken during ebb tide; obviously, theSalinity within the bay is relatively high. During wet lighter fresh water at the surface flows out faster, whileseason, sea surface salinity ranges between 22 and 26，close to the bottom, a salt wedge persists. In dry season,while at the bottom a salt wedge is found at the seawardstratification is weaker also in the seaward part (Fig.5B),edge with a salinity of 31 (Fig.4 B1). During dry season，as the fresh river water does not spread that far. The datafresh water at the surface does not spread as far, with a were taken during flood, hence, tidal current inducessignificant salinity gradient between the river mouths and turbulent mixing which causes equally distributed density,the southerm (outer) part of the bay, where the salinityconsequently, N at the bay mouth decreases to about 30increases to 31 (Fig.4 B2).cph (Stations 1 and 2). The higher N in Stations 3- -5 is .caused mainly by中国煤化ature rather3.2 Major Hydrodynamic Parametersthan lower salinityations weretaken during noon:THCNMHGerisaddi-3.2.1 Stratification/runt-Visili frequencyThe Brunt-Visili Frequency (BVF) N is a measure oftionally heated up by solar radiation, accounting for alarger vertical density gradient.SUN et al.1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-2217Temperature (C )@Depth (m)=TopTemperature (C )@Depth (m)=Bottom21.5832 21 58°NT用21.5621.54021.5421.5221.52921.50°21.502A0)109.02° 109.04° 109.06° 109.08°E09.04° 109.06° 109.08*ETemperature (C )@Depth (m)= Bottom3221.56°N2621.52021.50^21.50009109.020 109.04° 109.06° 109.080 109.1E09 109.029 109.040 109.06° 109.08 109E"Salinity@Depth (m)=TopSalinity @Depth (m)=Bottom21.589N[3221.58°N3021.56°21.56^2821 54022B(0)109.02 109.040 109.06 109.089E20109.029 109.040 109.06° 109.08°ESalinity@Depth (m)= Top21 .56°N-2621.50， 21.50°中国煤化工22BaYHCN MH G20109 109.029 109.04° 109.06° 109.08° 109.1E109° 109.02 109.04 109.06” 109.08” 109.1EFig.4 Temperature (A) and salinity (B) distribution in Lianzhou Bay in 2010/8/30 (1) and 2011/4/8 (2).自Springer18SUN et al. 1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-22speed generally occurred during late stage of flood tides(Fig.6D). Consequently, Ri is largest during periods of21935'Nlarge N and low current speed (Fig.6E). While during thehigher low tide, a stable stratification persisted for sometime, current velocities were large enough to keep Ri12:below 0.25 at all other times. During the lower low tidethe water depth was so small (less than 1 m), that thestratification was completely removed by turbulent mix-100ing.219301Relying on the calculated N and Ri, we conclude thatconditions in Lianzhou Bay are mostly dominated byturbulent mixing. This results in a vertically largely ho-mogeneous distribution of salinity and density, while it21935Nweakens the baroclinic flow. Considering the shallowness,we conclude that currents in Lianzhou Bay are largelybarotropic, i.e, in the same direction throughout the en-tire water column.一Depth(m)-一 Salinity Surface (%m)一Salinity-Bottom(%o) AR21930-自2.50+2:20 自「B君2:0109-0010905'10910E含1:510Fig.5 Distribution of N on the surface of the bay. (A))o2010/8/30, during wet season; (B) 2011/4/8, during dry- Depth(m) 一-p-srface(kgm) -一pO Bottom(kgm") B 1op2 E1024 日season1020 S3.2.2 Turbulent mixing/Richardson number-1014 日41012二A stable stratification with a fresh water surface layer-1010and a more saline deeper layer can be removed by turbu-lent mixing, provided that the potential energy of the4.0-Depth(m) -N-800。700stratification is smaller than the kinetic energy which is600_associated with a sheared current. This ratio between Epot目 2.5500 3and Ekin is defined as Richardson number Ri:400 5-300 7N2200i= .(du/dz)-一Depth(m) +Current Speed (ms')-0.35 inwhere N denotes the BVF, and du/dz the vertical gradient合 3,00.3 50.25 8of the current speed. If Ri<0.25, turbulent mixing break-三 2.00.2 ging up the stratification can be expected (Galperin et al,昌 1.50.15+0.12007). As current speed was measured only at the surface，0we use utrecH (with water depth H) as approximation0.40.054for du/dz, assuming zero current speed right at the bot-一Depth(m) + Ri---Critica. value司100tom.As current speed data under consideration are limited三 2.5-0.1to the 24h sampling, Ri is accessed only for these time- .500fseries. Turbulent mixing was prevalent during 66% of the口 1.040.001observation time, when Ri was smaller than 0.25, whereasduring 34% of the time (mostly during slack tide), Ri was0.0001larger than 0.25. As an example, Fig.6 shows data fromTimNo.5 24-h sampling. The potential density profile islargely determined by salinity (Figs.6A, 6B). The largestFig.6 24-h sampling No.5: Temporal variation of salinitydensity difference between surface and bottom occurred(A), potential density (B), N (C), current speed (D) and Riaround 14:00 on 2011/4/5 during the higher low water(E). Tides are indieagted hwthe. mwithin the sampling period. The density difference im-(grey bars)."中国煤化工e dephplies maximum N at the same time (Fig.6C). The smallstYHCNMH Gmeasured current speed was 0.09 ms ' and normally OC-3.2.3 Earth rotation/Rossby radiuscurred during slack tide at low water, while the highestOn large scales, oceanic currents are deflected to the包SpringerSUN et al.1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-229right (northern hemisphere) by the rotation of the earth. A with the measured water depth at the 24-h samplings, itcharacteristic length scale is given by the Rossby radius. was confirmed that the tides arrive at the mouths ofAs currents in Lianzhou bay are largely barotropic, the Nanliu River with a very short time lag compared to Bei-barotropic Rossby radius Lr is assessed here:hai Port (Fig.7). The predicted tides closely match theobserved ones; the only significant differences occurredVgHduring events of anomalous southerly wind stress.Lr=f’(3)500-■Beihai City450where g=9.81 ms ' denotes the acceleration of gravity, Hpredicted tideswater depth, and f = 0.521x10 4 s -1 denotes the Coriolis; 350-+ No.424-hparameter (Osinski et al, 2010).; 300water depth (m)With the measured depth between 1m and 5.6m, Lrranges from 58 to 141 km. This is far larger than the sizee 150of the bay (24 km), hence, current dynamics in Lianzhoubay are not significantly affected by the rotation of theearth, i.e, currents will not be focused to the coast on theright-hand side.40350{目300. No.5 24-h3.2.4 Tidal resonance6 250water depth (m)。Tides in bays can be amplified by interference between三200incoming and reflected tidal waves (e.g, in Bay of Fundy,点150-Greenburg, 1979; in the Strait of Georgia, Forman, 2005),三105provided the resonance period of the bay is close to the :tidal period.3138283圣圣圣The shortest possible basin length L which producesTimeresonances of tidal waves with period T (Leder and Orlic,Fig.7 comparison between tidal sampling data and pre-2004) is given by:dicted water depth. (A) Station No.4 24-hour sampling.4LNote the higher water level around midnight when south-T=(4)erly wind was prevalent for several hours (B) Station No.√gh5 24-hour sampling.with acceleration of gravity g and water depth H.With typical values for Lianzhou Bay, i.e., L=15 kmThe nearly simultaneous arrival of tides within Lian-and H=5m a resonance period of T=2.38h is found. Thiszhou bay is further supported by tide models which showis much shorter than the main tidal periods, which are 12the phases of the dominant tides at the Guangxi coastalh (S2), 12h 25 min (M2), 24h (S1), or 24h 50min (O2).areas, i.e, OI, Kr and M2 (Sun and Huang, 2001). Gener-To be in resonance with semidiumal tides (=12h 25 lly on the northem hemisphere, tidal waves roate coum-min), for instance, the length scale of the bay would needterclockwise around amphidromic points. In Lianzhouto be at least 78 km, which is by far larger than the actualBay, all tides progress from SE to NW, and it needs onlysize of Lianzhou Bay. Therefore, the tidal range will not8min for O1 and Ki or 4min for M2, respectively, to .be amplied within the bay. This is spored by the travel from Beihai city to the northwestem estuaries ofclose agreement between predicted tides in Beihai cityNanliu River. These are much shorter than the tidal peri-and the measured tides at the 24-hour samplings (Fig.7).ods, hence, high and low tides all occur almost simulta-Tidal waves are long gravity waves, which travel withneously throughout Lianzhou bay.a phase speed of√VgH 。Hence, the wave length λ isThese findings confirm that due to the small scale of thebay, the current dynamics are not considerably affectedgiven byby the earth rotation. Tide phase, amplitude and currentλ=T√gh.( 5) are found to be rather uniform over the entire bay area.Take again T=12h 25 min as an example, then λ equals 3.3 Coastal Currentsto 313 km. Referring to Sammari et al.'s work (2006),The sea water input into Lianzhou Bay stems from thewhen the length scale is smaller than one quarter of thewave length (L<312/4=78.2 km), then the semi-diurnalThose currents are part of the general wind driven watertide will be distributed almost equally everywhere in thecirculation in Beibu Gulf, of which Lianzhou Bay is abay. This is a plausible parameter to support the predic-sub-embayment(Sytion of tidal distribution dyamics in the next section.Cyclonic中国煤化工during winter.Though thereHC N M H Gmer circulation3.2.5 Tidal distribution dynamic(Yuan and Deng, 1999; Bao et al, 2005), newer studiesThrough comparing the predicted tides at Beihai City indicate a (weaker) cyclonic circulation also during sum-自Springer20SUN et al. 1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-22mer due to wind and tidal forcing (Cai et al, 2003), and and less distinguishable. Because of the shallow depth ofinflow through the Qiongzhou Strait between Hainan and the study site, and the largely barotropic character of theChina's mainland (Wu et al, 2008). The circulation in- circulation in Beibu Gulf (Wu et al, 2008), it is feasiblecludes mainly two components, i.e, a tidal residual cur-to assume that the surface current accounts for the waterrent and a wind driven current. The latter is evidently renewal in Lianzhou Bay. The velocity of the NW coastalstronger (Sun et al, 2001), with estimated current speeds currents at the mouth of Lianzhou Bay, summarized frombeing 3.5 times larger than the residual current inducedSun et al. (2001), is shown in Table 2.by tides. Due to the prevalent strong NE monsoon winds .Observations and calculations reveal that the coastalduring winter, the wind-driven current in Beibu Gulf current off Lianzhou bay is always directed to northwest,forms a cyclonic circulation. During summer, wind forc- but is 4 times faster during winter, associated with theing and hence wind driven circulation is generally weakerstronger wind stress during that season.Table 2, coastal current velocities of Lianzhou Bay'Wind driven currentTidal residual currentTotal currentSeasonCurrent directionvelocity (ms )velocity (ms")velocity (ms |)Winterto NW0.15.2Summer0.05Note: data obtained from Sun et al. (2001). .the scale of days, and is faster in winter than in summer.3.4 Residence TimeIn comparison, water exchange due to freshwater inflowResidence time is a useful value to estimate the contri-is much smaller, with residence times in the order ofmonths.T he longer residence time within the bay during sum-water exchange processes in the bay. It is defined as themer implies more accumulation of nutrients from aqua-period of time that a water parcel needs to leave a par-culture runoff.ticular area (Takeoka, 1984).If the water in Lianzhou bay were to be renewed onlyby the fresh water runoff from Nanliu river (average dis-charge 168m's，i.e., 0.4km' per month), it would take4.5 months until the entire volume of the bay of 1.79km’is replaced. However, water is also exchanged on the 2193SNseaward side of the bay. Tidal currents in Lainzhou bayare mainly flowing back and forth, as tides arrive simul-taneously everywhere across the bay, and the bay is toosmall for currents to be affected by the rotation of theearth. Hence, tidal currents themselves do not contribute21930+to water renewal, carrying the same water into and out ofthe bay. The coastal current is not expected to turn rightinto the bay as the bay is smaller than the barotropic108955'10901095'109 10'ERossby radius (as discussed above), and is shallower thanFig.8 Conceptual sketch of coastal current along thethe water in the offshore main pathway of the coastalmouth of Lianzhou Bay (thick arrows) and tidal currentcurrent. Nevertheless, during ebb tide water from Lian-in Lianzhou Bay (thin arrows). Water from the southernzhou bay is displaced seaward, where it is carried north-part of the bay is gradually displaced to the northernwestward with the coastal current (Fig.8). At the samepart (open arrows), while 'new' water enters the south-time, 'new' water is advected by the coastal current to theern part (black arrows).southern part of the bay, where it enters the bay with thenext flood. We predict that by this interaction between4 Conclusiontidal currents and coastal current, water from Lianzhoubay is gradually displaced to the northwest, and replacedThis study assessed water movement processes in theby 'new' water from the southeast.shallow Lianzhou Bay, Guangxi Province, China. TheDuring winter, water of the coastal current flows withfindings are based on in situ data of temperature, salinity,0.2ms , and thus needs about 1.4d to pass across the 24 current speed, etc, which are used to calculate severalkm width of the mouth of Lianzhou Bay. Hence, every hydrodynamic parameters, i.e, Brunt-Vaisala Frequency,1.4d, all the water flushing the bay due to the tides has Richardson Number, Rosby radius and Resonance Pe-been completely renewed by Gulf water from the coastal riod. Owing to theI 中国煤化工3re unifomcurrent. During summer, with the coastal current being throughout the entconditionsweaker at 0.05 ms , renewal may take 6 d. Although in the bay are fourMHCNMHGmixedduesome aspects are simplified in this concept, it suggests to high tides, and with only moderate freshwater runoff.that water renewal by tides and coastal current occurs at Water is primarily renewed by the combined action of包Springer22SUN et al. 1J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2014 13: 13-22nisia. Continental Shelf Research, 26: 338-350.Unnikrishnan, A. s., and Antony, M. K, 1990. On verticalSchlitzer, R., 2012. Ocean Data View. htp://dv.awi.de.velocity fluctuations and internal tides in an upwelling regionSun, H. L, and Huang, W. M, 2001. Observation, analysis andoff the west coast of India. Estuarine, Coastal and She!f Sci-numerical study of the tide and current in the Guangxi off-ence, 31: 865-873.shore area. Journal of Oceanography of Huanghai and Bohai Yuan, S, and Deng, J., 1999. A numerical study on circulationSeas, 14 (4): 12-21 (in Chinese with English abstract).in the Beibu Gulf. Nanhai Yanjiu Yu Kaifa, 12 (2): 41-46 (inSun, H. L, Huang, W. M., and Zhao, J. s, 2001. Three-dimen-Chinese with English abstract).sional numerical simulation of tide-induced, wind-driven and Wei, M. X, and He, B. M., 1998. A study on eutrophication andthermohaline residual currents in the Beibu Gulf. Oceanolo-red tide formation in Lianzhou Bay. Tropical Oceanology, 17gia Et Limnologia Sinica, 21 (5): 561-569 (in Chinese with(4): 66-72.English abstract).Wei, M. x, He, B. M., and Lai, T. H, 2004. The temporal andTakeoka, H，1984. Fundamental concepts of exchange andspatial distribution of pH value and DO and their relationtransport time scales I a coastal sea. Continental Shelf Re-with the environmental factors during the formation of thesearch,3 (3): 311-326.Algal Bloom in Lianzhou Bay. Guangxi Sciences, 11 (3): 221-Ulses, C, Grenz, C, Marsaleix, P, Schaaff, E., Estournel, C224.Meule, S, and Pinazo, C, 2005. Circulation in a semi-en- Wu, D. x., Wang, Y, Lin, X. P., and Yang, J. Y, 2008. On theclosed bay under influence of strong freshwater input. Jour-mechanism of the cyclonic circulation in the Gulf of Tonkinnal of Marine Systems, 56: 113-132.in the summer. Journal of Geophysical Research, 113: 1-10.(Edited by Xie Jun)中国煤化工YHCNMH G自Springer