SUN Ting, Andreas Macrander, and David Kaiser

1) College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, P. R. China

2) Marine Research Institute, Skúlagata 4, 121 Reykjavík, Iceland

3) Leibniz Centre for Tropical Marine Ecology, Fahrenheitstr, 6, D-28359 Bremen, Germany

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

SUN Ting1),*, Andreas Macrander2), and David Kaiser3)

1) College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, P. R. China

2) Marine Research Institute, Skúlagata 4, 121 Reykjavík, Iceland

3) Leibniz Centre for Tropical Marine Ecology, Fahrenheitstr, 6, D-28359 Bremen, Germany

This study investigates the physical conditions (water depth, current speed, salinity, temperature) in Lianzhou Bay, a shallow coastal bay in southern China, during two expeditions in the dry and wet seasons of 2011. Based on these expedition data, basic hydrodynamic parameters like Brunt-V?is?l? Frequency, Richardson Number, Rossby radius, and Resonance Period are calculated. The results show that Lianzhou Bay is characterized by comparatively small quantity of freshwater input and weak stratification. Strong tides, which are spatially uniform within the bay, cause turbulent mixing. Residence time of the water is shorter in winter due to a stronger coastal current in that season. Consideration of the water movement may help to reduce the harmful ecological impact of aquaculture waste water discharge.

water movement; water mixing process; Rossy radius; tidal resonance; residence time; Lianzhou Bay

1 Introduction

Water movement studies aim to characterize the physical oceanography of a given area regarding tidal regime, water mixing processes, water exchange processes,etc.Such studies have been carried out on different spatial scales,e.g., the Baltic Sea (Osińskiet al., 2010), the Gulf of Tonkin, which is also called the Beibu Gulf (Wuet al., 2008), the Tachibana Bay of Japan (Matsunoet al., 1999), Rio of Ferrol (deCastroet al., 2004), west coast of India (Unnikrishnan and Antony, 1990), and the estuary system of the Columbia River (Chawlaet al., 2008). In the management of aquaculture, sea water movement studies may be employed to minimize the ecological impact of wastewater. Moreover, it has been suggested that, an appropriate understanding of the water movement dynamics could be applied for any kind of anthropogenic activity in coastal areas (Páez-Osuna, 2001; Ulseset al., 2005; Matsunoet al., 1999).

The site of this study–Lianzhou Bay–is one of the coastal regions of China which accommodate intense aquaculture activities. The study assesses the physical oceanography of Lianzhou Bay and its driving factors, usingin situdata (temperature, salinity, pH, dissolved oxygen, water depth, current speed,etc.) from several expeditions during August to October in 2010 and March to May in 2011.

2 Methods

2.1 Study Site

Lianzhou Bay is a shallow coastal bay located on the northern coast of the Gulf of Tonkin, which is part of the South China Sea (Fig.1).

The Nanliu River delta borders the Lianzhou Bay. In its vicinity are Beihai city in the east and Beibu Gulf in the south (Fig.1). Lianzhou Bay is among the five largest bays in Guangxi province, Southern China. The width of its mouth and the length of its coastal line are 25 and 79 km, respectively (Chen, 2003; Lvet al., 1995; Liang and Li, 2002). The bay is rather shallow (average depth 5 m; Chen, 1996), and has extensive tidal flats in its northern half. There is intensive oyster farming in the shallow subtidal area of the bay.

Lianzhou Bay is typical for coastal bays with moderate freshwater input and large tides. Nanliu River is the largest fresh water source to the sea in Guangxi Province. It has three main branches: Nandong River in the west (which is the largest in terms of size and runoff), Nanxi River, and Nangan River in the east (which is the smallest). According to the data provided by the Changle Hydrological Station, the average fresh water discharge of Nanliu River from 1954–1982 reaches 168 m3 s?1. In general, the largest discharge occurs in summer during June to August (Gu and Wu, 2001; Jianget al., 2008). This runoff is small compared with estuaries of large riverslike,e.g., the Columbia River System which has a runoff between 2000 m3 s?1and 10000 m3 s?1(Chawlaet al., 2008). The sediments carried by the Nanliu River are the main material source for the Lianzhou Bay with the material transport direction from NE to SW (Gu and Wu, 2001).

The bay is affected by strong tides. The mean tidal difference is 2.54 m, and the largest tidal difference reaches 5.36 m (Lai and Wei, 2003). Fig.2 shows a typical tidal time series spanning the time of the dry season expedition, demonstrating that in Lianzhou Bay diurnal tides are mostly dominant over semi-diurnal tides. Previous studies concluded that tidal currents dominate water movement in the bay, where the primary pattern are reversing tidal currents (Lai and Wei, 2003; Jianget al., 2008; Sun and Huang, 2001; Liang and Li, 2002). Sea water rushes toward N and NE in flood, and returns sea-ward to S and SW after reaching the head of the bay. The average tidal current velocity in Lianzhou Bay is larger on the ebb than on the flood tide, being around 104 cm s?1and 88 cm s?1, respectively (Chen, 1996; Chen and Shi, 1996).

Fig.1 Map of Lianzhou Bay. The location is marked by the inset map. Local bathymetry data are obtained from a local bathymetry map in 1987 based on theoretical depth datum. Depths are referenced to mean sea level. Tidal flats are shaded.

Fig.2 Predicted water level at Beihai city from 2011/4/1 to 2011/4/30 during the dry season expedition. Mean sea level is 255 cm.

The ecology of Lianzhou Bay is affected by the economic activities of a population of about 320000 living within the catchment area of Nanliu River (Qiu and Lai, 2004). The land is mostly used for agriculture further upstream and for aquaculture closer to the coast. Local farmers discharge waste water from the aquaculture ponds into ditches, from where it flows into Nanliu River. The average annual runoff of nutrients (nitrogen, phosphorus, silicate) flushed from the river into Lianzhou Bay approaches 5900 t (Chen, 2001; Qiu and Lai, 2004). Consequently, the large load of nutrients and organic matter creates medium-polluted and eutrophicated conditions in Lianzhou Bay according to Sea water quality standard GB 3097-1997 and Environmental quality standard for surface water GB 3838-2002 (Chen, 2001). Algal (MicrocystisKuetz) blooms occurred in Lianzhou Bay in March 1995, and in February 2004 (Wei and He, 1998; Weiet al., 2004; Qiu and Lai, 2005).

2.2 Data Collection

Two expeditions have been carried out to collect in-situ data of physical parameters. To cover seasonal variability, expeditions have been done during wet season and dry season. The wet season extended from late August to early November in 2010. During this period, samples were taken once along each river tributary (not discussed in this paper), and twice on sections across the bay. During the dry season, an expedition was done during March and April in 2011 with samples taken once in each area. Sampling stations across the bay are shown in Fig.3. Typical parameters,i.e., temperature, salinity, pH, and dissolved oxygen were measured using a Hach HQ40d portable sensor and compatible probes (Hach Company, 2012). All measurements, except for DO, were made at both surface and bottom at each station of the bay and the three river branches. Water depths larger than 1 m were determined using a handheld echo sounder, while shallower depths were measured with a wooden stick and measuring tape. Current speed was measured with a simple and effective method recording the time for a handmade drift bottle to drift over a distance of 10 m along a moored boat or a fixed structure.

In addition to sampling cruises, there were five fixed sampling sites to observe the 24-h tidal changes. Two were at the mouth of Nangan River during dry season (Fig.3C), and the rest were in a ‘Clam house’ built by local fishermen around 1 km away from the other 24-h sampling station, more out to the sea (Figs.3A, C). The serial number instead of the date in Table 1 will be used hereafter. Sampling was done every 2 h during the wet season (Station 1) and hourly during the dry season (Stations 2–5). Procedures and parameters are similar to sampling cruises but taken at the same location.

Fig.3 Bay expeditions and 24 h sampling. In (A), (B), and (C), dark dots denote bay expedition cruises of 2010/08/ 30, 2010/10/05, and 2011/04/08, respectively. Numbers account for the direction of the cruise. Light squares denote the 24-h sampling stations for No.1 wet season, and Nos. 2 to 5 dry season.

Table 1 Description of the 24-h samplings

2.3 Data Processing

Calculation of parameters such as Brunt-V?is?l? Frequency (BVF), Richardson Number (Ri), and water volume differences related to tides are based onin situdata of Lianzhou Bay. The potential density was calculated using the UNESCO equation of state (Fofonoff and Millard, 1983). Data were visualized using Ocean Data View (Schlitzer, 2012) and overlain with a modified map provided by colleagues from the Guangxi Mangrove Research Center (GMRC) in Beihai, Guangxi, China. Numerical modelling of the current dynamics in the bay is beyond the scope of this study, as no high-resolution dataset of the local bathymetry is available to set up a high- resolution ocean circulation model for Lianzhou bay. Nevertheless, it is possible to determine the general characteristics of the water movement in the bay by hydrodynamic key parameters based on the data of the expeditions.

3 Results and Discussion

3.1 Physico-Chemical Profiles of Lianzhou Bay

Fig.4 shows the temperature and salinity distribution in Lianzhou Bay. Limited by the shallow water, the first wet season cruise on 2010/8/30 was conducted during a semidiurnal tidal flood and the first half of the ebb; the dry season cruise on 2011/4/8 was launched while a diurnal tidal flood started.

Fig.4A gives a clear example of the seasonal variation of water temperature. The average temperate was 30℃during the wet season (A1) and 23℃ during the dry season (A2). No significant temperature gradient was observed between surface and bottom water. During wet season, water temperature is rather uniform across the bay with values up to 31.75℃. During the cooler dry season, temperatures decrease from the river mouth (approx. 25℃) to the deep bay area (approx. 21℃).

Salinity within the bay is relatively high. During wet season, sea surface salinity ranges between 22 and 26, while at the bottom a salt wedge is found at the seaward edge with a salinity of 31 (Fig.4 B1). During dry season, fresh water at the surface does not spread as far, with a significant salinity gradient between the river mouths and the southern (outer) part of the bay, where the salinity increases to 31 (Fig.4 B2).

3.2 Major Hydrodynamic Parameters

3.2.1 Stratification/Brunt-V?is?l? frequency

The Brunt-V?is?l? Frequency (BVF)Nis a measure of the stratification of a water body and is proportional to the stability of stratification (Houryet al., 1987; Osińskiet al., 2010).

The Brunt-V?is?l? FrequencyNin a given depth is given by:

In Lianzhou Bay,Nranges between 91 cph (cycles per hour) during wet season and 59 cph during dry season. These values are somewhat lower than in,e.g., the Weser Estuary (North Sea), which is under a stronger riverine influence (average runoff 323 m3s?1, Grabemann and Krause, 2001). There, a meanNof 127 cph was found (Macrander, 2009), which indicates a stronger stratification, especially during ebb tide when the fresh water flows out faster than the bottom salt water (Geyer, 1993). In another highly stratified water system, Baltic Sea,Nis found between 126 and 198 cph within thermo- and haloclines (Osińskiet al., 2010). Compared to these examples, stratification in Lianzhou Bay is weaker.

Fig.5 showsNat the surface of the bay during wet season (2010/8/30) and dry season (2011/4/8). Towards the coast, salinity and density are lower, with the lowest salinities occurring close to the river mouths. Stratification in the rather shallow coastal areas is weak, too (smallNin,e.g., Station 3 of Fig.5A). In contrast, a higher saline bottom layer is found in the seaward Station 8 (Fig.5A), associated with a stronger stratification. Measurements at Stations 8–10 were taken during ebb tide; obviously, the lighter fresh water at the surface flows out faster, while close to the bottom, a salt wedge persists. In dry season, stratification is weaker also in the seaward part (Fig.5B), as the fresh river water does not spread that far. The data were taken during flood, hence, tidal current induces turbulent mixing which causes equally distributed density, consequently,Nat the bay mouth decreases to about 30 cph (Stations 1 and 2). The higherNin Stations 3–5 is caused mainly by higher surface water temperature rather than lower salinity. Measurements at these stations were taken during noontime, where the surface layer is additionally heated up by solar radiation, accounting for a larger vertical density gradient.

Fig.4 Temperature (A) and salinity (B) distribution in Lianzhou Bay in 2010/8/30 (1) and 2011/4/8 (2).

Fig.5 Distribution of N on the surface of the bay. (A) 2010/8/30, during wet season; (B) 2011/4/8, during dry season.

3.2.2 Turbulent mixing/Richardson number

A stable stratification with a fresh water surface layer and a more saline deeper layer can be removed by turbulent mixing, provided that the potential energy of the stratification is smaller than the kinetic energy which is associated with a sheared current. This ratio betweenEpotandEkinis defined as Richardson numberRi:

whereNdenotes the BVF, and du/dzthe vertical gradient of the current speed. IfRi<0.25, turbulent mixing breaking up the stratification can be expected (Galperinet al., 2007). As current speed was measured only at the surface, we useusurface/H(with water depthH) as approximation for du/dz, assuming zero current speed right at the bottom.

As current speed data under consideration are limited to the 24 h sampling,Riis accessed only for these timeseries. Turbulent mixing was prevalent during 66% of the observation time, whenRiwas smaller than 0.25, whereas during 34% of the time (mostly during slack tide),Riwas larger than 0.25. As an example, Fig.6 shows data from No. 5 24-h sampling. The potential density profile is largely determined by salinity (Figs.6A, 6B). The largest density difference between surface and bottom occurred around 14:00 on 2011/4/5 during the higher low water within the sampling period. The density difference implies maximumNat the same time (Fig.6C). The smallest measured current speed was 0.09 m s?1and normally occurred during slack tide at low water, while the highest speed generally occurred during late stage of flood tides (Fig.6D). Consequently,Riis largest during periods of largeNand low current speed (Fig.6E). While during the higher low tide, a stable stratification persisted for some time, current velocities were large enough to keepRibelow 0.25 at all other times. During the lower low tide the water depth was so small (less than 1 m), that the stratification was completely removed by turbulent mixing.

Relying on the calculatedNandRi, we conclude that conditions in Lianzhou Bay are mostly dominated by turbulent mixing. This results in a vertically largely homogeneous distribution of salinity and density, while it weakens the baroclinic flow. Considering the shallowness, we conclude that currents in Lianzhou Bay are largely barotropic,i.e., in the same direction throughout the entire water column.

Fig.6 24-h sampling No. 5: Temporal variation of salinity (A), potential density (B), N (C), current speed (D) and Ri (E). Tides are indicated by the measured water depth (grey bars).

3.2.3 Earth rotation/Rossby radius

On large scales, oceanic currents are deflected to theright (northern hemisphere) by the rotation of the earth. A characteristic length scale is given by the Rossby radius. As currents in Lianzhou bay are largely barotropic, the barotropic Rossby radiusLris assessed here:

With the measured depth between 1 m and 5.6 m,Lrranges from 58 to 141 km. This is far larger than the size of the bay (24 km), hence, current dynamics in Lianzhou bay are not significantly affected by the rotation of the earth,i.e., currents will not be focused to the coast on the right-hand side.

3.2.4 Tidal resonance

Tides in bays can be amplified by interference between incoming and reflected tidal waves (e.g., in Bay of Fundy, Greenburg, 1979; in the Strait of Georgia, Forman, 2005), provided the resonance period of the bay is close to the tidal period.

The shortest possible basin length L which produces resonances of tidal waves with period T (Leder and Orli?, 2004) is given by:

with acceleration of gravity g and water depth H.

With typical values for Lianzhou Bay, i.e., L=15 km and H=5 m a resonance period of T=2.38 h is found. Thisis much shorter than the main tidal periods, which are 12 h (S2), 12 h 25 min (M2), 24 h (S1), or 24 h 50 min (O2). To be in resonance with semi-diurnal tides (T=12 h 25 min), for instance, the length scale of the bay would need to be at least 78 km, which is by far larger than the actual size of Lianzhou Bay. Therefore, the tidal range will not be amplified within the bay. This is supported by the close agreement between predicted tides in Beihai city and the measured tides at the 24-hour samplings (Fig.7). Tidal waves are long gravity waves, which travel with a phase speed ofHence, the wave lengthλis given by

Take againT=12 h 25 min as an example, thenλequals to 313 km. Referring to Sammariet al.’s work (2006), when the length scale is smaller than one quarter of the wave length (L<312/4=78.2 km), then the semi-diurnal tide will be distributed almost equally everywhere in the bay. This is a plausible parameter to support the prediction of tidal distribution dyamics in the next section.

3.2.5 Tidal distribution dynamic

Through comparing the predicted tides at Beihai City with the measured water depth at the 24-h samplings, it was confirmed that the tides arrive at the mouths of Nanliu River with a very short time lag compared to Beihai Port (Fig.7). The predicted tides closely match the observed ones; the only significant differences occurred during events of anomalous southerly wind stress.

Fig.7 comparison between tidal sampling data and predicted water depth. (A) Station No.4 24-hour sampling. Note the higher water level around midnight when southerly wind was prevalent for several hours (B) Station No. 5 24-hour sampling.

The nearly simultaneous arrival of tides within Lianzhou bay is further supported by tide models which show the phases of the dominant tides at the Guangxi coastal areas,i.e., O1, K1and M2(Sun and Huang, 2001). Generally, on the northern hemisphere, tidal waves rotate counterclockwise around amphidromic points. In Lianzhou Bay, all tides progress from SE to NW, and it needs only 8 min for O1and K1or 4 min for M2, respectively, to travel from Beihai city to the northwestern estuaries of Nanliu River. These are much shorter than the tidal periods, hence, high and low tides all occur almost simultaneously throughout Lianzhou bay.

These findings confirm that due to the small scale of the bay, the current dynamics are not considerably affected by the earth rotation. Tide phase, amplitude and current are found to be rather uniform over the entire bay area.

3.3 Coastal Currents

The sea water input into Lianzhou Bay stems from the coastal currents which flow across the mouth of the bay. Those currents are part of the general wind driven water circulation in Beibu Gulf, of which Lianzhou Bay is a sub-embayment (Sunet al., 2001).

Cyclonic current circulation prevails during winter. Though there is some debate about summer circulation (Yuan and Deng, 1999; Baoet al., 2005), newer studies indicate a (weaker) cyclonic circulation also during sum-mer due to wind and tidal forcing (Caiet al., 2003), and inflow through the Qiongzhou Strait between Hainan and China’s mainland (Wuet al., 2008). The circulation includes mainly two components,i.e., a tidal residual current and a wind driven current. The latter is evidently stronger (Sunet al., 2001), with estimated current speeds being 3.5 times larger than the residual current induced by tides. Due to the prevalent strong NE monsoon winds during winter, the wind-driven current in Beibu Gulf forms a cyclonic circulation. During summer, wind forcing and hence wind driven circulation is generally weaker and less distinguishable. Because of the shallow depth of the study site, and the largely barotropic character of the circulation in Beibu Gulf (Wuet al., 2008), it is feasible to assume that the surface current accounts for the water renewal in Lianzhou Bay. The velocity of the NW coastal currents at the mouth of Lianzhou Bay, summarized from Sunet al.(2001), is shown in Table 2.

Observations and calculations reveal that the coastal current off Lianzhou bay is always directed to northwest, but is 4 times faster during winter, associated with the stronger wind stress during that season.

Table 2, coastal current velocities of Lianzhou Bay?

3.4 Residence Time

Residence time is a useful value to estimate the contribution of fresh water input and coastal current to the water exchange processes in the bay. It is defined as the period of time that a water parcel needs to leave a particular area (Takeoka, 1984).

If the water in Lianzhou bay were to be renewed only by the fresh water runoff from Nanliu river (average discharge 168 m3 s?1,i.e., 0.4 km3per month), it would take 4.5 months until the entire volume of the bay of 1.79 km3is replaced. However, water is also exchanged on the seaward side of the bay. Tidal currents in Lainzhou bay are mainly flowing back and forth, as tides arrive simultaneously everywhere across the bay, and the bay is too small for currents to be affected by the rotation of the earth. Hence, tidal currents themselves do not contribute to water renewal, carrying the same water into and out of the bay. The coastal current is not expected to turn right into the bay as the bay is smaller than the barotropic Rossby radius (as discussed above), and is shallower than the water in the offshore main pathway of the coastal current. Nevertheless, during ebb tide water from Lianzhou bay is displaced seaward, where it is carried northwestward with the coastal current (Fig.8). At the same time, ‘new’ water is advected by the coastal current to the southern part of the bay, where it enters the bay with the next flood. We predict that by this interaction between tidal currents and coastal current, water from Lianzhou bay is gradually displaced to the northwest, and replaced by ‘new’ water from the southeast.

During winter, water of the coastal current flows with 0.2 m s?1, and thus needs about 1.4 d to pass across the 24 km width of the mouth of Lianzhou Bay. Hence, every 1.4 d, all the water flushing the bay due to the tides has been completely renewed by Gulf water from the coastal current. During summer, with the coastal current being weaker at 0.05 m s?1, renewal may take 6 d. Although some aspects are simplified in this concept, it suggests that water renewal by tides and coastal current occurs at the scale of days, and is faster in winter than in summer. In comparison, water exchange due to freshwater inflow is much smaller, with residence times in the order of months.

The longer residence time within the bay during summer implies more accumulation of nutrients from aquaculture runoff.

Fig.8 Conceptual sketch of coastal current along the mouth of Lianzhou Bay (thick arrows) and tidal current in Lianzhou Bay (thin arrows). Water from the southern part of the bay is gradually displaced to the northern part (open arrows), while ‘new’ water enters the southern part (black arrows).

4 Conclusion

This study assessed water movement processes in the shallow Lianzhou Bay, Guangxi Province, China. The findings are based onin situdata of temperature, salinity, current speed,etc., which are used to calculate several hydrodynamic parameters,i.e., Brunt-V?is?l? Frequency, Richardson Number, Rossby radius and Resonance Period. Owing to the small size of the bay, tides are uniform throughout the entire area. The hydrographic conditions in the bay are found to be barotropic and well mixed due to high tides, and with only moderate freshwater runoff. Water is primarily renewed by the combined action oftidal currents and coastal current. Residence time in summer is markedly longer, as the mainly wind-driven coastal current is weaker during that time. Particularly during summer, nutrient-rich waste water is affecting the ecology of the bay, as the coastal region in Guangxi is heavily utilized for aquaculture. Further research is necessary to find out strategies that minimize the harmful effects of aquaculture effluents.

Acknowledgements

We acknowledge the financial support from the German Federal Ministry for Education and Research (BMBF) to the Leibniz-Zentrum für Marine Tropen?kologie GmbH (ZMT), Germany, for the project ‘The Role of Mangroves for Biogeochemical Fluxes into the Coastal Ecosystems under the Influence of Anthropogenic Alterations’ as part of the Sino-German Research Project ‘BEIBU-Holocene environmental evolution and anthropogenic impact of Beibu Gulf, South China Sea’, WTZ China (Grant No. 03F0607B) is acknowledged. The enduring logistical support from our colleagues from the Guangxi Mangrove Research Center in Beihai, Guangxi, enabled us to carry out the field study and is greatly appreciated. The authors are sincerely grateful for the comments and suggestions of the anonymous reviewers.

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(Edited by Xie Jun)

* Corresponding author. E-mail: entergoing_9@hotmail.com

(Received March 13, 2012; revised March 22, 2012; accepted July 26, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014