Seasonal Dynamics of Turbidity Maximum in the Muthupet Estuary, India

PRIYA K. L., JEGATHAMBAL P., and JAMES E. J.

School of Civil Engineering, Karunya University, Coimbatore, Tamil Nadu 641114, India

Seasonal Dynamics of Turbidity Maximum in the Muthupet Estuary, India

PRIYA K. L.*, JEGATHAMBAL P., and JAMES E. J.

School of Civil Engineering, Karunya University, Coimbatore, Tamil Nadu 641114, India

? Ocean University of China, Science Press and Spring-Verlag Berlin Heidelberg 2015

Results are presented of the longitudinal and vertical profiling of salinity and suspended particulate matter (SPM) at the Muthupet estuary, India, during a one year period under widely varying freshwater flow conditions. Freshwater flow was available during post-monsoon and monsoon. An up-estuary shift in the location of estuarine turbidity maxima (ETM) was observed during the transition from post-monsoon to pre-monsoon and further it shifted downstream during the transition from pre-monsoon to monsoon, thereby exhibiting a pronounced seasonal cycle. The salinity intrusion was dependent on the freshwater discharge and was expressed as a power function of freshwater flow, explaining 97% of the variance. The formation of a salt plug in Muthupet estuary and its seasonal dynamics were observed, which is not an identified feature of any of the Indian estuaries studied so far. The geographical positions of salt plug and ETM core were more or less the same during their formation. The occurrence of two ETM during the LW of post-monsoon and the absence of ETM during monsoon explains the strong seasonal variation in the formation of ETM. The primary factor affecting the formation of ETM was identified as the freshwater flow over an annual cycle; the resuspension of sediments by tidal current affecting the formation on a flood/ebb cycle was secondary. The extent of shift of ETM was found to be an inverse logarithmic function of the freshwater discharge. The separation between ETM intrusion and salinity intrusion increased two fold with the increase in ETM intrusion.

estuarine turbidity maximum; salinity intrusion; suspended particulate matter; salt plug; Muthupet estuary, India

1 Introduction

Estuaries are characterized by the dynamics of seasonal changes in salinity, suspended particulate matter (SPM), current velocity, etc. (Kennedy, 1984). They play a major role in transport and dynamic mixing of dissolved and suspended particles, which are either riverine or marine origin (Dyer et al., 1992; Schubel and Carter, 1984). Several factors affect the concentration of SPM in an estuary: fresh water flow, resuspension of sediments, tidal current, and flocculation of particles due to the dynamics of salinity. One of the distinctive features in an estuary is the occurrence of estuarine turbidity maximum (ETM) where the concentration of SPM is higher than the SPM concentration both seaward and landward (Dyer, 1988; Nicholas and Biggs, 1985; Schubel and Kennedy, 1984). ETM has an ecological significance as it has an impact on the primary productivity, pollutant flushing, and fish migration (Mitchell et al., 1998). The mechanism of ETM formation varies depending on the geometry (Brenon and Hir, 1999; Chen et al., 2005; Schoelhamer, 2001; Wang et al., 2001), tidal dynamics (Burchard and Baumert, 1998; Kineke andGeyer, 1995; Lindsay et al., 1996; Lopes et al., 2006; Mandang and Yanagi, 2009; Sanford et al., 2001; Schoelhamer, 2001; Uncles and Stephens, 1993; Wang et al., 2002) salinity stratification (Geyer, 1993; Hamblin, 1989; Sarin et al., 1985; Shi, 2004) fresh water flow (Ganju and Schoelhamer, 2009; Schubel and Carter, 1984; Shi, 2004) and wind and waves (Kessarkar et al., 2009; Rolinski, 1999; Rolinski and Eichweber, 2000; Weir and Mcmanus, 1987). Gravitational circulation induced by the fresh water discharge and tidal forcing can be related to the distribution of SPM concentration and hence the location of ETM (Geyer, 1993; Hamblin, 1989; Wolanski et al., 1995). Seasonal seaward or landward migration of turbidity maximum has been observed in some of the estuaries (Mitchell et al., 1998, 2003; Uncles et al., 1994). The formation of ETM has been related to the bed erosion, resuspension and deposition (Uncles et al., 1994; Wolanski et al., 1995). Each estuary is unique by its behaviour of SPM and has to be studied to have more understanding of the fate of pollutants (Althausen Jr. and Kjerfve, 1992).

Behavior of shallow estuaries is different from that of deep estuaries. The sedimentation of shallow estuaries has to be seriously viewed as they are more liable to get flooded due to the impact of sea level rise. Studies on the ETM of very shallow estuaries are limited (Hughes et al., 1998; Largier et al., 1998; Masselink, 2009; Ruhl andSchoellhamer, 2004; Uncles, 2002; Uncles and Stephens, 1996). Further, studies on the dynamics of Indian estuaries remain still scarse, except for Mandovi estuary (Kessarkar et al., 2009; Rao et al., 2011), Krishna estuary (Kumari and Rao, 2010), Hoogly estuary (Sinha et al., 2004), and Narmada estuary (Jena et al., 2008; Sinha et al., 2010). There is a general tendency for the shallow estuaries to become shallower due to the transport and deposition of sediments.

Point Calimere, located in the coromandel coast, is one among the 26 Ramsar sites in India. Ramsar sites are wetlands of international importance designated under the Ramsar convention. The Muthupet mangroves form a part of the Point Calimere Ramsar site and constitute the largest mangrove forest in Tamil Nadu state (Fig.1). The mangrove ecosystem consists of a shallow estuary, which is fed by the tributaries of Cauvery River. The freshwater discharge is controlled by barrage operations and there is no freshwater flow into the estuary for about 60% to 70% of the period of the year. The estuary has undergone some geomorphic changes in the past twenty years. There has been a reduction in the average depth of water and the width of the mouth has reduced. No investigation on the dynamics of SPM on a systematic seasonal timescale of this shallow estuary has been reported. This is the first time that the dynamics of SPM of this shallow estuary is visited on a seasonal timescale. The data during the 1 year period from January 2012 to December 2012 was used to study the seasonal behaviour of salinity and SPM concentration. The main objective of the study is to understand the driving forces causing the formation of ETM in the shallow estuary.

2 Study Area

The Muthupet estuary on the east coast of India is well known for its ecological, social and economic values. The estuary receives a major part of the freshwater from the tributaries of Cauvery River, namely, Korayar and Paminiyar. The discharge from these rivers vary from 0 to 120 m3s-1. The monsoon in the area occurs during the months from October to December. The freshwater flow is available only for 4–5 months, during monsoon and post-monsoon. For the remaining period, there is no flow and the salinity level in the estuary reaches as high as 35. The average depth of the estuary is 1 m, with the maximum depth of 1.7 m at the mouth. The geometry of the estuary is given in Table 1.

Table 1 Geometry of the estuary

Fig.1 The location of Muthupet estuary and the sampling stations.

The estuary is dominated with clay on the downstream side (inside the estuary and the mouth), while sand pre-dominates at the upstream river (about 20 km upstream the sea mouth). The tides entering are bifurcated into eastern and western directions, and 80% of the tidal discharge occurs in the eastern direction. The water spread area of the semi enclosed part of the estuary was 15.31 km2in 1971 and that in 1991 was 18.38 km2. An increase of 3.07 km2is evident over a period of twenty years (Fig.2). Hence, the theme of the paper has been formulated to study the dynamics of SPM in relation to the freshwater discharge over a spatial and seasonal scale.

Fig.2 Geomorphic changes of the estuary between 1971 (left) and 1991 (right).

3 Methods

Water samples were collected from four depths (surface, 0.2 D, 0.6 D, 0.8 D, D being the water depth for obtaining the vertical profile (Kumari and Rao, 2010; Hughes et al., 1998) from the sampling stations (Fig.1) for a duration of 12 hours (from 6 h to 17 h) during spring and neap tides during post-monsoon (January and February 2012), pre-monsoon (March and September 2012) and monsoon (December 2012). Thirteen sampling stations were observed for salinity and SPM, together with the water depth and current. Boats were anchored at all the stations during the field experiments. The water level was observed using a tidal gauge. The current speed was recorded using a Direct Reading Direction Finding current meter (EMCON, Cochin) at two depths (D/3 and 2D/3, D being the water depth). The samples were analyzed for salinity using a conductivity meter (Eutech-Cyberscan Con 11, range: 0 to 99.9 ppt, resolution: 0.05% full scale, accuracy: ±1% full scale + 1 digit) and for suspended sediment concentrations. For obtaining suspended sediment concentration, the following method was adopted (Wu et al., 2012; Yang et al., 2004). Around 50 water samples of different turbidity ranges were collected from the field and analysed for turbidity using a turbidity meter (Systronics Digital Nephelo-turbidity meter 132, accuracy ± 1% full scale) and suspended sediments by filtering the samples through 0.4 μm Whatman filter paper and then dried and weighed. A curve was drawn for turbidity vs suspended sediments. The suspended sediment concentration of any sample was determined from the curve once the turbidity of the samples is known. The data on freshwater discharge was collected from the Public Works Department, Nagapattinam.

4 Results

The estuary is micro-tidal with average tidal range less than 0.5 m at the mouth and it decreases upstream, with an average tidal range of 0.1 m at a distance of 6 km from the mouth. The average spring and neap tidal ranges are 0.5 m and 0.36 m at the mouth. The maximum currents at the downstream sea mouth are 0.65 m s-1and 0.5 m s-1during spring and neap tides respectively, while in the upstream region, the highest current ever recorded is 0.2 m s-1.

4.1 Freshwater Flow

The Muthupet estuary receives freshwater from the tributaries of Cauvery River, namely Korayar and Paminiyar rivers; the inflows from other rivers such as Kilathingyar, Maraikakorayar, and Kandankurichiyar are insignificant. The freshwater flow from the rivers is regulated on the upstream side by barrage operations. The flow is negligible during the pre-monsoon period (March–August). The annual average freshwater flow varied from 0–65 m3s-1during the study period. Freshwater flow was available during the months of January 2012 to February 2012, which are within the post-monsoon and during the months of October to December 2012, during which monsoon prevailed. During the remaining months, freshwater flow was negligible and consists of pre-monsoon. During the sampling days, the discharges were: postmonsoon – 64 m3s-1(January 2012), and 3 m3s-1(February 2012); pre-monsoon – no freshwater flow; monsoon: 20 m3s-1and 10 m3s-1(December 2012). The upstream salinity (at a distance of 30 km from the sea mouth) showed a seasonal variability from salinity less than 1 during high fresh water runoff to salinity greater than 20 during low run off conditions. Spring-neap cycle and runoff variations affected the salinity levels in the upstream reaches of the estuary.

4.2 Longitudinal Variations of Salinity and SPM

4.2.1 Variation of salinity

The salinity variations are distinct from the river to themouth of the estuary during spring and neap tides observed between January 2012 and December 2012 (Fig.3). The salinity intrusion limit was highly dynamic in nature and moved 34 km upstream during pre-monsoon, while it was only up to 9 km during post monsoon. In the present study, the longitudinal distances are specified with respect to the sea mouth. The salinity intrusion limit is taken as the upstream position where the salinity falls below 1. The salinity variation was highest during post-monsoon and decreased during pre-monsoon. The vertical and horizontal stratification of salinity showed a pronounced seasonal cycle with maximum stratification during postmonsoon and minimum during pre-monsoon. The highest salinity was observed at the mouth of the estuary during post monsoon and monsoon, while it occurred inside the estuary at a distance of 5 km from the mouth of the estuary with peak concentration exceeding 30 during pre monsoon (March 2012). During the dry and hot season, the occurrence of this salinity maximum zone, called as salt plug, is due to high evaporation and negligible freshwater discharge. For seaward salt plug, the water density decreases as in an inverse estuary, but for landward salt plug, the water density decreases as in a positive estuary. This zone of salinity maximum acts as a barrier that prevents the seaward flushing and the landward intrusion of ocean water (Valle-Levinson, 2011). The phenomenon of higher upstream salinity than downstream is commonly observed in some of the world estuaries:Spencer Gulf of South Australia (Nunes and Lennon, 1986) and Laguna San Ignacio of Mexico (Winant and De Velasco, 2003); but it is rarely seen in Indian estuaries. Further, a peak salinity of 36 was recorded at a distance of 7 km from the mouth during September 2012. The salinity intrusion moved by 25 km (from 9 km during January 2012 to 34 km during September 2012) up-estuary during the seasonal cycle. Thus, the movement of peak salinity in the upstream direction during the transition from post-monsoon to pre-monsoon is evident.

Fig.3 Longitudinal profile of salinity (a) Post-monsoon and spring tide (b) Post-monsoon and neap tide (c) Pre-monsoon and spring tide (d) Pre-monsoon and neap tide (e) Pre-monsoon and spring tide (f) Monsoon and spring tide (g) Monsoon and neap tide.

4.2.2 Variation of SPM

The SPM concentrations were higher at the downstream than in the upstream reaches of the estuary during the entire study period. The maximum SPM concentration of 1.4 g L-1was observed during pre-monsoon. The ETM was associated with higher salinity and moved up-estuary during pre-monsoon up to a distance of 6 km from the mouth. The geographical position of salt plug and ETM coincided more or less during its formation. The SPM concentration varied between 0 and 1.4 g L-1during the study period. Considering the range of SPM in this study, the location of ETM is defined as the geographical location with the highest SPM concentration during the tidal cycle concerned. The nose and tail of the ETM are defined as the upstream and downstream regions of the estuary with SPM concentration of 0.5 g L-1. The ETM intrusion limit is considered as the up-estuary position where SPM falls below 0.1 g L-1. During the spring tides of post-monsoon, the tail of the ETM was observed at the sea mouth. The shift of nose of the ETM core (ETM core is the region between the nose and tail of ETM) towards the upstream side was evident from post-monsoon to pre-monsoon transition, the location of nose during pre-monsoon being at 7 km upstream the sea mouth. The core of the ETM narrowed up during pre-monsoon, while it was staggered towards the sea during post monsoon. Another interesting feature is that during post-monsoon, two peaks of ETM within the ETM core were observed at LW, one near the mouth and the other at a distance of 5 km upstream the sea mouth (Fig.4(a) and 4(b), January and February 2012, LW). Further, during neap tides of monsoon, no ETM was observed at HW, while there was a tendency for the development of an ETM at LW, but only the formation of nose was evident, the tail being towards the sea. The nose was sharper than the tail during all spring tides. During the entire study, the rate of change of SPM concentration in the longitudinal direction within the ETM core was varying seasonally, with highest and lowest being ca. 0.3 g L-1km-1during pre-monsoon and ca 0.05 g L-1km-1during monsoon respectively. The ETM intrusion limit was shifted by a distance of 8 km in the upstream direction during transition from post-monsoon to pre-monsoon. From these observations, longitudinal displacement of SPM over a seasonal scale is evident. Further, the concentration of SPM at the ETM showed slight variations on a seasonal scale. The SPM showed vertical stratification throughout the study period, even though the salinity stratification was observed only during post-monsoon. The near-surface and near-bed maxima in SPM concentrations coincided spatially during pre-monsoon, while during post-monsoon, the near-bed maxima occurred at 3 km from the mouth and the near-surface one occurred at 5 km, showing an inclination towards down estuary (Fig.4(a) January 2012, HW). This may be due to the impact of freshwater flow causing a downward shift of maximum SPM at the surface of water column. The neap tide stratification of SPM in the ETM core was greater than spring tide stratification. SPM concentration decreased from the ETM nose towards the up estuary and was as low as 0.05 g L-1at a distance of 16 km from the sea mouth. Even though salinity stratification was less, SPM was stratified during spring and neap tides of premonsoon and monsoon.

During the post-monsoon, the formation of two ETM at the downstream reaches within the ETM core was observed at LW. Formation of two or more ETM, one at the downstream and the other at the upstream reaches are well documented (Biggs et al., 1983; Chen et al., 2005). During post-monsoon, the combined influx of freshwater and saline water during the ebb tide flushes the sediments out of the estuary, which is the reason for the location of ETM at the downstream reaches. When the ebb current weakens during LW, the SPM in the water column settles down. At the shallow reaches, the resuspension of the settled sediments may have caused a higher SPM in the water column (at 5 km from the mouth where the depth is ca 0.6 m), which is the reason for the upstream ETM. Towards the sea mouth, the tidal current is high enough to agitate the water column and cause a higher SPM, which is not prevailing at the upstream reaches, because the tidal current is less at the upstream reaches. This causes the formation of an ETM near the mouth. Thus, the formation of two ETM within the ETM core during the LW of post-monsoon is mainly caused by resuspension of sediments (at the shallow reaches) and tidal current (near the mouth). On the contrary, during flood tide, the freshwater flow and tidal flow in the mutually opposite directions cause shear in the bed, thereby resuspending the sediments. These sediments are carried upstream by the currents and during the subsequent slack water, they get settled down. As the freshwater flow continues in the downstream direction, the sediments are flushed out of the estuary, thereby causing higher SPM at the downstream reaches. This is evident from the HW of post-monsoon (Fig.4(a) January 2012). But during neap tide, the currents are weak to resuspend the sediments, resulting in a lower SPM concentration compared to spring tide (Fig.4(b) February 2012, HW and LW). Also, the tidal currents during neap tide are weaker than during the spring tide to cause resuspension, resulting in lesser SPM concentrations at the sea mouth. In the absence of freshwater flow, the flushing is weak in causing the sediments to be retained in the estuary for a longer period. This lack of flushing causes the ETM to get intruded further up-estuary during pre-monsoon. From these observations, it is understood that SPM, which is responsible for theformation of ETM, is carried from the sea into the estuary by the tidal water and not from the river.

During the onset of the next monsoon, the SPM which is retained during pre-monsoon is flushed out of the estuary. The larger storm runoff carries sediments from the upstream river and, depending on their settling velocities, they get deposited on the river path. As sandy silt predominates in the upstream river, they are not carried into the semi-enclosed portion of the estuary because of the large settling velocity; they get settled in the vicinity of the river itself. During the neap tide, the currents are not strong enough to resuspend the sediments, which is the reason for the absence of ETM during the neap tide of monsoon (Fig.4(g), December 2012). On the other hand, the strong spring currents cause resuspension of sediments, and the same phenomena observed during January 2012 was repeated, with ETM formed towards the downstream reaches of the estuary.

4.3 Tidal Cycle Behavior of Salinity and SPM

Fig.4 Longitudinal profile of SPM (a) Post-monsoon and spring tide (b) Post-monsoon and neap tide (c) Pre-monsoon and spring tide (d) Pre-monsoon and neap tide (e) Pre-monsoon and spring tide (f) Monsoon and spring tide (g) Monsoon and neap tide.

Fig.5 Tidal variation of salinity and SPM. (a) Post-monsoon and spring tide variation of salinity, (b) Post-monsoon and spring tide variation of SPM, (c) Post-monsoon and neap tide variation of salinity, (d) Post-monsoon and neap tide variation of SPM, (e) Pre-monsoon and spring tide variation of salinity, (f) Pre-monsoon and spring tide variation of SPM, (g) Pre-monsoon and neap tide variation of salinity, (h) Pre-monsoon and neap tide variation of SPM.

Fig.5 Tidal variation of salinity and SPM. (i) Pre-monsoon and spring tide variation of salinity, (j) Pre-monsoon and spring tide variation of SPM, (k) Monsoon and spring tide variation of salinity, (l) Post-monsoon and spring tide variation of SPM, (m) Monsoon and neap tide variation of salinity, (n) Monsoon and neap tide variation of SPM.

The behaviour of salinity and SPM were further delineated using flood-ebb profiling during the spring and neap tides in 2012. During post-monsoon, both salinity and SPM were stratified and the stratification was more evident at the up-estuary station S6 (Fig.5: Tidal cycle behaviour of S6 is only presented for reference). The stratification is higher during January 2012 than February 2012, because of the higher freshwater discharge in January 2012 (64 m3s-1). The flood-ebb variation of salinity was distinct during the period of study. The variation of salinity between the flood and ebb phases of the tide was apparent only during post-monsoon. During the premonsoon salinity was fairly well mixed in the water column when tidal force dominated over freshwater discharge. The absence of freshwater during the pre-monsoon resulted in the transition of the estuary into an extension of the sea. Similar observations were made in the Mandovi and Zuari estuaries, India (Shetye et al., 2007). Stratification of SPM was persistent throughout the flood and ebb phase, except during monsoon. The magnitude of SPM concentration was highest during the initial phase of flood tide (Figs.5(b), (d), f), (h), (j), (l)). This is attributed to the tidal advection carrying the sediments during the flood phase. The high SPM concentration persisted up to the HW due to gravitational circulation. The sediments carried into the estuary by the tidal water settles down during the subsequent HW slack, thereby reducing the SPM concentration in the water column. During the next ebb, the resuspensed sediments are carried by the ebb water and are removed by settling during the LW slack (Figs.5(b), (f), (h), (j)). ETM was observed close to LW during spring and neap tides of post-monsoon and monsoon, while it was associated with flood tide during pre-monsoon. Faster currents during spring tides caused vertical mixing in the water column, resulting in less stratification of SPM compared to neap tide stratification.

The SPM concentration was lower in the upstream region of the estuary throughout the study. The SPM carried up-estuary during flood tide got settled at HW slack. These settled sediments were subsequently resuspended during the strong currents of mid-ebb and added to the SPM carried down-estuary by the ebb water. The SPM, which are carried down-estuary by the ebb tide, undergoes settling, resulting in a reduction of SPM concentration in the upstream region, which is the reason for lower SPM concentration at the upstream stations. This phenomenon was observed during post-monsoon, when ETM was associated with ebb tide. On the other hand, during pre-monsoon, the ETM was associated with flood tide, when flood tide resuspension of bed sediments and the up-estuary movement caused a peak in SPM in the early to mid-flood. Such a high concentration of SPM in the flood water gets deposited on its path due to the weakening of the current, as a result of friction, bed shear stress and topography of the estuary, which leads to a reductionin SPM concentration in the water column at the upstream reaches of the estuary.

4.4 Seasonal Variation of ETM

Strong seasonal variations were observed at the salinity intrusion limit and ETM intrusion limit. The salinity intrusion limit occurred between 9 km and 34 km during the study period. The salt plug was evident only during pre-monsoon and it was located between 5 km and 7 km from the sea mouth. The location of ETM core was always at the downstream reaches of the estuary, spanning between 7 km to 0 km from the mouth. Longitudinal movement of ETM nose was within 0 to 7 km and it was shifted in the upstream direction during transition from post-monsoon to pre-monsoon. The fluctuation in freshwater discharge (Q) was the cause of the movement of salinity intrusion limit. The increase in salinity at the upstream river station, S0, occurred when the freshwater flow reduced to ＜ 20 m3s-1(Fig.6(a)). Thus, a minimum freshwater discharge of 20 m3s-1is required to control the intrusion of salinity further upstream of the river. Also, when the discharge falls below 20 m3s-1, the SPM concentration shows a marginal increase at the downstream reaches and a further intrusion at ca. 15 km from the mouth.

Fig.6 Seasonal variation of (a) salinity at station S0with freshwater discharge (b) salinity intrusion limit with freshwater discharge (c) ETM intrusion limit with salinity intrusion limit (d) ETM intrusion limit with SPM gradient (e) ETM intrusion limit with freshwater discharge (f) separation of ETM and salinity intrusion limits with ETM intrusion limit.

The salinity intrusion limit (Ds) is a logarithmic function of the freshwater discharge (Q) as given in Eq. (1),

where Dsis negatively correlated with Q (Fig.6(b)) with a coefficient of variance of 97%.

The intrusion of ETM (Dp), which corresponds to the upstream position where 0.1 g L-1of SPM contour intersects the bed, was positively correlated with the salinity intrusion limit, Ds. A logarithmic relation gave a coefficient of variance of 97% (Fig.6(c)):

A pronounced seasonal variation of the location of ETM, which corresponds to the maximum depth averaged SPM concentration, was observed during the one year period. The geographical location of ETM affects the magnitude of SPM stratification, which is quantified using SPM gradient (Gp). The SPM gradient, Gpwas obtained from the difference between top and bottom SPM concentration/ water depth. The currents are weakened at the upstream reaches due to friction and bed shear stress enablingstratification of SPM compared to downstream, where strong currents causes mixing in the vertical direction. The SPM gradient explained 97% variance when defined using a power law with Dp(Eq. (3), Fig.6(d)). It is interesting to note that there exists stratification of SPM despite the shallow depth of the estuary and that the salinity was fairly well mixed throughout the water column, except during post-monsoon.

The ETM intrusion occurred upstream up to 15 km during the pre-monsoon and the SPM stratification increased from 0.025 g L-1km-1in January 2012 to 0.088 g L-1km-1in March 2012. The ETM intrusion and fresh

water discharge were negatively correlated and the ETM intrusion was a logarithmic function of freshwater discharge explaining 91% of variance (Eq. (4), Fig.6(e)).

The separation between salinity intrusion limit and ETM intrusion limit (Ds–Dp) was always positive (Dpdown-estuary of the salinity intrusion limit). When the ETM intrusion was less than 10 km, it was less than 5 km down-estuary of the salinity intrusion; when the ETM intruded more than 10 km, the salinity intrusion limit was more than 5 km up-estuary of the ETM intrusion limit. A linear regression of Ds-Dpin terms of Dpexplained 93% of the variance (Fig.6(f)). As Dpmoved up-estuary, the separation between Dsand Dpincreased linearly as

5 Discussions

The salinity and SPM concentration showed a pronounced seasonal variation throughout the year. The salinity was stratified only during post-monsoon, while SPM was stratified throughout the study period. Thus, the SPM was stratified even though the estuary is shallow and salinity was well mixed. The locations of salinity intrusion limit and ETM were dependent mainly on the freshwater flow, while spring-neap effects were secondary. A fresh water induced turbidity maximum was reported in the Pearl River estuary (Wai et al., 2004) and the same phenomenon was observed in Muthupet estuary. In certain other estuaries in India, ETM consistently occurs at the same position, e.g., Mandovi estuary (Rao et al., 2011); seasonal migration towards upstream has been observed in the Zuari estuary (Rao et al., 2011). Similar seasonal migration in other estuaries like Tamar estuary (Uncles et al., 1994), Humber estuary (Mitchell et al., 1998) and Trent estuary (Mitchell et al., 2003) has been reported. During times of heavy freshwater flow, the flushing is good, causing the SPM to get flushed out of the estuary. When the freshwater flow is absent, the ETM was shifted upstream in the Muthupet estuary. This was mainly due to poor flushing characteristics causing sediments to retain in the estuary for longer periods. The tidal flow causes the SPM to be transported to the upstream reaches of the estuary. Similar migration of ETM towards upstream during the lean flow was reported in four Northern European estuaries, namely, Weser, Seine, Scheldt and Humber estuaries (Mitchell, 2013). During monsoon, the ETM is developed at the downstream reaches, due to good flushing by storm runoff. The absence of ETM during neap tides of monsoon in December 2012, was due to the weaker neap currents coupled with good flushing of sediments. The tidal cycle behaviour of ETM can be explained by the effect of tidal currents causing resuspension during LW. A reduction in magnitude of ETM is evident during sprig-neap transition, which is mainly attributed to the weaker currents during neap tides. Generally, estuarine circulation determines the occurrence of ETM in the estuary; it may occur near the fresh-salt water interface (Festa and Hansen, 1978; Postma, 1967), or away from it (Allen et al., 1980; Mitchell et al., 1998; Uncles and Stephens, 1989). In the present study, the ETM was away from the fresh-salt water interface and was consistent with the work of Allen et al. (1980); Mitchell et al. (1998) and Uncles and Stephens (1989). Downstream ETM formation is due to the transport of SPM from the sea by the tidal flow, whereas upstream ETM is due to the SPM carried by the river. Thus, the location of ETM indicates the source of SPM in the water column, whether riverine or sea borne, thereby shedding light on the direction of transport of SPM.

The ETM was consistently associated with high salinity reaches. The location of ETM intrusion limit was always down-estuary of the salinity intrusion limit. The ETM intrusion limit increased with increase in salinity intrusion limit and the separation between them was linearly related to ETM intrusion limit. Similar observations had been made in the Humber and Ouse estuary (Uncles et al., 2006). The up-estuary movement of ETM was further related to freshwater discharge and a negative logarithmic relation existed between them. Thus, freshwater discharge is identified as the primary controlling factor causing the ETM and its location on an annual basis. Similar observations have been made in some other estuaries (Ganju and Schoelhamer, 2009; Schubel and Carter, 1984; Shi, 2004). Periodic resuspension of sediments during flood/ebb tides due to tidal current aids the movement of sediments within the ETM core. The magnitude of SPM concentration is controlled by the variations in the spring/neap currents. Thus the effects of spring/neap and flood/ebb tides are secondary in controlling the ETM on a short temporal scale.

In our study, the vertical salinity stratification was persistent only during post-monsoon, while it was well mixed throughout the water column during pre-monsoon and monsoon despite the fact that SPM was always stratified. Similar observations were reported by Uncles et al. (2006). The shallow depth of the estuary along with less freshwater flow can be attributed to the well mixed conditions of salinity in the estuary. Further, the salinity inversions due to high evaporation coupled with negligible freshwater discharge was observed during pre-monsoon.The tidal discharge dominated the processes and the estuary was an extension of sea with a salt plug within 7 km from the mouth. Shetye et al. (2007) reported the Mandovi estuary in the west coast of India as the extension of sea during negligible freshwater discharge. But the occurrence of a salt plug has not been reported in any of the Indian estuaries and this report is the first of its kind and one of the important findings of the study. The SPM concentration was higher and associated with the salinity causing the formation of salt plug. So the combined effect of lack of freshwater, shallow depth and higher SPM concentration leads to the development of salt plug in Muthupet estuary, which makes it distinct and unique from other Indian estuaries. This location of high salinity and SPM has some ecological significance, as the estuary forms habitat for fish and other marine organisms. The marine species, which survive only in the sea, migrates to the salt plug formed within the estuary. During the onset of the next monsoon, the species shift outside the estuary finding their habitat in the sea. Thus, the formation of salt plug and ETM causes a pronounced seasonal cycle in the migration of diverse species. However, more studies are needed in this area to identify the effect of ETM and salt plug on species diversity.

6 Conclusions

Strong seasonal variation in salinity and SPM concentration was observed in the shallow micro-tidal estuary during the one-year period. The dynamics of freshwater greatly affected the salinity and SPM levels in the estuary. The salinity levels within the tidal limit varied between 1 and 30 at a distance of 15 km from the mouth. The vertical stratification of salinity was apparent only during post-monsoon, while it was fairly well mixed during monsoon and pre-monsoon. The lack of freshwater during pre-monsoon led to hyper-saline conditions, with salinity levels higher than that of sea. The formation of salt plug within 7 km was evident during pre-monsoon. There was a slight movement of the salt plug in the upstream region by about 2 km between March 2012 and September 2012, which was attributed to the enhanced evaporation and continuous tidal discharge and negligible freshwater flow.

The SPM concentration was stratified in the water column throughout the study period except during monsoon. During monsoon, good flushing characteristics cause the sediments to be washed out into the sea and thereby reducing the concentration within the estuary. The seasonal movement of ETM by about 8 km was observed during the post-monsoon to pre-monsoon transition. The absence of ETM during monsoon and the formation of two ETM during LW of post-monsoon were some of the observations made during the study. Storm water flushing during monsoon resulting in reduced SPM levels in the estuary and agitation of water column due to tidal current at the sea mouth and resuspension of sediments at the shallow reaches respectively were identified as the reasons for the specific seasonal behaviour of the estuary with regard to the formation of ETM. The study highlights the significance of a shallow estuary in the seasonal dynamics of ETM and salt plug; both have an impact on the migration of marine, estuarine and riverine species.

Good correlation was obtained for freshwater flow with salinity intrusion limit and ETM intrusion limit; lesser the freshwater flow, higher the intrusion limit of salt and SPM. As the salinity intruded upstream of the estuary, ETM was also found to be intruding as a logarithmic function of the salinity intrusion. The separation between salinity intrusion limit and ETM intrusion limit was a linear function of ETM intrusion limit. Further, under any condition of freshwater, flood/ebb or spring/neap tidal phase, the ETM intruded not more than 15 km upstream, while intrusion of salinity was up to 34 km. This explains the removal of SPM from the water column by settling. The resuspension of sediments was apparent only for this distance, because the currents were weak at the upstream reaches and they were as low as 0.1 m s-1. The effect of tidal current was not appreciable at the upstream reaches because during its flowing upstream, it is reduced by the shallow depth and bed topography.

Acknowledgements

The study was a part of the research project on the Management of Point Calimere wetland funded by Ministry of Environment and Forests, Government of India.

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(Edited by Xie Jun)

(Received October 10, 2013; revised January 14, 2014; accepted May 6, 2015)

J. Ocean Univ. China (Oceanic and Coastal Sea Research)

DOI 10.1007/s11802-015-2510-7

ISSN 1672-5182, 2015 14 (5): 765-777

http://www.ouc.edu.cn/xbywb/

E-mail:xbywb@ouc.edu.cn

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E-mail: klpriyaram@gmail.com