INFLUENCE OF RIVER FLOW ON OXYGEN DEPLETION IN INNER STRAITS

Maznah Ismail1, Noor Baharim Hashim2, a and Ziba Kazemi3

1 PhD Student, Universiti Teknologi Malaysia/Department of Hydraulic and Hydrology, Malaysia 2 Senior Lecturer, Universiti Teknologi Malaysia/Department of Hydraulic and Hydrology, Malaysia E-mail : [email protected] , [email protected] 3 PhD Student, Universiti Teknologi Malaysia/Department of Hydraulic and Hydrology, Malaysia E-mail: [email protected] a Corresponding author: [email protected]

The severity of dissolved oxygen (DO) depletion in the bottom waters of estuaries appears to range widely depending on a combination of factors, including morphometry, vertical density stratification, nutrient and organic matter inputs. Relationship among bottom water DO, vertical stratification and the factors responsible for stratification-destratification in inner Western Johor Strait were studied with vertical profiles and continuous monitoring data measurements on October 2009. The analysis data results indicate that stratification events and DO levels has a strong influence with estuarine circulation and variation in tidal amplitude.

Keywords: oxygen depletion; tidal range; estuarine circulation

Introduction The occurrence of severe oxygen depletion, either hypoxia (<2.0 mg/l, or <3.0 mg/l in some systems) or anoxia (0.0 mg/l), is a growing concern for estuarine and coastal areas around the world, leading to water quality problems (Kuo & Neilson, 1987; Diaz and Rosenberg, 2008; Kim et al., 2010; Zhang and Li, 2010; Brown & Power, 2011). Prolonged oxygen depletion not only can disrupt benthic and demersal communities but also can cause mass mortalities of aquatic life (Diaz and Rosenberg, 1995; Yin, 2004). The development of oxygen depletion involves the combination of several physical and biological processes. Many estuaries and coastal systems have a natural propensity for oxygen depletion due to their basin morphology, estuarine circulation, residence time of water and high freshwater discharge. Added to this structure is the cumulative load of point source inputs of organics and nutrients, as well as the more diffuse inputs of nutrients from nonpoint sources associated with the freshwater influx. The two principal factors that lead to the development of oxygen depletion are water column stratification, which isolates the bottom water from oxygen rich surface water, and decomposition of organic matter in the bottom water, which reduces oxygen levels. Both conditions must occur for oxygen depletion to develop and persist (Diaz, 2001; Lin et al., 2006). Oxygen depletion in the bottom water of an estuary can be periodic. It develops when the water column is stratified and weakens or disappears when the stratification is destroyed. The occurrence and longitudinal range of the salinity stratification phenomenon was, and continue to be, highly dependent on river flow. The inner portion of the Johor Strait, which encompasses the stretch of coastal waters from the Sungai estuary to the Sungai estuary (Figure 1.1), warrants special attention in light of the field observations. Because of recent active development along the Western Johor Strait, a comprehensive field program has been conducted in October 2009. Water quality conditions of the Johor Strait has been significantly deteriorated due to excessive organic loadings of domestic, industrial, agricultural and storm water origins. Heavy development along the coast to built industrial and residential complexes has worsened water quality problems into the strait. On the other hand, the presence of the Causeway which has effectively blocked the normal flow of the strait and resulted in standing wave propagation with no flow at the Causeway. This paper presents observations and analyses data in October 2009 showing oxygen depletion conditions not only in the Skudai River Estuary but also in the inner Western Johor Straits which is located closer to the Causeway Link. This paper examines the mechanisms that cause low oxygen concentrations, and the factors that restore oxygen concentrations to more moderate values.

Methods Data from intensive surveys of Western of Johor Straits were made to establish three water quality monitoring stations as shown in Figure 1. The measurements in this study were made with Yellow Springs Incorporated (YSI) sensors and sondes. Every 10 minutes the sondes measured temperature, conductivity (from which salinity was computed), dissolved oxygen, and pressure (from which the depth of the sensor was determined). Chlorophyll fluorescence was measured at the near surface depth at site with a YSI chlorophyll fluorescence. Vertical profiles were also collected at eight fixed stations and identified here by distance from Causeway Link to Second Link. All sampling stations were located at the middle of navigation channels. At each station, temperature, conductivity and dissolved oxygen were sampled at every 1 meter from surface to bottom.

Study Area The western Johor Strait is located at the southern tip of Peninsular Malaysia (Figure 1). A number of rivers from and Malaysia drain into it, while it connects with the Singapore Strait and the Strait of Melaka. Extensive reclamation has been undertaken along the strait and is currently being carried out in the vicinity of the study areas. A recent algal bloom in the Johor Strait has led to the damage of marine life in the water channel. This has led to heavy economic losses to the fish farms located along the water channel. Many factors such as weather patterns, tidal conditions and nutrient content in the water contribute to the extent of the eutrophication. Although the Johor Strait is heavily impacted, the habitats remain an important nursery ground for numerous fish including some commercial and recreational species.

S# S# S# S# S# S## # S# #SS S# S#S# S#S # ##S## # SS# S#S#S##SSS# # S# S#S# S## S S# S S# S#SS#S# S# S#S# S#S# S# S#S# S# # S# S# S# S#S# # S S# S # S# S# S# S# S# S# S# S# S# S# S# S# # S# S # S # S# S# S# S S# ð S# M03 S# ð S# M04 M02 M01ðð S# ð M05

S#

ð M06

S#

S# ð S# M07

M08 ð

Figure 1: Location of water quality sampling stations along Western Johor Straits.

The sampling locations for this study involve eight stations located along the Western Johor Straits, between the kelong near Second Link and the Causeway Link. These sampling stations were selected based on the criteria as the main area for fisheries and aquaculture activities. The coordinates of the sampling location is shown in Table 1. Stations M01 and M02 are located closer to the causeway which connects the island of Singapore to the Peninsular Malaysia. There is no flow of water between the two bodies of water separated by the causeway and results in an estuary like area near the causeway itself. Stations M03, M04, M05 and M07 are located closer to the main river estuaries (Sungai Skudai, Sungai Melayu, Sungai Perepat and Sungai Pendas). The flow in the strait is relatively uniform based on the simulation forecast of the currents in the channel. Station M06 is located near the rapid development area of at Nusajaya. Station M08 is located at kelong between Sungai Pulai estuary and Second Link.

Table 1: Sampling Stations and Coordinates Sampling Stations Coordinate M01 – Causeway Link 1° 27.350'N 103° 45.967'E M02- in front of Hospital Sultanah Zanariah 1° 27.317'N 103° 44.733'E M03- Sungai Skudai estuary 1° 27.933'N 103° 43.483'E M04 – Sungai Melayu estuary 1° 27.278'N 103° 42.032'E M05- Sungai Perepat estuary 1° 26.272'N 103° 40.682'E M06 – Nusajaya 1° 25.167'N 103° 40.100'E M07- Sungai Pendas estuary 1° 22.683'N 103° 38.512'E M08- Kelong near Second Link 1° 20.448'N 103° 36.506'E

Results Occurrence of Stratification The occurrence of salinity and DO stratification is illustrated for 12 days of low flow in the Skudai River and near Causeway Link. The variation in flow observed in the two selected stations facilitates the resolution of the interplay between flows, density stratification, and DO depletion in the bottom water. Clearly river flow was a strong regulator of the occurrence of the salinity stratification phenomenon. Salinity stratification was well established when the Skudai River flow at PUB was <1.0 m3/s. The increased vertical mixing associated with the increased flows and eliminated the stratification. The occurrence of substantial DO stratification (Figure 2c and 3c) corresponded to that observed for salinity stratification (Figure 2b and 3b) at the Skudai River site was lower than at the Causeway site. The DO was <5 mg/l (Class 2: Malaysian Marine Water Quality Criteria and Standard; beneficial uses for marine life, fisheries, coral reefs, recreational and mariculture) in the bottom water at stations Skudai River and Causeway for the most period of the October 2009.

5.0 0.0

5.0

4.0 10.0

15.0 /s) 3 (m

3.0 20.0 (mm) 25.0 Discharge

2.0 30.0 Rainfall

Flow 35.0

1.0 40.0

45.0

0.0 50.0 276 278 280 282 284 286 288

surface salinity bottom salinity surface DO bottom DO

30 14

25 12

10 20 (mg/l)

8 (ppt) 15 oxygen 6 Salinity 10 4 Dissolved

5 2

0 0 276 278 280 282 284 286 288 276 278 280 282 284 286 288 Figure 2: Paired temporal distribution in the Skudai River in October 2009. (a) daily average flow, as measured at PUB, (b) salinity at surface and near bottom depths, at a location 10 km downstream of PUB, and (c) DO at surface and near bottom depths, at the same location as for salinity.

surface salinity bottom salinity surface DO bottom DO

30 14

25 12

10 20 (mg/l)

8 (ppt) 15 oxygen 6 Salinity 10 4 Dissolved

5 2

0 0 276 278 280 282 284 286 288 276 278 280 282 284 286 288 Figure 3: Paired temporal distribution in inner Western Johor Strait in October 2009. (a) salinity at surface and near bottom depths, at a location nearest to Causeway Link, and (b) DO at surface and near bottom depths, at the same location as for salinity.

Temporal distribution In addition to the vertical profile data, time series of salinity, temperature, dissolved oxygen (DO) concentration and chlorophyll a data at fixed stations have been measured in Western Johor Strait. The data were measured at 10-minute intervals at a height of 1 m below the water surface. In this study, we established a continuous monitoring site at open boundary at Nusajaya, Danga Bay and Pengkalan Rinting at Skudai River Estuary from October 6-16, 2009 (Table 2 and Table 3). Three units YSI Water Quality Monitoring System Model 6600 Sondes were deployed. This site consists of conductivity, temperature, depth, dissolved oxygen and chlorophyll a optical sensor probes, at an upper elevation. Fouling and debris interference can reduce data yield from optical sensors, therefore all sensors were cleaned and checked with known standards during site visits (typically weekly).

Table 2: UTM Continuous Monitoring Stations (October 2009) No. Time Station ID Location Time interval Duration 1 October 4-18, 2009 WQ02 Danga Bay 10 minutes 15 days 2 October 3-16, 2009 WQ03 Pengkalan 10 minutes 14 days Rinting 3 October 4-16, 2009 WQ04 Nusajaya 10 minutes 13 days

Table 3: Calculated statistics of the observed data (October 2009) STATION Sample Minimum Maximum Median Mean Standard ID Size value value value value deviation WQ02 Temperature 2000 28.21 31.25 30.13 30.06 0.38 Salinity 2000 10.94 25.90 22.69 21.97 3.02 Dissolved 2000 0.15 13.24 1.73 2.52 2.06 Oxygen Chlorophyll 2000 1.40 98.8 7.70 16.30 8.12 WQ03 Temperature 1873 22.65 30.74 29.37 29.07 1.15 Salinity 1873 0.50 23.58 16.06 14.11 7.24 Dissolved 1873 0.17 9.64 1.72 2.40 1.88 Oxygen WQ04 Temperature 1710 29.49 30.86 29.97 29.99 0.20 Salinity 1710 21.69 24.69 23.62 23.59 0.56 Dissolved 1710 1.14 12.65 3.37 4.54 2.40 Oxygen

The relationship between oxygen depletion and estuarine circulation To consider the relationship between river discharge and estuarine circulation in detail, we measured currents using an acoustic doppler current profiler (ADCP) at station Danga Bay. The bottom current shown in Figure 4 was the average residual flow consisting of tide-induced residual currents, wind-driven currents, and density-driven currents. We inferred that the seawater exchange is controlled by estuarine circulation in inner Western of Johor Strait because the positive value currents were predominant (Sato, Nakayama, & Furukawa, 2012).

1.2

1.0

0.8

0.6

0.4

0.2

0.0 276 278 280 282 284 286 288

Figure 4: Time series of current velocity averaged over the bottom

The relationship between oxygen depletion and tidal range There are several important factors that lead to the oxygen depletion conditions observed on October 2009. The most important factors were availability of nutrients in the eutrophic zone to support phytoplankton blooms, meteorological conditions that enhanced water column stratification (freshwater input and surface heating), and variation in tidal amplitude. Our time series data show that tidal range has a strong influence on the oxygen content of water in inner Western Johor Strait. The relationship between oxygen depletion and tidal range is shown in Figure 5 from the Danga Bay, Pengkalan Rinting and Nusajaya data. During periods of low tidal range (around the neap tides), there is oxygen depletion in bottom waters. A decrease in bottom oxygen concentration was observed during neap tidal events. During periods of high tidal range (around spring tides), bottom oxygen concentrations are restored.

DO concentration profiles @ Nusajaya Surface salinity Bottom salinity Surface DO Bottom DO

14.0 35 15 14 13 12.0 30 12 25 11 10.0 10 (mg/L)

20 9 (ppt) 8.0 8 7 15 6 6.0 Salinity 5 concentration 10 4

4.0 3 DO 5 2 1 2.0 0 0

0.0 276 278 280 282 284 286 288 Time (dd/mm/yy) 276 277 278 279 280 281 282 283 284 285 286 287 288

DO concentration Profiles at Station Danga Bay Surface salinity Bottom salinity Surface DO Bottom DO

14.00 35 15 14 12.00 30 13 12 11 10.00 25

10 (mg/L)

20 9 (ppt) 8.00 8 7 15 6 6.00 Salinity 5 concentration 10 4

4.00 3 DO 5 2 2.00 1 0 0

0.00 276 278 280 282 284 286 288 Time (dd/mm/yy) 276 278 280 282 284 286 288

DO concentration profiles at Station Pengkalan Rinting Surface salinity Bottom salinity Surface DO Bottom DO

14.00 35 15 14 12.00 30 13 12 25 11 10.00

10 (mg/L)

20 9 (ppt) 8.00 8 7 15 6 6.00 Salinity 5 concentration 10 4

4.00 3 DO 5 2 1 2.00 0 0

0.00 276 278 280 282 284 286 288 Time (dd/mm/yy) 276 278 280 282 284 286 288 Figure 5: Surface and bottom oxygen and salinity values from (a) Nusajaya, (b) Danga Bay and (c) Pengkalan Rinting

The relationship between oxygen depletion and phytoplankton blooms As phytoplankton depends on photosynthesis to survive, the presence of chlorophyll a is a good indication of the presence of phytoplankton. The periods of high surface oxygen values, which are indicative of phytoplankton blooms, are sensitive to water column stratification. Blooms occur when stratification is enhanced during neap tides or increases in freshwater input. Blooms terminate during periods of increased vertical mixing caused by spring tides or storm induced wind mixing. Figure 6 shows evidence that changes in tidal range also affect the surface oxygen concentration. Based on the results from Gin et al. (2006), higher chlorophyll a levels were detected during the South-West Monsoon season (June-August) than during the North-East Monsoon season (December-February) in the Singapore Strait. A high level of chlorophyll a, which corresponds to an algal bloom, occurs frequently in the Western Johor Strait and seemed to be independent of the seasons. They were likely to be a result of anthropogenic inputs and favourable climate and tidal conditions.

2.5

2.0

1.5

1.0

0.5

0.0 276 278 280 282 284 286 288 ‐0.5

‐1.0

‐1.5

‐2.0

DO concentration Profiles at Station Danga Bay Chl a concentration profiles at Station Danga Bay

14.00 120

12.00 100

10.00 80 8.00 60 6.00 40 4.00

2.00 20

0.00 0 276 278 280 282 284 286 288 276 278 280 282 284 286 288 Figure 6: DO and chlorophyll a concentration profiles at Station Danga Bay

Discussion Skudai River estuary is a partially mixed estuary with thermohaline stratification. The water column density structure regulates advective exchange from the upper to lower layers. The primary factor controlling density structure is salinity which is graded vertically and longitudinally in the estuary. Rates of salinity changes with depth and distance are influenced primarily by fresh water inflow. With low river flow, sea water penetrates along the bottom of the estuary. River flow usually increases during heavy rainfall, freshening the surface waters and increasing the vertical salinity gradient. The data summarized here support this scenario but a quantitative cause and effect relation between river flow and oxygen depletion rate cannot be established because the relation between river flow and salinity is itself complex. The existing data do not permit us to ascertain whether oxygen utilization in the water column or the sediments dominates the loss process, but an estimate can be made. It seems clear that variability in the strength of the gravitational circulation is responsible for some of the difference in frequency, duration and severity of hypoxic conditions along the Western Johor Straits. Our interpretation of the data indicates that gravitational circulation is important for the Western Johor Strait and Skudai River Estuary. This finding has potentially important implications for management of the living resources along the Western Johor Strait system. For example, the oxygen patterns may explain, at least in part, why no living organisms were observed at stations nearest the causeway. Fecal coliform concentration levels monitored by the Department of Environmental (DOE) Malaysia are much higher in these areas than in areas further away from the causeway. These characteristics show the high demand of oxygen in near causeway water and sediment, due to oxidation of sewage wastes, and lack of flushing of sewage discharges out to the open sea.

Conclusions While oxygen depletion is not the only environmental issue of concern along the Western Johor Strait, it is certainly one of the most important. There are potential links between low oxygen and kills of fish and commercially valuable shellfish. Field observations indicated that bottom DO concentrations decreased when neap tidal events occurred. Limited water exchange with open ocean in semi enclosed intertidal water of Western Johor Strait which increased in flushing time and created a lot of anthropogenic pollutants via riverine inputs. In this period, stratification was still present and the vertical mixing was suppressed, which suggests that the DO supply in the bottom layer due to coastal water intrusion accompanied by estuarine circulation.

Acknowledgement The study described in this paper was conducted as a part of the E-Science Fund and FRGS project funded by the MOSTI and MOHE. We are grateful to Paul M. Craig and Prof. James L. Martin for their advice and comments on the handling of field observations and numerical models. Special thanks are due to the field crew in the Department of Hydraulics and Hydrology Laboratory at UTM Campus JB who collected the field data used in this study.

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