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INFLUENCE OF RIVER FLOW ON OXYGEN DEPLETION IN INNER JOHOR 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 Skudai estuary to the Sungai Tebrau 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 Danga Bay 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 Singapore 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 Iskandar Malaysia 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.

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