Samplings were carried out once a month in Ambong Bay, Sabah for 12 months period (September 2015-August 2016). Sampling stations were situated nearby Kampung Baru-Baru (Figure 1), one of the bivalve mollusc culture farms in the area which focus mainly on green-lipped mussel (Perna viridis) and Pacific oyster (Crassosstrea gigas) culture. Kampung Baru-Baru is also one of the villages participating in “Green-lipped Mussel Rearing Project” organized by the Sabah Fisheries Department. Coastal station (06˚17'59.2''N, 116˚17'36.22''E) was located at the inner part of the bay and was surrounded by mangrove forests, whereas open sea station (06˚18'16.9''N, 116˚18'16.6''E) was located approximately 2 km away from the coastal station.

Temperature (˚C), salinity (psu), pH, and dissolved oxygen (mg/l) of the seawater at 0.5 m below water surface were measured in-situ using a multi-function environmental sensor (YSI, Loveland. Co, USA). Water samples were collected at 0.5 m below water surface using a Van Dorn water sampler for total suspended solids (TSS) and phytoplankton cell density analyses.

In laboratory, water samples were filtered through GF/C glass microfiber filters (Whatman™, No. 1822-047) for TSS analysis. Filtered water samples were then stored in refrigerator (4˚C) for water nutrients analysis. The inorganic nutrients,

including total dissolved phosphorus (PO₄-P), total ammonia-nitrogen (NH₄-N), nitrate ( NO 3 − ) and nitrite ( NO 2 − ) were analyzed following [8] .

The filters for TSS analysis were then dried to constant weight in an oven for 24 hours at 105˚C. The difference in weight over that of the fully dried empty filters represents the TSS (mg/L) [9] .

For phytoplankton cell counting, the water samples were preserved with Lugol’s solution in the field. In the laboratory, samples were poured into a one litre measuring cylinder and left concentrated for 24 hours using Utermöhl sedimentation method [10] . The supernatant were then discarded and retained only about 50 ml. The phytoplankton cell densities were then quantified using a Sedgwick Rafter chamber under a Carl Zeiss microscope at 400× magnification [11] .

Data on rainfall for Ambong Bay area was obtained from Malaysia Meteorological Department.

Statistical analyses were performed using the SPSS statistics software. Tests were judged to be significant at p < 0.05 level. Pearson’s correlation coefficient test was performed to evaluate the relationship between the water parameters. One- way ANOVA test was applied to test for significant differences in environmental variables, water nutrients, and phytoplankton cell density among stations and months.

As shown in Figure 2, highest water temperature recorded in coastal and open sea stations was 33.2˚C in November 2015 and 33.1˚C in May 2016, respectively, whereas the lowest was 30.0˚C in August 2016 for coastal station and 28.3˚C in February 2016 for open sea station. The mean value of temperature for the two stations was in the range of 30.83˚C to 31.42˚C (Table 1). Ambong Bay is located on the west coast of Sabah, where it is experiencing equatorial climate and regular temperature throughout the year [12] . The range of temperature was probably due to the time of sampling, which affected by tide and heating from the sun [13] . Although there were variations in water temperatures between the two stations indifferent months, these variations are not significant (p > 0.05). According to [14] , water temperature for tropical aquaculture species is best at 25˚C - 32˚C. This suggests that the water temperature in Ambong Bay is still within the optimum level for aquaculture.

The pH measurements recorded in the two stations are presented in Figure 3. It was noticed that the pH in open sea station was highest (8.30) in October 2015 and lowest (7.56) in December 2015. For coastal station, maximum pH was recorded in October 2015, at 8.07 and minimum in July 2016 at 7.12. In general, pH value in open sea was higher, with mean of 7.89 ± 0.06, while coastal station was 7.67 ± 0.08 (Table 1). Although pH recorded in the two stations showed to have significant difference (p < 0.05), variations of pH within the station throughout the sampling period were insignificant (p > 0.05). The presence of fermentation process by anaerobic bacteria forming organic acids [15] [16] , decaying vegetation and acidic clays [17] [18] in the mangrove soils might explain the lower pH recorded in the coastal station. According to [18] and [13] , pH may also

be affected by water discharges from household and aquaculture activities. For marine organisms, the optimum pH is usually between pH 7.5 - 8.5 [14] . Below pH 6.5, some species may experience slow growth, fail to maintain its salt balance and reproduction [17] .

Salinity was the highest in February 2016, measured at 34.94 psu and lowest in June 2016, at 31.57 psu for coastal station (Figure 4). For open sea station, the highest salinity was recorded in March 2016, at 34.61 psu and lowest in August 2016, at 31.24 psu (Figure 4). The salinity in the two stations was not significantly varied (p > 0.05). However, salinity fluctuated significantly on temporal basis (p < 0.05). From September to October and November to March, Sabah experiences inter-monsoon and northeast monsoon seasons, respectively. During these periods, Sabah is expected to experience rainy season, or ‘wet season’. According to [19] and [20] , heavy rainfall should lower the value of salinity due to the large inflow of freshwater. However, salinity reading in both stations showed a gradual increase from September 2015 to January 2016, and then the values remained high with not much difference from February to April 2016. This was due to the El-Nino phenomenon that has affected rainfall pattern. As shown in Figure 5, there was a decrease of rainfall starting from September 2015 to January 2016. Similarly, no rainfall and salinity changes recorded from January to March 2016. This causes the salinity level to less fluctuating.

Maximum DO recorded in coastal and open sea stations was 6.60 mg/l and 6.80 mg/l in March 2016 respectively, whereas minimum DO was 3.12 mg/l in coastal station and 3.66 mg/l in open sea station during September 2015 (Figure 6). In general, DO at open sea station was higher than coastal station, with mean of

5.60 ± 0.29 mg/l in open sea and 5.04 ± 0.26 mg/l in coastal station (Table 1). Lower DO value in coastal station was most likely due to the decomposition of organic matters under aerobic conditions in mangrove and a decrease in the pH value of water [19] . It was found that DO between stations did not vary significantly (p > 0.05). However, variations in DO on the temporal basis seemed significant (p < 0.05). The significant variation in DO on temporal basis could be influenced by the water temperature, partial pressure of oxygen in atmosphere, salt contents in water and tides. Generally, DO decreases slightly as salinity increases. DO decreases more as temperature increases regardless of salinity [18] . Besides, DO in water is also affected by the tidal influence due to mixing of higher DO water from offshore to inshore water and current speed [16] [21] . Saturation level of at least 5 mg/l is required for good growth of marine fish. Values lower than this can put the organisms under stress and may result in mortality [14] .

Both coastal and open sea stations recorded highest TSS in March 2016 at 0.2672 mg/l and 0.2756 mg/l (Figure 7), respectively. Lower TSS was recorded in both stations in June 2016 at 0.0182 mg/l and 0.0133 mg/l, respectively. In general, TSS was higher in coastal station, with mean of 0.0515 ± 0.0198 mg/l, whereas mean for open sea station was 0.0454 ± 0.0243 mg/l (Table 1). Statistical analysis

showed the TSS in both stations did not vary significantly (p > 0.05) spatially but variation seemed significant (p < 0.05) when compared temporally especially in March 2016. Study by [22] has shown that monsoon season influences TSS concentration to increase during high rainfall. However, this may not be the case in this study since no rainfall was recorded in March 2016 due to El-Nino phenomenon [23] . In fact, the lowest TSS concentration was recorded in June 2016 where El-Nino has gone and rainfall started to increase. The main reason is due to the high phytoplankton cell density in March (Figure 12). TSS is contributed by the living microorganisms (e.g. plankton and nekton) and non-living matters (e.g. plant debris or suspended soil particles) moving in water, thus high concentration of phytoplankton in the water will also increase the TSS value. Another reason could be due to the dilution effect by the heavy rainfall which reduced the concentration, as suggested by [13] . Besides, tidal influences due to strong currents during sampling also caused resuspension of sediments [13] . High concentration of TSS could clog fish gills. However, this study suggests that the current TSS concentration in Ambong Bay is still under the standard values (50 mg/l) set for marine life, fisheries, coral reefs, recreational and mariculture in Malaysian Marine Water (Class 2) [24] .

In general, total ammonia-nitrogen (NH₃-N) in coastal station was slightly higher than in the open sea, with mean concentration of 0.0464 ± 0.0035 mg/l and 0.0443 ± 0.0046 mg/l respectively (Table 1). The highest concentration of NH₃-N was recorded during September 2016 in open sea station, at 0.0834 mg/l and lowest during December 2015 in coastal station, at 0.0278 mg/l (Figure 8). Nitrate ( NO 3 − ) concentration was the highest during February 2016 in coastal station, at 0.9898 mg/l and lowest during November 2015 in open sea, at 0.0305 mg/l (Figure 9). In general, NO 3 − concentration in coastal station was higher than open sea station, at 0.3091 ± 0.0823 mg/l and 0.1850 ± 0.0386 mg/l respectively (Table 1). These values were higher than the standard values (0.06 mg/l) set for marine life, fisheries, coral reefs, recreational and mariculture (Class 2), in Malaysian Marine Water [24] . The highest concentration of NO 2 − was recorded during December 2015 in coastal station, at 0.0023 mg/l and lowest during January 2016 in open sea station, at 0.0001 mg/l (Figure10). NO 3 − was considered to be less toxic when compared with other inorganic nitrogen compounds, but at high level it can affect osmoregulation, oxygen transport, eutrophication and algal bloom [18] .

The mean values of nitrite ( NO 2 − ) ranged from 0.0009 ± 0.0002 mg/l in coastal station to 0.0004 ± 0.0001 mg/l in open sea station (Table 1). As for total dissolved phosphorus (PO₄-P), highest concentration was recorded during November 2015 in coastal station, at 0.0188 mg/l and the lowest was recorded during March 2016 in open sea station, at 0.0007 mg/l only (Figure 11). In general, concentration of PO₄-P was slightly higher in coastal station than open sea station,

at 0.0072 ± 0.0013 mg/l and 0.0056 ± 0.0009 mg/l respectively (Table 1). NH₃-N, NO 3 − and PO₄-P were found to have significant temporal differences (p < 0.05), while NO 2 − showed to have significant spatial differences (p < 0.05).

Distribution of nutrients is mainly influenced by season, tidal conditions and freshwater flow from land source [25] . Dilution by heavy rainfall may be one of the reasons of decrease in nutrients concentration in water [20] [25] . However, in the present study, the opposite results were obtained. Changes in concentrations of NH₃-N, NO 3 − and PO₄-P in different months did not appear to have link with the changes of salinity and rainfall. This indicates that the variations of NH₃-N, NO 3 − and PO₄-P in different months are most likely influenced by tides.

NO 2 − is the intermediate product from NH₄-N to NO 3 − during the process of nitrification and denitrification [26] . It is not a stable product and its absence or presence is not so unusual in low quantities [27] [28] . However, it can be used as a pollution indicator in water. In the present study, significantly higher concentration of NO 2 − in coastal station could be due to higher NH₄-N and NO 3 − concentrations in the water, which encouraged more nitrification and denitrification processes. This is due to coastal station exposes to more organic matters and vegetation detritus from mangrove forest and anthropogenic wastes from land compared to open sea station.

A total of 34 phytoplankton genera, representatives of 28 families, were identified from the two stations: 33 genera and 25 families in coastal station (Table 2); and 34 genera and 28 families in open sea station (Table 3). Both coastal and open sea stations were dominated by Prorocentrum spp. with mean abundance of 16.23% and 24.44%, respectively, followed by Chaetoceros spp. (17.26%) and Thalassionema spp. (14.65%) in coastal station and Thalassionema spp. and Ceratium spp. (12.76%) in open sea station. In general, open sea station has higher phytoplankton cell density, with mean ± SE of 5.39 ± 2.36 cells/ml. However, significant difference (p < 0.05) in phytoplankton cell density was only apparent on temporal basis. Prorocentrum spp. is a dinoflagellate consisting several toxic and harmful species [29] [30] [31] . Ceratium spp. and Chaetocerous spp. are also potentially toxic algae species [32] . Besides, toxic phytoplankton species are relatively poor in nutrition. The balance in the nutritional values is an important features of food quality [33] , especially to shellfish or bivalve culture where most bivalve depends on phytoplankton as their primary food source.

Table 4 showed the results of Pearson Correlation analysis on surface water parameters data in Ambong Bay. NH₃-N was positively correlated with PO₄-P (r = 0.475, p < 0.05) but negatively correlated with salinity (r = −0.517, p < 0.01). Besides, salinity was positively correlated with DO (r = 0.505, p < 0.05) and TSS (r = 0.408, p < 0.05). In addition, DO was found to positively correlated with TSS (r = 0.451, p < 0.05). Phytoplankton cell density was also positively correlated with TSS (r = 0.644, p < 0.01).