The Study Area

The bay has a north-south orientation and narrows upstream. The length of the bay is approximately 35 km and the width varies from <1 km upstream to ~6 km at the mouth of the bay [1, 4-7, 27]. The mangrove area upstream and downstream of the swamp dominated coastal morphology. Settlement, industry, and other permanent infrastructure cover almost all the swamp areas in Balikpapan.

The Balikpapan Bay is influenced by seawater from the southern Makassar Strait. River runoff then occurs in the upper and middle areas. Significant river runoff comes from Sepaku, Semo, Wain, and Riko Rivers, with average discharges of 42.189 m³·s⁻¹, 83.496 m³·s⁻¹, 2.477 m³·s⁻¹, and 16.852 m³·s⁻¹, respectively [5-7, 28]. In addition, the Balikpapan Bay inland area experiences mean rainfall of 230.24±53.45 mm·month⁻¹. Rainfall varies from 108.51 mm·month⁻¹ to 317.00 mm·month⁻¹, with two peaks during November-December, and March-April. The dry months occur in August to October [3].

Data Collection and Sampling Process

The observations were conducted on October 14, 2021. This study focused on eight stations in the Balikpapan Bay (Fig. 1). The in-situ measurements and sample collection were performed at each station during the flood (07.00-12.00 local time) and ebb tides (13.00-17.00 local time) (Fig. 2).

The Conductivity Temperature Depth (CTD) profiler ALEC ASTD 678 and electromagnetic current meter (ECM) were vertically lowered at eight stations to measure temperature, salinity, turbidity, chlorophyll-a fluorescence, depth pressure, and current speed. The accuracy ranges of temperature, salinity, turbidity, chlorophyll-a fluorescence, and depth pressure were –3-45ºC, 2-42 PSU, 0-1000 NTU, 0-0.4 ppm, and 0-600 m, respectively. The current speed accuracy ranged from 0 to 500 cm·s⁻¹ and from 0° to 360° for the direction. The CTD and ECM were set to record data every 0.5 seconds.

The 3.5 l Nansen bottle was used to collect surface water samples from eight stations. Then, 2 l water was placed in the bottle for laboratory analysis of nitrate (NO_3­–N), phosphate (PO₄³⁻), and ammonia (NH₃).

A 30-micron plankton net with was used to filter 10,000 ml of water to a volume of 60 ml. The plankton sample was placed in a bottle and stored in a cool box.

In this study, all surface water parameters were analyzed, which included ocean dynamics (current speed and direction), physical parameters (temperature and turbidity), chemical parameters (salinity, nitrate (NO_3­–N), ammonia (NH₃–N), and phosphate (PO₄³⁻)), and biological parameters (phytoplankton abundance and chlorophyll-a fluorescence).

Fig. 1a).

Bathymetry map of Balikpapan Bay and its surrounding area. The sampling station shows in gray-black dots, Center Nusantara City, Penajam Paseur Utara Regency, Kutai Kartanegara Regency, and Balikpapan City are offered in a solid black box, Sepaku, Semoi, Wain, and Riko Rivers are shown in a solid black triangle. b) The inset map is the area of Indonesia, with the study area indicated by a white box.

Fig. 2.

Water elevation during sampling and measurement on neap tide at Balikpapan Bay.

Plankton and Nutrient Sample Analysis

Plankton and nutrient (NO_3­–N, NH₃–N, and PO₄³⁻) samples were analyzed at the water quality laboratory belongs to Samarinda Industrial Research and Standardization Centre, Ministry of Industry of the Republic of Indonesia. Phytoplankton samples were identified under a light microscope using a Sedgwick Rafter Counter Cell (SRCC), where cell morphology and color were observed. In addition, cell counts were analyzed for abundance and biodiversity data. Phytoplankton species were identified according to Indonesian standard method SNI 06-3963-1995. This method is equivalent to the standard methods for examining water and wastewater part identification of the type and counting phytoplankton abundance part 1002 F.2 [29].

The nitrate (NO_3­–N) concentration was measured using a spectrophotometric method. The Brucine method was applied using a spectrophotometer at a wavelength of 410 nm. This study followed the Indonesian standard method for measuring nitrate concentrations in open water SNI 06–2480–1991. The procedure was similar to the standard method for examining water and wastewater part 419D [30]. Ammonia (NH₃–N) was analyzed using the indophenol blue method with a spectrophotometer at 640 nm. These steps were based on the Indonesian standard method for measuring ammonia concentration in open seawater (SNI 19–6964.3–2003). This technique was adopted in the standard method for examining water and wastewater Part 3112 [31]. Subsequently, the phosphate (PO₄³⁻) concentration was analyzed using the ascorbic acid method with a spectrophotometer at a wavelength of 880 nm. This procedure was based on the Indonesian standard method for measuring phosphate concentrations in open water and wastewater (SNI 06–6989.31–2005). The SNI 06–6989.31–2005 standard is equivalent to the standard methods for examining water and wastewater [32].

Data Analysis

Principal Component Analysis

Principal component analysis (PCA) was used to evaluate the relationship between the ocean dynamics, physical, and chemical parameters of marine water and the abundance and diversity of phytoplankton. This analysis identified the vigorous physical and chemical parameters that affected the abundance and diversity of phytoplankton at all observation stations during ebb and flood tides. The XLSAT 2019 v.3.2 software is used to perform PCA.

Water Quality Parameters and Ocean Dynamics

The Balikpapan Bay area in general can be divided into three sub-areas, that is downstream (S1, S2, and S3), middle (S4, S5, and S6), and upstream (S7 and S8). The average sea surface current velocity was 0.022±0.101 m·s⁻¹ (0.088±0.069 m·s⁻¹) for the u-component and -0.079±0.062 m·s⁻¹ (-0.177±0.075 m·s⁻¹) for the v-component during flood (ebb) tides. The highest and lowest current speeds were observed at S4 in front of the Riko and Wain Rivers. The highest is 0.27 m·s⁻¹ southward during the flood, and the lowest is 0.04 m·s⁻¹ westward during the ebb. In the upstream and some of the middle sub-areas, the surface current direction is almost similar during flood and ebb tides. Meanwhile, in the downstream sub-areas, the surface current direction was significantly different and almost in the opposite direction, namely southwestward during flood and southeastward during ebb tide. However, at all stations, the path moved permanently downstream (Fig. 3). The lower dominance of tidal effects during the neap tide may have caused this movement. Thus, river discharges were more dominant than tides during neap tide, especially in the upstream and middle sub-areas.

Fig. 3.

Surface current during flood and ebb on neap tide in Balikpapan Bay.

Water quality parameters measured in Balikpapan Bay are shown in Fig. 4. The average temperature, salinity, turbidity, and concentration of chlorophyll-a, nitrate, ammonia, and phosphate during flood (ebb), respectively are 29.89±0.28°C (30.47±0.32ºC), 28.28±2.24 PSU (27.41± 1.76 PSU), 5.24±1.8 NTU (4.91±1.7 NTU), 44.83±14.4 ppm (43.68±14.8 ppm), 0.368±0.21 mg·l⁻¹ (0.352±0.13 mg·l⁻¹), 0.004±0.004 mg·l⁻¹ (0.023±0.054 mg·l⁻¹), and 0.003±0.002 mg·l⁻¹ (0.002±0.002 mg·l⁻¹. The standard deviation revealed that variations during the flood and ebb tides were generally the same. Variations during floods and ebbs exhibited spatially similar patterns. These patterns indicate that the water quality during the flood and ebb tides did not change significantly. This state may prevent the dynamically driven current from being dominated by tides during the neap tide.

The sea surface temperature (SST) ranges from 29.59ºC to 30.56ºC during flood tides and 29.85ºC to 30.97ºC during ebb tides. The bay SST in the downstream and middle sub-areas was higher during the ebb tide and lower during the flood periods. However, in the upstream sub-area, there is almost no gap. The differences in the SST in the downstream, middle, and upstream sub-areas were approximately 0.90ºC, 0.52ºC, and 0.15ºC, respectively. The sea-air interactions likely caused the difference in SST during flood and ebb tides. The measurements were performed in the morning during flood tide, while the ebb tide occurred in the afternoon. The effect of sunlight dominantly influenced the SST. However, the stations located in front of the river (S4), near the floating settlement (S5), and under the vast bridge (S7 and S8) had small SST gaps, with values lower than the other stations. The direct effects of various water inputs and the amount of protection from sunlight might have contributed to these conditions.

The sea surface salinity (SSS) gradually decreased from downstream to upstream, from approximately 33.00 PSU (30.20 PSU) in the downstream sub-area, 28.30 PSU (27.70 PSU) in the middle sub-area, and 24.70 PSU (24.40 PSU) in the upstream sub-area during the flood (ebb) tide. The SSS during the flood tide was always higher than during ebb tide at all stations. This finding indicates that the effect of the tidal cycle on SSS persisted. However, the difference was moderate.

The differences in SSS during the flood and ebb tides in the downstream, middle, and upstream sub-areas are 1.23, 0.53, and 0.8 PSU, respectively. The SSS gap in the downstream sub-area was relatively high. This condition may have caused the seawater during the flood in the morning and the freshwater effect during the ebb tide in the afternoon. As seen in Fig. 2 and Fig. 3, SSS in S1 during the ebb tide measured at 14:10 local time with a decreasing water level. Thus, a freshwater effect was induced until S1. In contrast, the SSS differences between flood and ebb tides in the upstream sub-area were relatively small. This condition corresponds to a surface current variation that mostly flows southward but at a greater speed during ebb tide. Therefore, the influence of fresh water is more dominant and lowers SSS. However, along the middle to upstream sub-areas, SSS at S7 is the lowest, which is also related to surface current during ebb tide. At S7, the surface current speed during ebb tide is the highest; therefore, the influence of fresh water from the Semoi and Sepaku Rivers is stronger compared to S8.

The turbidity from upstream to downstream sub-areas is almost steady during both ebb and flood tides. However, at station S4 a significant increase was found, from approximately 4 to 7 NTU. This increase occurred during flood tide and is likely due to the interaction between tidal current and river flow, which potentially lifts the sediment in front of the shallow and muddy Riko River.

Fig. 4.

Water quality parameters during flood and ebb on neap tide in Balikpapan Bay.

Nutrients (nitrate, ammonia, and phosphate) exhibited unique patterns. However, only nitrate showed a significant difference between flood and ebb tides. The nitrate concentration during ebb tide was significantly above the average in S4 (0.53 mg·l⁻¹), S6 (0.54 mg·l⁻¹), and S7 (0.65 mg·l⁻¹), which means that these areas received more input from the rivers. In the downstream sub-area, the nitrate concentration during ebb tide is lower than during the flood tide due to significant differences in the surface current direction. The interaction between tides and river runoff spread nitrate concentrations throughout this sub-area.

Ammonia exhibited different patterns. Plateaued, from downstream to upstream. Ammonia concentrations at each station during the ebb and flood periods were almost similar, except at S4. The highest ammonia concentration of 0.17 mg·l⁻¹ was observed at S4 during the ebb tide; meanwhile, the concentration at other stations was approximately 0.001 mg·l⁻¹. We found that high concentration of ammonia at S4 was strongly associated with high turbidity observed at that station. Phosphate showed a persistent pattern during ebbs and floods at all stations. The highest concentrations were recorded at S5, with values of 0.009 mg·l⁻¹ during the flood tide and 0.006 mg·l⁻¹ during the ebb tide. The other stations varied from 0.0009 mg·l⁻¹ to 0.0037 mg·l⁻¹. S5 is in the vicinity of floating fishing settlements, which are thought to be the source of high phosphate concentration.

Chlorophyll-a concentration gradually increased from downstream to upstream, varying from 0.41 ppm to 1.52 ppm during the flood and 0.70 ppm to 1.38 ppm during the ebb tide. On average, the chlorophyll-a concentration during flood tide was lower than ebb tide and increased from downstream to upstream sub-areas. The differences in concentration during flood and ebb tides in the downstream, middle, and upstream sub-areas were approximately 0.21 ppm, 0.36 ppm, and 0.18 ppm, respectively. The chlorophyll-a concentrations in the downstream and upstream sub-areas were lower than that in the middle sub-area. Generally, the concentration of chlorophyll-a is related to phytoplankton abundance. However, this is not well illustrated from observations. The correlations between the chlorophyll-a concentration and phytoplankton abundance during the flood and ebb tides were 0.001 and 0.5, respectively. This condition revealed that chlorophyll-a concentration could be used as a proxy for phytoplankton abundance only during the ebb tide. Meanwhile, it failed as a proxy during flood tide as shown in Fig. 4.

Structure of Phytoplankton Community in Balikpapan Bay

The phytoplankton community in Balikpapan Bay consisted of 12 species with two classes during flood tide and 13 species with three classes during ebb tide (Fig. 5). The class identified during flood were Bacillariophyceae (diatoms) (93.94%) and Dinophyceae (6.06%). In contrast, diatoms (88.68%), Dinophyceae (9.43%), and Chlorophyceae or green algae (1.89%) were identified during ebb. The diatom was the most dominant class both during flood and ebb tides, with 10 species, of which three potentially caused HAB. The species identified were Chaetoceros sp., Nitzschia sp., and Pseudo-nitzschia sp. The composition of these three species was 42.5% during the flood and 21.6% during the ebb tide. The Dinophyceae class consists of two species that have the potential to cause HAB, namely Ceratium sp. and Dinophysis sp., with a total composition of 6.0% during flood and 9.4% during ebb. The green algae class existed only in Balikpapan Bay during ebb, with only one species, Spirogyra sp. (1.9%). Known as a freshwater saline algae, Spirogyra sp. has probably been transported by the river flow to Balikpapan Bay. This state was also observed for S4 (Table 2), which is in front of the Riko and Wain Rivers.

Phytoplankton Abundance and Diversity in Balikpapan Bay

The average phytoplankton abundance was 85±79 cells·l⁻¹ during flood, and 125±133 cells·l⁻¹ during ebb. This indicates that phytoplankton abundance varied significantly, with approximately 40 cells·l⁻¹ to 680 cells·l⁻¹ during flood and 80 cells·l⁻¹ to 880 cells·l⁻¹ during ebb. The highest abundance was observed at S4 (680 cells·l⁻¹) during flood, and S6 (880 cells·l⁻¹) during ebb. The lowest abundance was observed in S5 (40 cells·l⁻¹) during flood and S1 (80 cells·l⁻¹) during ebb. This finding indicates that during ebb tide, the abundance is more varied compared to flood tide.

Individual abundance varied among the 40 cells·l⁻¹ to 480 cells·l⁻¹ in each station. During the flood tide, only one species phytoplankton was identified in S1, namely Ceratium sp. However, Ceratium sp. was observed also in S4 and S6 but only during ebb tide. The species that was observed at almost all stations was Cyclotella sp., except in S1. Furthermore, the species that only existed once at S6 were Gyrosigma sp. and Nitzschia sp., while in S7, Pseudo-nitzschia sp. was observed.

The highest abundance species over stations was Cyclotella sp., with a total abundance of 2,000 cells·l⁻¹. The lowest abundance was Ceratium sp. with a total abundance of 40 cells·l⁻¹. The species with the highest number of cells were Chaetoceros sp. (760 cells·l⁻¹) and Dinophysis sp. (360 cells·l⁻¹).

Generally, the phytoplankton species and abundance change more significant during the ebb tide. The abundance in S1 included four species with a total of 320 cells·l⁻¹. S1 had only one species during the flood tide with an abundance of 40 cells·l⁻¹. S5 contained only one species, with an abundance of 40 cells·l⁻¹. Nitzschia sp. was not present during the flood tide. Overall, the highest abundance at all stations was observed for Chaetoceros sp. and Cyclotella sp., with total abundances of 720 cells·l⁻¹ and 720 cells·l⁻¹ respectively.

Fig. 5.

Percentage of phytoplankton class a) and species b) in Balikpapan Bay during flood and ebb on neap tide.

The phytoplankton diversity index (H′) was generally greater than 1 for both flood and ebb tides, except in S1 during flood and ebb tides and S5 during ebb tide only (Fig. 6). This result indicates that phytoplankton diversity in the Balikpapan Bay is low and has a small community size. The dominance index (C) indicates that there are dominant species (C>0.5) found at S1 and S7 during both flood and ebb tides, and S5 during the ebb tide only. The stress quality also might affect the survival of phytoplankton species.

Relationships between Seawater Parameters and Phytoplankton Abundance and Diversity

Based on PCA, cumulative eigenvalues of 57.75% (58.25%) during flood (ebb) tides and a variable minimum of 0.5 squared cosines illustrated that there were three groups of data observed. The three groups were in axes F1 and F2, positive for both flood and ebb tides, F1 was negative at flood tide, and F2 was negative at ebb tide (Fig. 7). The first group included S4, S2, salinity, current speed, and H′ during flood tide, and S4, S6, ammonia, turbidity, and chlorophyll-a at ebb tide. This finding is shown in Fig. 7 as a solid black line. The dash line represents the second group, consisting of S3, turbidity, phytoplankton abundance, and nitrate concentration during flood tide and S5, phosphate concentration and dominance index during ebb tide. The dot dash line marks the third group, which consists of S7, S8, chlorophyll-a, and dominance index during floods (Fig. 7a). The dot line consists of S3, salinity, and diversity index (Fig. 7b) during ebb tide.

Fig. 6.

Phytoplankton diversity (H′) and dominance (C) indexes during flood and ebb at neap tide at Balikpapan Bay.

Fig. 7.

Correlation between physical parameters with phytoplankton abundance at Balikpapan Bay during the flood a) and ebb b) on neap tide.

Dissimilarity analysis was performed based on the Bray-Curtis dissimilarity index (Fig. 8), which was determined by the diversity (H′) and abundance of phytoplankton at all stations for each flood and ebb tide. The results were significantly similar to those of the strong flood and ebb tides. A dendrogram of the eight observation stations is shown in Fig. 8a) during flood and Fig. 8b) during ebb. The three classes (C1, C2, and C3) obtained from the analysis showed a similarity level of approximately 96.50% (93.25%) during flood (ebb) tides.

Fig. 8.

Bray-Curtis′s dissimilarity index of phytoplankton abundance and diversity (H′) at Balikpapan Bay during the flood a) and ebb b) on neap tide.

The highest values were observed in C1 during the flood tide (98.02%) and C2 during the ebb tide (100%). The C2 class followed this trend during the flood (96.95%) and C1 during the ebb (94.52%) tides. The lowest values were observed for C3 (95.96%) and ebb (91.98%) tides. During the flood tides, C1 class was observed in S1 and S8. The C2 class was found in S3, S4, S5, S6, and S7. The next class was C3 in S2. During ebb tide, the C1 class was found in S1, S3, S4, S6, and S7. The C2 class was found in S2 and S8. The last class was C3, which was found in S5. These results are highly similar in almost all stations. Environmental conditions influenced the similarity of the source of phytoplankton diversity (H′) and abundance in Balikpapan Bay.