The equipment used for plankton sampling included a 5 L bucket, a plankton net with a 20 µm mesh size, and 250 mL sample bottles. Water quality parameters were measured using a thermometer, refractometer, and pH indikator strips. The materials used for preservation and storage of plankton and water samples included Lugol’s solution, permanent markers, labels, and a cooler bag.

Plankton samples were collected at each sampling station in two replicates. Approximately 75 L of seawater was collected using a water bucket and filtered through a plankton net with a mesh size of 20 µm to concentrate the plankton. The concentrated samples were then transferred into 250 mL sample bottles and preserved with Lugol’s solution (approximately 3–5 drops) to prevent cell degradation and maintain structural integrity. Plankton identification and analysis, as well as physico-chemical parameters, were conducted at the Water Quality Laboratory, Faculty of Fisheries and Marine Sciences, Mulawarman University.

Plankton abundance was calculated and expressed as individuals per liter. Community structure was analyzed using ecological indices, including the Shannon–Wiener diversity index (H′) and evenness index (E). Prior to statistical analysis, the normality of the data was evaluated. As the data did not meet the assumption of normal distribution, differences in plankton abundance among sampling stations were tested using the non-parametric Kruskal–Wallis test at a significance level of 95% (p < 0.05).

The results of the study conducted in three different ecosystems in the waters of Beras Basah Island showed a relatively similar plankton species diversity. A total of 13 plankton species were identified across all observation sites, with individual abundances ranging from 1574 to 1732 individuals per liter. The plankton community was dominated by phytoplankton, comprising 10 species belonging to two classes, namely Bacillariophyceae and Dinophyceae. In addition, zooplankton were also recorded, consisting of only three species belonging to the classes Ciliata and Crustacea. Detailed information on plankton species composition in each ecosystem of Beras Basah Island is presented in Table 1.

The Shannon–Wiener diversity index (H′) of plankton across the three ecosystems in the waters of Beras Basah Island ranged from 2.17 to 2.30. The highest value was recorded in the seagrass bed e, while the lowest was observed in the coral reefs. The Evenness Index (E) ranged from 0.94 to 0.98 across all sampling stations. The highest evenness value was found in the mangrove ecosystem, whereas the lowest value was recorded in the seagrass ecosystem. The values of diversity and evenness indices were comparable among the three ecosystems.

Based on the measurement results of several physico-chemical parameters of the waters at three different sampling stations around Beras Basah Island, the environmental conditions were relatively uniform. This condition indicates that there were no significant differences among the sampling stations in terms of the characteristics of the aquatic environment. The detailed results of the physico-chemical parameter measurements at each sampling station are presented in Table 2.

Measurements of physico-chemical parameters across the three ecosystems showed relatively similar values. Water temperature ranged from 42 to 43 °C, salinity ranged from 25 to 30 ppt, and pH ranged from 7.9 to 8.1. Nutrient concentrations were relatively low, with nitrate values ranging from 0.018 to 0.022 mg L⁻¹ and phosphate ranging from 0.005 to 0.018 mg L⁻¹. The Kruskal–Wallis test indicated that there was no significant difference in plankton abundance among the three ecosystems (p > 0.05), suggesting that the observed variations in abundance values were not statistically different.

Based on the results of this study, phytoplankton were found to be more dominant than zooplankton. This finding is consistent with (Nontji, 2008), who stated that these two phytoplankton classes are commonly found in marine waters. In addition, (Padang, 2023) reported that these classes have a wide distribution across various aquatic environments. Similar patterns have also been reported in several marine waters, where phytoplankton generally dominate over zooplankton. This dominance is closely related to the role of phytoplankton as the base of the marine food web and their rapid response to changes in nutrient availability in aquatic ecosystems (Fadhilah et al., 2022). The high abundance of phytoplankton is attributed to their function as primary producers that directly utilize light and nutrients for photosynthesis (Maya et al., 2021). Consequently, phytoplankton tend to dominate the structure of plankton communities in various marine environments.

Pleurosigma sp. was found at all research stations. This phytoplankton species is considered cosmopolitan, indicating a high level of ecological tolerance to various environmental conditions (Padang, 2023). The presence of this species across different coastal ecosystems is often associated with its ability to adapt to fluctuations in physicochemical water parameters, such as water transparency, salinity, and nutrient availability (Wang et al., 2019)(Haviludin et al., 2023). In contrast, Rhizosolenia bergonii was only recorded in the coral reef ecosystem. (Padang, 2023) reported that Rhizosolenia is a plankton genus commonly found in marine waters, as it belongs to the oceanic plankton group. Similar findings have been reported by (Reynalta et al., 2025), who documented the occurrence of the genus Rhizosolenia in marine environments. In the present study, the limited occurrence of Rhizosolenia bergonii in the coral reef ecosystem may be temporal in nature, possibly influenced by water currents and the relatively close proximity between research stations.

Dinophyceae, particularly Ceratium furca and Ceratium tripos, were evenly distributed across all ecosystems, with relatively high abundances recorded in the coral reef ecosystem. Dinoflagellates of the genus Ceratium are known to exhibit diel vertical migration and high tolerance to variations in light intensity, enabling them to persist in waters with diverse environmental conditions (Glibert et al., 2018). The high abundance of Ceratium furca in the coral reef ecosystem is presumed to be associated with relatively stable water conditions and moderate nutrient availability, which represent optimal conditions for dinoflagellate growth (Maherezky et al., 2023).

In contrast, zooplankton were found in relatively low abundances. This condition is influenced by the timing of sampling, which was conducted during the daytime. (Jimny et al., 2023) explained that most zooplankton tend to migrate away from the surface toward deeper water layers through diel vertical migration in order to avoid high light intensity and predation pressure. During feeding activities, zooplankton may migrate to depths of up to 200 meters. The limited distribution of zooplankton species, such as Tintinnopsis directa, which was only found in the coral reef ecosystem, as well as Acartia clausi and Acartia omorii, which were recorded in sea and seagrass ecosystems, indicates the influence of micro-environmental factors such as currents, turbulence, and habitat structure on zooplankton distribution ((Kiørboe, 2018); Maharezky et al., 2023).

Plankton species diversity across the three ecosystems in the waters of Beras Basah Island showed Shannon–Wiener (H′) values ranging from 2.17 to 2.30 (Figure 2). An index value of around two is classified as indicating moderate species diversity. This condition suggests that the waters of Beras Basah Island remain relatively stable, with environmental pressures that have not yet resulted in the dominance of one or two plankton species (Magurran, 1988). Moderate diversity levels are generally found in productive waters that still maintain good environmental carrying capacity to support the structure of plankton communities ((Wang et al., 2019); (Padang, 2023)).

Evenness index values at all research stations were relatively similar, ranging from 0.94 to 0.98 (Figure 3). Evenness index values greater than 0.6 indicate a uniform distribution of plankton species across the research stations (Magurran, 1988). This suggests that higher evenness values reflect a more equitable distribution of individuals among the observed plankton species at the study sites.

Variations in the diversity and evenness index values in a water body, according to (Maya et al., 2021), are influenced by physical water factors as well as differences in nutrient availability and nutrient utilization by each organism. The temperature measurements at each sampling station, including the sea, coral reef, and seagrass ecosystems, showed relatively similar values ranging from 42–43 °C. The relatively high temperature values were likely influenced by the time of sampling, which was conducted during the daytime, resulting in increased water temperatures. According to (Nontji, 2008), sea water temperature is strongly influenced by solar radiation, weather conditions, and the timing of data collection, with temperatures generally being higher during the daytime. Furthermore, (Effendi, 2003) stated that fluctuations in water temperature are a physical factor that is highly sensitive to environmental changes and directly influence the biological processes of aquatic organisms. This finding is consistent with (Reynalta et al., 2025), who reported that daytime sampling resulted in plankton diversity and abundance patterns that were comparable to those observed in the present study.

The results of the acidity (pH) measurements at all research stations showed relatively stable values ranging from 7.0 to 8.1. These conditions indicate that the waters remain within the optimal range for marine organisms and comply with the quality standards specified in Appendix VIII of Government Regulation No. 22 of 2021. These values are also consistent with the range suitable for plankton growth and abundance reported by (Reynalta et al., 2025), which is between 6.2 and 8.6. Despite these favorable chemical conditions, the low diversity of zooplankton observed in this study is likely influenced by the sampling time, which was conducted during the daytime. The relatively high water temperature during the sampling period may act as a limiting factor affecting the vertical distribution of zooplankton.

The in situ measurements of salinity showed values ranging from 25–30 ppt, which are relatively lower than the national seawater quality standards. Based on the field measurements, the salinity levels at the research site ranged between 25 and 30 ppt. Although these values are below the seawater quality standard threshold for marine biota as stipulated in Government Regulation No. 22 of 2021, ecologically they remain within the tolerance range of plankton. As stated by (Padang, 2023), planktonic organisms in marine waters are generally still able to survive and distribute in environments with salinity levels above 20 ppt

The nitrate concentration at all sampling stations ranged from 0.018 to 0.022 mg/L (as presented in Table X), indicating values well below the maximum threshold of 0.06 mg/L as stipulated in Appendix VIII of Government Regulation No. 22 of 2021. This condition suggests that the waters are in a relatively stable oligotrophic state and remain within a safe tolerance range for plankton growth (Padang, 2023). As one of the limiting nutrients in aquatic ecosystems, the availability of nitrate at measurable concentrations plays a crucial role in regulating plankton diversity and abundance, considering its function as a primary component in the synthesis of proteins and chlorophyll in phytoplankton. Adequate nitrate availability in the waters supports primary productivity, increases phytoplankton abundance, and indirectly supports zooplankton populations through trophic interactions within aquatic ecosystems. However, excessive nitrate concentrations resulting from domestic waste discharge may lead to eutrophication, which can trigger algal blooms and reduce plankton diversity (Paerl, 2014)(Glibert et al., 2018).

The orthophosphate concentration at the research site ranged from 0.005 to 0.018 mg/L. The concentrations recorded at the marine and seagrass stations were 0.018 mg/L, exceeding the seawater quality standard for marine biota of 0.015 mg/L as stipulated in Government Regulation No. 22 of 2021. Orthophosphate is the most essential form of dissolved inorganic phosphorus for primary productivity, as it can be directly absorbed by phytoplankton for photosynthesis and cellular metabolic processes (Hamuna et al., 2018). The relatively high orthophosphate concentration at these stations is likely influenced by anthropogenic activities around the coastal area of Beras Basah Island as well as the decomposition of organic matter accumulated on the seabed. According to (Padang, 2023), elevated phosphate levels in coastal waters are often triggered by land-based runoff that transports nutrients derived from human activities. Despite the relatively high phosphate concentration, the availability of nitrate during the daytime remained relatively low (0.018–0.022 mg/L), which may act as a limiting factor preventing the occurrence of massive algal blooms. The orthophosphate concentration exceeding the established threshold may contribute to the total phytoplankton abundance observed in this study, where the average abundance reached 13,777 ind/L. This finding is consistent with the study of (Reynalta et al., 2025), which reported that optimal phosphate availability can stimulate the growth of diatoms (Bacillariophyceae). In the present study, this group was identified as the most dominant phytoplankton group across all sampling stations.

Overall, the environmental conditions observed during this study suggest that the waters surrounding Beras Basah Island remain relatively suitable for supporting marine life. The relatively clear water conditions, stable pH, and moderate salinity provide favorable habitats for plankton communities and other aquatic organisms inhabiting coral reef, seagrass, and open-water ecosystems. The Kruskal-Wallis test results revealed that the different ecosystems (marine waters, coral reefs, and seagrass beds) did not significantly influence plankton abundance. This is presumably due to the relatively close proximity between stations and the influence of tidal currents, which facilitate water mixing. The homogeneous environmental conditions, particularly in pH and salinity, support a uniform distribution of plankton throughout this conservation area, suggesting that no single ecosystem is extremely superior in supporting plankton populations compared to others.