This study was conducted from May to December 2023 in the Ujung Genteng Coastal Area (7°22’54.2’’ S; 106°24’25.2’’ E), a popular local tourist destination. Data were collected using the line transect-quadrat method (English et al., 1994). Observation sites were determined using a purposive sampling method covering three coastal zones, based on the physical landscape structure and distinct substrate types (Zebua, 2017); (Aprilia et al., 2023), i.e., the West Zone (the western part of the coast), the South Zone (the southern part), and the East Zone (the eastern part) Figure. 1.

Biotic and abiotic data were collected at the designated zones/stations (West, South, and East) following the procedure suggested by (Handayani et al., 2020). In each zone, five haphazardly placed 100-meter transect lines were established perpendicular to the coastline Figure. 2. Along each transect, five quadrats (size of 1 m x 1 m each) were positioned at regular 20 m intervals to quantify macroalgal cover and species composition (Ariani & LOA, 2017). A total of 25 macroalgal samples were collected from each zone.

Macroalgal samples were collected from each quadrat along the transect lines during low tide using photographic and destructive sampling techniques (Nurmiyati, 2013); (Piazzi et al., 2019). The photographic method was used to document dominant taxa and morphological groups, while the destructive method is widely recognized as appropriate for assessing assemblage structure, biodiversity, and biomass (Piazzi et al., 2019). Both approaches are considered suitable and cost- effective for monitoring macroalgal assemblages and conducting environmental impact assessments (Cecchi et al., 2014). During sampling, macroalgal assemblages were photographed for documentation, and then carefully removed from the substrate using a sharp knife to ensure collection of the complete assemblage, including the holdfast. Samples were carefully placed into labeled, airtight plastic bags without compression and preserved in 70% ethanol prior to transport to the laboratory for further processing and analysis.

In the laboratory, macroalgal samples were identified to species level based on morphological characteristics of the thallus, including overall form, size, color variation, branching pattern, growth form, texture, holdfast type, and reproductive structures. Identification was conducted using standard and credible taxonomic references, including (Coppejans et al., 2009); (Sukiman, 2011); (Al-Yamani et al., 2014), and (Handayani et al., 2014).

Abiotic parameters such as substrate type, temperature, current velocity, water clarity, pH, and salinity were measured in situ(Sukiman, 2011). Substrate type was visually assessed and documented directly. Water temperature was measured using a thermometer, current velocity was assessed with a current drogue, water clarity was determined using a Secchi disk, pH was measured with a pH meter, and salinity was gauged using a salinity meter.

The data collected during the study were analyzed using descriptive statistics to describe, illustrate, and summarize the data. It was expected that a specific pattern might emerge in tables and graphs, describing the macroalgal distribution in Ujung Genteng. The spatial distribution of macroalgae was analyzed using one-way ANOVA in SPSS version 16.0 to investigate significant differences in distribution patterns and community structure among the three coastal zones. The ecological functioning of macroalgae communities was assessed using several biological indices proposed by Indarjani and (Indarjani & Nurhayati, 2022), including Shannon-Wiener Index (H’) for measuring taxa diversity, the Evenness Index (E) to analyze the distribution of taxa within the community, and the Simpson Dominance Index (D) to identify dominant taxa associated with the assemblages.

Ujung Genteng coastal area (7°22’54.2’’ S; 106°24’25.2’’ E) is relatively protected from wave breaks with a wide intertidal area reaching up to about 150 m toward the center and a depth of up to 85 cm. The area features a diverse range of habitats, including seagrass meadow, edge-of-the- sea coral beds, and coral flats in the mid-intertidal area near the wave break zone, where macroalgae were abundant. The coast is characterized by diurnal tides, with two high tides and two low tides each day. Variations of land use in Ujung Genteng ranging from plantations and agriculture, fishery ports, shrimp farms, tourism areas, and conservation areas. Anthropological activities around this coastal area highly affect the macroalgal communities.

For benthic organisms, substrate types greatly influence the presence of macroalgae. The West Zone is dominated by sand and seagrass meadow, with some coral, while the South Zone has substrate type consisting of sand, seagrass meadow, and reef crevices. The East Zone has a substrate characteristic dominated by sand, coral, and a little seagrass. This finding aligns with other research (Johan et al., 2015); (Effendi et al., 2024), which showed that the substrate types influence the structural community of macroalgae assemblages.

Diverse marine environmental conditions significantly influence the composition of macroalgae in eachzone. Toexplore this connection, measuring key factors such as temperature, pH, current velocity, water clarity, salinity, and substrate types was crucial. Comprehensive details regarding the environmental parameters are presented in Table 1.

Generally, environmental conditions along the Ujung Genteng Coast support the growth rates and community composition of benthic macroalgae. pH ranged from 7.90 to 8.69 is ideal, as most benthic macroalgae can thrive at pH 6.6 to 9.9 (Ulfah et al., 2017). pH influences the metabolic and respiratory processes of macroalgae existence of organisms by disrupting metabolic and respiratory (Kartikasari et al. 2025). Additionally, water temperatures at all observation sites ranged from 28.1 °C to 32.8 °C, which was within the normal range. Temperature affects the photosynthesis process of benthic macroalgae growth. (Arfah & Simon, 2016) suggested that the optimal temperature for macroalgae growth in tropical waters was between 15 °C and 30 °C, with a growth threshold at 34.5 °C.

Salinity along the Ujung Genteng Coast varied from 20.0 to 39.5 ppt, falling within the optimum range for benthic macroalgae. Salinity influences metabolic process and reproductive capacity, which in turn determines their distribution and abundance in the intertidal zone (Larsen & Say-Jensen, 2006). Luning (1990), as cited in (Arfah & Simon, 2016), indicated that many species of benthic macroalgae can thrive across a broader salinity range. This was supported by (Palalo, 2013), who observed that macroalgae grow in waters with salinity levels from 13 to 37 ppt. Variation in salinity across the three coastal zones of Ujung Genteng waters may be attributed to differences in depth during measurement, as salt concentration tends to be higher at lower depths (Yulianda, 2009). The current research reported that salinity levels in two different observed zones along Ujung Genteng waters were 34 and 35 ppt (Effendi et al., 2024).

Abiotic factor potentially detrimental to macroalgae growth is current velocity, which influences the distribution of nutrients necessary for photosynthesis. In this study, current velocity ranged from 7.7 cm/s to 25 cm/s, considered ideal for macroalgae growth. Current velocity above 40 cm/s may damage cultivation structures and break macroalgal assemblages (Marianingsih et al., 2013).

Sunlight availability is essential for photosynthesis. Water depth and clarity influence light penetration, determining which species can thrive. Our study found that water transparency at the three coastal zones was 100% at depths of 5 – 70 cm, indicating that macroalgae can grow optimally since sunlight can reach the bottom. Previous research reported that the depth for photosynthetic activity in macroalgae ranges from 0.6 to 6.5 meters or more (Arfah & Simon, 2016).

Variation of substrate types was observed across the three coastal zones of Ujung Genteng waters. The substrates primarily consisted of sand, coral fragments, reef crevices and seagrass. Different macroalgae possess varying attachment mechanisms. Rocky substrates support diverse communities, while seagrass can trap nutrients while limiting the growth of macroalgae that require full sunlight (Han et al., 2014). In contrast, sandy substrates, which are often unstable, may restrict macroalgal attachment; however, some opportunistic species can thrive in nutrient-rich sandy environments (Ko et al., 2024).

Variation of types, species, and forms of macroalgae reflects the rich diversity of macroalgae in Ujung Genteng coastal waters Table 2 &3. Benthic macroalgae collected during the study in Ujung Genteng comprised three divisions (Rhodophyta, Chlorophyta, and Phaeophyta), 19 families, 28 genera, and 41 species, totalling 1,909 individuals. This number was relatively comparable to a previous study in Ujung Genteng conducted by (Johan et al., 2015), who found 24 genera, 34 species from Rhodophyta, Chlorophyta and Phaeophyta, and the current study (Effendi et al., 2024), which reported 19 genera, 23 species, and 3,884 individuals of macroalgae assemblages in Ujung Genteng waters.

Variations of macroalgae organisms were presumably due to the cover area of sampling sites, the plot site, and time. Macroalgae assemblages collected in Ujung Genteng waters were still considerably higher than in other places in Indonesian waters. A study in Ambon Bay, Moluccan waters, identified 21 species of macroalgae, comprising 10 species of Rhodophyta, six species of Chlorophyta, and five species of Phaeophyta (Litaay, 2014). In addition, a study in Batu Putih National Park, Belitong Island, identified 18 species, comprising 15 families, across three divisions (Rhodophyta, Chlorophyta, and Phaeophyta) (Langoy et al., 2012). (Meriam et al., 2016) observed macroalgae assemblages in Mantehage Island waters, North Sulawesi, which consisted of green algae (Chlorophyta) comprised 3 orders, 6 families, 11 genera, and 23 species, while brown algae (Phaeophyta) included 3 orders, 3 families, 5 genera, and 5 species. Red algae (Rhodophyta) had 4 orders, 8 families, 10 genera and 16 species. The difference in the number of species was due to varying local natural conditions and environmental factors, which influence the presence of macroalgae in a specific location.

Rhodophyta division was found to be more diverse than Chlorophyta and Phaeophyta. The Rhodophyta Division (red algae), considered having the highest diversity, consisted of 22 species across 13 genera and 10 families and contributed 56% to the macroalgae assemblages, which were dominated by Gracilaria coronopifolia. In comparison, the Chlorophyta (green algae) comprised 13 species, distributed across 11 genera and six (6) families and made up 32% of the assemblages, which were dominated by Boergesinia forbesii. The division with the lowest diversity was Phaeophytan (brown algae), with 6 species from 4 genera and 3 families and dominated by Padina australis.

These findings align with research conducted by (Johan et al., 2015), which identified 15 species of Rhodophyta, 14 species of Chlorophyta, and 5 species of Phaeophyta in the waters of Ujung Genteng beach.(Mudrikah et al., 2024) noted that the significant diversity of Rhodophyta may be attributed to its effective sexual and asexual reproduction methods, which facilitate rapid growth and reproduction. Additionally, its adaptability to different environments allows it to flourish in both intertidal zones and deep-sea regions.

Conversely, B. forbesii from Chlorophyta was observed as the dominant species, with 312 individuals, or 16.8% of the total macroalgae found in this area. This situation was comparable with a previous study in Ujung genteng that Chlorophyta had the highest coverage (46.40%) compared with Rhodophyta and Phaephyta (Johan et al., 2015) and a similar situation with the current study (Effendi et al., 2024). This species is commonly found in intertidal areas where fluctuating environmental conditions are often occur. Besides, this algae species typically attaches to hard substrates like crevices, fragmented dead coral, rock and even other algae (Guiry et al., 2014) which was also observed in this study.

In the intertidal zone, Chlorophyta plays a crucial role in aquatic ecosystems as primary producers in the food chain, with their growth heavily reliant on high levels of sunlight. Besides this, greean algae was able to grow in various types of substrate, as observed in Ujung Genteng waters.

This study observed that the East Zone exhibited the highest relative abundance of macroalgae, which contributed 48% of the total abundance, followed by the South Zone with 30% and the West Zone was the site with the lowest one and contributed only 22% of the total individuals collected during the study.

The distribution pattern of macroalgae contains data regarding the presence of species and the interaction among individulas of the same species or between different species within their habitat. Studies investigated how various factors, such as environmental conditions, influence the distribution and diversity of macroalgae. As benthic organisms, the substrate characteristics play a crucial role in shaping the composition and distribution of macroalgae by affecting their ability to attach, grow, and compete, which in turn determines the marine ecosystem dynamics and biodiversity. (Ariani & LOA, 2017) observed that the variation of macroalgae composition was due to the type of holdfast and substrate suitability.

In the case of the Ujung Genteng areas, the spatial distribution of macroalgae was represented by the zoning division, specifically the West Zone, South Zone, and East Zone Figure. 3.

The East Zone exhibited a significantly different pattern of community structures compared to the other two zones Figure. 3 and was primarily characterized by a substrate dominated by coral and reef crevices. Substrates in the West Zone and South Zone mainly consisted of sand and seagrass beds. These findings align with the study of (Kadi, 2006), who noted that substrate structures have a significant influence on the diversity of macroalgae species. In coral-dominated areas, a greater variety of macroalgae was observed, including tubular and filamentous macroalgae of various shapes and larger sizes (Palalo, 2013). Additionally, our study identified species such as B. forbesii, G. coronopifolia and P. australis, which were found to be common across all three zones. These species are well- adapted to rock and coral substrates through disc-shaped holdfasts that securely attach to hard surfaces, allowing them to withstand waves and ocean currents, and tolerate drought during low tide (Palalo, 2013); (Johan et al., 2015);(Arfah & Simon, 2016).

Given the abundance of these macroalgal species, bioprospecting of these common species in Ujung Genteng warrants consideration. Boergesinia forbesi contains bioactive compounds such as steroids, flavonoids, phenolics, and saponins with antioxidant properties, indicating potential applications in medical and pharmaceutical fields (Gazali et al. 2023 in (Effendi et al., 2024). Gracilaria coronopifolia, a widely utilized seaweed in Indonesia, has potential as an alternative food resource, including gelatin production, and exhibits antiviral, antifungal, and antibacterial activities (Indarjani & Nurhayati, 2022); Li et al. 2019 in (Effendi et al., 2024). Padina australis contain alkaloids, flavonoids, saponins, tannins with antioxidant, antimicrobial, and antifungal properties (Rachmawati et al. 2021b in (Effendi et al., 2024). Acantophora muscioides exhibits promising bioactive potential for pharmaceutical applications (Sunarpi et al., 2018).

The community structure of macroalgae assemblages was analysed using biological indices. to examine patterns of biodiversity, community dynamics, and ecological relationships in Ujung Genteng coastal waters Figure. 4.

Biological indices reveal significant spatial variations in the macroalgal community structures along the Ujung Genteng coastal area, particularly in terms of species diversity and species dominance. The diversity index (H’) was classified as moderate, with average values ranging from 2.5 to 2.70, indicating that the communities were relatively stable and could maintain disturbances within the habitat. The East Zone had the highest diversity index value (H’ = 2.7), which was significantly different from the other two zones, while the West Zone had the lowest value (H’= 2.52). Environmental conditions and the suitability of a habitat influence macroalgal growth, as evidenced by the high diversity index (H’). The type of substrate in the East Zone, which was dominated by coral fragments and coral crevices, created a heterogeneity of species composition. A stable substrate of coral fragments, rocks, and sand supported the growth and species diversity of benthic macroalgae compared to places with a substrate consisting of sand and mud, which created an unstable substrate (Imchen, 2015), such as in the West Zone.

The distribution pattern of macroalgae explains information about the existence of species and relationships between the same species or between species and their habitat. The Evenness Index (E) within the three coastal zones showed no difference; the scores were categorized as high and ranged from 0.73 to 0.82, indicating that the distribution of individuals across species in the macroalgal community within the habitat was balanced. (Mudrikah et al., 2024) also found a similar pattern at Sarangan Beach, where macroalgae assemblages were characterised through a more balanced distribution of individuals of macroalgae and a few dominant species within the communities.

The dominance index (D) is an important ecological index used to assess the structure of macroalgae communities. The Dominance Index (D) measures the concentration and distribution of dominant species. A higher index value indicates that dominance is concentrated in a single species, while a lower value suggests that multiple species share dominance.(Indarjani & Nurhayati, 2022). The West Zone had the highest value (D = 0.17) and a significant difference from the other two zones, while the East Zone had the lowest value (D = 0.09) Figure. 4, indicating that there was a concentration of certain species at each observation point on the western part of the coast.

Our study showed that locations with a high diversity index (H’) likely to have a low dominance index (D). Conversely, stations with a low diversity index (H’) have a high dominance index (D). The East Zone had the highest average diversity index (H’) and the lowest average dominance index (D). This indicates that species diversity at each observation point was accompanied by a relatively even distribution of individuals of each species. In addition, the environmental conditions and substrate types at this location were relatively suitable for the growth of various macroalgae species. Conversely, the West Zone had the lowest average diversity index (H’) and the highest average dominance index (D). This was due to the substrate types dominated by seagrasses, sand and less fragmented corals. Additionally, environmental factors such as relatively strong current velocity, significantly influence the development of macroalgae assemblages, causing the survival of only a few macroalgae species.