Vertical Distribution of Chl-a in Relation to Environmental Factors and Water Column Stratification in the Downstream Section of the Air Bengkulu River

Abstract

The Bengkulu Watershed, which spans two administrative districts, plays a crucial role in supporting ecological and socio-economic functions in Bengkulu Province, Indonesia. Its main river, the Air Bengkulu, has experienced environmental pressures from land conversion, coal mining activities, domestic waste discharge, and industrial effluents, which may alter downstream water quality and disrupt estuarine ecological processes, including primary productivity. Historically, the adjacent coastal area was utilized by local fishers as a fishing ground in the early 2000s, but this function has since declined. However, studies on the ecological implications of these pressures on phytoplankton production in the Air Bengkulu estuary remain limited. This study aimed to assess phytoplankton productivity using Chl-a concentration as a proxy and examine its relationship with key environmental parameters. Fieldwork was conducted during the peak dry season from June to August 2025, with two sampling sessions per event in the morning and afternoon under spring tide conditions. Observations were carried out at three stations on three sampling events with measurements taken at three depth levels. Salinity, current velocity, dissolved oxygen, and nitrate concentration were measured at all depths, while temperature, pH, and water transparency were limited to surface levels. Chlorophyll-a (Chl-a) concentration was analyzed using spectrophotometric methods and its relationship with environmental parameters was evaluated using correlation analysis and quantile regression. The results showed that Chl-a concentrations were consistently very low across stations, ranging from 0.025 to 0.139 µg/L, indicating ultra-microtrophic to oligotrophic conditions. Among the measured parameters, nitrate concentration and current velocity exhibited the strongest relationships with Chl-a, suggesting their role as primary limiting factors, while other parameters showed comparatively weaker influences. These findings indicate that low nutrient availability and hydrodynamic conditions constrain phytoplankton biomass development and primary productivity in the Air Bengkulu River estuary, potentially limiting its capacity to sustainably support the food web.