Diatom-Dominated Phytoplankton Community and Water Quality across Hydrological Zones of Sumber Jenon, East Java, Indonesia

Groundwater-fed springs remain poorly documented in many tropical settings used for tourism and local water supply. This study characterized phytoplankton and physicochemical conditions across the inlet, central pool, and outlet of Sumber Jenon, East Java, Indonesia. Sampling was conducted on 10, 20, and 30 July 2025. At each station, 15 L of water was filtered through a plankton net, concentrated to 50 mL, preserved with Lugol solution, and examined using an improved Neubauer hemocytometer. Each unicell, colony, or filament was counted as one natural unit. Twenty-three genera from six major groups were recorded. Mean total abundance was 2,266.7 ± 83.6 ind/L at the inlet, 2,270.0 ± 44.7 ind/L in the central pool, and 2,165.3 ± 20.6 ind/L at the outlet. Bacillariophyta dominated all zones, contributing 55.71–57.79% of total abundance. Cyclotella led at the inlet and central pool, whereas Navicula led at the outlet; Ulnaria and Cymbella were also prominent. Mean Shannon diversity increased from 2.855 at the inlet to 2.970 at the outlet, while Simpson dominance declined from 0.065 to 0.057. The Secchi disk remained visible at the bottom, indicating transparency of at least 1.6, 1.7, and 6.0 m at S1, S2, and S3. Dissolved oxygen remained high at 8.07–8.37 mg/L. Temperature, pH, and total dissolved solids were similar among zones, while ammonia, nitrate, and total organic matter varied more across dates. In the exploratory CCA, the outlet lay apart from the inlet and central pool primarily along Axis 1, which accounted for 93.04% of constrained variation and defined a spatial pattern for broader seasonal testing. These findings provide a short-term genus-level baseline for monitoring Sumber Jenon.