Seasonal rainfall shifts, localized nutrient enrichment, and low bottom oxygen in Batan Bay, Philippines

Batan Bay supports shellfish production and has recorded toxin-positive harmful algal blooms whose drivers remain poorly understood, and its rainfall, nutrient, and oxygen conditions have not been characterized together. We combined 2003–2023 rainfall and sea-surface temperature records with surface nutrients and water quality from 51 stations on 14–15 September 2023 and depth-resolved oxygen profiles on 24–29 September 2024. Compared with 2003–2012, the later decade received about 100 mm less May and 70–80 mm less July rainfall, and gained about 3 June wet days per month, with no resolved change in annual total. Entrance temperature rose by 0.036 \(^\circ \)C per year, with 60 marine heatwaves. A rank-based index placed 11 of 51 stations in its upper fifth; three stations 9.7–12.1 km from the inlet were simultaneously enriched in ammonium, oxidized nitrogen, phosphate, and silicate, so enrichment was localized rather than a single landward gradient. Surface salinity fell away from the inlet while temperature, chlorophyll-a, and turbidity rose, and oxygen was lower where turbidity was higher. Within individual casts, oxygen declined from the upper to the lower third of the water column on both tides: median station means fell from 5.88 to 4.57 mg L\(^{-1}\) at 22 flood stations and 6.63 to 5.88 mg L\(^{-1}\) at 42 ebb stations. Bottom-third means were below 5 mg L\(^{-1}\) at 17 of 22 flood and 12 of 42 ebb stations. The rainfall shift, and the sustained inlet warming could widen the window for future harmful algal blooms.