The study’s foundation rests on two decades of satellite-derived climate records. The researchers combined rainfall data from the Climate Hazards Group InfraRed Precipitation with Station dataset and sea-surface temperature measurements from the Group for High Resolution Sea Surface Temperature Level-4 Multi-scale Ultra-high Resolution analysis, both spanning 2003 to 2023. When they compared the most recent decade against the first, a clear seasonal redistribution emerged. May rainfall declined by roughly 100 millimeters, and July rainfall dropped by 70 to 80 millimeters in the later period. At the same time, June gained about three wet days per month. Crucially, the annual total rainfall showed no resolved change, meaning the bay is not simply drying out or getting wetter overall. Instead, the timing of freshwater delivery to the estuary is shifting, a distinction that matters enormously for how nutrients are flushed, diluted, and concentrated within the bay’s waters.

Temperature told its own story. At the bay’s entrance, sea-surface temperature rose by 0.036 degrees Celsius per year over the study period, a steady warming trend that may appear modest on a graph but accumulates meaningfully over two decades. The team also identified 60 marine heatwave events using a hierarchical detection framework, underscoring that episodic thermal stress is superimposed on the long-term warming. For a bay that already experiences harmful algal blooms, these thermal conditions are significant because many bloom-forming species respond strongly to temperature, with warmer waters often accelerating growth rates and extending the seasonal windows during which blooms can establish and persist.

To understand how these climatic shifts interact with local water quality, the researchers conducted an intensive field campaign, sampling surface nutrients and water quality parameters at 51 stations across the bay on 14 and 15 September 2023. They then returned a year later, from 24 to 29 September 2024, to collect depth-resolved oxygen profiles. This two-pronged approach allowed them to map the horizontal geography of enrichment while simultaneously capturing the vertical structure of oxygen in the water column, a combination rarely achieved in small tropical estuaries of this kind.

One of the study’s most striking findings concerns the spatial pattern of nutrient enrichment. Using a rank-based index to synthesize nutrient concentrations across all stations, the team found that 11 of the 51 stations fell within the index’s upper fifth. But rather than forming a single gradient that intensified toward the land, as classical estuarine theory might predict, the enrichment was strikingly localized. Three stations situated between 9.7 and 12.1 kilometers from the bay’s inlet were simultaneously elevated in ammonium, oxidized nitrogen, phosphate, and silicate. This means that the most enriched waters were not at the head of the bay but in its interior, a pattern that points to localized sources or retention zones rather than a simple land-to-sea delivery of nutrients. For managers trying to control eutrophication, this localization changes the calculus entirely, because interventions would need to target specific zones rather than assuming a uniform dilution gradient.

The horizontal structure of other water quality variables reinforced this picture of a bay organized by its connection to the sea. Surface salinity decreased with distance from the inlet, as expected as marine water mixes with fresher landward inputs. Meanwhile, temperature, chlorophyll-a, and turbidity all rose moving away from the inlet, indicating that the inner bay harbors more phytoplankton biomass and more suspended particles. The relationship between turbidity and oxygen proved particularly telling: oxygen concentrations were lower where turbidity was higher, consistent with the idea that particle-rich waters block light, alter primary production dynamics, and fuel microbial respiration that consumes oxygen. In shallow tropical bays where sediments are easily resuspended by wind and tide, this coupling between turbidity and oxygen can create self-reinforcing stress on bottom-dwelling organisms.

The vertical oxygen profiles collected in September 2024 revealed perhaps the study’s most consequential pattern. Within individual measurement casts, oxygen consistently declined from the upper third to the lower third of the water column, and this stratification held on both flood and ebb tides. At 22 stations sampled during flood tide, the median station-mean oxygen concentration fell from 5.88 milligrams per liter in the upper third to 4.57 milligrams per liter in the lower third. At 42 stations sampled during ebb tide, the corresponding decline was from 6.63 to 5.88 milligrams per liter. These are not trivial differences. Bottom-third means dropped below 5 milligrams per liter at 17 of the 22 flood stations and at 12 of the 42 ebb stations, crossing a threshold widely regarded as stressful for many marine organisms. In a bay where shellfish grow on racks and in cages near the seabed, sustained low bottom oxygen can impair feeding, growth, and survival, translating environmental degradation directly into economic losses for fishing communities.

The mechanism behind this vertical oxygen structure likely involves the interplay of stratification, organic matter decomposition, and restricted ventilation of bottom waters. In shallow, semi-enclosed bays, density differences between fresher surface water and saltier bottom water can limit vertical mixing, trapping respired oxygen in the lower layer. The localized nutrient enrichment identified by the rank-based index may stimulate phytoplankton production in surface waters; when that biomass sinks and decomposes, it consumes oxygen precisely where concentrations are already lowest. The finding that this pattern persisted across both tidal phases suggests it is a persistent feature of the bay rather than a transient artifact of a single tidal state, raising concerns about chronic hypoxic stress during periods of high water-column demand.

Batan Bay is not an isolated case. Globally, coastal waters have been losing oxygen as warming reduces oxygen solubility, strengthens stratification, and stimulates biological consumption, a trend documented across estuaries, bays, and open oceans. Harmful algal blooms, meanwhile, have been expanding in the Philippines and across Southeast Asia, with researchers increasingly linking their spread to climate-driven changes in temperature and hydrology. What makes the Batan Bay study valuable is its integration: rather than examining rainfall, nutrients, or oxygen in isolation, the team characterized all three together, revealing how a shifting monsoon regime, patchy enrichment, and bottom-water oxygen depletion coexist within a single productive embayment. The bay’s toxin-positive bloom history, documented through monitoring bulletins from the Philippine Bureau of Fisheries and Aquatic Resources, gives these environmental conditions immediate public health relevance, since paralytic shellfish poisoning remains a serious risk in Philippine coastal communities.

The authors conclude that the seasonal rainfall shift and the sustained warming at the bay’s entrance could widen the window for future harmful algal blooms, a warning that carries weight for the thousands of people who depend on Batan Bay’s shellfish harvests. The study’s analysis code and derived data have been made publicly available through a GitHub repository, and the satellite datasets underpinning the climate analysis are freely accessible, lowering the barrier for other researchers and managers to replicate the approach in comparable tropical estuaries. As climate change continues to redistribute rainfall and warm coastal waters across the Coral Triangle and beyond, the Batan Bay findings offer both a caution and a template: the environmental conditions that precede toxic blooms are measurable, their spatial structure is knowable, and with sustained monitoring, the communities that live with these risks can be better prepared for what the changing climate brings to their waters.

Subject of Research: Climate-driven rainfall shifts, nutrient enrichment, and bottom-water oxygen depletion in a Philippine shellfish aquaculture bay

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

Article References: Eladawy, A., Nakamura, T., Herrera, E. C., Basina, R. M., Hernandez, B. C. B., Primavera-Tirol, Y. H., & Nadaoka, K. (2026). Seasonal rainfall shifts, localized nutrient enrichment, and low bottom oxygen in Batan Bay, Philippines. Environmental Monitoring and Assessment, 198(11), Article 1140. https://doi.org/10.1007/s10661-026-15953-3

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15953-3

Keywords: Batan Bay, harmful algal blooms, eutrophication, dissolved oxygen, marine heatwaves, shellfish aquaculture, estuary, Philippines, rainfall variability, sea surface temperature, nutrient enrichment, coastal monitoring