Temporary Shift from Net CO2 Uptake to Net CO2 Release During the Southwest Monsoon in a Tropical Mangrove Forest

Mangrove forests are important blue-carbon ecosystems, but their atmosphere-surface CO2 exchange can vary over short time scales in response to environmental conditions. This study examined CO2 exchange in the Del Carmen Mangrove Reserve, Siargao Island, Philippines, using PhilFlux eddy covariance and micrometeorological observations from March 2025 to February 2026. We analyzed diurnal, daily, and monsoon-season patterns of gross primary productivity (GPP), ecosystem respiration (RE), and net ecosystem exchange (NEE). The tower footprint showed net CO2 uptake during most of the observation period. August 2025 was the only month with positive aggregated NEE (19 g C m−2 month−1; 95% CI: −22 g C m−2 month−1 to 62 g C m−2 month−1). During August, monthly GPP was 158 g C m−2 month−1; 95% CI: 130–189 g C m−2 month−1 and monthly RE was 177 g C m−2 month−1; 95% CI: 135–226 g C m−2 month−1. Sixteen of 31 days showed positive NEE, mainly from 8 to 24 August. We therefore interpret this as a temporary positive-NEE episode rather than a month-wide or lasting reversal of CO2 uptake. The episode coincided with dry conditions during the Southwest Monsoon. Environmental analyses identified several variables associated with variation in NEE, GPP, and RE, but these relationships are interpreted as statistical associations rather than causal effects. Since the study covers only one annual cycle, longer-term observations are needed to determine whether similar events recur. Continued flux monitoring is needed to better understand the stability of mangrove CO2 exchange through time.