Sources and sinks of dissolved oxygen in eutrophic tropical earthen fishponds

Dissolved oxygen is the main limiting factor in pond aquaculture production, yet the relative contributions of its various sources and sinks remain poorly quantified in eutrophic earthen pond systems. Photosynthetic oxygen production, surface exchange, sediment oxygen demand and water respiration were assessed in greenwater earthen fishponds using a combination of in situ measurements, bottle incubation experiments and laboratory batch trials. Dissolved oxygen levels followed a diel cycle with near-anoxic conditions before sunrise and hyperoxic conditions in the late afternoon. Nocturnal respiration rates exhibited oxyconformity, declining >5-fold from hyperoxic to moderately hypoxic conditions. Photosynthetic oxygen production was confined to a shallow photic zone, and gross primary production rapidly declined with depth, becoming negligible at 50 cm, with a compensation depth of 21 cm. Photosynthesis increased O2 saturation by up to 16% min−1 at high light intensity, but oxygen evolution was suppressed under hyperoxic conditions regardless of irradiance. Sediment oxygen consumption ranged from 110 to 288 mg O2 m−2 h−1 and was correlated with sediment organic carbon content. Oxygen consumption of the pond water was the dominant nocturnal oxygen sink. A theoretical oxygen budget for a 1000 m2 pond indicated that daytime photosynthesis generated sufficient oxygen to meet pond requirements, but lack of mixing, atmospheric losses and nocturnal respiratory demand deplete all available oxygen, resulting in pre-dawn anoxia. These findings underscore the importance of phytoplankton biomass management for high light penetration, and the potential benefits of installing mixing devices to eliminate stratification.