Seasonal hydrology drives shifts in production sources supporting fishes in the Lower Mekong River Basin

Background/Question/Methods

By creating water diversions and impoundments, humans have altered river flow regimes worldwide. Knowledge of river food web ecology is essential for management of these impacts on fluvial ecosystems and their valuable services and resources. Tropical rivers are ideal for studying food web structure and dynamics because seasonal rainfall and hydrology results in relatively predictable dynamics of nutrients and primary and secondary production. Previous research on the Lower Mekong River indicated that, during the dry season, fine particulate organic matter is mostly derived from algae, and, during the wet season, it is mostly derived from vascular plants. We used stable isotopes of carbon and nitrogen to estimate the primary production sources supporting fish biomass in four large rivers within the Lower Mekong River Basin in Cambodia – three rivers unimpacted by dams and one dammed river. We used the MixSIR model to estimate the relative contribution of four alternative production sources – seston, benthic algae, riparian C₃ macrophytes, and riparian C₄grasses.

Results/Conclusions

Seston and benthic algae were the most important production sources supporting fish biomass during the dry season, and riparian macrophytes appeared to be the most important production source supporting fishes during the wet season. We also detected a potential effect of river impoundment on contributions of basal production sources to fish biomass. In the river with strong flow regulation from an upstream impoundment, seston and benthic algae were even more important production sources supporting fishes during the dry season.