Phytoplankton production in a large, regulated river: A modeling and mass balance assessment

Phytoplankton production in riverine systems is regulated by hydrologic processes and coupled optical dynamics, which determine the light dosages experienced by phytoplankton during transit within a defined reach. We used data on river stage, discharge, and channel geomorphometry to model changes in light availability experienced by phytoplankton during transit within a 122-km navigational pool of the Ohio River. Whole-pool estimates of phytoplankton production were derived from photosynthesis-irradiance relationships and modeled values of light availability. Derived estimates of primary production showed good agreement with whole-pool mass balances for algal carbon. The sum of upriver inputs and autochthonous production agreed to within 10% of downriver export. During a summer with above normal discharge (1998), phytoplankton production within the pool corresponded to <10% of phytoplankton inputs from upstream and tributary sources. During lower flows in 1999, phytoplankton production in the pool exceeded external inputs of algal carbon. Modeled estimates of primary production were used to predict seasonal and longitudinal variation in algal abundance assuming a constant C: chlorophyll ratio. Model results showed good agreement with measured chlorophyll values and supported the hypothesis that biomass development was constrained by light availability and transit time within the pool. The model overestimated chlorophyll in late summer when grazing might limit biomass accumulation. The cumulative irradiance experienced by phytoplankton during transit within the pool was found to be a good predictor of autotrophic potential and for interpreting complex interactions arising from seasonal hydrologic cycles and the influence of water regulation structures.