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 Ͻ10% 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.

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Trihalomethanes (THMs) are byproducts produced during the disinfection of drinking water. We combined survey and experimental approaches to identify factors that influence THM formation potential (THMFP) in the Ohio River drainage basin. Two surveys of the Ohio River and its five principal tributaries were conducted to characterize spatial variation in THMFP in relation to algal abundance and suspended organic matter. We performed three experiments by placing Ohio River water in 2000-L outdoor mesocosms and manipulating algal senescence and bloom development by shading. Increases in THMFP among high- and low-light and dark tanks suggest that algal production, algal senescence, and possibly photolysis increased THMFP by as much as 50% over 3–6 days. Comparable yields of THMs (per unit of chlorophyll) were observed in both survey and experimental settings. Comparison of input waters with outputs indicates that the Ohio River at times acts to attenuate downstream transport of THM precursors. Our findings suggest that both watershed-scale and internal processes regulating THMFP should be considered as utilities develop strategies to meet new drinking water guidelines.