The Ohio River has no effective water quality standard for nutrients because this deep, channelized river is too turbid to allow light penetration sufficient to allow the potamoplankton to grow.  In this light-limited condition only under conditions of drought and high zebra mussel attachment (2005 and 2007) is the water clear enough to allow the suspended algae to accumulate.  Based on our surveys of nutrients and biomass every 5 miles for 250-981 mile transects in early August, the algal biomass averaged 5-10 ug chlorophyll a per liter in early August, when point source enrichment is the major source of nutrients.  In our highest flow summer (2001) the achieved algal biomass was low.  In our mid-ranged years the algal biomass was the highest with a compromise between nutrient delivery and light penetration, reaching 20-30 ug chlor a /liter.  In our lowest discharge year (2007) the algal biomass was clearly nutrient limited as the estuaries effectively removed all nutrients before delivery to the Ohio.  With projected warmer, drier summers and wetter winters with climate change, nutrient TMDLs for nutrients should be considered seasonally.