EFFECTS OF NAVIGATION DAMS, TRIBUTARIES, AND LITTORAL ZONES ON PHYTOPLANKTON COMMUNITIES IN THE OHIO RIVER

Phytoplankton and water chemistry variables were sampled monthly along a 361-km stretch of the Ohio River. We compared (i) regions from Ohio through Kentucky, (ii) sites above, below, and within tributaries, (i ii) nearshore and channel habitats, and (iv) sites away from and near navigation dams. Densities of suspended cells were lowest during colde r, high-flow periods (31 000 cells/mL) and greatest during warmer, low -flow summer (104 000 cells/mL). Of 134 taxa identified, > 60% were di atoms, comprising 6% of cell numbers and 28% of total biovolume. Summe r assemblages were dominated by colonial Aphanothece saxicola, Merismo pedia punctata, Microcystis aeruginosa, and unicellular Synechococcus species. Diatoms Melosira jurgensii and Melosira distans were abundant in summer and Cyclotella spp. in spring. Concentrations of dissolved inorganic phosphorus (approximate to 40 mu g/L) and dissolved inorgani c nitrogen (approximate to 1100 mu g/L) were high; neither was positiv ely correlated with any phytoplankton variable. Interpool declines in picoplankton correlated with greater copepod nauplii densities. Naviga tion dams significantly reduced current velocity and densities of bent hic forms; dissolved inorganic phosphorus levels were significantly gr eater. Near-dam sites had increased biomass of larger phytoplankton (> 2 mu m) in winter, but lower densities during summer. Multiple regres sion identified temperature (+) and current velocity (-) as the main f actors affecting phytoplankton densities in this large river.