PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON lll(3):720-730. 1998. Phytoplankton composition within the tidal freshwater region of the James River, Virginia Harold G. Marshall and Lubomira Burchardt (HGM) Department of Biological Sciences, Old Dominion University, Norfolk, Virginia 23529-0266, U.S.A.; (LB) Department of Hydrobiology, Adam Mickiewicz University, Marcelinska 4, 60-801, Poznan, Poland Abstract. — Based on a 10.5 year data set, 271 taxa were identified at a single tidal freshwater station in the James River The mean monthly concentrations of major algal categories, total biomass and productivity are given. Diatom maxima were associated with peak periods of river discharge, with chloro-phytes, cyanobacteria, and autotrophic picoplankton abundance and productiv-ity greater during reduced river flow and more stable water conditions. Within the tidal stretch of estuarine riv-ers, there is a unique and little studied re-gion dominated by freshwater plankton, yet it is subjected to daily tidal influence and the occasional intrusion of estuarine spe-cies. These waters are referred to as tidal fresh. The channel divisions between fresh-water, tidal fresh, and oligohaline sections of a river are not constant, but will move longitudinally within the river basin in re-sponse to changes in the amount of river flow and tidal influence. During a period of drought, the range of the tidal fresh area moves farther upstream, whereas, during the seasonal rains, it would extend farther downstream. The abundance of tidal fresh algae in this region has been associated to hydrodynamic events in the river, with cell concentrations inversely related to fresh-water input and directly related to the wa-ter's residency time (Jackson et al. 1987, Schuchardt & Schirmer 1991, Jones et al. 1992). Other major environmental factors influencing abundance in this region in-clude turbidity, nutrients, tidal cycles, and light availability (Cole et al. 1982, Cloem et al. 1983, Cloem 1987, Haas et al. 1981, Jones 1991). The extent of dominance by freshwater algae downstream varies within different estuaries, but the downstream flora will be dominated by estuarine species (Haerte et al. 1969, Forester 1973, Jackson et al. 1987, Marshall & Alden 1990). In tid-al freshwater (<0.5 ppt), the algae are mainly composed of chlorophytes, cyano-bacteria, and diatoms (Forester 1973, Opute 1990, Marshall 1994), with diatoms often predominating (Haertel et al. 1969, Jackson et al. 1987, Schuchardt & Schirmer 1991). Diatoms are more common during periods of high river discharge in contrast to chlo-rophytes, cyanobacteria, and phytoflagella-tes, which are more dominant during low river discharge (Farrell 1994, Schmidt 1994). The James River is a major tributary of the Chesapeake Bay, with a drainage basin of 26,440 km-, a length of 545 km and a mean monthly discharge rate of approxi-mately 7100 ft' sec' (Belval et al. 1995). Within the southern Chesapeake Bay trib-utaries, several studies have compared phy-toplankton spatial and temporal distribution within the tidal fresh and saline sections of these rivers (Marshall & Alden 1990, Mar-shall & Affronti 1992, Marshall 1994). These studies indicate the transport of es-tuarine species upstream via sub-surface waters and the transition to estuarine spe-cies dominance downstream. Marshall and
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