Autotrophic Picoplankton in a Regulated Coastal River in New South Wales

Cell density, cell type and vertical distribution of autotrophic picoplankton (APP, cell size 0.2-2 μm) were examined for a year from September 1992 at three freshwater sites in the Hawkesbury-Nepean River. During the study, mean cell density of APP at 1 m deep varied seasonally two orders of magnitude from 2.2 x 10³ to 3.2 x 10⁵ cells mL^-1 At upstream sites of Penrith and North Richmond, higher cell density was observed from summer to autumn. There were three cell types of APP (i.e. coccoid, ellipsoid and rod-shaped). Proportionally, coccoid cells increased downstream from 25 to 52 % of total cells, whereas ellipsoid and rod-shaped cells decreased downstream from 64 to 47 % and from 11 to 1 % of total cells, respectively. The vertical distribution of APP (1 and 4 m deep), examined for 9 months at North Richmond, showed that overall mean density at 1 m was significantly higher than overall mean density at 4 m. Overall, the cell density of APP at 1 m deep was positively correlated with temperature and total chlorophyll a. The present results suggest that APP may need to be incorporated into a conceptual model of river plankton food webs.