SPATIAL AND TEMPORAL CHARACTERISTICS OF ALGAE IN THE RIVER GREAT OUSE.1. PHYTOPLANKTON

The phytoplankton of the middle reaches of the River Great Ouse was ex amined over a period of five years (1989-1993). Ecosystem variables th at were measured included, phytoplankton chorophyll a, cell number and the underwater light climate in relation to discharge. The underwater light climate was examined using a suspended solids sensor and irradi ance sensors. Turbidity is caused by allochthonous solids during the w inter but rapid development of phytoplankton in the eutrophic water in spring causes continued high light attenuation until late in the summ er in most years. The hydrology of the river plays an important role i n determining the extent and timing of the phytoplankton populations. The onset of the spring phytoplankton populations is determined by the discharge pattern in the late winter and early spring. In dry years l arger populations develop with increasing numbers of Cyanobacteria and Chlorophyceae. However, the crash in the spring population during Jun e and July and, generally, the lack of any major regrowth between July and September does not appear to be caused by nutrient limitation, po or underwater light climate or flush-out. Possible causes are examined . In contrast, areas of restricted flow (marinas) develop large phytop lankton populations that generally persist throughout the summer. (C) 1997 by John Wiley & Sons, Ltd.