Spatial and temporal characteristics of algae in the River Great Ouse. I. Phytoplankton

Abstract

The phytoplankton of the middle reaches of the River Great Ouse was examined over a period of five years (1989–1993). Ecosystem variables that 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 irradiance sensors. Turbidity is caused by allochthonous solids during the winter but rapid development of phytoplankton in the eutrophic water in spring causes continued high light attenuation until late in the summer in most years. The hydrology of the river plays an important role in 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 larger populations develop with increasing numbers of Cyanobacteria and Chlorophyceae. However, the crash in the spring population during June and July and, generally, the lack of any major regrowth between July and September does not appear to be caused by nutrient limitation, poor underwater light climate or flush-out. Possible causes are examined. In contrast, areas of restricted flow (marinas) develop large phytoplankton populations that generally persist throughout the summer. © 1997 John Wiley & Sons, Ltd.