A Study of the Phytoplankton of a Calcareous River

Compared with lakes, relatively little is known of the ecology of phytoplankton populations in rivers in Britain. A few of these have been investigated qualitatively, but none, apparently, quantitatively. Records were begun on the Thames by Fritsch (1902, 1903, 1905) and the work was continued by Rice (1938) with observations over the period 1928-32. Here, however, only the larger plankton algae were considered owing to the method of net sampling. Several rivers were studied by Butcher (1932 onwards), but mostly with regard to the benthic populations. Williams (1950, 1954) included nannoplankton forms in lists of phytoplankton from the River Dee at Chester. The Shannon was investigated extensively by Southern & Gardener (1938), but this river is not really comparable with those in England as it connects a series of large lakes and the flora contains many typical lake algae. The Severn and Stour (Suffolk) are currently under investigation and are proving to have considerable amounts of phytoplankton at times. The study of the River Lee, Hertfordshire, the subject of this paper, extended over two years and was undertaken in order to obtain quantitative information of phytoplankton populations in relation to chemical and physical data. This tributary of the Thames is 56 miles (90 km) long and its flow is considerably retarded by canalization. The river and its main tributaries (Fig. 1) arise in springs in the chalk escarpments of the Chilterns of Hertfordshire and Cambridgeshire and together drain an area of about 1500 km2 (Houston 1922). For most of its course the river flows through regions of Eocene clays of the London Basin, overlying chalk (Sherlock 1960). The bed is composed of small stones, gravel and mud and the water carries much suspended silt in times of high rainfall. The largest tributary, the Stort, enters at Feilde's Weir, near Hoddesdon. The part of the river covered by the present study is canalized, the first lock on the Lee being at Hertford, and on the Stort at Bishop's Stortford. At Feilde's Weir the average daily flow is about 763 000 m3 (Houston 1922). The upper part of the Lee receives a considerable amount of sewage effluent during its passage through Luton. During the period of the investigation a new sewage treatment plant was opened at Rye Meads, near Hoddesdon, which serves Welwyn Garden City and the new towns of Harlow and Stevenage. The effect of its effluent on the algal population was considered. Also observed was the influence of the warm water discharged at Dobb's Weir from Rye House Electricity Generating Station, which extracts water for cooling purposes from the river below Feilde's Weir.