LONG-TERM CHANGES IN MACROPHYTES OF BRITISH RIVERS - 1 - RIVER WEAR

A survey of flowering plants, bryophytes and macroscopically recogniza ble algae in each 0.5-km length of River Wear in summer 1996 permitted comparisons to be made with earlier surveys in 1966, 1976 and 1986. T he initial survey was made at the end of a decade in which marked envi ronmental changes due to closure of various coal mining and coke works operations had taken place. Some species showed similar distributions down the 90 km of river throughout the 30-year period, whereas others showed marked changes. Using net changes of 20 or more records within either the upstream 40 km or downstream 50 km as evidence of change b etween 1966 and 1996, the following were found: upstream increase in C ladophora glomerata; downstream increase in Myriophyllum spicatum, Ran unculus fluitans, Elodea canadensis, Potamogeton pectinatus and Sparga nium erectum; downstream decrease in Zannichellia palustris. Hydrodict yon reticulatum, first noticed in 1995, had extended for 18 km in late summer 1996. Callitriche platycarpa, C. stagnalis and Potamogeton ber chtoldii are also invasions since 1966, although all seem likely to ha ve been present in the river in the pre-industrial era. Possible envir onmental causes are suggested for the changes. The increase in Ranuncu lus fluitans and decrease in Zannichellia palustris mostly occurred be tween 1966 and 1976. The former was probably influenced by the decreas ed sediment load in the river; this may also be true for Potamogeton p ectinatus. The decrease in Zannichellia at some sites may be due to a decrease in saline effluents. Elodea canadensis was probably influence d by flood events rather than any consistent long-term change. One fac tor likely to have influenced some of the other changes is the occurre nce of a number of growing seasons with predominantly low river flows, conditions likely to have adverse effects on some upstream species, b ut enhance light penetration for some downstream species. Decreased up land mining activities leading to decreased aqueous zinc may also have favoured upstream growth of Cladophora glomerata. (C) 1998 Elsevier S cience B.V.