Long-term changes in macrophytes of British rivers: 1. River Wear

A survey of flowering plants, bryophytes and macroscopically recognizable 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. The initial survey was made at the end of a decade in which marked environmental 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 between 1966 and 1996, the following were found: upstream increase in Cladophora glomerata; downstream increase in Myriophyllum spicatum, Ranunculus fluitans, Elodea canadensis, Potamogeton pectinatus and Sparganium erectum; downstream decrease in Zannichellia palustris. Hydrodictyon reticulatum, first noticed in 1995, had extended for 18 km in late summer 1996. Callitriche platycarpa, C. stagnalis and Potamogeton berchtoldii are also invasions since 1966, although all seem likely to have been present in the river in the pre-industrial era. Possible environmental causes are suggested for the changes. The increase in Ranunculus fluitans and decrease in Zannichellia palustris mostly occurred between 1966 and 1976. The former was probably influenced by the decreased sediment load in the river; this may also be true for Potamogeton pectinatus. The decrease in Zannichellia at some sites may be due to a decrease in saline effluents. Elodea canadensis was probably influenced by flood events rather than any consistent long-term change. One factor likely to have influenced some of the other changes is the occurrence of a number of growing seasons with predominantly low river flows, conditions likely to have adverse effects on some upstream species, but enhance light penetration for some downstream species. Decreased upland mining activities leading to decreased aqueous zinc may also have favoured upstream growth of Cladophora glomerata.