At least 5,000 fish died in the Medway incident. The Agency described it as "extremely serious", but its investigation revealed that the only discernible cause was lack of dissolved oxygen - due to low flows, an algal bloom and unseasonably high temperatures. All three factors are the result of lengthy warm and dry periods which are becoming common in southern Britain.
Iain Barker, the Agency's local water quality officer, told ENDS that similar fish kills occurred on the Medway in 1995 and 1996. But water quality was actually improving, he said: "We now get fish kills because there are fish there to kill." Up to the early 1990s, high loads of industrial and sewage effluent meant that the estuary was "septic and would go anoxic for months on end." A major factor in the improvement was a £35 million investment in effluent treatment by the paper companies concentrated in the area.
Mr Barker believes that the Medway's problems could be solved by improved modelling and prediction of depleted oxygen levels to allow emergency re-oxygenation. But he conceded that nutrient enrichment and lack of flushing by high winter flows - regarded by the water industry as fair game for water supply abstractions - could be part of the problem.
Another notable deoxygenation incident occurred in the Trent catchment in July 1995. Widespread public concern was caused by the deaths of thousands of fish, but no source of pollution was discovered other than storm sewage discharges which followed heavy thunderstorms.
The Agency's latest report on pollution incidents notes 12 serious incidents due to "natural events" in 1996 (see p 6 ). The worst affected region was Anglian, where seven sizeable fish kills were caused by low flows and algal blooms. The worst incident resulted in the deaths of 6,000 fish at a Rutland fishery following an algal bloom in April, while 2,000 coarse fish were killed by low dissolved oxygen levels in the West Fen Catchwater in Lincolnshire in August.
Low flows and high temperatures reduce the ability of watercourses to cope with organic loads, and favour the development of algal blooms. In order to ensure that aquatic life is protected in a warmer and drier climate, the Agency will be forced to consider tighter regulation of organic effluents and nutrient levels in discharges and to impose flow conditions on abstractions.
The National Rivers Authority produced a research report on the implications of climate change for the water environment in 1994. Discharge consents would need to be reviewed regularly and possibly tightened up, it concluded.
Limited progress has apparently since been made. Jim Cursey, the Agency's Sustainable Development Manager, told ENDS that a Climate Change Focus Group has been set up, but a strategy is still being formulated. The debate on water issues had been held back by the need to consider all parts of the environment together, he said.
This year, Phaeocystis blooms have already appeared off extensive areas of the north and west coasts. At Prestatyn and Rhyl in the north, the Agency has reported that these blooms have made way for very large blooms of two other species - Noctiluca and Chaetocerous. Although these are not toxic, advice from the Department of Health is to avoid contact with the scum and water close to it.