The Environment Agency is warning that climate change could increase the risk of eutrophication in English rivers, reducing water quality.
The warning comes in a new study conducted by the Centre for Ecology and Hydrology for the Environment Agency’s Research, Analysis and Evaluation group.
The study shows how climate change may increase the risk of eutrophication (a process in which too much nutrient in water causes algae and higher plants to grow excessively) in slow flowing English rivers, reducing water quality. Climate change is expected to alter water quality in rivers, but where and when this may happen is uncertain.
The Phase 1 study of the project in 2016 demonstrated that climate change impacts on river flow would increase phosphorus concentrations by 2050 and beyond. However, climate-driven changes in river temperature regime and light, and plant responses to these, are also important in altering the future risk of excess algal growth.
Algal blooms can be toxic to people and animals as well as requiring additional treatment for drinking water. The study found that residence time, water temperature and exposure to sunlight are particularly important. Algal blooms tend to occur only in rivers with a residence time (the time water takes to travel from an upstream distance to a site) of over 4 days.
Such long residence times in the UK tend to occur in canals, and slow flowing and shallow gradient rivers (often in their lower reaches). Using this residence time threshold of 4 days, a total of 26 sites in England on 24 different rivers with available data for analysis of trends were identified out of the 115 sites from Phase 1.
Management implications
The results suggest that phosphorus reduction strategies will only be effective in reducing the incidence of algal blooms if concentrations can be lowered sufficiently to be below trigger thresholds.
The researchers are suggesting that management strategies focusing on reducing sunlight and thermal interactions (both through river shading by trees) may be particularly effective in reducing the risk of blooms on some rivers in the future.
The methods developed in the study could help to evaluate the most effective options for reducing bloom risk and preventing deterioration in water quality.
The report says:
“By understanding future risk, people involved in water quality management can implement a range of cost-effective management solutions to ensure that improvements in water quality in England continue to be achieved.”
The results suggest that eutrophication risk will generally increase in the future (relative to the baseline period of 1961 - 1990) at the studied sites. The study found that:
- By the 2050s, the number of days where blooms are likely to occur increased at 95% of sites
- By the 2080s, the number of days where blooms are likely to occur increased at only about 50% of sites. This appears to be due to the limited number of days the sunlight duration threshold is exceeded by this time.
Exposure to sunlight may be the most important factor in preventing algal blooms
The researchers have concluded that with the lowest number of threshold days at the greatest number of sites, exposure to sunlight may be the most important factor in preventing algal blooms, the report says.
The study also concludes that phosphorus management strategies may not be effective in reducing the risk of algal blooms occurring in slow flowing rivers, an observation confirmed by the fact only 3 sites showed a reduction in risk using an improved phosphorus treatment scenario.
The study has flagged up residence time as a useful high level indicator of eutrophication risk which could be used in future as part of river basin management planning. Residence time is the primary control on the occurrence of an algal bloom: there needs to be sufficient river length and time for algal biomass to develop.
It also highlights that better estimates of future water quality will depend on better information about how water temperature will change.
However, currently there is considerable uncertainty in the estimation of future water temperature, which was derived from air temperature using simple regression methods.
The researchers are also proposing that finding a better way of estimating water temperature would be particularly useful for helping to model future water quality.
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