The results of research commissioned by the Wye & Usk Foundation (WUF) and
carried out by Cardiff University have been released and we are now a step closer to
understanding what is causing the Wye to resemble pea soup at times.
It has long been thought that elevated levels of phosphate entering the river was
generating these harmful blooms, but despite phosphate levels dropping in the
middle and lower Wye over the past 3 decades, the algal blooms have been getting
worse. WUF commissioned Cardiff University to try and and out why; you have to
understand a problem to solve it.
The team at Cardiff University have spent 2 years monitoring a whole host of factors
along a 156 mile long stretch of the Wye and Ithon and a further year crunching the
data. The full report will be out towards the end of 2024, but they have now identified
various suspects and it’s not just phosphate. Prof Roo Perkins said, “Current
phosphate levels in the River Wye itself are mostly within Special Areas of Conservation
(SAC) targets, lower than historical records and unlikely to be the primary cause of algal
blooms in the river.”
Algae comes in various forms, but is essentially a single celled plant that needs
sunlight and nutrients (fertiliser) to produce energy through photosynthesis and so by
divide and bloom. In the river this manifests in 2 ways: Suspended algae – that floats
freely in the water and turns it green or brown and Benthic algae – the slimy coating
that plasters the river bed and other aquatic plants.
The team has found over 200 different species of algae in the river and they all have their own ecological niches. This is why sometimes the bloom is green and other times it is brown; sometimes the bloom is on the bed of the river, sometimes it is in the water column.
Nutrients come in many forms. There are 3 main forms of phosphorus and 2
main forms of nitrogen, and each species requires its own unique combination of the
above nutrients to bloom. This is why just focussing on phosphate is wrong.
Algal blooms are regulated by grazing invertebrates, nutrient levels, temperature,
sunlight and the îow of the river (i.e how many times it can divide before it is washed
into the sea). When those factors are out of balance, we can get an abundance of
algae clouding the river shutting out the sun from other plants and in some cases
producing toxins. Just such events happened in 2020 (suspended) and 2022 (benthic).
The research points to a combination of factors – from weather patterns and water
conditions, to the ratios of nutrient types that can form the perfect storm, enabling
algae to multiply in huge numbers.
OTHER FACTORS WE NEED TO CONSIDER:
HIGH FLOWS
Rain events which run rapidly over ground straight into the river, rather than
infiltrating slowly into the water course via the soil, makes floods more extreme. If
they are too big they rip out the plants that house the algae eating invertebrates.
Overland flow also brings in large amounts of nutrients from the wider catchment.
LOW FLOWS
If the rain all runs oì and is not stored in the catchments soils and aquifers it means
that there aren’t enough reserves to keep the river flowing strongly should the rain
stop. Less water means the river is shallower – and the water takes much longer to go
down the river. Shallow, slow-moving water will heat up much quicker. Algae loves
slow warm water (take a look at a bird bath or a dog bowl left out in the sun) The
photo synthesis process to goes into overdrive, and the algae numbers double every
day as it moves slowly down the river.
OTHER NUTRIENTS
Testing for phosphate is relatively simple; its source is usually regular and easy to
predict. Other nutrients like organic phosphorus and ammonia can be missed partly
because they are not being widely tested for but also because they tend to spike in
short bursts when it rains meaning they can easily be missed by a governmental
monitoring programme that, at best, takes 1 sample a month.
WHERE THE BLOOM STARTS
Algae blooms move downstream and the amount of algae doubles daily if conditions
are right. The higher in the catchment the bloom starts, the more days it takes to get
to the bottom and so the worse the bloom will be. In recent years the conditions that
can create algal blooms has moved upstream by 116km.
SO WHAT CAN BE DONE ?
WUF and partners are working on a suite of solutions to tackle the various issues:
Natural flood management to reduce high flows and boost low flows. Flooding is not just caused by a large volume of rain, more important is the amount of time it takes for that volume of water to reach the river. Too much, TOO QUICKLY and we will get floods. We can slow the flow by allowing the rain to permeate through the soil surface and be lagged as it moves through the catchments soils, reducing soil compaction, planting trees and other mitigations like attenuation ponds and bunding flow lines can help reduce the flood risk.
Advise government agencies on monitoring and legislating for ALL the nutrients that contribute to algal blooms. If we can get decision makers to agree to monitor and legislate for the various nutrients that can enter the river, we can work with all sectors to reduce the major energy supply for algal blooms.
Use this study, citizen science and WUF monitoring to determine where the nutrients are coming from and work with farmers, wider agri-supply chain and water companies to reduce the levels of all nutrients getting into the water.
Plant trees on riverbanks to cool the water and reduce sunlight.
