Blue-green algal bloom on the River Stort. May 2. 2020.

At the end of April, there was a report of a blue-green algal bloom on the River leading to the death of fish and other wildlife. The bloom had been recorded as being between Lower Sheering and Roydon Park. The river was seen to be a dark khaki colour.

A sample was taken using a very fine plankton net and the sample then centrifuged to concentrate and looked at under a microscope at x100, x400 and x1000 magnifications.

 Here are the trillions of microscopic cells at x100 making the water khaki coloured.

 There was also quite a variety of other different types of algae such as this green one which is very much related to the green mould you get on the North face of a tree.

 There were some diatoms, but not anywhere near as numerous as would be expected.

 This is almost certainly a colonial form called Synura with each cell having beating filaments.

 This is a desmid called Closteruim.

 The actual identification of the golden/yellow/brown cells has been found to be quite difficult. In some cases the cells are joined in chains.

 In the majority of cases the chains seem to have broken up into individual cells which are shown in different aspects. The oval structures inside the cells are chloroplasts which the alga will use for photosynthesis.

Further Investigation will be needed to prove that these are the cause of the algal bloom but what could be causing the bloom anyway?

The reports put it down to a 'natural phenomenon' but I would suggest it is a natural phenomenon aided by Man.

Natural causes:

One of the driest April is on record = low river flow rate.

Unusually high temperatures for the latter part of April.

Man made causes:

7 sewage treatment works discharging effluent into the River rich in phosphates.

Low flow rate from low rainfall made worse by abstraction from the chalk aquifer in the Bishops Stortford area.

 Numerous narrowboat passages continually stirring up organic silt rich in phosphates from the treatment works.

This organic silt is smothering of water weeds which should be growing in the river and thus reducing their ability to oxygenate the water.

Road and hard surface run-off in times of heavy rain bringing pollutants from road traffic.

Is this something which we must expect on a regular basis because it is causing an ecological disaster on the river?

One of the solutions would be to have phosphate stripping at the treatment works which would remove the fertilising effect of the phosphates, thus reducing the organic silt and nutrients which is what is promoting algal growth along with the higher temperatures.