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Oxygen crashes put Hampstead Heath fish and wildlife at risk

Matthew Pierce

22 Jan 2025

HAHAS monitors oxygen levels and stands ready with emergency aeration equipment

The ponds on Hampstead Heath all suffer to varying degrees from low dissolved oxygen (DO) during the summer months. Sometimes DO drops low enough to kill water life. Such an “oxygen crash” happened on Hampstead No 2 pond in 2023, causing the deaths of 15 large carp.

Crashes often occur when a bloom of algae dies off, indicated by green water turning clear. Bacteria feeding on it then strip oxygen from the water. Sometimes daphnia feed on the algae as well, also consuming oxygen. They can turn the water red.

It is difficult to predict or prevent oxygen crashes. What’s important is to spot them as quickly as possible and deploy aeration equipment (see photo above). This can keep fish alive until DO levels get back to normal.

Last summer HAHAS took steps to detect and respond to oxygen crashes by:

  • Monitoring DO levels twice-weekly on all 5 fishing ponds (see results below) and keeping an eye out for green water turning clear or red, or fish in distress

  • Following the Hampstead Heath Fishing Facebook page in case local anglers reported problems, also staying in touch with the City of London Corporation (CoLC) ecologist

  • Asking anglers to report problems to CoLC and the Environment Agency (EA), both of which have emergency aeration equipment

  • Investing in our own portable oxygen pump and assembling a team of HAHAS volunteers to deploy it at short notice

There weren’t any significant oxygen crashes in 2024, however we are now better prepared to deal with them in future.

OXYGEN LEVELS ON THE FIVE FISHING PONDS

The graph below shows our oxygen test results from last summer. Note that dissolved oxygen (DO) is measured in milligrams of oxygen per litre of water (mg/l). Generally we interpret test results as follows:


  • Over 5 mg/l – good

  • Below 4 mg/l – borderline

  • Below 2 mg/l - fish deaths may occur


As a general rule, DO levels are high during the day when algae is photosynthesising (releasing oxygen) but very low at daybreak when it has been digesting nutrients overnight (consuming oxygen). For this reason, we usually do oxygen testing at daybreak, and all the data shown below is from early morning tests.


A summary of these results is as follows:


The Men’s Pond had consistently good levels of DO.


No 2 Pond had mostly good DO, but it dropped very low – to 0.63 mg/l - in early July. This did not last long and there were no dead fish as far as we know. No 2 pond had a lot more algae than the others over the season, often turning green and carrying thick surface scum (see photo below). These algae blooms caused DO levels to drop, but not to fish-killing levels.


The Model Boating Pond had mostly good DO, but the reading of 3.3 mg/l in early September was exceptionally low for this pond. We saw more algae than in recent years. There was a pollution incident from a house above the bird sanctuary pond on 18-Jun. This is still under investigation by Thames Water. It’s possible that it may be affecting water quality in the Model Boating Pond.


The Vale of Health had consistently low DO levels, but not low enough to kill fish.


The Viaduct Pond was consistently very poor, but surprisingly there were no fish kills. One explanation is that this pond only contains small fish (max size around 5lb) which are better able to deal with low DO than larger ones.



POTENTIAL HELP FOR NO 2 POND

Of all the five fishing ponds, CoLC regards No 2 as being most at risk of oxygen crashes in future. They have applied for funding to install permanent oxygen diffusion equipment on the bed of this pond in 2025, to run all year round and help keep DO levels up. We will have to wait until later this year to find out if this project is going ahead.

Hampstead and Highgate Angling Society 2022

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