Work experience with the National Trust – river pollution and invertebrates

(Before I begin this blog post, I’m just going to apologise for the lack of content over the past few weeks – at this time of year and with my current location it is quite difficult to gather photos and videos, however as we head into Spring I have some exciting new ideas and potential adventures that I can’t wait to share!)

Yesterday I participated in the second session of my National Trust work experience programme. The day began with… pouring rain. Once again it occurred to me that investing in a raincoat with taped seams might be a good idea, because within the first couple of hours I had damp shoulders and arms.

This session was focused on the River Bollin again, but this time we were conducting a couple of different surveys on the river, the data from which will then be collated and used by the National Trust in their conservation projects.

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The first survey involved us walking along a stretch of the river, identifying and recording any potential sources of pollution. An obvious example of this is pipes which empty into the water – these are likely to contain waste substances which could have a negative impact on aquatic life forms.

Unfortunately the National Trust can only control what enters the river in the section that they own, and there is a sewage treatment plant upstream that releases waste into the water body. Under normal circumstances there is a limit to how much can be deposited in the river, but if there is a period of high rainfall then the limit is discounted and the treatment plant can release as much as they want to. This clearly has a significant impact on the health of the river: the water turns to a cloudy green-brown colour and gives off quite an unpleasant smell.

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Another slightly less obvious source of pollution that we noted was the presence of tributaries flowing into the river. Whilst they just seem to merge in as part of the water body, many of them travel down from farmland – if farmers have been using pesticides then these may run off the fields into the water system and could result in the death of aquatic organisms.

If fertilisers enter the river, they can cause eutrophication – this is where plants (particularly algae) in the river grow excessively, leading to overcrowding and competition for resources. The plants then die and much of the oxygen supply is consumed by decomposers as they break down the dead matter – the shortage of oxygen then causes other creatures to die.

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An additional problem that the National Trust have to deal with is the damage caused to the river banks by people and their dogs – allowing a dog to go down to the water and have a paddle is a really fun thing to do as an owner, and we saw several dogs enjoying this. The downside to this activity is that it accelerates the rate of erosion of the banks – large amounts of sediment are deposited into the water; this is another way in which pollutants can enter the system.

In the photo below you can see a particularly popular spot for dog walkers – we were allowed to walk onto it to check for pipes, and the effects of dogs were very visible. The National Trust can plant trees along the bank to prevent this erosion from happening (this blocks the dogs’ access and the roots stabilise the soil) but are reluctant to do this in all locations as they do not want to discourage dog walkers altogether.

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The second survey we conducted was investigating the life forms in the river – once kitted out in our waders we each took a turn going into the water with a net to take a sample. Facing downstream, we had to disturb the river bed with one foot whilst holding the net in front of us to catch any escaping creatures. Once three minutes had passed, we then carried the net and its contents to the bank where they could be examined more closely in order for us to record what was present.

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For the most part we just discovered freshwater shrimp and mayfly larvae (plus another species of invertebrate that we couldn’t identify) – however these are a good sign that the levels of water pollution in this area are low. Other species such as the water louse and the sludgeworm are indicative of high levels of water pollution – thankfully we didn’t uncover any of these!

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A small fish was also caught – however we were focusing on invertebrate life so after a few photos were taken it was returned to the water. The rangers were unsure of the species but due to the flattened shape of its underside it was likely to be a bottom feeder.

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Once we had completed the survey, all material and creatures were returned to the water and we headed back to Manchester. The sun finally made an appearance just as we were leaving on the bus – typical!

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