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How Clean is The Ghyll Stream?

posted 30 Jun 2012, 15:43 by Haley Whittall   [ updated 30 Jun 2012, 15:47 ]

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For some time now I have been curious about the water quality of the stream at The Ghyll so I recently took advantage of some rare sunny weather to carry out an informal survey of the aquatic life.  This is a useful method of assessing water quality because different families of invertebrates vary in their sensitivity to organic pollution, so pollution levels can be inferred from the animals present.  Each invertebrate family is assigned a score from 0 to 10 on indices such as the BMWP (Biological Monitoring Working Party) biotic index.  For example, stonefly larvae are only found in very clean water and some families are allocated a maximum BMWP score of 10 whereas most worms are very tolerant of polluted water and score just 1.  A proper survey would employ a strict sampling methodology to calculate an overall score for the stream which could be compared to similar water bodies.  The biotic index only works for running water.

Wading upstream, I was immediately struck by the variety of habitats down in the water.  In some places the banks towered above my head, sheer walls of dark wet mud shaded by trees where nothing grows.  The stream winds through deep pools, more open gravelly stretches, undercut banks skirted with mosses and liverworts, and sparkling waterfalls where the water rushes over sandstone as flat as paving slabs.  The stream width, depth and water flow speed varies, and the substrate ranges from large boulders through gravel to silt and mud.  Tangled roots and fallen trees add to the complexity of the micro-habitats.  I didn’t notice any higher plants growing in the water.

These were the main animals found;-

Golden-Ringed Dragonfly (Cordulegaster boltonii)

Two of these were found in the process of emergence from the stream, proving that they breed in the Ghyll.  This species normally prefers open unshaded streams so the Ghyll habitat does not seem typical, especially considering the lack of emergent vegetation in the stream here, although they are also present in similar habitat at the Country Park.  In each case, the larvae had climbed a steep wall of mud to about 5 feet and the adults were clinging to their exuviae whilst pumping up their wings.  They had not yet achieved their adult colouration as can be seen by comparing the photographs of the emerging teneral with another adult that I found resting under a bank further upstream.

Golden-Ringed Dragonfly in emergence at The Ghyll (28th June 2012)

Golden-ringed dragonfly in emergence at The Ghyll (28th June 2012) 

Adult Golden-Ringed Dragonfly resting under an overhanging bank at The Ghyll

Adult Golden-ringed dragonfly resting under an overhanging bank at The Ghyll 

Exuvia of Golden-Ringed Dragonfly recovered from The Ghyll. This predatory larva may live buried in the gravel underwater for up to 5 years, ambushing passing prey

Exuvia of Golden-ringed dragonfly recovered from The Ghyll
This predatory larva may live buried in the gravel underwater for up to 5 years, ambushing passing prey. 

The transformation of this ferocious mud-encrusted beast crawling from the water to emerge as a beautiful adult insect is truly awe-inspiring. Golden-ringed dragonflies score a healthy 8 on the BMWP index.

Bullhead (Cottus gobio)

I was a little surprised to find a fish!  The Bullhead, otherwise known as 'Millers Thumb', is a European Protected Species that lives in fast flowing, well-oxygenated water.  It does not tolerate pollution so its presence at The Ghyll is further indication of high water quality.  I saw other fish darting amongst the rocks in the deeper pools which may also have been Bullheads.  They are well camouflaged on the gravel bottom and feed on invertebrate larvae and small crustaceans.

Bullhead found at The Ghyll 28th June 2012

Bullhead found at the Ghyll 28th June 2012 


Mayfly larvae superficially resemble damselfly larvae as they have 3 'tails' but differ in that their gills are conspicuous on the sides of their body.  Mayflies generally fall into three groups - swimmers, burrowers and crawlers - each group with body shape adapted to living in the micro-habitats created by different flow rates.  Crawlers inhabit fast flowing water where their flattened body shape enables them to cling to stones without being washed away in the current.  Swimmers inhabit medium flow and have a torpedo shaped body suited to moving through the water.  Burrowers bury themselves in the soft sediments associated with slow moving water with their gills raised over their abdomen into the current.  I found plenty of swimming mayflies of the family Baetidae (Score 4), but there were also other unidentified families represented (Score 7+)

Mayfly larva from The Ghyll.  Note the three tails and the distinctive gills at the sides of the abdomen. Damselfly larvae lack the gills and have three flattened paddle-shaped 'tails'

Mayfly larva from the Ghyll
Note the three tails and the distinctive gills at the sides of the abdomen.  Damselfly larvae lack the gills and have three flattened paddle-shaped 'tails'

Caddis Flies

Some families of Caddis fly larvae construct protective cases from pieces of vegetation or grains of sand and gravel.  Two species of cased caddis fly were found (Score 7).  Identification of cased Caddis Flies to family level is not particularly easy without removing them from their case which I wasn’t prepared to do.

A Species of cased Caddis Fly larva from The Ghyll (Probably Limnephilidae).  Legs can be seen protruding from the specimen on the left

A species of cased Caddis Fly larva from The Ghyll (Probably Limnephilidae)
Legs can be seen protruding from the specimen on the left


Stonefly larvae have two long segmented tails and appear to be quite common in the Ghyll stream. They are particularly pollution intolerant with most families scoring 10 on the BMWP biotic index, although I wasn’t able to identify the specimens found to family level.

Stonefly larva from The Ghyll stream

Stonefly larva from The Ghyll stream 

In addition, I found freshwater shrimp (Gammaridae) which score 6, a single leech (4) and Chironomid midge larvae (2), as well as a few other creatures that I couldn’t identify.  I didn’t find any damselfly larvae although I have seen adult Beautiful Demoiselles near the stream earlier this year.

Overall, I was pleasantly surprised to find so many pollution intolerant animals in The Ghyll stream, especially Bullhead, Golden-ringed dragonfly, and Stonefly larvae.  I’m no ecologist but this would appear to suggest that the water quality is pretty good, at least with regard to organic pollution. However, freshwater bodies are always vulnerable to one-off pollution incidents.

All the animals were returned unharmed.

By Pete Meiners