Download Chironomid populations within the Exmoor Mires

Chironomid populations within the Exmoor Mires:
Observations of invertebrate life
Introduction
The Exmoor Mires project is a partnership initiative with the objective to restore and re-wet upland areas in an attempt to
restore hydrological systems to optimise freshwater retention, support water quality through natural filtration, halt peat
erosion and boost carbon sequestration.
At the Aclands site, temporal pools were observed with numerous rashes of raised tubes constructed from peat sediment,
both within the pools – sometimes mere puddles - and exposed above standing water. On closer inspection these were
confirmed as insect larval tubes for Chironomid species. What are these seemingly insignificant insects and where do they
feature within the ecosystem or food webs of the Exmoor landscape?
Chironomid lifecycle
Adaptations
There are essentially 4 main stages to the Chironomid
lifecycle: Eggs are laid as a mass, usually on the water
surface sinking to the bottom where they hatch in less
than a week. Larvae quickly burrow into substrate and
may construct tubes in which to live.
Chironomids globally are diverse and numerous; in excess
of 15,000 separate species are thought to exist, which
have exploited a range of river or still water environments.
1
2
In certain circumstances, Chironomids
may comprise the dominant life in a
pond, due to their ability to adapt,
and survive where competitors may
not.
Temporal pools or puddles are by
nature
unpredictable
–
the
combination of using the refuge of a
mud tube to reduce the risk of drying
out, plus haemoglobin to absorb and
store oxygen appears to be a
successful strategy. 1
These tubes as observed on the Exmoor Mires, effectively
provide shelter, protection from predators and
camouflage. The larvae take on a pink / red colouring
(hence are also commonly known as blood worms), due to
presence of haemoglobin in their blood. This unusual, and
believed unique, characteristic means that Chironomids
can survive in very low dissolved oxygen levels – such as
muddy pools or shallow scrapes.
After a number of weeks pupation occurs; still in the
protective tubes from which the pupae emerge and swim
to the surface. Within a matter of hours, hatching as fully
mature adults ready to breed and die in just days – similar
to other ephemeral fly life such as mayflies.
Furthermore, numerous hatchings are possible in summer
months – but development may be suspended over winter
in readiness for early spring. 1
The form of sediment tubes may
differ; certain species have been
observed to raise the opening of
tubes above the level of sediment
due to lower oxygen levels (ie. the
lower the oxygen content the higher
the tube). 2
In terms of ability to support greater
population
densities,
deeper
sediment is required to provide this
which is evidenced through more
vertical tubes. 2
Food chains and food webs
Chironomids may be categorised as a ‘collector – gatherer’ under the system known as Functional Feeding Groups (FFG), this
is one way of assessing the role that individual species play within for example the wider communities or an ecosystem3.
This allows for consideration of the community structure and linkages to illustrate energy flows between trophic levels.
Chironomids, particularly the larval stages, play an important role in the food chain. They feed on detritus and bacteria in the
waterbody, directing it into their mouths using a fan-like arrangement of hairs 1. By doing so this forms a connection from
very small dissolved or particulate organic matter (DOM / POM) collected by such organisms and converted into energy to
feed and support them, but also passing further up the food chain as they in turn provide food (and hence energy) for higher
trophic levels; particularly predators.3
Interpretation
What can we infer from the presence of Chironomid populations in these temporal pools?
•
It has been shown that Chironomids, as one of many freshwater invertebrate species potentially present in the pools
and wetted areas restored under the Exmoor Mires project, could be considered as one of the best placed to quickly
colonise and exploit these available habitats.
• They represent a link in the food chain transferring energy from particulate matter up to higher trophic levels 3.
Ongoing mire restoration works will produce numerous small pools providing suitable habitat for numerous aquatic or
semi-aquatic species, in which Chironomids play a supporting role, e.g.. as a food source for other invertebrates such as
Dragonflies.
• Anecdotal evidence particularly among anglers suggests that Chironomids (known as ‘Buzzers’) form a high percentage
of the food resource of wild Brown Trout (Salmo trutta) 4 & 5. This appears to be more so in lakes, ponds and still waters .
The connectivity of the Mires with the headwaters of the Exe mean that Chironomid metapopulations could help
support local fish stocks.
• Their wide distribution, short lifecycles and tolerance to uncertain
conditions has meant that they are considered a key species for
inclusion in the ongoing assessment and biological monitoring
programmes for water quality. 2
• The ability to tolerate variable conditions of low dissolved oxygen,
enrichment or react to temperature (ie. by prolonging pupation) ,
adds to their value when assessing temporal site conditions.
Preserved larval material in lake sediments has also been utilised in
assessment of historical change (land use / inputs / temperature
FSC
etc) 6 & 7, and topically this could provide an insight into issues of
climate change.
References
1. Field Studies Council, Life on Land http://www.lifeonland.org.uk/SpeciesImages/Inverts/Midges%20mosquitoes.htm )
2. Armitage P.D., Cranston P.S., Pinder L. C. (Eds.) 1995. The Chironomidae: Biology and Ecology of Non-Biting Midges
3. Wotton R.S., Life in Water. 2001-2012. http://www.ucl.ac.uk/~ucbt212/
4. Elliott J.M. 1967. The food of trout (Salmo trutta) in a Dartmoor stream. The Journal of Applied Ecology Vol. 4, No. 1.
5. Schmid-Araya J. M., Hildrew A.G., Robertson A., Schmid P.E., Winterbottom J. 2002. The importance of meiofauna in food webs; evidence from an acid
stream. Ecology Vol. 83
6. Brooks S. J. & Birks H. J. B. 2000. Chironomid-inferred Late-glacial air temperatures at Whitrig Bog, Southeast Scotland. Journal Quaternary Science Vol.
15.
7. Lang B., Bedford A., BROOKS S.J., Jones R.T., Richardson N, Birks H.J.B. & Marshall J.D. 2010. Early-Holocene temperature variability inferred from
chironomid assemblages at Hawes Water, northwest England. The Holocene Vol. 2
Hazel Kendall, Westcountry Rivers Trust & Upstream Thinking Project Partner
www.wrt.org.uk