Natural History of Upper Weardale
7 - FRESHWATER LIFE
By Martyn Kelly, Michael Gardner, Paul Frear, Paul Atkinson: the Editor's synopsis
In 1903 Angustus Grimble wrote of the River Wear as a salmon river "ruined by pollution" so that, "there is nothing much to be said about its angling."
Image: Migratory Sea Trout (c) G M Heeley
Today native Brown Trout dominate the upper catchment from the extreme headwaters and tributaries to Cowshill Waterfall, the natural barrier to migratory fish. Salmon, Sea Trout (the migratory form of Brown Trout), and Eel are present below Cowshill and throughout the river system to the sea.
Other fish species are present and the non-migratory Brook lamprey (Lampetra planeri) is found in the main river and tributaries in lower Weardale. Times can change for the better.
While fish are at the top end of the food chain, it is the plant life that sustains the aquatic ecosystem. The slime covering rocks on the river bed are home to microscopic algae called ‘diatoms’ and any sample from the upper parts of the River Wear is likely to contain large numbers and a range of different forms.
Image: Tufts of Lemanea fluviatilis on a boulder removed from Daddryshield Burn (c) Martyn Kelly
In winter and early spring this algae-rich layer (the ‘biofilm’) can be several millimetres thick, with diatoms as the most abundant group of algae. In late winter and early spring, the bed of the river can turn bright green due to filamentous algae such as Ulothrix zonata, Stigeoclonium tenue and Draparnaldia glomerata. These organisms, using sunlight to make food, form the base of the many food chains that sustain the aquatic animals. Larger and more stable boulders may also host aquatic bryophytes – principally Fontinalis antipyretica and Platyhypnidium (formerly Rhynchostegium) riparioides though several other species can also be found fully-submerged in the river. Apart from these, true aquatic plants are rare in the upper part of the Wear.
The way that the river and tributaries is shaped by underlying geology is explained along with the contribution that water acidity and chemistry has on its ecology. The presence, diversity and distribution of invertebrates is greatly affected by the water quality. Although abandoned long ago, the lead mines of Weardale still have an impact on wildlife - there are fewer types of invertebrate and also fewer individuals of each type in the metal impacted streams.
Moorland streams host ‘shredders’, invertebrates which feed on autumn-shed leaves and bankside plant detritus that has entered the channel. Freshwater shrimps such as Gammarus which would normally perform this role are sensitive to heavy metals, but the role is performed by metal-tolerant stoneflies such as Amphinemura sulcicollis and Protonemura meyeri (Early Brown) and Leuctra spp. (needle flies). The ubiquitous mayfly Baetis rhodani (Large Dark Olive) can be found on the wing all year. The mayfly Rhithrogena semicolorata (Olive Upright) is also abundant and feeds predominantly by scraping algae from the substrata.
Image: Caperer Caddis Fly (Halesus sp.), adult (c) Lesley Hodgson
Further down the Dale, more nutrients and a drop in zinc concentrations, results in a more diverse stonefly community including Dinocras cephalotes, a large predatory species which can be relatively abundant. As the river flows along, further invertebrate communities appear and all are food for other animals along the food chain, ending in fish.
This chapter concludes with comments on the relative ecological merits of the Wear and tributaries and the threats to its biodiversity. Times can also change for the worse.
Preparing for the online Chapters on Flora, Fauna and Freshwater Life we created a PDF reference list for the hundreds of species name-checked in the book. For this Chapter, entries for Freshwater Life are included in the FAUNA list, by both Vernacular and Scientific names, with hand links to what an internet search might add to wider information on each.Download Chapter in PDF