Ecological Background

2.1. Topography

The area studied (Figure 2) is geologically very uniform, consisting mainly of sedimentary Silurian grits and shales, except where anticlinal folding and faulting brings out Ordovician strata in the eastern region. The region as a whole is traversed by the Teifi and Towy anticlines. Weathering has resulted in an area that is predominantly bleak, high moorland with a capping of poor, calcium-deficient soil. As a result, except in the valleys, land is poorly developed, supporting only a sparse population. Both rivers under consideration have their sources at high altitudes in peat bogs or lakes in upland moors characterised by rough, yellow mountain pasture in dry places and Sphagnum in the wetter regions.

The Teifi has its source in Llyn Tefi, one of six upland lakes at an altitude of 455 m OD on the southern edge of the Plynlimmon plateau in the western region of the Cambrian Mountains. It is approximately 117.5 km long with a catchment of approximately 1007 km2. After being joined by over 70 tributaries it flows into Cardigan Bay. In its first 7 km the channel falls 215 m but for the next 15 km, as it passes through the Tregaron Bog (Cors Goch Glan Teifi), the channel gradient is negligible. In this raised bog the river wanders sluggishly through a vast quagmire with a substratum consisting mainly of fine gravel and silt. When not in flood the water flow is barely detectable and it consists of a long series of deep stagnant pools connected by short stretches of shallow water. At the southwest end the river swells into broad and muddy shallows. Along the river banks the vegetation forms a well-defined river terrace, the most abundant plants being Juncus effusus, Phalaris arundacea, Deschampia caespitosa, Carex acuta, Galium palustre, Ranunculus acris and Equisitum species (Godwin and Conway, 1939). Submersed vegetation is also abundant. Nuphar lutea grows by the edges of deep pools while Potomogeton natans, Myriophyllum, Ranunculus penicillatus (Dumart) var. penicillatus and Callitriche obtusangula thrive in the shallows. Glyceria fluitans grows patchily on the margins. According to Brooker (1984) Apium inundatum, Luronium natans and Nuphar lutea are restricted to the Tregaron Bog. Stations 1 and 2 for the faunistic studies and Station A for the fish collection are located on the southern end of the bog (Figure 2).

Figure 2 Map showing the river systems investigated. The grid references and site landmarks for Stations 1-8 on the Teifi where the invertebrate samples, including intermediate hosts, were collected are as follows: (1) SN 6750 6186; Tregaron Bog; shallows 1.5 km N of Pont Eynon. (2) SN 6727 6148; 200 m north of Pont Eynon. (3) SN 6638 5775; below Tyndomen. (4) SN 6645 5717; near mouth of River Carfan. (5) SN 6528 5691; near Pontllanio Bridge. (6) SN 6489 5685; at bend below Pontllanio Bridge. (7) SN 6468 5636; near Old Mill, Llanio Isaf. (8) SN 6423 5440; at bend below Pontgoyan Bridge. Fish samples used to collect helminth parasites were obtained from station A in the Tregaron Bog area which encompasses Stations 1-2 above and also from Station B located below the bog area which encompasses Stations 3-7 above. Station D is located on the Pysgotwr, a tributary of the River Towy.

Figure 2 Map showing the river systems investigated. The grid references and site landmarks for Stations 1-8 on the Teifi where the invertebrate samples, including intermediate hosts, were collected are as follows: (1) SN 6750 6186; Tregaron Bog; shallows 1.5 km N of Pont Eynon. (2) SN 6727 6148; 200 m north of Pont Eynon. (3) SN 6638 5775; below Tyndomen. (4) SN 6645 5717; near mouth of River Carfan. (5) SN 6528 5691; near Pontllanio Bridge. (6) SN 6489 5685; at bend below Pontllanio Bridge. (7) SN 6468 5636; near Old Mill, Llanio Isaf. (8) SN 6423 5440; at bend below Pontgoyan Bridge. Fish samples used to collect helminth parasites were obtained from station A in the Tregaron Bog area which encompasses Stations 1-2 above and also from Station B located below the bog area which encompasses Stations 3-7 above. Station D is located on the Pysgotwr, a tributary of the River Towy.

Downstream of the bog the gradient is relatively constant falling at a rate of about 150 m in 100 km. The river, now swollen to a width of 20-30 m by numerous tributaries, meanders through farmland dominated by sheep and dairy farming, forming a succession of pools, riffles, fast reaches and backwaters. The substratum consists of cobbles, pebbles, coarse gravel and silt depending on the water type. Submersed macrophytes consist mainly of ubiquitous Ranunculus penicillatus, Potomogeton natans, Myriophyllum sp., Callitriche hamulata and C. stagnalis on gravel beds and Fontinalis antipyretica on stony substrate. Over the last 50 years both the submersed macrophytes and riparian trees, such as the alder and willows, have become less abundant. As a result the main river and particularly some of the tributaries, which have been subjected to canalisation, have become more erosive in character. Stations 3-8 for the faunistic studies are located in the upper region of the Teifi between Tyndomen and Pont Goyan. Salmonid fish were collected at Station B (Stations 3-7 in faunistic study).

The Pysgotwr, one of the small headwaters of the River Towy (Figure 2), is situated at an altitude of 430 m on impermeable mudstones and shales of the Silurian series (Station D). During the 1950s its catchment, which consisted of peat bogs and rough, virtually treeless mountain pasture, was used for sheep farming. At that time the characteristic bankside species were Drosera rotun-difolia, Molinia caerula and Juncus squarrosus. Often the smaller feeder streams were almost completely shaded by the latter species. As a result of the steep gradient the small 1-4.5 m wide stream had many small waterfalls with scour pools followed by fast reaches and riffles. The substrate was strongly erosive in character, consisting mainly of small boulders, cobbles and coarse gravel usually colonised by epilithic algae and Fontinalis antipyretica.

Since then, however, virtually the entire catchment has been planted with coniferous woodland.

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