ROTIFERS of the Cwm Llwch

• PlaceNant Cwm Llwch
• Coordinates51° 53′ N, 3° 26' W
• Type
  • 1st to 3rd order gravel streams
  • median grain-size: 14.1 mm
  • pH: slightly alkaline

The Cwm Llwch catchment, located in the Brecon Beacons in south Wales, originates from the peaks of Pen Y Fan and Corn Du and drains pasture land and patches of riparian vegetation. The streambed surface and subsurface of these sandstone streams is inhabited by 332 invertebrate species1.

The streambed sediments, which are mostly composed of red sandstone, have a median grain-size of 14.1 mm and a mean porosity of 32%, which enables an extensive hyporheic interstitial.

These gravel streams are characterized by abundant rotifer assemblages, with 27 monogonont and 25 bdelloid species1. Monogonont rotifers display density maxima during the winter period in both 1st and 3rd order streams, while bdelloids peaked in the 1st order stream in summer. However, the species number of bdelloids was highest in the 2nd order stream in summer.

Among bdelloid rotifers, notable is the presence of the rare species Henocerus falcatus (Milne) in the 1st order stream and the high densities of Philodinavus paradoxus Murray particularly in the streambed sediments of the 2nd order stream. In the Cwm Llwch catchment, the monogonont rotifers were dominated by Cephalodella incilla Wulfert and the widely distributed species of Monostyla lunaris (Ehrb.) and Lepadella ovalis (O.F.Muller).

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