The Ecology of the Lough Ine Rapids with Special Reference to Water Currents

Abstract

The bed of the Lough Ine Rapids is covered by large laminarian seaweeds, mainly S. bulbosa. The fronds form a dense canopy, which has a typical fauna associated with it. A study has been made of the speed of the water current at the level of the canopy throughout the tidal cycle (12.5 hrs.) at a series of selected stations. The total lengths of water-columns passing and the maximal speed of the current at each station are given. The various stations range, in maximal speed of current, from a very low value up to 2.5 m./sec. An investigation has been made of the distribution of animals associated with the canopy. This investigation comprises both general observations made throughout the Rapids area and a detailed study of samples of Saccorhiza collected at the stations. Of the animals studied 6 spp. (3 polyzoans and 3 gastropods) were confined to situations where they were exposed to weak currents only, that is, on the fronds of Saccorhiza at stations of quiet water and in some cases in the shelter of the crenulated margins ("frills") of the stipes at stations of stronger current. Two spp. (the polyzoan Membranipora membranacea and the limpet Patina pellucida) favored moderate current and were less abundant in weak or strong current. Two spp. (the amphipod Fassa falcata and the hydroid Tubularia bellis) favored proximity to strong current but not direct exposure to it. In expts. with the gastropod Gibbula cineraria it was shown that the adult animal was washed off Saccorhiza by strong currents.