The Intertidal Ecology of Bardsey Island, North Wales, with Special Reference to the Recolonization of Rock Surfaces, and the Rock-Pool Environment

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Abstract
The ini-tial stages in the recolonization of rock surface are characterized by the appearance of green algae (particularly Ulva and Cladophora) except in the case of the most exposed surface, where there was some growth of Porphyra. This initial growth only covered a small proportion of the rock surface and usually soon disappeared, to be followed by a more extensive and more permanent growth of fucoids (again with the exception of the most exposed surface, where there was a more transient fucoid growth). This later growth, though more rapid on rocks nearer low-water mark, was gradual and did not suggest intense competition for the area exposed. In 2 cases the flora of the rock was different from that of adjacent areas. The rock-pool environment was studied by a series of diurnal readings, supplemented by daily readings over longer periods. O2 conc. pH, chloride content and temp. were recorded. The density of the flora is the most important factor governing the range of O2 and pH variations so far as observations and expts. have been made, the particular species of alga present has little influence. Daily readings indicate a fairly regular rhythm in pools situated between tide marks, but a less regular series of changes in those near high-water mark. The salinity of such pools may also vary widely, particularly if a spring tidal period coincides with heavy rain. Under these circumstances fluctuations comparable with those recorded under. estuarine conditions may occur in the chloride content, these being most marked at the surface, but also affecting the deeper water. Recovery from such conditions may take several days. The salinity of pools above high-water mark spring tides also varied considerably, but the variations were more gradual and periods of high or low salinity more prolonged. The diurnal changes of tidal pools are only slightly affected by the time during which contact with the sea occurs: the configuration of the neighboring shore, with its effect on the way in which water enters the pool, may be a factor of some importance.[long dash]Auth. summ.