XIII.—The Brackish‐water Lochs of Orkney

Abstract

The Lochs of Stenness and Harray on the mainland of Orkney are the only known stations in Scotland for Neritina fluviatilis L. The Loch of Stenness has a salinity varying between 9 and 26·8 parts per thousand; 24 species of invertebrates live in the loch, as well as several kinds of fish. The distribution of H. jenkinsi Smith and Peringia ulvœ Pennant is peculiar and only partly explicable by the salinity. The Loch of Harray is almost fresh and only slightly affected by spring tides, the salinity varying between 0·6 and 4·3 parts per thousand. Twenty-seven species of invertebrates are to be found in it, most of which are fresh-water species. The calcium content of the lochs varies from 56 to 139 mg. per litre.