The results of our first experiment in applying fertilizers to a small sealoch, Loch Craiglin (Gross, 1947; Marshall, 1947; Orr, 1947; Raymont, 1947; for references see No. V), were sufficiently encouraging to justify an extension of the work to a larger area. Loch Craiglin provided a good site for preliminary investigations because it could be easily enclosed and, for all practical purposes, isolated from the main loch. The conditions in Loch Craiglin were, however, atypical in many respects, and it seemed desirable that the results obtained there should be tested by investigations in a body of water (1) sufficiently large to reduce the extreme fluctuations in salinity, pH and other factors, encountered in Loch Craiglin; (2) sufficiently deep to reduce the proportion of seaweed to phytoplankton in the productivity of the cultivated area; and (3) with its connection with the main loch or the sea left undammed, to test the possibilities of fertilizer application in open, i.e. not artificially enclosed, sea-lochs.