1. The phytoplankton in Kyle Scotnish and Sailean More, two parallel arms of Loch Sween, was studied and the seasonal variations described.
2. Artificial fertilizers were distributed in Kyle Scotnish and their effects on the phytoplankton were followed up. It was found that on some occasions the phytoplankton increased rapidly after the distribution of fertilizers, but that on other occasions there was no apparent response.
3. The zooplankton populations in Kyle Scotnish and Sailean More were studied and the seasonal variations described.
4. It was found that after fertilizers had been distributed the zooplankton in Kyle Scotnish became considerably richer than that in Sailean More, and its density remained higher until it was reduced in September 1945 by the feeding of young herring. Even so, the average density in Kyle Scotnish was twice that in Sailean More.
5. Experiments were done to determine the effect upon the phytoplankton of the grazing of the zooplankton. It appears that the zooplankton removed a large portion of the phytoplankton by grazing, and that the apparent lack of response of the phytoplankton to the distribution fertilizers was at least on some occasions due to the effects of grazing.
6. It was shown that the productivity of Kyle Scotnish, in terms of plankton, has been doubled by the distribution of fertilizers.
This paper was assisted in publication by a grant from the Carnegie Trust for the Universities of Scotland.