SynopsisA study has been made of the supply and utilisation of nitrogen and phosphorus in Loch Leven, a shallow eutrophic lake receiving discharges from sewage treatment plants, industry and agricultural sources. Calculations of the nutrient budgets over a period of several years indicate that nitrogen enters primarily as nitrate derived from agricultural fertilisers, in quantities more than sufficient to sustain the dense algal blooms which occur. Much of the nitrate appears to be removed by benthic algae, or other processes associated with the sediments, and a high proportion of the incoming nitrogen is retained in the lake.Phosphorus enters the lake mainly from industry, with some contribution from sewage and agriculture, and a high proportion is discharged at the outflow. The algal blooms require more phosphate than is present in solution initially, and utilise it at rates greater than that of the inflowing sources, and it is deduced that the additional phosphate is derived from the sediments. A mechanism for such a release, occurring in the absence of stratification, is postulated, and the processes involved in the return of phosphorus to the sediments are discussed.

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SYNOPSISThe Outer Hebrides comprise approximately 1-3% of the land area but 15-8% of the area of standing waters of Great Britain. Streams there are generally small and only three are as large as fifth order. The majority of the lochs occur below 200 m altitude and are less than 25 ha in area. A total of 1375 stream systems enter the sea, and drain the land mass through 9240 stream segments. There are just over 6000 lochs, of which less than half are connected to these networks. Most of the water systems are on geologically base-poor rock or soil, and less than 3" ", situated on machair, are base-rich and eutrophic. The islands, due to their situation, receive precipitation in the form of very dilute sea water (with some non-marine sulphate probably derived from fossil fuel combustion) and this leads to unusual concentrations of chloride in the inland waters. Metamorphic rocks are resistant to weathering, so there is usually little variation in the composition of fresh waters except in the machair areas, where calcareous sands affect calcium concentrations and alkalinities.The flora and fauna reflect the aqueous conditions and there are some unique successions of species from high to low salinities even in the same water body. The lochs provide a wide range of habitats for aquatic vegetation, ranging from the rich machair lochs to the oligotrophic peaty lochs (which show marked similarities with their counterparts on the mainland). Three broad categories can be delineated on the basis of their water quality and vegetation: brackish lochs with varying degrees of marine influence, calcareous machair lochs which may also be affected by sea connections, and the peaty lochans of low alkalinity. A few brackish water animals are sufficiently euryhaline to exist in the machair lochs, which are otherwise dominated by freshwater species. The occurrence of chloride in inland waters enables some crustaceans, e.g. Neomysis integer, Gammaru.s duebeni and Diaptomus wierzejskii to extend into what are poor freshwater habitats. The main constituents of the freshwater fauna are molluscs and arthropods, and a few species of euryhaline fishes, i.e. salmon, sea and brown trout, three-spined and ten-spined sticklebacks and eels. There are some populations of non-migratory charr. No amphibians are present.

SUMMARY The distribution of DDT in brown trout poisoned by low aqueous concentrations of a labelled form of the insecticide has been studied. A direct‐mount technique for determining the carbon‐14 activity extracted from various organs is described. The DDT was rapidly removed from the water by the fish and by detritus, and relatively high concentrations were found in all organs and tissues examined. Storage occurs principally in the stomach, pyloric caeca, intestine, spleen, muscle and skin. The concentrations found in lipid extracts from the various organs generally showed much more uniformity than concentrations in terms of unit weight of tissue. It is suggested that the concentration of DDT present in the lipids may determine the toxicity to fish, thus explaining the greater susceptibility to DDT of fish in poor condition or of low fat content. DDT extracted from ovaries and testes indicates the possible danger to reproduction. The use of static water for fish toxicity tests involving insecticides is criticized.