The distribution of about 80 species of medusae from the Scottish and neighbouring areas over the periods 1935–1939 and 1946–1968 is given. A method of charting has been adopted to give an average picture over the 28-year period so as to reduce as far as possible the effects of variations in annual and seasonal sampling intensity. When the occurrence of a species has, in any one year, been particularly different from the average this has been noted in the text and it is hoped that the average picture given here can be used as a basis for comparison in future years.

In feathers of a wide variety of unrelated birds non-iridescent blue and related shades are produced by structures. In these, each colour-producing cell of a barb medulla contains a cavity whose outline tends to parallel the cell-boundaries (‘polyhedral’ cavity).

This contrasts with the situation in some other birds with corresponding colour structures where each medullary cell contains a smaller ‘spheroidal’ cavity left by the pycnotized nucleus.

Three different processes of moulding polyhedral cavities are described which modify the thickness of the colour-producing cloudy medium and possibly represent independent evolutionary trends.

The location and major characteristics of Loch Leven and land use of its catchment are briefly described. The water level was lowered in 1830 affecting migrant fish. Many changes have taken place in the fauna and flora in the present century with serious depletions to the macrophytes, Odonata and Ephemeroptera. The loch is of international importance for waterfowl and was made into a National Nature Reserve in 1964. An IBP project was started in 1966 to measure production and production processes in the food chain to fish and diving duck. Energy and nutrient input, production of phytoplankton, chironomids and the larger trout and perch have been measured for the whole loch. Many other related studies have been carried out. The numbers of several groups have been shown to fluctuate greatly from year to year, as have the principal components of phytoplankton, zooplankton and zoobenthos.

The historical formation of the loch is described, using geological and geomorphological evidence. The changing areal dimensions from the seventeenth to nineteenth centuries are discussed using historical cartographic and documentary evidence. Finally, a comparison is made between the results of the three bathymetrical surveys that have now been carried out in an attempt to determine the amount of silting.

The sediments of shallow and deep water areas of Loch Leven are, respectively, medium to very fine-grained sands and silty clays (muds). The largest area of more or less uniform sediment type (sands) occurs on the north-eastern shelf. The sediments with the finest grain size are found in an area to the south and east of Castle Island and Reed Bower. Here, the median grain size is less than 4 microns. The amount of sand in a sediment is very highly correlated with the median grain size of the sediment and may therefore most usefully be used to describe the general distribution of sediments in the loch. The distribution of organic carbon in the loch sediments is largely confined to deep-water areas. The southern deep area is more organic rich than the northern deep area. The amount of organic carbon in a sediment is positively correlated with the amounts of clay and silt in the sediment.

This paper describes the environment, structure and internal physical conditions of the loch. The first section deals with structure, land use and climate of the catchment area. The structure of the loch itself is then considered together with a brief description of the sediments. The bulk of the paper is devoted to the effect of sun, rain and wind on the loch, e.g., radiation and water temperature, water balance and hydraulic conditions. It ends with a summary of the influence of environmental factors on phytoplankton production and higher trophic levels.

A 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.

Data on the evolution of nitrogen gas and methane, and the utilisation of nitrate were obtained from sediment-water interface core samples obtained from Loch Leven, Kinross, and Windermere Esthwaite Water, Coniston Water and Ennerdale Water in the English Lake District. Differences in the rate of evolution and total quantity of the gases are discussed in relation to the trophic state of the lakes. The potential importance of nitrogen gas loss to the nitrogen budget of the lakes is considered.

The species composition and succession of the algal plankton are described. Population density changes, as indicated by chlorophyll a, cell volume and numbers are discussed in relation to physical, chemical and biological factors in the environment.

The quality and the amounts of the plankton vary considerably from year to year. However, for most of the period reviewed it has been dominated by dense developments of nanno and other small algae belonging to the diatom and blue-green groups.

Factors of particular importance affecting the high observed biomass and production include the water depth, adequate mixing and a rich supply of nutrients from the inflows and the sediments. Competition from other green plants appears to be low. Utilisation of living algal cells in the plankton by crustacean and protozoan grazing and fungal parasitism is rarely important.

The composition of the phytoplankton is discussed in relation to the eutrophic nature of the loch.

Photosynthetic productivity of phytoplankton in Loch Leven was studied over a 4-year period (1968–71), using the oxygen light and dark bottle technique. Marked seasonal changes in hourly and daily rates of gross photosynthetic productivity are described within the range 0·02 to 1·59 g O2/m2.h and 0·4 to 21·0 g O2/m2.day respectively. Hourly rates are shown to be relatively insensitive to variations in surface light intensity, whereas daily rates are influenced to a considerable extent by the duration of incident radiation (daylength).

The phytoplankton itself exerts a dominant influence on underwater light penetration, accounting for ca 75 per cent of light extinction at highest crop densities. This self-shading effect contributes to the poor correlation observed between crop density and areal gross productivity. The chlorophyll a content per unit area in the euphotic zone often approached its estimated theoretical limit of 430 mg/m2.

In general, increase in photosynthetic capacity (per unit content of chlorophyll a) accompanied increase in water temperature. During certain periods an inverse relationship between photosynthetic capacity and population density was evident. Reduction in photosynthetic capacity is attributed, in part, to the high pH values (> 9·5) with concomitant CO2-depIetion associated with dense phytoplankton crops.

Estimates of net photosynthetic productivity were frequently zero or negative, even over periods when algal populations were increasing and dissolved oxygen and pH values were above their respective air-equilibrium values. Underestimation of gross photosynthesis due to photochemical oxidation, photorespiration or the use of stationary bottles could not account for this apparent anomaly. The most probable sources of error in the estimates of net photosynthetic productivity are discussed.

During 1970 a pilot-scale water treatment plant was operated at Loch Leven to investigate the feasibility of producing a potable water from such a source.

The study showed that while no particular problems were encountered in producing a water of satisfactory chemical quality it was rarely possible to produce a water of satisfactory biological quality. This was due to the fact that the water treatment processes investigated were unable to retain sufficiently large numbers of the small algae so characteristic of Loch Leven and as a consequence the finished water nearly always contained unacceptably high numbers of algae.

The distribution and production of submerged macrophytes in Loch Leven has been investigated using drag-rake surveys and biomass measurements. Survey data suggest that changes in the species diversity of the loch have occurred since the earlier records. There appears to have been a decline in some species with an increase in others.

In general there has been an overall loss in species diversity which may be correlated with the increasingly eutrophic status of Loch Leven. At present the macrophyte flora is dominated by a few species, notably Potamogeton filiformis, Zannichellia palustris, Nitella opaca and algal periphyton, with declining, though still significant amounts of Chora aspera. It is probable that the species diversity is in the process of adjusting to the eutrophic status of the loch and that P. filiformis in particular is in an expanding condition.

The extent of emergent vegetation at Loch Leven has declined considerably within historical times. In 1972 only 5 per cent of the shoreline and less than 0·01 per cent of the area of the loch was occupied by emergent vegetation, the principal species being Phragmites communis Trin. and Polygonum amphibium L. The production of Phragmites communis was estimated from standing crop measurements and is low for a eutrophic lake. The maximum aerial shoot biomass varies from 1148 to 513 g dry wt/m2 in the different beds. The total production of all species of emergent vegetation is insignificant when compared to other sources of primary production. Possible factors causing the low production and decline in emergent vegetation are discussed. These include water level changes, soil types, exposure to wind and wave action and grazing by wildfowl and domestic stock.

The available information, both published and unpublished, on the zoobenthos of Loch Leven has been studied and tabulated. There have been a number of major changes in the community over the last 100 years—notably a reduction in diversity of the fauna with the disappearance of many species of invertebrates, particularly Ephemeroptera, Odonata and Coleoptera. The existing macro-invertebrate fauna is dominated by larval Chironomidae, but Nematoda, Mollusca, Annelida and Crustacea are also of importance. Dramatic changes have taken place in the chironomid population during the present study (1968–71), notable among which has been the disappearance of Endochironomus. Production studies of the two dominant larval Chironomidae (Glyptotendipes and Stictochironomus) in the sandy littoral area (42 per cent of the loch bed) gave annual estimates of 40·5 and 1·2 g (dry weight)/m2 respectively for 1970 and 5·0 and 10·2 g/m2 for 1971. Speculative estimates of the entire zoobenthos production in 1970 gave a value of 46·5 g/m2. The significance of these results is briefly discussed.

The production of four dominant species of larval Chironomidae (Chironomus anthracinus, Glyptotendipes paripes, Polypedilum nubeculosum and Limnochironomus pulsus) was measured and compared between different sections of the mud zone covering a total area of 662 ha (50 per cent of the loch bed) from March 1971 to March 1972. The results showed that the performance of Chironomus was very similar throughout the area in respect to numbers, growth and production but it was more variable in other species. The mean annual dry weight production of Chironomus over the whole area was 26 g/m2 and the four species together raised this to 29 g/m2 (equivalent to 579 k J/m2). The numbers of Chironomus increased and Glyptotendipes decreased through the sampling period. Estimates of biomass are given for other larval Chironomidae present in the mud. The accuracy of the results is discussed, as well as the variations in the seasonal growth of larvae and the changing composition of the community; some tentative estimates are given of assimilation by larval Chironomidae. The methods used for analysing these data are described in an appendix.

Utilising data on biomass, mean weight and oxygen consumption, the energy requirements for four genera of larval chironomid populations in Loch Leven have been calculated on a monthly and annual basis. Applying the formula of McNeill and Lawton (1970), estimates for annual production have also been derived.

The species composition of ciliates and nematodes in the mud of Loch Leven was investigated: 59 species of Ciliophora and 14 species of Nematoda were recorded. The ciliate species composition varied widely throughout the 11-month sampling period, whereas that of the nematodes was more constant.

The vertical distribution of ciliates and nematodes at four sites is described and related to the findings of other workers. Two distinct patterns of vertical distribution occurred, one typical of ciliates, the other of nematodes.

The planktonic movements of larval Chironomidae in Loch Leven, Kinross, are described, together with a provisional list of the species and age groups involved. The importance of this type of behaviour in determining the distribution of larval Chironomidae is discussed.

Important qualitative changes that have occurred in the zooplankton of Loch Leven within the present century are described, together with the methods and results of routine quantitative sampling during 1969 and 1972. In 1969, the crustacean zooplankton was a virtual monoculture of the carnivorous cyclopoid copepod Cyclops strenuus abyssorum and showed a marked absence of filter-feeders. This contrasts drastically with the situation in 1972 when the filter-feeding cladoceran Daphnia hyalina var. lacustris co-dominated with Cyclops. The standing crop data for the 2 years are compared and the important rotifers mentioned.

The annual production of trout in Loch Leven lies between 2·2 and 3·9 g(wet).m−2, and of perch between 5·1 and 67·4 g(wet).m−2. Between June and September adult trout consumed 3·7 times their own wet body weight of food, and adult perch 6·58 times, the daily intake decreasing from the beginning to the end of the period. The principal dietary components of the two species were the same, Asellus and perch fry being quantitatively the most important. Comparisons are made between field estimates of food consumption, laboratory estimates of food requirement, and field estimates of food production.