The Clyde River Purification Board (Clyde RPB) began its water quality studies of Loch Lomond in 1966 as part of its duty 'to promote the cleanliness of the rivers and other inland waters ... and to conserve so far as practicable the water resources of their areas'.
Other studies of the loch in recent years include Maulood and Boney (1980). Maitland et al (1981), Beveridge (1982) and Tippett (1988). Tippett (1988) studied the loch intensively during the period 1984-1986 and found that the north basin was considerably less rich in ammonia, nitrate, phosphate and silicate than the south basin. Beveridge (1982) reviewed previous work, including the survey data of the Clyde RPB, and concluded that all studies of water chemistry and flora/fauna showed Loch Lomond to be oligotrophic and the south basin to be less trophically poor than the north. Maitland (1981) concluded that the north basin was oligotrophic and the south basin mesotrophic on the basis of biological classifications of phytoplankton, invertebrates and fish. However water quality sampling was restricted in that study to a single site in the north basin. Maulood and Boney (1980) reported that the bulk of phytoplankton production in 1972/1973 occurred in the south basin compared with the rest of the loch. This report attempts to describe the trophic level of Loch Lomond and its basins based on the Clyde RPB's most recent work.
Slack (1957) deduced that Loch Lomond has three major basins. Also shown are the two largest rivers which flow into the loch, the Endrick Water and River Falloch, and the River Leven which flows out at its southern tip. In addition there are three minor rivers, the Fruin Water, Luss Water and Douglas Water. There are also numerous small streams. Some geographic background data is given. The calculation of phosphorus contributions offer an introduction to the subject of nutrient loadings.
Eutrophication is regarded as a complex problem. Loch Lomond experiences some nutrient enrichment due to river inputs and sewage effluents which in other lakes has been severe enough to interfere with the beneficial uses of water and in turn caused significant economic loss. The loch is an important drinking water supply and 46 million gallons per day is abstracted for the domestic requirement of about 1 million people. In addition Loch Lomond has a large and active angling association which produced a revenue of £91,000 in 1988 alone. There are two species of rare aquatic plants, three species of nationally or internationally rare invertebrates, and an unusual fish called the powan (Palmer, 1986), all of which have an economic value as yet unmeasured. The loch was designated a regional park on 29 April 1988 and has been officially recognised as an environmentally sensitive area with the object of encouraging farmers to continue with, or adopt, specific farming methods to protect or enhance the conservation value of their land. The south eastern part of the loch is a national nature reserve run by the Nature Conservancy Council. However Loch Lomond's role as a tourist centre gives rise to concerns about pollution. The local tourist industry sold over 75,000 bednights in the 1986 season (March-August inclusive). Since most developments around the loch discharge sewage, the Clyde RPB has placed stringent standards on these effluents and frequently the Royal Commission Standard has been applied to prevent pollution; in these instances developers have installed biological sewage treatment plants at considerable expense. To avoid damage to the water environment the Clyde RPB will also examine other aspects of a development such as the construction phase, car parking and fuel storage, to minimise contamination and protect the interests of other water users.
In June 1986 the Clyde RPB received a report from the Central Scotland Water Development Board (CSWDB) that a drinking water treatment process at Ross Priory, Gartocharn was clogging. The volume of sludge collected from microstrainers had doubled since April of the same year. A single sample was examined by the Clyde RPB's freshwater biologist who reported that the sludge consisted largely of diatoms, mostly Asterionella. the event, which was short-lived, provoked a fresh consideration of the loch's waster quality particularly a possible increase in nutrient levels, and led to a special eutrophication survey programme. The long-standing routine survey work continued. This report presents the first set of results from the eutrophication surveys for a 24 month period which began on 27 April 1987.
Following a short literature study the report of the Organisation for Economic Co-operation and Development (OECD 1982) was considered as a suitable model for this work with appropriate modifications suggested by various specialists at our East Kilbride office. The OECD report was the outcome of several years of effort by 18 member countries to provide a guide for eutrophication control and inland waters worldwide. Other organisations involved in lake studies may find the method attractive since it relies on a minimum of sampling points consistent with a proper limnological investigation. Samples were taken as far as possible once a month at 7 sampling stations covering every basin. Table 4 is a tabulation of the criteria used to select each sampling point. Only the epilimnion was examined and the results were expected to give an adequate picture of the average conditions in the surface of the loch. Sampling stations were chosen over deep water, away from point source nutrient inflows such as rivers or sewage treatment plants. The far north, around Ardlui, was not sampled since the hinterland is the least populated and least cultivated in the Loch Lomond catchment. The station at Central Scotland Water Development Board is nearest to the shore at 250 metres. However the shore zone, that part of loch bed affected by wave action which may alter the chemical characteristics of the water, was estimated to be about 93 metres in the basin (Smith, Lyle and Rosie, 1981). This sampling station was therefore, beneficially, outwith the immediate influence of the shore. However it differed in one respect from the other sampling stations in that the water has a significant horizontal speed induced by the nearby water abstraction. This speed has previously been estimated at up to 381 metres per day in the vicinity of the abstraction point (Poodle, 1970).
Other sampling stations were chosen for their position in basins, or beyond physical boundaries such as islands, or where the geology changed beyond the Highland Boundary Fault. This is summarised. In this way with only 7 sampling stations, it was hoped to deal with the spatial variations in water quality without the problems of a much larger sampling programme.
Loch Lomond has a large storage capacity and the water residence time was calculated approximately as about 1.5 to 2 years although the figures are somewhat notional since the flow patterns of the whole loch are unknown. Since the survey period was 2 years the water sampled on our first survey in the north basin may have entered or left the south basin at the time of the last survey in March 1989.