Sources and utilisation of nutrients in the Loch of Strathbeg, Aberdeenshire
1. A phosphorus budget was calculated for the Loch of Strathbeg from 24/4/1978--24/4/1980. The loch is on an exposed site in the north-east corner of Scotland, has 2 an area of 2.01 km2 and a mean depth of 0.75 m. The loch is 'L' shaped and conveniently divided into long and short arms: the former draining to the sea. The catchment of 55.7 km2, is glacial ground moraine over-lying metamorphic rocks, on which grazing and arable fanning is carried out. There was no major human sewage input to the loch. 2. From October April an average of 1760 Grey Lag and 2740 Pink-footed Geese roosted on the loch. Each night a single Grey Lag deposited a mean of 332 mg P after feeding on barley grain and 211 mg P after feeding on grass, while a Pink-foot deposited 234 mg P and 172 mg P respectively. Extrapolations related to body weight were made to the less common species feeding on similar food. Published measurements of phosphorus input by gulls were discussed and new predictions were made from published energetic equations. The mean phosphorus input to the loch from birds totalled only 295 kg P yr-1 or 5.7% of the total input, However faeces probably fell directly to the sediment, where they were responsible for about 16% of the phosphorus loading. 3. There are five streams draining into the loch and a few small areas drained by groundwater. 83% of the area drains into the shallow short arm causing accelerated flushing and uneven loading. Separate phosphorus predictions were made for each stream, groundwater, rainfall and tidal input. Of these 90% (4,386 kg P yr-1) entered the short arm. 97% was from the catchment, 0.4% from rainfall and 2.3% from tides. The long arm had no direct tidal input, and of the total (482 kg P yr-1), 93% was from the catchment and 7% from direct rainfall. 4. The loch was shown to be a net sink for both reactive and non-reactive phosphorus. The output of reactive phosphorus as a percentage of the input varied from 3% in the summer to 34% in the winter and total phosphorus 55% to 60% respectively. The loch was also a sink for monomeric silica and total oxidised nitrogen. Ammonium was generated within the loch during the autumn, and the concentration of major ions was related to marine influence. 7. Microcosm experiments investigated the change in concentration of dissolved nutrients in the presence of sediments and primary producers. Winter conditions were simulated and showed that reactive phosphorus and oxidised nitrogen were removed from solution but that ammonium and silicate increased in concentration. Simulated summer conditions with growths of rooted macrophytes in the light resulted in a rapid removal of all the above mentioned nutrients from solution. This showed the effect of primary producers in stripping nutrients from solution and maintaining low concentrations throughout the summer. 8. After a spring bloom of the diatom Tabellaria fenestra, the water column cleared and most of the loch sediment was covered by growths of rooted macrophytes. The submerged vegetation was surveyed and mapped. Generally the less rich long arm was covered by Chara whilst the short arm held a more diverse flora. During August 1979 the macrophytes in the long arm had a significantly higher biomass and lower phosphorus concentration than the short arm. This reflected the water depth, the availability of reactive phosphorus in the sediment and in the water column. Mid-summer water concentrations of high dissolved oxygen and low carbon indicated intense photosynthesis, with the possibility of temporary carbon limitation. Between mid-August and January, avian herbivores consumed about 75 T dry weight of macrophytes (47% of August standing-crop). About 2 T was washed onto the banks, and the remaining 83 T decomposed in the loch. Reactive phosphorus released quickly became bound. 9. Total phosphorus results from this study were tested using published nutrient budget models. The parameters theta (theta = mpm/lp) and O (O = z/ℓ ) were devised. A relationship between the two was derived using data from this study and published sources. Assuming that the mean depth, rate of flushing, and mean spring concentration were known, the loading could be predicted. From a known, increase or decrease in loading, the ultimate spring concentration and the number of years taken to reach that concentration could be calculated. 13. It is concluded that an increase in summer phosphorus loading will largely destroy the macrophyte community especially Chara. This would prove detrimental to macroinvertebrates, the trout fishery and waterfowl feeding within the loch.