The recent diatom history of Loch Leven, Kinross

Diatoms in the top metre of lake sediment have been studied for indications or recent changes in the environment of Loch Leven. The diatoms indicate a eutrophic, alkaline lake with Melosira italica ssp. subarctica, Tabellaria Flocculosa and Stephanodiscus astraea the dominant taxa at the base of the 1-m section. There is then an appeance of Melosira ambigua and Stephanodiscus hantzschii suggesting a richer environment. Post A.D. 1816 changes in taxa suggest possible correlation with the lowering of lake level in 1830 and the dominance of Diatoma elongatum, Cyclotella pseudostelligera, Stephanodiscus astraea var. minutula, S. tenuis, S. hantzschii, and Synedra rumpens and var. soctica in zone 4 agrees with present-day plankton records.     

Complex effects of habitat loss on Golden Eagles Aquila chrysaetos

The development of commercial forests presents potential threats to large raptors that rely on prey caught in open country. We examined the effect of afforestation of breeding habitat used by a population of Golden Eagles Aquila chrysaetos in Scotland where, over the last 50 years, extensive stands of exotic conifers have been planted. Using data for 31 years on territory occupancy and breeding success, together with spatiotemporally dynamic mapping of forest cover and predicted areas of territory‐use in a Geographical Information System, we examined relationships between forest cover and Eagle ecology at landscape and individual territory scales. Several territories were abandoned during the earliest phases of forest planting, but relatively few were apparently lost to later plantings. Territories with poorer breeding productivity appeared to be more vulnerable to abandonment than territories with better breeding productivity. At the landscape scale, temporal differences in breeding productivity were negatively related to the extent of forest cover, although productivity of individual territories showed no clear relationship with forest cover. Several territories with less than a 5% increase in forest cover experienced reduced productivity; however, territories least constrained by neighbouring pairs of Eagles showed an increase in productivity. Territories experiencing the greatest increases in forest cover showed a greater use of spatially separated nest‐sites by occupying pairs. Hence, pairs that were less constrained by neighbours appeared to compensate for loss of open habitat by shifting their territory‐use, whereas pairs that were more constrained could not compensate for open habitat loss and suffered reduced productivity (and, probably in some cases, abandoned the territory). We suggest that simple guidelines based on the extent and locations of habitat loss are inadequate when predicting effects on large territorial raptors such as Golden Eagles. Consideration should also be given to the ‘quality’ of a territory or occupying pair, as well as the extent to which territory‐use is constrained by neighbouring pairs or other ‘unsuitable habitat’ which may have been affected by previous episodes of open habitat loss.     

Diet specificity is not associated with increased reproductive performance of Golden Eagles Aquila chrysaetos in Western Scotland

Amongst raptor species, individuals with specialized diets are commonly observed to have higher reproductive output than those with general diets. A suggested cause is that foraging efficiency benefits accrue to diet specialists. This diet specificity hypothesis thus predicts that diet breadth and reproductive success should be inversely related within species. We highlight, however, that a prey availability hypothesis also makes the same prediction in some circumstances. Hence, when high diet specificity results from high encounter rates with an abundant, preferred prey, then prey availability may affect reproductive success, with diet specialization as an incidental correlate. Using three insular study areas in western Scotland, we examine diet specificity and reproductive success in Golden Eagles Aquila chrysaetos. Diet breadth and breeding productivity were not negatively related in any of our study areas, even though birds with specific diets did tend to have a higher incidence of preferred prey (grouse and lagomorphs) in the diet. Indeed, in two study areas there was evidence that diet generalists had higher breeding productivity. Our results therefore failed to support the diet specificity hypothesis but were consistent with the prey availability hypothesis. We highlight that although many other studies are superficially consistent with the diet specificity hypothesis, our study is not alone in failing to provide support and that the hypothesis does not provide a generic explanation for all relevant results. Diet specificity in predators can be at least partially a response to prey diversity, availability and distribution, and benefits associated with different prey types, so that being a generalist is not necessarily intrinsically disadvantageous. We suggest that the available evidence is more consistent with variation in prey abundance and availability as a more influential factor explaining spatial and temporal variation in breeding productivity of ‘generalist’ species such as the Golden Eagle. Under this argument, prey abundance and availability are the main drivers of variation in reproductive output. Diet specificity is a consequence of variation in prey availability, rather than a substantial cause of variation in reproductive success.     

The Changes in the Composition of the Diatom Assemblages Found in the Surface Sediments of Blelham Tarn in the English Lake District During 1973

A study has been made of the accumulation of diatom frustulesin the surface layers of lake sediments, taking advantage ofthe appearance of a new diatom bloom in Blelham Tarn in theEnglish Lake District. Results from monitoring the top 5 cmof sediment and seston trap material, and comparison with phytoplanktonrecords show that there is good agreement between all threeand that sediment diatom assemblages change in response to theliving populations so that the record of large ‘blooms’can be quickly diluted. The centric diatom, Stephanodiscus astraeavar. minutula appeared to sink out of the phytoplankton quicklyand mix down into older sediments fairly readily, circumstanceswhich may be assisted by the compact shape of its frustule.     

A comparison of diatom phosphorus transfer functions and export coefficient models as tools for reconstructing lake nutrient histories

1. We compared the baseline phosphorus (P) concentrations inferred by diatom‐P transfer functions and export coefficient models at 62 lakes in Great Britain to assess whether the techniques produce similar estimates of historical nutrient status. 2. There was a strong linear relationship between the two sets of values over the whole total P (TP) gradient (2–200 μg TP L^?1). However, a systematic bias was observed with the diatom model producing the higher values in 46 lakes (of which values differed by more than 10 μg TP L^?1 in 21). The export coefficient model gave the higher values in 10 lakes (of which the values differed by more than 10 μg TP L^?1 in only 4). 3. The difference between baseline and present‐day TP concentrations was calculated to compare the extent of eutrophication inferred by the two sets of model output. There was generally poor agreement between the amounts of change estimated by the two approaches. The discrepancy in both the baseline values and the degree of change inferred by the models was greatest in the shallow and more productive sites. 4. Both approaches were applied to two lakes in the English Lake District where long‐term P data exist, to assess how well the models track measured P concentrations since approximately 1850. There was good agreement between the pre‐enrichment TP concentrations generated by the models. The diatom model paralleled the steeper rise in maximum soluble reactive P (SRP) more closely than the gradual increase in annual mean TP in both lakes. The export coefficient model produced a closer fit to observed annual mean TP concentrations for both sites, tracking the changes in total external nutrient loading. 5. A combined approach is recommended, with the diatom model employed to reflect the nature and timing of the in‐lake response to changes in nutrient loading, and the export coefficient model used to establish the origins and extent of changes in the external load and to assess potential reduction in loading under different management scenarios. 6. However, caution must be exercised when applying these models to shallow lakes where the export coefficient model TP estimate will not include internal P loading from lake sediments and where the diatom TP inferences may over‐estimate TP concentrations because of the high abundance of benthic taxa, many of which are poor indicators of trophic state.