Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia. The predominant broad-scale pattern seen at all sites is a major shift in moisture regimes from wet to dry, or vice versa (depending on location), that occurred after a period of relative stability. These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region.
Drought is a recurring natural feature of climate that has had dramatic environmental, economic, and social impacts on modern (1) and ancient (2) civilizations. Decade-to centuryscale episodes of prolonged drought or high rainfall have been recorded from North America in continental archives as diverse as tree rings (3), tree stumps (4), lake sediments (5), and river deposits (6, 7). The prairie region of North America is particularly susceptible to extreme droughts (8). However, few highresolution proxy records of climate exist from the prairies. Tree-ring records on the prairies typically are spatially limited to wooded areas at the periphery of the prairies (9) and temporally limited to, at best, Ϸ500 years (10). Sediments from closed-basin lakes can provide high-temporal-resolution paleoclimatic information from prairie regions for much longer periods.Here we provide evidence from high-resolution (subdecadal to decadal) sediment core records from six lakes on the Canadian and northern U.S. prairies (Fig. 1). All of the sites indicate that shifts in drought regimes have been a prevalent feature of this region, occurring on decadal through multicentennial scales. Inferred changes in climatic conditions over the past two millennia are based on analysis of diatom assemblages preserved in sediment cores, a commonly used technique for tracking past climatic conditions (11). Our focus here is on long-term dynamics and broad-scale similarities among the lake records. This approach is in part undertaken because of the inherent difficulties of comparing short-term dynamics across records constrained by carbon-dated chronologies.
Materials and MethodsDiatom Inferences. Diatom remains in sediment cores from six lakes were used to reconstruct variables influenced by climatic conditions. Diatom-inferred salinity estimat...
1. A diatom-phosphorus weighted averaging (WA) transfer function, derived from a training set of currently oligotrophic to mesotrophic European Alpine lakes, was applied to a high-resolution sediment core with armual laminae from Mondsee, an Austrian prealpine lake, in order to reconstruct the eutrophication history of the lake. 2. The water chemistry records of total phosphorus (TP) available for Mondsee were compared with the diatom-inferred TP from the model for the period 1975-93. The trend in TP values as inferred by the model paralleled the monitored trend in TP values closely, with matching peaks in 1979/80, a decrease in values from the early 1980s, a second smaller peak in 1986/7, and a further reduction in concentrations in the last 6 years. 3. However, there was a clear mis-match between the actual timing of the major TP peak, with the water chemistry records reporting its occurrence in 1979, and the diatom model indicating a small peak in 1980 and the highest concentrations in 1982. This can be attributed to the uncertainty of the sediment chronology for this section of the core, and possibly to the inconsistency between the core resolution and the resolution of the diatom model. 4. In terms of the actual concentrations of TP inferred by the model, they compared reasonably well with the measured data, although the model tends to underestimate for the lower core section owing largely to poor diatom assemblage analogues. In the upper part of the core, the diatom-inferred TP values were in extremely close agreement with the monitored chemical data. 5. This validation study indicates that diatom-phosphorus transfer functions are robust and are able reliably to infer past-TP concentrations from fossil diatom assemblages in sediment cores. Despite the natural intra-and interannual variability in diatom assemblages and epilimnetic water chemistry, the technique can provide accurate estimates of TP with an annual resolution. The model can be applied to selected sites with suitable sediment records to reconstruct lake TP histories, thus providing a pragmatic management tool for addressing lake eutrophication problems.
1. We studied the distribution of epilithic diatoms in streams subjected to different degrees of human impact in order to evaluate their potential as bioindicators for environmental changes such as nutrient enrichment and acidification.
2. Three descriptors of the diatom assemblages were tested with respect to their potential to predict environmental changes: species composition, genus composition and size distribution.
3. Water colour and pH explained the largest amount of variation in diatom assemblages. According to ordination analyses, water colour explained variations in size distribution (42%) better than those in generic (25%) or species composition (8%). On the other hand, pH was not correlated with size distribution while a significant fraction of variation was explained by species (11%) and especially generic (18%) composition. Only species composition responded to changes in phosphorus and grazer biomass, however.
4. Size distribution and coarse (genus level) taxonomic analyses sometimes outperformed fine taxonomy in describing the response of diatom assemblages to colour and acidity. In view of the simplicity of these alternative descriptions of diatom assemblages, their potential for routine stream monitoring should be further explored.
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