COMPARISON OF DIATOMS, FOSSIL PIGMENTS AND HISTORICAL RECORDS AS MEASURES OF LAKE EUTROPHICATION

1. Analysis of fossil diatoms and pigments was used to examine the eff ects of land-management practises on the trophic status of Williams La ke, a eutrophic lake in central British Columbia, Canada. Published we ighted-average (WA) models were used to infer changes in total phospho rus concentration (TP) during the past 200 years. 2. Diatom-inferred T P (DI-TP) was compared to 20 years of direct chemical TP measurements to determine the accuracy of diatom-TP models in inferring mean summer TP in Williams Lake. Plant pigments were measured using high performa nce liquid chromatography (HPLC) to quantify historical changes in gro ss algal community composition and abundance and to evaluate further d iatom-TP inferences.3. Palaeolimnological analyses showed that William s Lake has been productive throughout the last 200 years. Diatoms char acteristic of alkaline, eutrophic conditions were continuously present c. 1765-1990 AD. Carotenoids from filamentous cyanobacteria (myxoxant hophyll, aphanizophyll) were regularly present in Williams Lake sedime nts, although cryptophytes (alloxanthin), diatoms (diatoxanthin), chlo rophytes (lutein-zeaxanthin b-phorbins), and siliceous algae (diatoms, chrysophytes) or dinoflagellates (fucoxanthin) were also important co mponents of past algal communities. Terrestrial disturbance (railway a nd road constructions, cattle ranching) increased lake production, but resulted in relatively little permanent environmental change. 4. Comp arison of DI-TP with measured TP (1972-91) showed that inferences from simple WA models were similar to average summer TP (39.1 vs. 35.2 mu g TP l(-1)). Inferences resulting from data manipulations that down-we ighted eutrophic lakes (outlier elimination, bootstrapping) or diatom species (square-root transformation, tolerance-weighting) were weakly and negatively correlated with measured TP, introduced bias into infer ence models, or underestimated measured TP. These patterns suggest tha t, when using diatom-TP models developed from sparsely populated regio ns, accurate palaeoecological inferences for TP in eutrophic lakes sho uld avoid data manipulations which down-weight the most productive sit es and taxa. 5. Comparison of DI-TP and fossil-inferred algal abundanc e during the past 200 years suggested that changes in nutrient inputs accounted for relatively little variation in past algal abundance. Alt hough past changes in total algal biomass (as p-carotene) and DI-TP we re broadly similar, the two variables were not significantly correlate d (alpha = 0.05). In contrast, changes in DI-TP were significantly cor related with mean concentrations of diatom-specific carotenoids (diato xanthin), although the explanatory power was low (r(2) = 0.16). These patterns suggest that the DI-TP model reflects more closely environmen tal conditions in Williams Lake during periods of diatom growth, and n ot necessarily those when total algal biomass is greatest.