Long-term trends in the profundal chironomid-fauna in nitrogen-limited Lake Esrom, Denmark: a combined palaeolimnological/historical approach

Abstract

A 90-cm sediment core from Lake Esrom was analysed for subfossil diatoms and Chironomidae. Quantitative diatom-inferred reconstruction of total phosphorus showed an increase in TP levels from 84 μg P/l around 1600 AD (extrapolated 210 Pb chronology) to 200μg P/l in 1975, followed by a decrease to 165 μg P/l at the present day. Chironomid-inferred chlorophyll-a concentration ranged from 5--15 μg chl-a/l 1 throughout the core. Analysis of historical oxygen data and long-term zoobenthic population dynamics from the profundal zone of Lake Esrom suggested that the period of oxygen depletion in the hypolimnion increased during the 20 th century. However, the chironomid record showed only minor variations in the subfossil assemblages over the last ∼400 years and there was no chironomid indication of significant changes in the hypolimnetic (oxygen) conditions. Because primary production in Lake Esrom is nitrogen limited during the stratification period, the chlorophyll concentration is lower than commonly found for Danish lakes with comparable phosphorus concentrations. However, both the diatom-[TP] and chironomid-[chl-a] inferred values for recent periods agree well with monitored data. The two models work because the diatoms are reflecting the ([TP]) spring situation before nitrogen becomes the limiting factor whereas the chironomid fauna is reflecting the average summer (chlorophyll) situation both before and during the period of nitrogen limitation. This study illustrates the importance of multi-proxy analyses for quantitative palaeolimnological reconstruction of lake trophic state, particularly for lakes that behave differently from the more predictable responses to nutrient enrichment.