Tracing lake trophic history with a chironomid-total phosphorus inference model

1. In the absence of historical water chemistry data, predictive biological indicator groups preserved in lake sediments can be employed to reconstruc t the history of lake eutrophication. Diatoms are well established in this role, but to augment diatom-based inferences of nutrient status we investig ate the potential use of chironomid midges (Insecta: Chironomidae). 2. Canonical correspondence analysis (CCA) of modern chironomid assemblages in surface sediments from 44 lakes in the English Midlands and Wales, U.K. , shows that five environmental variables (total phosphorus (TP), bottom di ssolved oxygen, maximum lake depth, Secchi depth and surface water temperat ure) make a statistically significant (P < 0.05) contribution to explaining the variance in the chironomid data, of which TP makes the largest contrib ution (29%). 3. The relationship is used to develop a series of weighted averaging (WA) and partial least squares (PLS), (WA-PLS) models to infer log(10)TP. The mo dels are evaluated by leave-one-out (jack-knifing) cross-validation. The si mplest minimal adequate model is provided by WA with unweighted inverse des hrinking of root mean square error of prediction (RMSEPjack = 0.34 and r(ja ck)(2) = 0.60). 4. Using this model, the trophic history of Betton Pool, Shropshire, U.K., is reconstructed from the mid-19th century to the present day and the resul ts from the chironomid-TP model are compared with inferences from a diatom- TP model (Bennion, Juggins & Anderson, 1996). Both reconstructions suggest that there was a gradual rise in TP since 1850 AD until about 1974, followe d by a more pronounced and rapid increase that has continued until the pres ent. Inferred TP values from the WA chironomid inference model agree with d iatom-inferred values. 5. The study demonstrates that fossil chironomid assemblages can be used to investigate quantitatively the trophic history of lakes.