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 reconstruct the history of lake eutrophication. Diatoms are well established in this role, but to augment diatom-based inferences of nutrient status we investigate 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 dissolved oxygen, maximum lake depth, Secchi depth and surface water temperature) make a statistically significant (P < 0.05) contribution to explaining the variance in the chironomid data, of which TP makes the largest contribution (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 models are evaluated by leave-one-out (jack-knifing) cross-validation. The simplest minimal adequate model is provided by WA with unweighted inverse deshrinking of root mean square error of prediction (RMSEPjack = 0.34 and r(jack)(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 results 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, followed by a more pronounced and rapid increase that has continued until the present. Inferred TP values from the WA chironomid inference model agree with diatom-inferred values. 5. The study demonstrates that fossil chironomid assemblages can be used to investigate quantitatively the trophic history of lakes.