Chironomid-based inference models for estimating end-of-summer hypolimnetic oxygen from south-central Ontario shield lakes

1. Subfossil chironomid head capsules were sampled from surficial sediments from 86 boreal shield lakes in south-central Ontario, Canada. Lake charact eristics ranged from shallow to very deep (> 80 m), ultraoligotrophic to me sotrophic, and with end-of-summer hypolimnetic oxygen conditions ranging fr om near-saturation to anoxic. 2. Subfossil chironomid assemblages, comprising 44 taxa from 59 lakes, were analysed using multivariate ordination techniques such as redundancy analy sis (RDA) and canonical correspondence analysis (CCA). Forward selection in RDA and CCA both showed that measures of oxygen, such as end-of-summer vol ume-weighted hypolimnetic oxygen concentration (VWHO) and bottom oxygen con centration (botO(2)), were the strongest explanatory variables for the chir onomid data. Maximum depth and major ion chemistry were also important expl anatory variables. 3. Oxygen inference models were developed using partial-least-squares regre ssion (PLS), weighted-averaging partial-least-squares regression (WA-PLS), and weighted averaging regression (WA). Models were developed using both th e full 44 taxa assemblage (which included littoral taxa) and using only 15 profundal-type taxa. 4. Cross-validated models (jackknifing) using full-assemblage or profundal- only taxa had similar statistical power (similar root mean squared error of prediction, RMSEP). The best models had moderate predictive power, with an r(jack)(2) as high as 0.56, and an RMSEP as low as 2.15 mg L-1 for [VWHO], and an r(jack)(2) of 0.49 and an RMSEP of 0.24 for log([botO(2)] + 1). 5. Reconstruction of [VWHO] and [botO(2)] using a previously published chir onomid profile that showed strong lake response to land-clearance and loggi ng suggests that oxygen inference models are reliable and accurate, reflect ing the qualitative changes occurring in subfossil assemblages. However, th e profundal-only models may be misleading in situations where the ratio of littoral-to-profundal subfossils changes drastically in response to lake di sturbance.