Pii: S1463-4988(99)00065-2

Hamilton Harbour, a heavily urbanized and polluted embayment, has been selected for environmental remedy by an International Joint Commission. Ecosystem restoration efforts, however, require an understanding of harbour water balance and in particular the influence of recently enhanced exchange with the more dilute waters of Lake Ontario via the Burlington Canal. Here we provide a 6000-year quantitative reconstruction of hydrologic communication between Hamilton Harbour and Lake Ontario, based on the carbon isotope composition in the cellulose of the lake sediment, as a tracer of dissolved inorganic carbon. Results indicate that excavation of the canal has led to mixing levels 30–100% greater than the natural hydrologic state, conditions comparable to the Nipissing Flood when Upper Great Lakes drainage was diverted through Lakes Erie and Ontario roughly 5000 years ago. The effects of elevated exchange are clearly displayed by abrupt attenuation of anthropogenically driven eutrophication in the uppermost sediments from Hamilton Harbour. Thus, restoring the harbour water balance to predisturbance status would generate unfavourable environmental conditions in the harbour unless the effluent discharge to the harbour is eliminated entirely. q 2000 Elsevier Science Ltd and AEHMS. All rights reserved.