Paleolimnological evidence of limnetic nutrient concentration equilibrium in a shallow, macrophyte-dominated lake

Paleolimnological techniques were used to infer long-term changes in trophi c status of Lake Opinicon (Ontario), a mesotrophic, shallow, macrophyte-dom inated lake, and to assess whether these patterns support the hypothesis of 'alternative equilibria' proposed for shallow lake systems. Analysis of the diatom assemblages indicated cold, oligotrophic to mesotrop hic, alkaline conditions during the period shortly following deglaciation ( >11000 yr BP). With the establishment of an open spruce-woodland (ca. 11000 yr BP), benthic diatoms dominated, indicating shallow waters and the prese nce of macrophytes. Pieces of aquatic macrophytes also became common in the core. An increase in lake productivity to mesotrophic levels (diatom-infer red [TP] of 23 mu g/L) occurred following the transition to a deciduous for est in the catchment ca. 7500 yr BP. Paleolimnological data did not indicat e any significant change in limnetic nutrient concentrations resulting from the hemlock decline (ca. 4800 yr BP) or from extensive flooding with the c onstruction of the Rideau Canal (1828-1832). Modest nutrient enrichment of the lake has occurred more recently with increasing agricultural and reside ntial/recreational activities, with a diatom-inferred [TP] increase to 32 m u g/L. However, compared to other, similarly-disturbed deep lakes in the ar ea, Lake Opinicon showed much smaller changes in lakewater conditions, stil l maintaining a clear-water, mesotrophic state and a large macrophyte commu nity. The apparent inertia of Lake Opinicon to changes in trophic status, as comp ared to other deep, near-by, stratifying lakes in southeastern Ontario, sup ports the 'alternative equilibria' hypothesis. Paleolimnological evidence s uggests that Lake Opinicon has been in a clear-water, equilibrium state dom inated by macrophytes since early in its development, and has displayed hys teresis to nutrient enrichment. These results may provide lake managers wit h important information for effective management strategies of shallow wate r systems in southeastern Ontario.