Temporal trends and spatial patterns in the temperature and oxygen regimes in the Bay of Quinte, Lake Ontario, 1972–2008

Abstract

The temperature and oxygen regimes of the Bay of Quinte have been monitored, weekly or biweekly, in the open water season at a series of offshore index sites since 1972. Since 2001, temperature has been monitored in situ using loggers at both offshore and nearshore sites. This study was undertaken to assess the potential role of morphometry, climate, nutrient loading, and invasion by dreissenids in temporal trends and spatial patterns of temperature and oxygen regimes. The thermal regime was assessed using summer surface temperatures, Schmidt's vertical stability values, and vertical stratification characteristics. When the Quinte studies began taking measurements in the hypolimnion, deeper stations were attaining low oxygen levels by the end of summer, raising concerns that the cumulative impacts of eutrophication might lead to anoxia. The bottom oxygen regime at sites that stratified was assessed using the hypolimnetic warming rates and the apparent oxygen depletion rates one meter off the bottom. Quinte's maximum summer surface waters have warmed almost 1oC from 1972 to 2008 in line with climate warming. Nearshore sites in the shallow upper Bay have similar temperatures to offshore index sites while those in the lower Bay are warmer. The warming of surface waters has increased the maximum Schmidt's stability values at the deeper sites but thermocline depth or bottom temperatures have not changed. Hypolimnetic warming rates showed no temporal change but were inversely correlated with mean summer flow from the main river draining into the Bay. Oxygen depletion was variable but exhibited a downward trend in response to reduced phosphorus loading with neither climate warming nor the driessenid invasion having a detectable effect. Projected 21st C climate warming poses a threat to the Bay of Quinte ecosystem.