AAquatic Research and Monitoring Section, Ontario Ministry of Natural Resources and Forestry, 2140 East Bank Drive, Peterborough, Ontario K9J 7B8, Canada
BDepartment of Ecology and Evolutionary Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario M5S 3G5, Canada
CGreat Lakes Laboratory for Fisheries and Aquatic Science, Fisheries and Oceans Canada, PO Box 5050, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada
Nearshore regions of the North American Great Lakes are biologically productive and are important for municipal (e.g. water intake plants) and recreational (e.g. beaches) purposes. There is evidence of warming nearshore water temperatures in the Great Lakes (McCormick and Fahnenstiel 1999; Austin and Colman 2008) and therefore a need exists to project how climate change could affect future conditions. Nearshore water temperatures in large systems like the Great Lakes are inherently complex due to processes like thermal bar formation in the spring (Mortimer, 2006) and hypolimnetic upwelling during the summer stratified period (Casanady, 1977).
The approach was to (1) compile data sources of nearshore and offshore water temperatures, (2) develop an index to characterize site-specific patterns of nearshore-offshore water temperature differences, (3) link site-to-site variation in this index to easily measured site characteristics such as depth and fetch, and (4) use these results to develop a tool to scope possible future midsummer conditions at nearshore sites under a climate change scenario.
- Daily average water temperatures from 28 water intake sites (4-58 years of data/site) situated across the five Great Lakes were the source of nearshore water temperatures. Daily average lake-wide offshore surface water temperatures for each Great Lake were estimated using a previously developed model linking air temperatures to offshore mean surface water temperatures (Trumpickas et al., 2009) (Figure 1).
- The daily differences between nearshore and offshore water temperatures were plotted against day of year for each site. Two distinct patterns were apparent (Figure 2): shallow sites showed a fixed width of daily temperature differences while deep sites showed a wider distribution of daily temperature differences in the summer. Quantile regression was used to develop a Seasonal Variation Index (SVI) that captured the range of these patterns.
- Linear regression was used to determine the relationship between SVI and depth, fetch, exposure (a measure of the fetch of a site combined with wind patterns) across sites, and the annual maximum offshore summer water temperature of each lake.
- The distribution of midsummer neashore-offshore water temperature differences were characterized using quantile regression. A model predicting benchmark values of the low, mid, and high end of this distribution from each site was developed using depth, fetch, exposure, and the annual maximum offshore surface water temperature of each lake as candidate independent variables. Previously developed projections of offshore water temperatures (Trumpickas et al., 2009) under the IPCC A2 scenario were applied to give one set of possible future conditions for a set of nearshore sites across a range of depth and fetch values.
A regression between SVI values and site characteristics showed that sites with greater fetch, exposure and depth showed a wide distribution of summer daily temperature differences. These relationships indicate that deep, open sites are impacted by upwellings of hypolimnetic waters in the summer. Shallow, enclosed sites were warmer than offshore surface waters in the spring and gradually became cooler than offshore temperatures by the fall. A model with depth, mean and maximum fetch as independent variables explained 73% of the variation in SVI.
A model with depth, mean, and maximum fetch and an interaction term between maximum offshore summer surface water temperature and maximum fetch explained 59-70% of the variation in the midsummer neashore-offshore water temperature difference benchmarks. Projections of future distributions of midsummer water temperatures showed that shallow, open sites would show the greatest increase in water temperatures by 2071-2100 while deep, enclosed sites would show the least increase (Figure 3).
Our approach showed that we were able to characterize general patterns of complex nearshore-offshore temperature patterns and link these patterns to easily measured site characteristics (depth, fetch and exposure). This approach contributes to understanding whether a given site on the Great Lakes is susceptible to upwellings, particularly at sites without routine temperature monitoring. Site characteristics may also be related to how water temperatures will be affected by climate change in the future. Although our simplistic approach has ignored sub-basin variation in water temperatures; changes in thermocline depth, hypolimnetic temperatures and length of the stratitified period; and relies on only one climate change scenario, future empirical and mechanistic work will refine projections of future conditions. Projections of future nearshore conditions can be useful for lake managers assessing concerns such as future habitat availability for species of concern and changes in overall biological productivity.
- Austin, J., Colman, S., 2008. A century of temperature variability in Lake Superior. Limnol. Oceanogr. 56, 2724–2730.
- Csanady, G.T., 1977. Intermittent ‘full’ upwelling in Lake Ontario. J. Geophys. Res. 82 (6C0717), 397–419.
- McCormick, M.J., Fahnenstiel, G.L., 1999. Recent climatic trends in nearshore water temperatures in the St. Lawrence Great Lakes. Limnol. Oceanogr. 44, 530–540.
- Trumpickas, J., Shuter, B.J., Minns, C.K., 2009. Forecasting impacts of climate change on Great Lakes surface water temperatures. J. Great Lakes Res. 35, 454–463.
Full study published in the Journal of Great Lakes Research, Volume 41, Issue 1, March 2015, pp. 53 – 64.
Featured Image: The Great Lakes region as seen from space. (Credit: NASA)