Regular water quality monitoring provides snapshots of what is happening in a lake. Collecting sediment cores provides diatoms, a type of algae with cell walls made of silica, that can be identified to the species-level and used to re-construct the composition of a lake’s algae communities going back hundreds of years. The species composition of those historic communities lends insights to past environmental conditions in a lake (e.g., temperature, acidity, oxygen levels).
Recent studies have documented potential changes in boreal lakes that include a longer ice- free season, stronger stratification between warm surface waters and cold deeper waters, shifts in algal communities, and increased frequency of harmful algal blooms. In the Great Lakes region, annual water quality monitoring and studies of diatoms from sediment cores collected from park lakes are revealing evidence of changes in algal communities over time. Can the information diatoms provide about how lakes responded to past environmental changes give us an indication of how lakes will respond to future climate changes?
NPS used the program MINLAKE2012 to develop daily temperature models for eight inland lakes in two Great Lakes Network national parks, four lakes in Voyageurs National Park and four in Isle Royale National Park, from 1960 to 2001. Water quality data, including temperature profiles, gathered during routine monitoring by Great Lakes Inventory and Monitoring scientists and park staff since the mid-2000s allowed us to check modeled results against actual in-lake measurements.
Overall, there was good agreement between modeled and observed temperature profiles, especially in the deeper lakes. The shallow lakes at Isle Royale did not fit the model quite as well, possibly due to their smaller volumes and factors related to their size, such as increased sheltering from wind, sunlight reaching the lake sediment and heating it up, and being proportionately more affected by water flowing in and out of the lake.
NPS also used the models to look for trends in shallow, and deep, water temperatures and timing of temperature gradient (thermocline) formation between two time periods (1962–1986 and 1987–2011). The most common significant trend was the increase in shallow-water temperatures across all eight lakes during the summer. We also found an increase in how often and for how long thermoclines in deep lakes equaled or exceeded 2°C–3°C per meter.
Diatom community shifts in the shallow lakes at Voyageurs National Park suggested slight increases in lake pH; in Isle Royale National Park shallow lakes, the shifts suggested an increase in the number of days that the lakes completely mixed. Changes in diatom communities were more pronounced in the deeper lakes and tracked with the modeled increased frequency and duration of a stronger (2°C or 3°C) thermocline.
This study is a proof of concept that past meteorological data and lake characteristics can accurately model past physical responses of lakes to weather and climate conditions and predict future responses. The ultimate aim is to use simple lake parameters to predict the sensitivity of different lake types to future climate change. Knowing how sensitive lakes are to change can help park managers identify potential management issues such as determining what lakes are most likely to support a cold-water fishery, or which lakes might be more susceptible to harmful algal blooms.
This work is a cooperative effort of the Great Lakes Inventory and Monitoring Network and the St. Croix Watershed Research Station.