Winter presents a logistical obstacle to our understanding of lake ecosystems. A recent collaboration of the PIs with the Canadian- and U.S. Coast Guards and their icebreaking programs has facilitated annual winter surveys of Lake Erie since 2007. Conducted during times of expansive ice cover, these surveys have documented high phytoplankton biomass, often in discrete formations and dominated by a filamentous diatom, Aulacoseira islandica. Whereas Lake Erie is characterized by a high annual median ice cover [AMIC] consistent with its relative shallow bathymetry, it also shows extremes in maximum ice extent ranging from ~10% in low ice years to > 99% in high ice years. While maximum ice cover on Lake Eries has reached ~95% each winter from 2007-2010, the winter of 2011-12 is shaping up much differently, with unseasonably warm conditions and almost no ice cover. The PIs will use a Rapid Response Research (RAPID) grant to investigate the changes in phytoplankton community structure and function during this warm and practically ice-free winter. Specifically, they will test the hypothesis that the warm monomictic mixing regime that occurs in the absence of expansive ice cover suppresses diatom growth in Lake Erie's central basin. Suppression of abundant winter diatom growth may have important implications for events occurring during summer in Lake Erie. The documentation of abundant winter diatom growth, combined with low measured rates of bacterial decomposition results in net accumulation of algae on the lake bottom. As summer progresses and the hypolimnion warms, bacterial remineralization of the exported diatom biomass accelerates, depleting the hypolimnion of oxygen. These observations are consistent with a new hypothesis on lake function, namely that winter phytoplankton production drives Lake Erie summer hypoxia. Oxygen depletion in Lake Erie's central basin is well documented with effects ranging from enhanced internal nutrient loading to loss of habitat for macrofauna. At its full expanse, the area can exceed 10,000 km2, comparable in surface area to the low oxygen 'dead zone' in the Gulf of Mexico. Thus, deviation from the high phytoplankton biomass accumulation associated with 'typical' winter ice cover may be reflected in higher hypolimnetic dissolved oxygen measured during summer. This research opportunity can help define environmental changes that might be expected in a warming climate.
Broader Impacts: Bowling Green State University (BGSU) has established an ongoing educational collaboration with the U.S. Coast Guard (USCG). The goal of this unique program is to engage Coast Guard crew members in limnological surveys intended to increase both temporal and spatial resolution of sampling during winter in Lake Erie, a period when extreme weather conditions challenge our ability to sample. BGSU scientists will visit the ship fortnightly to lead classroom discussions and pick up samples. Crew members have the opportunity to earn university credit with tuition costs covered by the Navy, Marine Corps, and Coast Guard Tuition Assistance Program.
Lakes and reservoirs serve as rapid responding sentinels of human influence on the natural environment rendering them powerful tools to advance our understanding of a changing climate on ecosystem structure and function. The Laurentian Great Lakes are especially valuable in this respect in that they share characteristics of both oceans and closed basin systems such that knowledge gained from their study can be used to gain insights for our coastal oceans. The effects of climate change have been especially pronounced in the Great Lakes where winter ice cover has declined by 71% over the past 4 decades. The decline is not constant; rather it is driven by high interannual variability combined with an increase in the frequency of years with low ice cover. The manifestations of declining ice cover have likely far-reaching effects on biogeochemical cycles and ecosystem functioning in lakes. Further, the ecological integrity of aquatic systems is intimately tied to the activities of microbial populations and consortia. Whereas we possess a baseline knowledge of microbial diversity in the Great Lakes, we know little about how these communities respond to the manifestations of climate change. This RAPID project to understand the effect of low ice extent on phytoplankton community structure and function in Lake Erie addressed manifestations of climate change as factors predicted to drive shifts in the phytoplankton community. The project exploited a nearly ice-free winter season on Lake Erie driven by a warm positive Arctic Oscillation in winter 2012 (Fig. 1a). Surveys conducted during high-ice years in 2010 and 2011 as well as follow-on activities in winter 2013 provided a comparison with 'normal' ice years (Fig. 1b). With this project, we showed that dramatic changes in annual ice cover were accompanied by equally dramatic shifts in phytoplankton community structure (Fig. 2). Expansive ice cover documented for Lake Erie in winters 2010 and 2011 supported ice-associated phytoplankton blooms dominated by physiologically robust, filamentous centric diatoms. By comparison, coincident with nearly ice-free conditions in winter 2012 were pronounced declines in microplankton (> 20 μm) chlorophyll a biomass. The phytoplankton community in winter 2013 was dominated by microplankton- sized filamentous diatoms, coincident with expansive ice cover and thus returning to the size structure of the 2010 and 2011 communities. Reduced size is recognized as a universal ecological response to global warming in aquatic systems although it usually marks a response to climate warming over multiple years, not a single season as shown here. Additional insights into microbial community dynamics were gleaned from short 16S rRNA tag Illumina sequencing which demonstrated that diatoms are the dominant taxa (phytoplankton + microbes) during winter in the lake regardless of surface ice cover (Fig. 3). Thus, whereas cell size of the phototrophic community was biased towards cells smaller than 20 µm, diatoms still persisted. Further, UniFrac analysis of iTag sequences showed clear separation of microbial communities related to presence or absence of ice cover. These changes are expected to have important consequences for Lake Erie's food web.
