Synergistic impact of climate induced acidification, temperature, total alkalinity and nutrients on cyanobacteria HABs in the Great Lakes

Synergistic impact of climate induced acidification, temperature, and nutrients on cyanobacteria HABs in the Great Lakes The global oceans have absorbed the increase in atmospheric carbon dioxide (CO2) concentrations, leading to chemical changes in the dissolved inorganic carbonate (DIC = CO2 + H2CO3 + HCO3-+ CO32-) system and causing a decline in oceanic pH, known as ocean acidification. Over the past 30 years, measurements of the carbonate system associated with ocean acidification have been a high priority and have indicated a global decline in pH. Though the uptake and chemical processes are similar, freshwater systems have received less focus. It is projected that large freshwater systems are expected to experience a similar decline in pH leading to acidification, though presently there are only a handful of monitoring programs capable of detecting long-term trends in pH across freshwater systems. Other large lake systems worldwide with similar geology as Lakes Michigan, Huron, and Erie have seen a decrease in pH of 0.3 units over the past 40 years. Presently, it is unclear if the Great Lakes have experienced a similar decrease in pH due to lack of long-term, consistent inorganic carbonate chemistry database. While the U.S. EPA manages a long-term water quality monitoring program, the methods employed do not adequately address trends in pH as they use less accurate potentiometric measurements and lack the spatial and temporal sampling frequency needed to assess long-term seasonal and geographic variation and trends. The Laurentian Great Lakes support an economy exceeding $60 billion annually, and face numerous challenges including shifting land use, prevalent invasive species, harmful algal blooms (HABs), hypoxia, and loss of native populations. Intertwined through each of these threats is the multi- faceted impact of climate change. Though physically interconnected, the Great Lakes encompass five distinct freshwater systems in terms of biogeochemistry and productivity. All the Great Lakes are projected to experience pH declines at rates similar to that of the oceans. Biogeochemistry of the five-lake system varies significantly, suggesting that the impact of acidification will also vary between systems. Lake Superior is the most well-studied Great Lakes system for carbonate chemistry. While research has shown that Lake Superior’s alkalinity, temperature, and subsequently pH has increased over the last 20 years, it is geologically distinct from the other basins, consisting of a bedrock composed of granites and gneisses, as opposed to the other Great Lakes whose basins are carbonate based (sedimentary limestones, dolomites, and sandstones). In addition to shifts in pH, the rise in aquatic temperatures is impacting the rate of chemical processes, precipitation, and lake stability. The synergetic impacts of acidification, warming, and shifts in nutrient availability due to changes in precipitation and stratification offer a new stress to aquatic systems and may lead to changes in phytoplankton community dynamics and promote new areas of concerns for cyanobacteria HABS (cHABs) within the Great Lakes. While cHABs have been extensively studied over the last decade, the Great Lakes have been widely under sampled for acidification and little effort has been invested in monitoring or understanding potential impacts of acidification on the ecosystem. Acidification of aquatic systems leads to an increase in dissolved inorganic carbon (Ci) availability, which reduces stress and the energy costs of carbon uptake for many photosynthetic organisms, including cyanobacteria. Photosynthetic organisms transport Ci into the cell through an assortment of enzymes and cell membrane transporters, known collectively as a carbon concentrating mechanism (CCM). Variation in the use and effectiveness of the CCM in algae have been indicated at the group, genera, species, and strain levels. In Lake Erie, initiation of toxic Microcystis blooms is typically characterized by slightly acidic conditions, but as the bloom progresses Ci availability drops and the pH increases, leading to potential Ci limitation. A recent analysis of Microcystis ecophysiology, by members of this research group, noted that strains with the potential to produce microcystin have a CCM makeup which favors high Ci conditions, suggesting Ci availability is connected to toxic cyanobacteria succession within the Great Lakes. Warming associated with the increase in atmospheric CO2 is expected to lengthen and strengthen lake stratification, leading to increased lake stability and conditions that favor phytoplankton species that can take advantage of vertical migration in the water column, such as cyanobacteria. Preliminary data from our group indicates a toxic Lake Erie strain of Microcystis has a significantly higher growth rate at elevated pCO2 conditions and temperatures greater than 24?, suggesting both acidification and higher temperatures promote cyanobacterial growth. Changes in precipitation will also influence nutrient conditions in the lake, changing the nutrient type and stoichiometry, which are connected to cyanobacteria growth and toxin production. These changes will have a direct impact on the Canada-U.S. Great Lakes Water Quality Agreement to reduce cyanobacteria blooms. There is an urgent need to understand acidification and other factors associated with climate change in the Great Lakes region and their impact on existing and emerging cHAB bloom formation and succession. Contributions of the University of Kentucky (UK) group will focus on the analyses of short (~1 m) sediment cores from three focal lakes - Lakes Erie, Huron and Superior (5 cores per lake = 15 cores max total). We will characterize the sedimentology of these cores (microscopy, grain size), and measure a suite of short-lived fallout radionuclides (137Cs, 210Pb) that will allow us to date each core at high resolution. From these, and supporting core data provided by the National Lacustrine Core Facility (LacCore) at the University of Minnesota, and our project collaborators, we will develop a high-resolution core chronology, determine quantitative estimates of lake sediment accumulation rates, and determine the fluxes (burial rates) and inventories (standing stocks) of particulate organic carbon at these settings. We will also participate in student training, outreach to the public and relevant stakeholders, and the broad dissemination of these results at professional meetings and via the peer-reviewed literature.