On 04 March 2021, the world lost its freshwater colossus—a man who led the development of Limnology from an esoteric academic discipline in the 1950s to the keystone of environmental science 50 yr later. His masterful blend of natural history, ecosystem experimentation, biogeochemistry, and science advocacy fundamentally changed our world. It is unlikely we will see another who will so profoundly shape our understanding of inland waters as David William Schindler has done (Fig. 1). Born 03 August 1940, Dave grew up on and around lakes of western Minnesota where the tall grass Prairies meet the eastern deciduous forest, near the town of Barnesville, Minnesota. In fact, it was his love of lakes in forested landscapes, combined with the plain-spoken practicality of a prairie kid, that characterized much of Dave's life and scientific career. Eldest of four children, Schindler was an avid reader, outdoorsman, student, and athlete (football, wrestling), favoring biology under the tutelage of high school teacher, Rudolfs Malta. He initially enrolled in engineering and physics at University of Minnesota in 1959, but came under the mentorship of a 44-yr-old WWII veteran, Gabriel Comita, who had started a faculty position at North Dakota State University 5 yr earlier and needed someone to set up a bomb calorimeter in his lab and sample local lakes during the summer. While in Comita's lab, Dave read Charles Elton's Ecology of Invasions by Animals and Plants, Niko Tinbergen's Animal Behaviour, and G. Evelyn Hutchinson's then brand-new A Treatise on Limnology Vol 1: Geography, Physics and Chemistry. This trio foretold of Dave's next move—a Rhode's Scholarship to study at Oxford, initially with Tinbergen, then with Elton, whose field-based research appealed more to the young Ph.D. student. He also published his undergraduate work with Comita in Science! Schindler's Ph.D. on the flow of energy in lake food webs had three profound effects on his approach to research. First, it focused his mind on the study of entire ecosystems—their physics, chemistry, and biology. Second, it imbued a life-long preference for empirical and experimental approaches over modeling or theoretical exercises. Third, his informal daily discussions with other Oxford students and faculty (Tinbergen, Richard Dawkins) sharpened his analytical skills and made him a formidable debater. Each of these became hallmarks of Dave's 60-yr scientific career and highlights the profound influence of mentors and student colleagues in crafting one's path. In a life marked by purposeful if abstruse change, Dave's career started in 1966 at Trent University, a new school in rural Ontario, Canada, having turned down elite opportunities at Yale and University of Michigan. He simply preferred the boreal landscape. He also published his second paper, in Nature, on the use of liquid scintillation to measure rates of photosynthesis. However, having become interested in nutrient pollution and eutrophication, Dave was soon enticed to join the Freshwater Institute (FWI) in Winnipeg by Jack Vallentyne, head of the Eutrophication Section of the Fisheries Research Board of Canada (FRBC). Vallentyne was himself a champion of the environment and public education (latterly as “Johnny Biosphere”), having received his Ph.D. with Hutchinson, before taking on academic positions at Queen's and Cornell universities. Upon joining FRBC, Vallentyne wanted to set up an “experimental lakes area,” modeled in part on the 1950s whole-lake experiments of Wisconsin's Arthur Hasler and FWI's Director Wally Johnson. After initially recruiting some of the world's best international limnologists (e.g., Richard Vollenweider, Kazimierz Patalas, Mitsuru Sakamoto, et al.), Jack set his sights on Dave, convincing him in 1968 to lead what later became ELA, The Experimental Lakes Area. Both Vallentyne and Schindler were dismayed by the eutrophication of the Laurentian Great Lakes, as well as by the raging debate about its causes. On one hand, prior lake surveys showed clearly that the phosphorus (P) content of lakes was the strongest predictor of algal growth, whereas bottle-scale fertilization experiments, heavily supported by the soap industry, suggested that phytoplankton were rapidly limited by the supply of atmospheric CO2 after fertilization. Schindler and his team resolved the issue by surveying hundreds of undisturbed lakes in the ELA region near Kenora, Ontario, selecting the lake (Lake 227) with the lowest levels of dissolved inorganic carbon, and fertilizing it at rates equivalent to those seen in the lower Great Lakes. Within weeks, phytoplankton biomass increased 10-fold, favoring green algae and cyanobacteria. In contrast, diffusion of CO2 was only limiting for a short period at midday, and was rapidly compensated by the CO2 respired by microbes from terrestrial organic matter. However, the experiment did not produce the surface blooms of N2-fixing cyanobacteria seen in polluted waters, so Dave altered the nitrogen (N) inputs to Lake 227 to demonstrate that diazotrophic species only predominated at high P concentrations and low N : P ratios. Further fertilization experiments were conducted in shallow lakes, as well as the capstone study in Lake 226 where basins of the narrow lake were hydrologically separated by a curtain and differentially fertilized with C, N, and P (Fig. 2). The resulting bloom in the C + N + P treatment, but not the C + N basin, was captured by an aerial photograph and became one of the most iconic environmental images of all time. That photo, combined with the peerless science that accompanied it, and Dave's clear, direct and ever-present engagement with policy makers, resulted in a phosphate ban from detergents, as well as P removal from the wastewater of many inland cities. These experiments have protected freshwaters from extreme nutrient degradation for over half a century. Despite these successes, it was not smooth sailing for ELA. By the early 1970s, the FRBC was disbanded and the Freshwater Institute and ELA were moved eventually to the Canadian Department of Fisheries and Oceans, a unit with a fish-habitat mandate that was less supportive of ecosystem experiments, particularly within a charged political arena. Recognizing the need to adapt, Dave started a series of whole-lake acidification experiments to identify the mechanisms by which acidic precipitation damaged lakes and their food webs, particularly the emblematic lake trout. As with the eutrophication work, the ELA team started with a single basin acidification experiment (Lake 223) before progressing to a split-basin study (Lake 302) to contrast effects of H2SO4 and HNO3. Again, a photo of emaciated lake trout, combined with numerous publications in key aquatic journals (Canadian Journal of Fisheries and Aquatic Sciences, CJFAS; Limnology and Oceanography) and high-profile papers in Science, provided essential evidence to convince policy makers of the urgency to reduce industrial air pollution. Even so, science may not have been enough to effect change without Dave's dogged and effective promotion of scientific evidence, through tireless public presentations, legislative hearings, media interviews, and engagements with policy makers. The culmination of this work was a series of changes to key international laws (1990 Clean Air Act, 1991 Acid Rain Treaty) whose effects continue to protect the environment today. As the 1980s closed, increasing federal government restrictions, combined with limited employment prospects for his partner Dr. Suzanne Bayley, herself a world-class wetlands scientist, forced Dave to do the unthinkable—leave ELA. In 1989, he and Bayley accepted professorships at University of Alberta, thereby opening the next chapter in his remarkable life. Several strands of Dave's life combined to make his time at ELA so impactful. As noted above, Dave's farming background made him immensely practical. When faced with obstacles, he simply removed them, whether it was by developing better field methods to measure primary production or capture zooplankton (Schindler-Patalas trap) or by resolving public and scientific controversy (whole-lake experiments). Second, Dave's prairie upbringing led to a directness and simplicity of communication that was well suited for public outreach and government engagement. Third, Dave learned to argue with the full force of evidence. Blessed with a terrific memory and deep devotion to learning (everyone has a story of Dave reading Nature or Science while hunting, fishing, training his dogs, etc.), Schindler thrived at the Freshwater Institute where his superb colleagues were encouraged to be merciless in their critiques of manuscripts. As Vallentyne noted, “if you think your friends' comments are bad, wait until your enemies get it.” Critically, Schindler was never satisfied with short-term, singular, or superficial analyses to support his position. Instead, scientific topics at ELA were addressed through multiple or sequential experiments, often at different scales, each refining knowledge until a more comprehensive understanding had been reached. Usually, these early findings were published in Journal of the Fisheries Research Board of Canada (JFRB; later CJFAS), not because it was the most prestigious outlet, but because, in the pre-Internet era, JFRB was the worlds' most widely distributed freshwater journal. Dave wanted his science to be seen. Schindler's transition back to university life did not end his involvement at ELA. Instead, after 21 yr of monitoring, effects of slowly developing phenomena such as climate change and biodiversity loss were becoming apparent, so Dave and his former ELA colleagues pivoted again to address these now critical issues (Fig. 3). Even 30 yr after his departure, Dave's presence is felt throughout ELA, notes recent director Michael Paterson. In Alberta, Dave started what would become a 20+ yr long-term ecological research program on the effects of introduced exotic fishes on mountain lakes, perhaps reflecting his lifelong admiration for Elton's book (Fig. 4). At the same time, he expanded his research activities to include the pollution ecology of northern rivers, protection of the Boreal Forest ecozone, effects of Alberta tar sands development, atmospheric deposition of mercury, agrochemicals and industrial pollutants, and protection of Indigenous Treaty rights. However, instead of conducting this work with his governmental colleagues and elite international researchers, Dave's ideas became the incubator for generations of student and early career scholars, most of whom have continued freshwater and environmental research at dozens of universities and agencies. Transition to academe also had its challenges. University politics, small research grants, inaccessible local leadership, and, of course, teaching were new to Dave's operating model. In typical Schindler fashion, Dave's solution was to do the stuff he liked, the way he liked it, and to bin the rest. The definition of a rugged outdoorsman, Dave quickly found like-minded people and built a network that allowed him to conduct whole lake experiments in Banff National Park—a nearly impossible feat at the time. In this case, he proved that biotic impoverishment from nearly a century of fish stocking could be reversed by removing non-native fish and replenishing invertebrates from fishless lakes. The image of billions of bright red copepods being hauled over the Rocky Mountains in a helicopter cannot be easily forgotten. Schindler also became engrossed with the diversity of mountain lakes, leading him to study the role of dissolved organic matter, UV radiation, and trophic cascades in these transparent alpine waters. Research by him and his students on organochlorine contamination of the snowpacks and glaciers in alpine and high-latitude lakes demonstrated how past atmospheric pollution left a legacy of environmental contamination in cold regions whose damage was being unleashed by atmospheric warming and meltwater transport (Fig. 5). Life in Alberta also allowed Dave and his family to fully embrace their passion for sled-dog racing, eventually building up a 90-dog kennel and winning multiple championships. One of the most terrifying questions that his then students could be asked was the seemingly innocuous, “can you feed the dogs this weekend?” Sled-dog racing allowed Dave to regularly reconnect with the land, while also clearing his mind to think the “big thoughts” that elude many of us. The 21st century saw Dave's environmental advocacy increase markedly—a fact barely credible given his career to that point. Dave often remarked that he could have published twice as many scientific papers (350 to date) if he had forgone public presentations and popular press writings, but that he felt that it was the obligation of scientists to bring their work to the public. Through his numerous awards (Stockholm Water Prize; Volvo, Tyler, and Rachel Carson awards; all major ASLO and SIL awards; national academy memberships; Order of Canada; 15 honorary doctorates), Dave became the public face of environmental science in Canada and around the world, lending him the authority to credibly challenge regional and national governments, admonish industry, develop effective environmental policy, and improve protection of his beloved forest and its lakes. Still there were frustrations. Dave felt profoundly that freshwaters were dying from a 1000 small cuts, propagated by unending human population growth and resource consumption. He saw the rise in nonpoint source eutrophication and was frustrated that the public, government, and some scientists had failed to heed the lessons of the past. This recognition of the importance of history has imbued much of his work—from his deep reading of older literature and use of paleolimnology, to his advice that students, “talk to the old people.” However, in typical Dave fashion, he merely redoubled his efforts and focused more recent papers on syntheses of climate change impacts, eutrophication, aquatic biodiversity, prairies ecosystems, and, of course, the Boreal Ecozone. Dave died surrounded by family. In the end, a heart infection got him—medically the result of broken ribs, but metaphorically because of a broken heart for the environment. As with everything, he fought right to the end, confident in himself and his vision, while still recognizing the enormity of what needed to be done. He will be missed by all who knew him. A passionate family man, inspirational mentor, quiet introvert, fierce combatant, stubborn farm boy, and peerless scientist. Dave. In the end, one wonders what Dave's career might have been like had he joined G. Evelyn Hutchinson at Yale rather than taking a teaching position at a small, newly formed university. Did we lose the “next G.E. Hutchinson?” We think not. Instead, we gained the first D.W. Schindler—the scientist who made limnology matter. Peter R. Leavitt, University of Regina Karen A. Kidd, McMaster University Rolf D. Vinebrooke, University of Alberta Marguerite A. Xenopoulos, Trent University on behalf of his former students and postdocs.

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