Preface: paleolimnology and lake management

Abstract

Paterson AM, Köster D, Reavie ED, Whitmore TJ. 2020. Preface: paleolimnology and lake management. Lake Reserv Manage. 36:205–209.

Paleolimnology uses information preserved in lake, river, and wetland sediments to understand past environmental conditions. Paleolimnologists access and analyze records of environmental change that have been temporally and spatially integrated over decades to centuries. These data provide a powerful complement to monitoring programs or shorter term studies that are unable to evaluate predisturbance conditions. The present-day environment is a product of the natural geologic setting and past human influences, and environmental stressors affect lakes over long time periods. Consequently, lake managers have recognized the value of paleolimnology for assessing long-term impacts from environmental stressors, and for establishing management baselines or reference conditions. This special issue on paleolimnology and lake management explores 7 examples from lakes across North America that show the value of paleolimnology in providing a long-term perspective on environmental change.

Keywords:

• Climate change
• eutrophication
• lake sediments
• long-term data
• multiproxy
• stressors

Paleolimnology is a multidisciplinary science that uses information preserved in lake, river, and wetland sediments to understand and interpret past environmental conditions. Interest in paleolimnology increased in the mid 20th century as researchers used lake sediments to develop and test ideas about lake classification and ontogeny (Battarbee and Bennion 2011). Fueled by advances in coring techniques (Glew et al. 2001, Figure 1), dating methods for establishing core chronologies (Appleby 2001), and quantitative analyses (Birks et al. 2012), paleolimnologists pivoted in the late 1970s and early 1980s to applied topics about the impacts of human-caused stressors on aquatic ecosystems. Many of the approaches in use today were refined during the acid rain debate of the 1980s and early 1990s, as paleolimnological data were crucial for distinguishing human impacts from natural processes as drivers of pH change in acid-sensitive regions (Charles and Smol 1990, Cumming et al. 1992, Battarbee et al. 2010). New advances and techniques in paleolimnology continue to emerge as the field melds with other scientific specialities, such as molecular biology and ecotoxicology. These examples and others are described in detail in many excellent reviews on paleolimnology and its applications (e.g., Smol 2008, Whitmore and Riedinger-Whitmore 2014, Saulnier-Talbot 2016, Korosi et al. 2017).

Figure 1. Sediment cores collected from (a) Lake of the Woods, Ontario, using a Glew-type gravity corer, and (b) Mille Lacs, Minnesota, using a Livingstone-type piston corer. The photos were taken by Kathleen Rühland (Lake of the Woods) and Heidi Rantala (Mille Lacs).

Lake managers have long recognized the value of long-term data for assessing impacts from environmental stressors, and for establishing management baselines or reference conditions (Smol 2019). Environmental stressors affect lakes over decades to centuries, and current conditions reflect past human impacts (Renberg et al. 2009). Because monitoring programs are rarely long enough to understand natural variability in aquatic ecosystems, or to reveal conditions prior to significant human influence, information archived in lake sediments complements monitoring efforts and fills data gaps (Reavie 2020). As clearly articulated by Smol (2010), paleolimnology can be used to address key management questions, such as: (1) Has an aquatic ecosystem changed from its natural state? (2) If so, what are the drivers of this change? (3) What does restoration look like and is it achievable? Paleolimnology provides evidence-based answers to questions like these, and is useful when making decisions regarding management approaches (Sayer et al. 2012).

It is not uncommon for several paleolimnological papers to be published every year in Lake and Reservoir Management, and special sessions and keynote addresses on paleolimnology are regularly included in the North American Lake Management Society (NALMS) Annual Symposium. The first mention of paleolimnology in the NALMS literature appears to come from the 1984 Proceedings of the Fourth Annual Conference and International Symposium of Lake and Reservoir Management: Practical Applications, in a paper by Smeltzer and Swain (1985). In a study of 2 lakes in Vermont that show evidence of eutrophication, the authors demonstrated that paleolimnology is a valuable tool for lake diagnostic studies, and that “when historical limnological data are lacking on a lake, paleolimnology offers a means to evaluate the magnitude and timing of recent changes in water quality.” Since then, countless studies have demonstrated the value of paleolimnology to lake management, including the examples presented in this special issue.

Highlights from the special issue

This special issue on paleolimnology and lake management includes studies from across North America, from remote subarctic regions, to boreal and north-temperate lakes, and to warm-temperate ecosystems in southern Florida. Many of the studies presented here (e.g., Korosi et al. 2020, Nelligan et al. 2020, Simmatis et al. 2020, Whitmore et al. 2020) use multiple proxies (or environmental indicators) to interpret past environmental conditions. This is important because there are often several temporally overlapping drivers of ecosystem change. This can lead to multiple interpretations of past conditions, and without the benefit of a time machine, paleolimnologists must re-create the past from the information they collect in the present day. A multiproxy, multidisciplinary approach allows scientists and lake managers to tease apart this complexity by weighing supporting lines of evidence (Whitmore and Riedinger-Whitmore 2014). In addition, multidisciplinary approaches allow investigators to more effectively assess the nature of changes because aquatic ecosystem perturbation is a multidimensional phenomenon that involves many communities and key ecosystem functions (Davidson et al. 2018). For example, although management is often concerned about changes in nutrient loading to establish mitigation guidelines, eutrophication is not a univariate process, and mitigating nutrients alone will not necessarily restore a system. Multidisciplinary paleolimnological studies, therefore, provide a more complete assessment of the nature of changes that have occurred (Bennion et al. 2015).

A clear advantage of paleolimnology for lake management is the ability to characterize natural variability and baseline conditions prior to recent impacts. If predisturbance conditions are unknown, there is uncertainty when establishing restoration targets, and thus baseline or reference conditions can serve as useful management benchmarks (Jacques et al. 2020). These data can also be used to evaluate what is achievable or realistic from a management perspective. For example, Korosi et al. (2020) note that limnological changes in Lac de Gras in the Northwest Territories (Canada) occur prior to the onset of diamond mining operations in the region. These changes, consistent with climate change, may confound results from an aquatic effects monitoring program designed to assess mining-related impacts. By quantifying trajectories of change over decades, Korosi et al. (2020) provide meaningful data that can help distinguish mining from climate-related impacts.

In some cases, aquatic ecosystems have been dramatically altered from predisturbance baselines, and this has resulted in disruptions to natural hydrological and nutrient cycles. In an examination of 6 lakes in the Kissimmee Chain in south-central Florida, Whitmore et al. (2020) maintain that reestablishment of large natural fluctuations in water levels might provide a solution to some key management concerns. This may, for example, lower ongoing costs associated with active management, such as water drawdowns and sediment scraping. Hydrological restoration of these lakes would also reduce the downstream nutrient cascade that contributes to complex and costly restoration of Lake Okeechobee and the Florida Everglades.

Other important findings arise when paleolimnological analyses reveal that lakes are naturally acidic or naturally elevated in nutrients. Muskrat Lake in eastern Ontario, Canada, commonly experiences cyanobacterial blooms that are, in part, a result of eutrophication from land use activities. However, paleolimnological analyses also show that the lake was moderately enriched in nutrients before European settlement in the watershed (Simmatis et al. 2020), so management targets need to reflect the natural baseline condition of the lake. Attempting to mitigate nutrients to levels lower than natural conditions would not be cost-effective or practicable.

In lakes and reservoirs across the globe, warmer air temperatures are resulting in warmer surface water temperatures (O’Reilly et al. 2015), stronger thermal stratification (Woolway et al. 2019), shorter periods of ice cover (Yao et al. 2013), and shifts in algal species composition (Wagner and Adrian 2009). Precipitation is becoming more variable, and severe weather events (e.g., record-breaking storms, floods, droughts and heat waves) are increasing in frequency (Stockwell et al. 2020). While the effects of climate change on inland lakes are complex and difficult to predict, clear patterns are beginning to emerge, and climate is now considered an overriding driver of limnological change in many aquatic ecosystems. For example, in a discussion of the resilience of aquatic ecosystems in Yellowstone National Park, Chraïbi and Fritz (2020) report that diatom assemblages in 3 of 4 study lakes respond more to regional recent climate change, via its impact on lake physical structure, than to fish stocking. This illustrates the importance of considering multiple stressors that may act on different temporal and spatial scales. It also highlights the need for adaptive management frameworks that consider climate impacts that are neither static nor easily predicted. Similarly, Nelligan et al. (2020) and Simmatis et al. (2020) show that recent changes in north-temperate lakes are influenced strongly by recent warming, which may have exacerbated recent harmful algal blooms and negatively impacted coldwater fisheries habitat. With increased unpredictability in nutrient loading, water quality conditions, and algal bloom frequency and intensity in a warming world, a precautionary approach to management is advisable.

Fortunately, as argued by Reavie (2020), paleolimnology may provide an early warning of future degradation from environmental stressors, including climate change. This may be of particular use in remote or relatively undisturbed regions (Jacques et al. 2020), or when monitoring data are sparse. Moreover, when multiple stressors act in concert, paleolimnological data analyzed over decades to centuries can be used to tease apart their relative impacts on water quality and biological communities.

Paleolimnological data integrate information from airsheds, watersheds, and waterbodies, and provide continuous records of long-term environmental change. These retrospective data are crucial when the historical context of a waterbody is needed to inform decisions on aquatic management or policy. Finally, lake sediment can be archived and revisited in the future as new methods become available, or as new questions arise. These strengths and well-developed and proven methodologies have established paleolimnology as a useful and complementary tool in lake management. Coupled with new and emerging subdisciplines (e.g., paleo-ecotoxicology), paleolimnology offers a bright future that will continue to provide new insights for lake and resource managers.

Acknowledgements

The authors thank Andrea Smith and John Smol for providing helpful comments that improved this preface. We are grateful to the authors who contributed articles to the special issue on paleolimnology and lake management, and to the reviewers whose comments improved the quality of the papers.