Rachel A. Hovel

Autumn is an important transition time for seasonally stratified lakes where summertime stratification shifts to cooler, mixed periods. While ponds can also experience prolonged summertime stratification, their relatively low water volume accelerates mixing, suggesting they could have distinctly different autumn mixing dynamics compared to lakes. This study examined the mixing dynamics of mostly temperate, small, shallow, and summertime stratified waterbodies. We show that, in contrast to most lakes, ponds have long and distinctive stratified to mixing transition seasons, characterized by repeated mixing and restratification, which can occur for days to months, before sustained isothermal conditions. Transition season mixing starts earlier in shallower and low‐nutrient ponds, with most ponds having at least one continuous week of restratification in autumn. Ultimately, describing pond transition seasons provides an important update to established turnover paradigms for lakes and indicates that these periods have the potential for high biogeochemical and ecological activity.

Seasonality in environmental conditions plays a fundamental role in shaping lake ecosystems. However, patterns of seasonality vary worldwide, and these patterns are shifting over time amid global change. Thus, it is increasingly important to evaluate how seasons and seasonality are represented in lake ecosystem research. Here, we used a literature review and global data analysis to synthesize approaches for conceptualizing seasons and seasonality in lakes. We found that a wide range of criteria are used to delineate discrete seasons in published literature, including fixed dates (e.g., months, solstice/equinox), environmental thresholds (e.g., temperature and precipitation cutoffs), and lake‐specific indicators (e.g., ice cover, plankton phenology). Analyzing data from lakes worldwide, we found that using different criteria to define the same season resulted in divergent interpretations of ecosystem states. Based on our synthesis, we offer recommendations for how to incorporate seasonality into lake research and communications amid global change.

Research on climate change refugia in aquatic systems frequently emphasizes cold‐water habitats and resistance to increasing temperature. Higher‐elevation locations are often identified as important for preserving conditions suitable for cold‐water organisms or communities. However, this concept remains understudied in lentic compared to lotic systems, even as lakes variably experience pronounced climate‐related impacts including ice loss and higher temperatures. Lake responses to climate depend on characteristics such as landscape position, but the role of elevation is not well‐documented and associated biological responses are unclear. Here, we describe spring ice and thermal dynamics in small remote lakes in Maine, USA, ranging from 76 to 955 m above sea level, and how temperature influences zooplankton phenology and community composition. Ice persisted on average 8 days longer in high‐elevation (>500 m) lakes and, after ice breakup, high‐elevation lakes warmed faster (0.3°C/day) than low‐elevation lakes (0.2°C/day), and reached maximum temperatures 45 days earlier on average. Zooplankton phenology was driven by water temperature, but zooplankton taxa varied in response to lake conditions, shaping different zooplankton assemblages in high‐ and low‐elevation sites. This suggests that refugial high‐elevation lakes with prolonged ice and cold spring conditions could present an important regional conservation priority.

As the impacts of anthropogenic climate change increase, conservation of climate‐change refugia has become a key strategy for effective environmental stewardship. Over the last 5 years, the field of climate‐change refugia conservation has made exciting advances, shifting from concepts and theory to refugia mapping and implementation. However, few studies have advanced to action on the ground; while 84% of studies identified and mapped refugia, only 4% involved implementing management action. Moreover, taxonomic and geographic gaps remain, with most studies focused on terrestrial plants and vertebrates in Europe and North America. Here, we outline impediments to implementation following the steps of the Climate‐Change Refugia Conservation Cycle. Based on a systematic literature review, we elucidate advances and obstacles with examples from a diversity of systems and sectors from across the world and highlight emerging work bridging the gap between research and implementation.

End of summer stratification and onset of ice‐on are shifting later across the mid‐latitudes, but the lake‐specific and climatic drivers of lake thermal structure in autumn remain unclear. We used high‐frequency water temperature data (2007–2021) from nine sub‐alpine (>600 m ASL) lakes in the Northeastern US to investigate the drivers and coherence of ecologically important autumn processes. The end of summer stratification was highly variable across lakes and mediated by DOC and lake depth. The timing of end of stratification affected the onset of continuous autumn mixing; however, mixing was decoupled from ice‐on. Ice‐on was highly coherent and driven by air temperature. Extreme weather events with high winds, heavy precipitation, and anomalous air temperatures drove early onset of mixing, re‐stratification events, and freeze‐melt events. The variability of thermal structure during autumn demonstrates that climate change will not uniformly shift the start and end of autumn season later in the year.

Northern Indigenous communities are experiencing rapid climate change and disrupted seasonal transitions. The Teetł'it Gwich'in use a five‐season calendar to measure the year, indicating the timing of seasonal events and associated cultural practices. From trapping in the spring, to fishing in the summer and fall, and hunting in the fall and winter, the Gwich'in have moved upon the land with the changing seasons. However, disrupted seasonal synchrony can disconnect cultural practices from suitable conditions, creating risks to self and culture. With warming temperatures, communities have observed slower river freeze‐up in the fall and faster spring thaw, which has impacted the timing of when fishers can safely set their nets under river ice. Historically, freeze‐up occurred in October, providing several weeks when fishers could set nets under ice while łuk dagaii (broad whitefish, Coregonus nasus) traveled downriver. Today, freeze‐up often begins in November, and fishing during the łuk dagaii migration requires setting nets while the ice is thinner and the river is not completely frozen. This presents risks to individuals working to maintain a fundamental cultural practice. Here, Arlyn Charlie, a Teetł'it Gwich'in artist whose career focuses on culture and language, uses personal narrative to explore impacts of climate change on Gwich'in culture. Arlyn notes how these changes are making the traditional seasonal calendar unreliable, and explores how changing patterns among animals and the landscape no longer provide consistent, safe harvesting conditions. With a growing risk of working on thin ice, ongoing cultural practices are threatened.

Many small-scale fisheries are remote in nature, making data collection logistically difficult. Thus, there is a need for accessible solutions that address the data gaps present in these fisheries. One possible solution is to incorporate photography into community- or harvest-based monitoring frameworks and employ these images to estimate biological data. Here, we test this approach using łuk dagaii, or broad whitefish, Coregonus nasus (Pallus, 1776) in the Gwich'in Settlement Area, a remote region in the Mackenzie River system in Canada's Northwest Territories. We used photographs taken by Gwich'in collaborators using a simple, standardized set-up to ask the question: how accurately can weight be estimated from a photo? Using random forest models based on morphometric photograph measurements as well as season and location of harvest, we predicted broad whitefish weight to within 13% of true weight (257 g, for fish weighing an average of 2036 g). The model predictions were well distributed in their residuals for most fish, though we discuss biases at low and high weights. Image analysis is a simple, low cost, and accessible method that may contribute to ongoing, community/harvest-based fishery data collection where fish length (measured) and weight (predicted) can be tracked through time.

Article impact statement: Indigenous land users’ experiences of global ecological change can help researchers understand variable climate impacts and adaptations. This article is protected by copyright. All rights reserved