The complexity of defining autumn in limnology

In part, autumn research in limnology is hindered by the lack of consistency amongst definitions, which complicates direct comparisons among studies and may not capture key limnological conditions. Autumn definitions primarily fall into two categories: astronomical seasons and meteorological forcing. Astronomically, autumn begins at the autumn equinox and ends at the winter solstice. A meteorological focus on autumn is defined as the period between September 1st to November 30th in the Northern Hemisphere and March 1st to May 31st in the Southern Hemisphere [5]. This standardization ensures comparability across studies, particularly for climate analysis [5]. However, within limnology, the interest in autumn lies in the change in a lake’s thermal structure from stable stratification to a period of turnover prior to winter ice cover, which is not always appropriately captured by astronomical or meteorological definitions.

Limnological definitions of autumn vary from single-day to multi-day events. Definitions that include single-day events, such as the onset of an isotherm (e.g., consistent temperature throughout the water column, specifically the date surface-to-bottom water temperature differences are less than 2°C) [6], provide a clear and quantifiable event that allows for direct interannual comparisons and trend analyses over time. However, these definitions treat autumn as a single point, which does not facilitate the understanding of the gradual nature of autumn as a period. In contrast, multi-day definitions (e.g., the duration from breakdown of stratification to ice formation) in lakes capture the entire transition, offering a more ecologically relevant period for assessing lake ecosystem changes [7]. Defining autumn with a beginning and end more accurately represents ecological phenomena (e.g., leaf senescence) [3], and introduces variability, as the duration of this period may change significantly between years and locations. The challenge limnologists face is to identify which definition is the most relevant for a given study and which definition permits comparisons across individual studies. For example, do we favor a simple definition that remains consistent through time or a complex and data-intensive definition that targets a specific ecosystem of interest?

The importance of autumn for limnology

Despite being a critical transitional period, autumn has historically been less well represented in the published literature and sampling events (Fig 1). We conducted a structured literature survey in Web of Science to assess the number of seasonal limnological studies published between 2000 and 2024. Annually, a maximum of 10% of limnological studies investigated focus on autumn (Fig 1a), and this rate has only marginally increased since 2000 (Fig 1b). Furthermore, publicly available limnological data shows diminished sampling following spring and summer, implying limited capacity to synthesize existing autumnal data [8] (e.g., secchi depth; Fig 1c). Among all seasons, summer dominates research, likely because peak biological productivity occurs in summer. Consequently, researchers often prioritize spring monitoring to anticipate summer conditions, after which sampling effort substantially declines. Similar levels of attention have not yet been paid to autumn as a critical transition season. Unlike spring, which often has sudden and apparent changes (e.g., ice breakup and spring leafout), autumn is characterized by gradual shifts in phenology, increasing the difficulty to identify key events [3].

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Fig 1. Four Web of Science searches for titles and abstracts were conducted using the search string (lake* OR reservoir* OR pond) AND season (winter, autumn, spring, or summer) for studies published between 2000 and 2024.

Search results for: A) the proportion of total limnological studies published in each year focusing on spring, summer, autumn, and winter, and B) the cumulative number of publications for each season. To illustrate that limnological data collection is uncommon relative to summer and spring, we highlight: C) the number of secchi disk samples collected in the United States by month based on data available in the Water Quality Portal (De La Torre et al. 2025). Together, these results imply that not only is autumn less well studied in the limnological literature, but also publicly available data necessary to synthesize autumnal change are lacking.

https://doi.org/10.1371/journal.pclm.0000648.g001

The study of autumn is becoming increasingly important as global environmental changes accelerate. Climate change contributes to shorter winters, longer summers, stronger and longer periods of stratification during the open-water season, and altered mixing regimes [9]. While autumn climate may not be changing as rapidly as other seasons [10], subtle variations in autumn climate result in measurable shifts in autumn limnological conditions. Furthermore, autumn climate plays a critical role in winter conditions such as the timing of ice formation [11], and strong autumn storms can enhance primary production and increase chlorophyll concentrations under the ice [12], highlighting the importance of autumn conditions to under-ice ecological processes. Biological communities may also be impacted by seasonal changes because zooplankton rely on autumn feeding for winter survival, and the resuspension of dormant stages during this period can shape seasonal population dynamics [13]. A greater emphasis on investigating autumnal processes would highlight how lake ecosystems are shaped in autumn and how long-term changes may influence cross-seasonal dynamics.

Opportunities for future directions

Cross-seasonal interactions shape ecosystem function, yet the role of transitional seasons (i.e., spring and autumn) remains poorly understood. Future research in limnology could address these gaps by placing greater emphasis on interseasonal dynamics to recognize autumn as a critical, but understudied, component of ecological memory. Given that ecosystem functions progress over extended periods rather than discrete moments [14], understanding how autumn dynamics shape winter conditions and carry over to subsequent seasons is essential for capturing the full complexity of ecosystem processes. Establishing reference points and baseline conditions for autumn is essential to understanding long-term changes because future shifts may be more pronounced than contemporary observations. Moreover, establishing a baseline requires typifying features of autumns across a range of biomes. While understanding of autumnal limnology is already limited in otherwise well-studied northern temperate regions [15], understanding of limnological autumn dynamics is likely even more limited in tropical and non-freezing lakes. In these regions, seasonal transitions differ fundamentally from those in temperate systems, thereby restricting the ability to generalize seasonal patterns across different lake types. At present, the lack of autumn research leaves significant knowledge gaps that limit the ability to understand year-round limnological dynamics and predict future ecosystem changes. Addressing these gaps would ultimately enhance the ability to assess how lakes will respond to global environmental change.

Acknowledgments

We thank Anne Timm, Wilson Salls, and Ryan Riggs for their comments on earlier version of our manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.