Michael Frederick Meyer , Benjamin M Kraemer, Carolina C Barbosa, Davi GF Cunha, Walter K Dodds, Stephanie E Hampton, César Ordóñez, Rachel M Pilla, Amina I Pollard, Joshua A Culpepper, Alexander K Fremier, Tyler V King, Robert Ladwig, Dina M Leech, Shin-ichiro S Matsuzaki, Isabella A Oleksy, Simon Topp, R Iestyn Woolway, Ludmila S Brighenti, Kate C Fickas, Brian P Lanouette, Jianning Ren , Mortimer Werther, Xiao Yang

For over a century, ecologists have used the concept of trophic state (TS) to characterize an aquatic ecosystem’s biological productivity. Because measuring productivity can be challenging within an ecosystem and across landscapes, multiple TS classification schemes, each relying on a variety of proxies for productivity, have emerged to meet use-specific needs. Most commonly, chlorophyll a, phosphorus, and Secchi depth are used to discriminate TS based on autotrophic production, whereas phosphorus, dissolved organic carbon, and true color are used to discriminate TS based on autotrophic and heterotrophic production. Both classification schemes aim to characterize an ecosystem’s function broadly, but the relative emphasis on heterotrophic and autotrophic processes masks nuances in how an ecosystem’s function is understood. Moreover, differing classification schemes can create inconsistent understanding and can lead to narrowed interpretation of ecosystem integrity. For example, the U.S. Clean Water Act focuses exclusively on threats to autotrophic water quality, framed in terms of eutrophication in response to nutrient loading. This usage lacks information about non-algal threats to water quality, such as dystrophication in response to dissolved organic carbon loading. Consequently, the TS classification schemes used to identify eutrophication and dystrophication may refer to ecosystems similarly (e.g., oligotrophic and eutrophic), yet these categories are derived from different proxies. These inconsistencies in TS classification schemes may be compounded when interdisciplinary projects employ varied TS frameworks. Even with these shortcomings, TS can still be used to distill information on complex aquatic ecosystem function into a set of generalizable expectations, which can then be used to contextualize, compare, and project ecosystems across scales. However, to emphasize the consequences of using multiple TS classification schemes, we present three scenarios for which an improved understanding of the TS concept advances freshwater research, management efforts, and interdisciplinary collaboration. To increase clarity in TS, the aquatic sciences could benefit from including information about the proxy variables as well as the spatiotemporal domains used to classify TS. As the field of aquatic sciences expands and climatic irregularity increases, we highlight the importance of re-evaluating fundamental concepts, such as TS, to ensure their compatibility with evolving science.