High variability in carbon stable isotopes in the macroalga Ulva spp. and implications for eutrophication indicators.

Ulva spp. are among the most commonly encountered macroalgal species forming algal blooms and are responsible for many Green Tide events worldwide. Numerous adaptations have provided this genus with broad plasticity in carbon acquisition and metabolism, allowing rapid responses to nutrient inputs and sustained growth during the limiting conditions that arise during large bloom events. Because of this plasticity, the carbon isotopic composition of Ulva spp. may be highly variable. A compilation of 1,382 observations of Ulva δ¹³C signatures from literature sources and systematic surveys revealed an unusually wide range of values, much broader than what is typically reported for other marine macroalgae, with a bimodal distribution. This pattern suggests two distinct growth modes in Ulva spp.: one characteristic of non-limiting conditions, and another associated with bloom conditions where rapid growth and carbon limitation drive higher δ¹³C signatures. Analysis of environmental drivers in New England sites further showed that enriched Ulva δ¹³C values arise under the limiting conditions that develop within dense macroalgal canopies in eutrophic estuaries. Excess nutrient inputs stimulate large macroalgal accumulations, which then alter the surrounding physicochemical environment and shift isotopic signatures. These results confirm that Ulva δ¹³C is a valuable tool for assessing the trophic status of coastal environments.