Intraspecific trait variability is a key feature underlying the association between community‐weighted mean and abiotic gradients

Functional plant traits that influence fitness or performance provide a powerful tool for understanding and predicting how plants interact with their environment, respond to ecological changes, and contribute to ecosystem processes. To this day, most trait‐based ecological studies in both terrestrial and aquatic ecosystems have assumed that species turnover is the main driver of trait variation, by using a single species‐specific value for each trait. Evidence is growing that intraspecific trait variation (ITV) is non‐negligible, but we are lacking a comprehensive framework on the mechanisms that shape the extent of ITV at the community level and the consequences of ITV for indices of plant community structure. This is a major bottleneck for functional ecology when considering ecosystems characterized by relatively low species richness, which typically exhibit higher variation within traits of a species (ITV) at local scales. We conducted a field survey in August–September 2023, where we sampled 20 soft bottom sites dominated by different aquatic vascular plants, along a strong (50 km) gradient of environmental variables (i.e. exposure and salinity) in the northern Baltic Sea. Sampled traits included specific leaf area, maximum height, root depth, and root‐to‐shoot ratio, which capture key variation in plant life‐history strategies. ITV explained a large proportion of trait variability in CWM patterns; however, the importance of ITV was trait specific and potentially modulated by abiotic context. ITV's response to environmental stress was highly variable across sites, but for most singular traits, the extent of ITV was positively correlated with environmental stress. That the extent of ITV for multivariate phenotypes across plant organs did not respond to environmental stress, could imply trait integration. Our study illustrates that the assumption of negligible ITV and species turnover being the main driver of trait variation, may not hold true for ecosystems characterized by low species richness and high micro‐environmental heterogeneity. This highlights that ITV is not trivial and that it should be taken into consideration if one wants to make decisions based on trait‐based studies in these ecosystems.