Seasonal trait variation and functional niche overlap of macrophyte growth forms

Macrophytes provide many ecosystem processes and functions which support freshwater ecosystem services, and the ecological role of a macrophyte is related to its growth form (emergent, free-floating, floating rooted, submerged). Differences between growth forms and the relationships between ecosystem functioning and environmental conditions can be described by functional traits. Seasonal variation in functional trait expression can lead to alterations in ecosystem functioning. As such, when inferring trait-environment relationships, a species' functional niche should capture this temporal variation. However, it is unknown how functional traits in macrophytes vary seasonally, and the importance of between-growth form variation. Using hypervolume analysis and linear mixed effect modelling, we demonstrate that seasonal trait variation within-growth forms is stronger than between-growth form variation over time. We found that emergent macrophytes have significantly (p <0.01) higher specific leaf area in June compared to September. Whereas leaf nutrients (total nitrogen and phosphorus), are significantly higher early in the growing season for emergent, floating rooted, and submerged plants (p < 0.05). We show that the large functional niche of submerged macrophytes is shaped by seasonal variation, and that the functional niches of all macrophyte growth forms overlap, suggesting redundancies in the maintenance of ecosystem functions. Together, this study demonstrates the influence of seasonal variation on macrophyte functional traits. Thus, seasonality is relevant to our understanding of aquatic ecosystem functioning and must be considered when determining the ecological role of macrophytes across a season. This study provides rationale for further examinations of between-growth form redundancies in the ecological role of macrophytes.