Aquatic macrophytes provide numerous functions in lotic ecosystems. For instance, plants that grow within the channel and along the banks are known to mediate nutrient and sediment transport from the land into the watercourse (Valkama et al., 2019; Walton et al., 2020), while at the same time they stabilize the channel and the banks preventing erosion. Most importantly, aquatic plants can provide foraging and reproduction habitats for fish, amphibians and invertebrates (Lind et al., 2019; Cole et al., 2020). They can also influence the physical, chemical and flow characteristics within the channel (Gurnell, 2015; Preiner et al., 2020), which in turn may affect fish and invertebrate communities.

Because of their importance for stream and riverine ecosystems, aquatic macrophytes have been widely used as indicators of ecosystem health and ecological integrity (Aguiar et al., 2014; Rodrigues et al., 2019; Szoszkiewicz et al., 2020). Aquatic macrophytes are one of the four biological quality elements that are used for the assessment of ecological status of streams and rivers of Europe, following the implementation of the Water Framework Directive 2000/60, and several national assessment approaches have been developed by the EU Member States (Birk and Willby, 2010). Most ecological assessment schemes consider the composition of aquatic macrophytic communities (Szoszkiewicz et al., 2020; Stefanidis et al., 2022) since the increased occurrence and abundance of certain plants is related with environmental factors that indicate anthropogenic degradation of aquatic ecosystems (e.g. eutrophication and hydromorphological alteration) (Szoszkiewicz et al., 2014; Stefanidis and Papastergiadou, 2019; Manolaki et al., 2020; Stefanidis et al., 2021). However, there are studies that have shown that the responses of macrophytes to environmental gradients can be complex and difficult to decipher (Steffen et al., 2014; O’Hare et al., 2018; Son et al., 2018; Gyosheva et al., 2020). Thus, freshwater ecologists have shown increased interest in studying multiple facets of aquatic biodiversity, including aquatic macrophytes (Fu et al., 2014a; Schneider et al., 2015; Alahuhta et al., 2017; Elo et al., 2018; Son et al., 2018; Stefanidis et al., 2019).

The functional diversity which Tilman (2001) defined as ‘those components of biodiversity that influence how an ecosystem operates or functions’, has emerged as a facet of biodiversity and a step beyond species richness. It has become a powerful tool to link community composition to ecosystem properties and then to ecosystem services by quantifying the value and range of functional characteristics and thus ecosystem functioning (Díaz et al., 2007). In addition, the impact of the global loss of biodiversity is increasingly attributed to the loss of functional rather than taxonomic groups (Bellwood et al., 2002). Further, species are not equal in their effects on ecosystem functioning since their functional traits matter to ecosystem processes (Mouchet et al., 2010). The study of trait distributions can be used as a more powerful conceptual model for understanding broad-scale patterns in assemblage structure since organisms with similar traits will share similar niche requirements and will select the same habitat (Olden et al., 2010).

Besides species richness (alpha diversity) and changes in species composition among communities (beta diversity), investigating patterns of functional alpha (α) and beta (β) diversity may provide invaluable information and a better understanding on how environmental gradients affect aquatic biodiversity processes (Zhang et al., 2018; Wang et al., 2021). Considering that assembly processes influencing natural communities may differ depending on the spatial scale considered, separating functional diversity in within-community (α) and among-community (β) components will improve the detection of all processes influencing community assembly. Furthermore, trait-based approaches are more likely to indicate an early response to environmental change than community-based approaches because a change in the functional structure is easier to detect than a change to community composition (Degen et al., 2018). In addition, such approaches can be applied to all species regardless of geographic region and location (Dolédec et al., 2006).

Previous studies have linked morphological and life history traits of aquatic plants with ecological and biogeochemical processes showing the response of aquatic plant functional structure to eutrophication and other environmental changes (Fu et al., 2014b; Baattrup-Pedersen et al., 2016; Stefanidis and Papastergiadou, 2019). Therefore, exploring the patterns of functional trait composition in relation to environmental change is crucial for a better understanding of the response of aquatic communities to pressure and identifying key trait characteristics that can serve as indicators of anthropogenic pressures. Although there is a general consensus that taxonomic diversity is shaped by environmental filters, including anthropogenic changes (Bornette and Puijalon, 2011; Dybkjaer et al., 2012), there are relatively few studies that have explored the patterns of functional α- and β- diversity of aquatic macrophytes in lotic ecosystems.

The main objective of this study is to investigate the patterns of functional alpha and beta diversity of aquatic macrophytic assemblages in Greek lotic ecosystems and to assess whether these patterns are subject to environmental change. Our hypotheses are that a) indices of a- functional diversity decline with increased levels of nutrient pollution and degraded water quality, b) the change of functional compositional structure increases with water quality impairment and c) α- and β- functional diversity show significant variations among different types of hydromorphological conditions and different degrees of hydromorphological modification.

The study of plant ecomorphological traits attempts to link morphological characteristics (e.g. habitus size, shape and morphology) with species functions. In this article, we assessed functional diversity patterns and changes of functional composition of aquatic macrophytic communities of riverine ecosystems in Greece. We identified notable variations of the functional composition in terms of key trait characteristics, such as life growth form and preference to light conditions. We also managed to distinguish a few significant relationships between functional diversity (alpha and functional dissimilarity) and environmental variables.

Our results indicated that growth form and light preference are key trait characteristics that grouped the aquatic macrophytes into five discrete groups based on their functional trait composition. Macrophytes were distinguished along a gradient that reveals light availability as a key driver that shapes macrophytic communities in the investigated river reaches. The PC1 has a high affinity with rooted submerged macrophytes and macrophytes with high EIV L value (which indicates preference to higher light intensity) and thus plants that were positioned at the left part of the PCA plot are those that could require waters with high clarity and undisturbed conditions (e.g. oxygenated waters and less turbidity). Previous studies have highlighted the role of light availability as an important environmental filter that restricts certain functional traits of aquatic macrophytes (Fu et al., 2014b; Su et al., 2019) and promoting others that can help plants to persist to the environmental conditions. However, when we examined the relationship between functional diversity indices and environmental variables, including water quality features that are related with light availability (e.g. turbidity and nutrient concentrations), we did not find evidence that could explain a possible effect of water clarity on functional diversity. Specifically, we did not find any significant effect of water quality parameters on functional richness, but only a significant effect of pH and oxygen saturation on both functional dispersion and evenness. Similarly, we did not find any significant effect of water quality variables on the taxonomic richness. Previous studies in Greek rivers have noted relatively moderate and high levels of nutrients in several sites, indicating signs of eutrophication (Stefanidis et al., 2021). Narrow gradients of nutrient concentrations and high occurrence of macrophytes that can be found in a wide range of trophic conditions (e.g. Myriophyllum spicatum, Potamogeton nodosus, Stuckenia pectinata) may explain the difficulty in finding specific patterns of responses along water quality gradients. These results are similar with those published by Zelnik et al. (2021), who also found Myriophyllum spicatum as the most common aquatic macrophyte species in watercourses in Slovenia, despite relatively different environmental conditions from those in Greece. Among the submerged macrophytes or macrophytes with floating leaves, the most common species were: Myriophyllum spicatum, Potamogeton nodosus, Potamogeton perfoliatus, Elodea canadensis, Potamogeton crispus, Stuckenia pectinata. This is another evidence for wide ecological amplitudes of these species, which make the interpretations more difficult. Furthermore, excessive nutrients are related with eutrophication processes that reduce light availability and may have substantial effects on taxonomic and functional diversity in standing waters (Stefanidis and Papastergiadou, 2019; Lindholm et al., 2020). However, water transparency in rivers can be affected by geohydromorphological factors such as erosion, sediment load, geology, land uses and rainfall intensity (Chalov and Prokopeva, 2022; Lu et al., 2023) that are not necessarily related with water chemistry (e.g. nutrients). Our findings also showed a significant differentiation of the functional diversity indices among the types of bed stability (stable, solid, soft and unstable) and the types of habitat (pool, rifle, run and slack), which suggests that hydromorphological conditions (e.g. substrate rigidness, depth and flow type) are important drivers of certain plant traits. Functional richness, dispersion and evenness were higher at slack and run habitats, which are characterized by deeper waters and slow or fast water flow respectively. All three indices were also higher at soft substrates (mostly sand, silt and mud) than more stable substrates (e.g. bedrock, gravel, cobbles and boulders). Similarly, functional evenness was higher at fine substrates (p=0.002) which confirms a possible association of increased functional diversity with soft and fine river substrates. Previous studies have shown that geomorphological features such as river bottom type, substrate structure and riverbank stability are important factors for explaining macrophyte composition in lotic ecosystems (Peternel et al., 2022). Rooted aquatic plants in particular, prefer fine sediments (Willby et al., 2000; Hrivnák et al., 2010) while bryophytes usually occur at coarser substrates, such as boulders and cobbles. Thus, life forms are differentiated among various types of substrates. Moreover, macrophytes are considered to be ecological engineers because macrophyte assemblages have a positive impact on fine sediment accumulation on the river bottom, modifying the channel bed and facilitating plant colonization (Jones et al., 2012). We did not find any other significant differentiation of α diversity indices among the remainder hydromorphological factors, except for a significant effect of hydrological alteration and water abstraction on functional dispersion and evenness respectively. Although aquatic plants are known to respond to hydromorphological changes (Szoszkiewicz et al., 2014; Baattrup-Pedersen et al., 2016; Turunen et al., 2016; Birk et al., 2020; Gyosheva et al., 2020), only a few studies have highlighted the role of hydromorphology as an important driver of aquatic macrophytic diversity, including functional diversity (Manolaki et al., 2020; Stefanidis et al., 2021; Vukov et al., 2022). Besides substrate, hydrology plays a major role in promoting species with traits that enable them to persist droughts and low flow conditions in Mediterranean rivers (Manolaki et al., 2020). In our case, we used nominal and ordinal hydromorphological variables that limited our capability to fully explore how functional diversity changes across hydromorphological gradients (e.g. hydrological alteration). Still, we were able to capture the effect of different substrates and river habitats and extract useful conclusions on the conditions that favor increased functional diversity.

Another issue that might explain why we did not find significant relationships between certain environmental parameters and diversity indices, is the intraspecific trait variability that many plants exhibit, including aquatic plants (Fu et al., 2014b). Some plants may show high phenotypic plasticity that provides them with various adaptations to large environmental changes (Fu et al., 2018; Lindholm et al., 2020). For instance, a submerged macrophyte in China, Potamogeton maackianus, can form large monospecific beds across wide environmental gradients (Fu et al., 2018). Similar macrophyte assemblages with a few species that occur across wide ranges of nutrients and physico-chemical gradients are common in Greece (Stefanidis et al., 2019), such as Myriophyllum spicatum and Potamogeton nodosus. Other plants can show adaptations to water level fluctuations showing various growth forms (e.g. emergent, rooted with floating-leaved plants, or rooted submerged) that can help them offset environmental limitations such as limited light availability. It is likely that the inclusion of intraspecific trait variability in trait-based studies could further elucidate the functional responses to environment gradients.

An additional finding of the current research concerns the relationship between taxonomic and functional diversity. The relationship between taxonomic and functional richness has been previously used to investigate the functional redundancy of communities (Ricotta et al., 2016; da Silva Camilo et al., 2018). In this study, we found a positive significant relationship between functional and taxonomic richness that indicates low functional redundancy. This means that the loss of a few species from the macrophyte communities is more likely to lead to loss of certain functions that are strictly related with those species. This finding is particularly important for conservation scientists and environmental managers because it emphasizes the need to include the monitoring of functional diversity (besides taxonomic diversity) in order to better assess the impact of environmental changes (Ricotta et al., 2016; Biggs et al., 2020).

Separating turnover and nestedness-resultant contributions to the overall β-diversity could provide further insights into mechanisms shaping community composition with respect to β-diversity. We found that the taxonomic β-diversity was mainly governed by turnover, i.e., replacement of disappearing species by new emerging species along the gradient, which is a typical finding in freshwater studies (Perez Rocha et al., 2019). Studies that explore the relationship between taxonomic and functional beta diversity in freshwaters are not common, and their results are rather ambiguous (Perez Rocha et al., 2019; Teittinen and Virta, 2021). In the current research, we found that functional β-diversity was higher than taxonomic β-diversity. The high functional dissimilarity may partly stem from low number of taxonomic species versus considerably higher number of functional traits or functions. A discrepancy was also revealed in the decomposition of functional β-diversity, where the higher levels of functional β-diversity were mainly due to a higher nestedness-resultant component compared to taxonomic β-diversity decomposition, where the taxonomic and functional β-diversity turnover were at similar levels. Thus, replaced species in functionally poor assemblages held traits already included in the functional space of functionally rich ones, resulting in increased functional nestedness. The results of the GDM did not show strong indications of environmental effects on the taxonomic and functional dissimilarity rate and thus our study does not provide support for the role of environmental filtering as a driver of neither functional nor community dissimilarities. The PERMANOVA results showed significant variation of the overall β-diversity and the turnover among the majority of hydromorphological factors but the R² values were quite low indicating a large share of unexplained variation. It is likely that regional spatial processes (dispersal limitation with increasing geographic distance) are more important factors than local environmental descriptors for aquatic macrophyte taxonomic and functional composition changes among sites (Oikonomou and Stefanidis, 2020; Stefanidis et al., 2021).

This article provides new information filling the gap of knowledge of the functional responses of aquatic macrophytic assemblages to environmental gradients in an extended network of running waters from mainland Greece. With this article we quantified the a- and β- functional diversity of aquatic macrophytic communities of river reaches and we attempted to look for significant responses to environmental parameters related with water quality gradients and hydromorphological factors. We found that the trait characteristics that contributed most to explaining the total variance of the functional space were the macrophyte growth form and the preference to light conditions which indicates that light availability plays a major role in filtering traits of aquatic plants. We did not find any clear indication of strong relationships between functional diversity and water quality gradients. We found significant variations of alpha and beta functional diversity among hydromorphological factors - mainly substrate and river habitat - which suggested that lotic systems with fine substrates and deep waters (run and slack habitats) promoted functional diversity. We consider likely that further studies to explore the effects of additional hydromorphological gradients could reveal significant responses of functional plant communities. An important finding was the positive relationship between species richness and functional richness which implies that the loss of taxonomic richness could lead to a loss of functions. Overall, our study provides useful insights and recommendations concerning the study of functional diversity of aquatic plant assemblages within the frame of freshwater monitoring and conservation.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

KS, AO and EP contributed to the conception and design of the study. EP, GD, MS, DT and KS contributed to data compilation. KS and AO analysed the data and wrote the manuscript. KS, AO and EP authors contributed to the final version of the manuscript.