1. Macroalgal (seaweed) beds and forests fuel coastal ecosystems and are rapidly reorganizing under global change, but quantifying their functional structure still relies on binning species into coarse groups on the assumption that they adequately capture relevant underlying traits. 2. To interrogate this 'group gambit', we measured 12 traits relating to competitive dominance and resource economics across 95 macroalgal species collected from the UK and widespread on NorthEast Atlantic rocky shores. We assessed the amount of trait variation explained by commonly used traditional groups-(a) two schemes based on gross morphology and anatomy and (b) two categorizations of vertical space use-and examined species reclassification into post hoc, so-called emergent groups arising from the functional trait dataset. We then offer an alternative, emergent grouping scheme of macroalgal functional diversity. 3. (a) Morphology and anatomy-based groups explained slightly more than a third of multivariate trait expression with considerable group overlap (i.e. low precision) and extensive mismatch with underlying trait expression (i.e. low accuracy). (b) Categorizations of vertical space use accounted for about a quarter of multivariate trait expression with considerable group overlap. Nonetheless, turf species tended to display attributes of opportunistic forms. (c) A nine-group emergent scheme provided a highly explanatory and parsimonious alternative to traditional functional groupings. 4. Synthesis. Our analysis using a comprehensive dataset of directly measured functional traits revealed a general mismatch between traditional groups and underlying traits, highlighting the deficiencies of the group gambit in macroalgae. While existing grouping schemes may allow first order approximations, they risk considerable loss of information at the trait and, potentially, ecosystem levels. Instead, we call for further development of a trait-based approach to macroalgal functional ecology to capture unfolding community and ecosystem changes with greater accuracy and generality.

Ecologists use a functional trait-based approach to seek a general understanding of organism-environment interactions, but, among primary producers, the empirical basis rests on vascular plants. We hypothesised that with increasing intertidal elevation, traits of large brown macroalgae would reflect a resource acquisition vs. conservation (stress tolerance) trade-off at species and community levels. Across the elevation gradient at four UK sites of varying wave exposure, we: (1) screened species' relevant morphological traits, using principal component analysis to reduce dimensionality; and (2) up-scaled species' traits using community weighted trait means (CWMs). The first principal component (PC1) strongly related to specific thallus area and thallus dry matter content, representing an acquisition-conservation trade-off. Although species generally shifted to the conservative end of this axis as elevation increased, mid-shore Ascophyllum nodosum sat at the extreme conservative end. PC2 related to holdfast ratio, thickness and length, with A. nodosum scoring higher than other mid-shore species. CWMs of PC1 decreased with elevation at two sites indicating a shift from 'fast' to 'slow' ecosystem functioning, but this relationship was disrupted by A. nodosum at the sheltered site, and by the up-shore extent of Laminaria digitata at the most exposed site. The anomalous traits of A. nodosum reflect its unique competitive strategy (slow, persistent growth) in the relatively stressful mid-shore. Seaweed functional traits show promise in linking species' identities to their strategies and ecosystem contributions. However, because resource conservation traits can be related to competitive as well as stresstolerance strategies, predicting seaweed trait responses to environmental stress gradients is challenging.

Marine habitat formers such as seaweeds and corals are lynchpins of coastal ecosystems, but their functional diversity and how it varies with scale and context remains poorly studied. Here, we investigate the functional diversity of seaweed assemblages across the rocky intertidal stress gradient at large (zones) and small (quadrat) scales. We quantified complementary metrics of emergent group richness, functional richness (functional space occupied) and functional dispersion (trait complementarity of dominant species). With increasing shore height, under species loss and turnover, responses of functional diversity were scale- and metric-dependent. At the large scale, functional richness contracted while—notwithstanding a decline in redundancy—emergent group richness and functional dispersion were both invariant. At the small scale, all measures declined, with the strongest responses evident for functional and emergent group richness. Comparisons of observed versus expected values based on null models revealed that functional richness and dispersion were greater than expected in the low shore but converged with expected values higher on the shore. These results show that functional diversity of assemblages of marine habitat formers can be especially responsive to environmental stress gradients at small scales and for richness measures. Furthermore, niche-based processes at the small—neighbourhood—scale can favour co-occurrence of functionally distinctive species under low, but not high, stress, magnifying differences in functional diversity across environmental gradients. As assemblages of marine habitat formers face accelerating environmental change, further studies examining multiple aspects of functional diversity are needed to elucidate patterns, processes, and ecosystem consequences of community (dis-)assembly across diverse groups.

9 1. Macroalgal (seaweed) beds and forests fuel coastal ecosystems and are rapidly 10 reorganising under global change, but quantifying their functional structure still relies on 11 binning species into coarse groups on the assumption that they adequately capture relevant 12 underlying traits. 13 2. To interrogate this "group gambit", we first measured 12 traits relating to competitive 14 dominance and resource economics across 95 macroalgal species collected from UK rocky 15 shores. We then assessed trait variation explained by traditional grouping approaches 16 consisting of (i) two highly-cited schemes based on gross morphology and anatomy and (ii) 17 two commonly-used categorisations of vertical space use. To identify the limitations of 18 traditional grouping approaches and to reveal potential alternatives, we also assessed the 19 ability of (iii) emergent groups created from post hoc clustering of our dataset to account for 20 macroalgal trait variation. 21 3. (i) Traditional groups explained about a third of multivariate trait expression with 22 considerable group overlap. (ii) Classifications of vertical space use accounted for even less 23 multivariate trait expression. Notwithstanding considerable overlap, the canopy vs. turf 24 scheme explained significant differences in most individual traits, with turf species tending to 25 display attributes of opportunistic forms. (iii) Emergent groups were substantially more 26 parsimonious than all existing grouping approaches. 27 4. Synthesis: Our analysis using a comprehensive dataset of directly measured functional 28 traits failed to strongly support the group gambit in macroalgae. While existing grouping 29 approaches may allow first order approximations, they risk considerable loss of information 30 at the trait and, potentially, ecosystem levels. We call for further development of a trait-based 31 approach to macroalgal functional ecology to capture unfolding community and ecosystem 32 changes with greater accuracy and generality. 33 3 KEYWORDS 34 Canopy; emergent groups; functional diversity; functional traits; Littler and Littler; seaweed; 35 Steneck and Dethier; turfs 36 37 38

A functional trait-based approach seeks a general understanding of organism -environment interactions, but, among primary producers, its empirical basis rests on vascular plants. We hypothesised that with increasing intertidal elevation, traits of large brown macroalgae would reflect a resource-acquisition vs. conservation (stress tolerance) trade-off at species and community levels. Across the elevation gradient at four UK sites of varying wave exposure, we: i) screened species' relevant morphological traits, using principal component analysis to reduce dimensionality; and ii) up-scaled species' traits using community weighted trait means (CWMs). The first principal component (PC1) strongly related to specific thallus area and thallus dry matter content, representing an acquisition -conservation trade-off. Although species generally shifted to the conservative end of this axis as elevation increased, mid-shore Ascophyllum nodosum sat at the extreme conservative end. PC2 associated with holdfast ratio, thickness and length, with A. nodusum scoring higher than other mid-shore species. CWMs of PC1 decreased with elevation at two sites indicating a shift from 'fast' to 'slow' ecosystem functioning, but this relationship was disrupted by A. nodusum at the sheltered site, and by the up-shore extent of Laminaria digitata at the most exposed site. The anomalous traits of A. nodusum reflect its unique competitive strategy (slow, persistent growth) in the relatively stressful mid-shore. Seaweed functional traits show promise in linking species' identities to their strategies and ecosystem contributions. However, because resource conservation traits can be related to competitive as well as stress tolerance strategies, predicting seaweed trait responses to environmental stress gradients is challenging.