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A long‐standing rule in ecology is that structural complexity increases abundance and diversity of organisms, but this paradigm glosses over potential trait‐specific benefits of habitat structure across different regional species pools. We tested this idea using multiple response variables emphasizing taxonomic and functional diversity in seagrass‐vegetated, edge, and unvegetated habitats across three estuaries in Washington State (USA). We also used these variables in tandem to evaluate functional redundancy as a proxy for ecosystem resistance and resilience. The estuaries spanned a twofold range in richness of mesopredatory fishes and decapods. Increases in per‐sample abundance with habitat structure were confined to three of seven functional groups, specifically those occupying the water column or directly associated with seagrass shoots. Consequently, seagrass reduced mesopredator diversity by reducing evenness. Habitats differed in mesopredator assemblages despite their spatial proximity (~3 m), supported by multivariate analyses performed at functional and taxonomic resolutions, but site differences were less apparent functionally than taxonomically. Functional redundancy did not differ by habitat, but increased with the richness of the regional species pool. Edge habitats were generally intermediate in community structure, per‐sample abundance, and diversity between seagrass and unvegetated habitats. Structural complexity provided a trait‐specific enhancement of abundance, and this pattern applied across species pools. Because seagrass benefits species with certain traits, management focused on the low‐intertidal estuarine habitat mosaic, rather than a particular habitat type, and on places where redundancy is already low, best supports mesopredator diversity and function. [ABSTRACT FROM AUTHOR]

Structure provided by temperate seagrasses is expected to reduce encounter rates with predators and therefore benefit small nekton most in summer, when predation is intense and seagrass reaches peak biomass, and in the day relative to night, when darkness limits visually-oriented predators regardless of habitat. Based on seines in eelgrass (Zostera marina), unvegetated habitat, and edges in Willapa Bay, Washington, USA, nekton abundances varied across habitats and on both diel and seasonal time scales, yet only a few time-by-habitat interactions were observed, in which habitat distinctions were most pronounced at certain times. One explanation is that four of the six most abundant species disproportionately occupied unvegetated habitat or were habitat generalists, but our expectations were based on eelgrass-associated taxa. Multivariate community structure responded separately to season, habitat, and diel period, in order of importance. Total abundance showed a significant season-by-habitat interaction. A summer peak in vegetated habitats was largely driven by shiner perch and sticklebacks, two eelgrass-associated fishes, while unvegetated habitat showed year-round uniform abundances due to taxa like English sole and sand shrimp with winter and spring peaks, and no strong habitat associations or association with unvegetated habitat, respectively. In a single diel-by-habitat interaction, shiner perch were eelgrass-associated during the day but not at night. No evidence emerged of differences in body size across habitats for any species measured, but many taxa grew seasonally, likely as cohorts migrating out of the estuary. Seasonality was thus the strongest signal governing patterns of community structure, abundance, and body size across time and space, while habitat structure and diel period were less important. Our results are largely consistent with the other studies showing the primacy of seasonality in structuring estuarine nekton communities, but also show that this pattern is highly dependent on the seasonal recruitment patterns and habitat associations of abundant nekton. Unlabelled Image • Abundance in seagrass peaked in summer corresponding to peak seagrass biomass. • Abundance in unvegetated habitat did not change seasonally. • Season, habitat, and diel period affected community structure separately. • No diel-by-habitat interaction was observed for abundance or community structure. • Overall, seasonality was the most important driver of community dynamics. [ABSTRACT FROM AUTHOR]