Abstract
Kelp holdfasts form dense 3D habitat that supports diverse biological communities, but the relative importance of different structural components and the methods used to measure them are poorly understood. Here, we compare multiple 3D imaging approaches spanning different levels of cost and accessibility including CT scanning, photogrammetry and depth sensing, to quantify the structural complexity of holdfasts in the kelp Ecklonia maxima (Osbeck) Papenfuss. Furthermore, we link the taxonomic and functional diversity of both macrofaunal and macroalgal holdfast assemblages to specific structural metrics. Blender interstitial volume was positively associated with macrofaunal abundance and taxonomic richness, but negatively associated with functional evenness. Conversely, haptera density was positively associated with taxonomic diversity and evenness, as well as functional distance and divergence. The macroalgal assemblages responded primarily to the availability and quality of interstitial volume. These contrasting responses demonstrate that habitat complexity is multidimensional, with different components governing taxonomic and functional diversity. Accordingly, the ecological relevance of structural metrics is context dependent, highlighting the need to align measurement approaches with research questions or employ complementary metrics. By advancing both conceptual and methodological understanding, this study quantifies habitat complexity to better predict biodiversity and functional responses related to environmental change and habitat homogenisation.