Tidal height normalization (THN)—a new method to study the local vertical distribution of benthic intertidal organisms

Vertical zonation of intertidal benthic organisms is a key concept in marine ecology, strongly influenced by tidal range, which determines habitat availability. However, there is no standardized methodology to quantitatively measure and compare zonation across sites with varying tidal influences. Under different tidal regimes, a given height on the shore will not experience the same emersion times, leading to differences in the vertical distribution of intertidal organisms across sites. To address this, we developed an innovative indicator, tidal height normalization (THN), which normalizes the heights of canopy-forming macroalgae relative to a local tidal reference (mean sea level). Six species of Fucales were targeted at 4 sites representing 3 different macro- and megatidal regimes. A global navigation satellite system equipped with real-time kinematic positioning was used to record centimeter-level elevation data for 4790 individuals across the 4 sites. THN redefines the intertidal zone on a scale from 1 to -1. By normalizing the tidal effect, THN highlights local colonization patterns, with 3 main groups of species, under the influence of site-specific environmental drivers such as hydrodynamics and geomorphology. This approach provides a new baseline for studying intertidal zonation, offering a normalized and transferable method applicable to diverse coastal environments.