Eelgrass Structural State Is Associated with Climate- and Terrain-Dependent Coastal Thermal Sensitivity A multi-scale open-data analysis of Zostera marina thermal microclimates, natural structural loss, bathymetry, predictive validation, and global scenario projection

Abstract

Seagrass meadows are shaped by temperature, light, exposure, tidal exchange, and bathymetry, yet whether eelgrassstructural state is related to the transmission of regional thermal forcing into local meadow temperatures remainsunresolved. We introduce a thermal-sensitivity framework, λ = ΔTlocal/ΔTregional, and test whether Zostera marinastructural state is associated with this transfer across eastern-Pacific meadows. The principal density × regional-heat ×background-climate interaction was negative (β = -0.166, 95% CI -0.261 to -0.071, p = 0.000595), and leave-oneregion-out estimates remained negative. A site-mean geographic permutation yielded p = 0.0027. Meadow thermalmicroclimates were persistent, while day-night analyses did not support simple shading as the primary explanation.Natural structural-loss events shifted toward greater thermal sensitivity relative to matched low-change controls, butinference weakened after explicit ETOPO 2022 terrain adjustment (mean signed DiD = -0.124; 10,000-randomization p= 0.054). A leakage-resistant chronological holdout against a separately fitted reduced model produced only 1.41%RMSE improvement, not exceptional under structure-label permutation (p = 0.329). An exploratory regime searchsuggested more frequent buffering-direction responses in warmer settings, but the globally corrected search wasnonsignificant (p = 0.153). We therefore propose a specific new framing: eelgrass structural state is associated withclimate- and terrain-dependent regional-to-local thermal transmission, rather than universal coastal cooling. Keywords: Zostera marina; eelgrass; thermal microclimate; coastal warming; shoot density; hydrodynamics;bathymetry; thermal sensitivity; marine heatwaves