Wave-driven plant reconfiguration modifies light availability in seagrass meadows

Abstract

Seagrasses are considered foundation species in marine and estuarine ecosystems by contributing biomass, providing habitat, and damping waves and currents. Globally, seagrass health and primary productivity are threatened by factors that affect light availability, such as shading by algae and epiphytes, self-shading, and increased water column turbidity. This study focuses on how plant motion and reconfiguration lead to shading of an individual plant by itself and its neighbors, and how wave conditions, plant material properties, and shoot density affect light availability along a seagrass blade. We use a simple ray-optics shading model with the plant motion model of Zhu et al. (2020; Journal of Geophysical Research: Oceans 125:e2019JC015517) for a flexible blade under wavy flow to understand how phase-resolved plant behavior affects light availability as a function of vertical location in the water column. Results show that that shading of a plant by its neighbors occurs more under wave crests and troughs, and that factors that increase blade tip excursion (large wave height or wave period, or high plant flexibility) reduce light exposure. We develop a simplified theory and parameterization for average light exposure as a function of flow and plant conditions (as captured by the Cauchy number, buoyancy parameter, and ratio of stem spacing to blade length). These results help delineate optimal conditions for maximizing light exposure to seagrass' photosynthetic tissue in restoration projects, and facilitate the inclusion of flow-vegetation interactions in biological models of seagrass production.