Understanding the seagrass-sediment-light feedback to guide restoration planning: A case study using Zostera muelleri

Seagrass ecosystems are ecologically important but are declining worldwide, prompting restoration efforts. However, restoration success is partially reliant on ensuring that donor material and planting methods are suitable under the environmental conditions expected within restoration sites. This may require planting at critical densities needed to initiate favorable environmental feedbacks, ensuring plants are pre-adapted to local conditions, and setting realistic restoration goals. An understanding of the relationship between local environmental conditions (e.g. the local light environment and seagrass structural complexity) can be particularly important in guiding restoration decision-making. Here, we investigate how sediment-light conditions interact with seagrass structural complexity (density and above-ground morphology) to guide restoration planting approaches for the intertidal seagrass Zostera muelleri. Using generalized additive models and empirical data, we identified significant relationships between the local sediment-light environment and the structural complexity of Z. muelleri meadows present in Western Port, Victoria, southeast Australia. We found a decrease in shoot density and leaf length with decreasing light availability, potentially reflective of an adaptive change in the species morphology in response to low-light environments. We also found a decrease in sediment sorting and increased fine particles with increasing meadow structural complexity, suggesting that seagrass structural complexity increases sediment stability, and accretion, and may contribute to local water clarity via ecological feedbacks. These findings suggest that understanding both environmental drivers and the potential for ecological feedbacks to occur is needed before large-scale planting begins and that restoration targets should reflect the meadow form most likely to occur under the environmental conditions present.