OCE-PRF Effects of below-ground complexity on seagrass sediment structure and function

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).

Marine sediments control the fate of carbon in the ocean by either metabolizing (“breaking down”) organic matter or burying it away. However, sediments are somewhat of a “black box”. Chemistry and ecology are closely connected in sediments, making it difficult to identify which processes control metabolism.

This problem is even more pronounced in seagrass beds which are important coastal ecosystems and sites of very high carbon burial. Seagrass roots create complex structures that exchange nutrients and oxygen with the surrounding sediment, and burrowing animals can increase sediment metabolism by mixing and flushing water through sediments. This results in variability in seagrass sediment metabolism that is currently not well explained.

The proposed research will link differences in seagrass root structure and animal communities to variation in sediment metabolism rates. The results of this research will inform strategies to manage and protect seagrass beds and anticipate changes in their valuable ecosystem services. The findings of the project will be adapted into a middle/high school level lesson plan on “Hidden Complexity” to teach students about the relationships between physical structure, chemistry and ecology in nature.

The overarching goal of the proposed research is to integrate the complex physical structure and geochemical activity of seagrass roots and rhizomes into a model of the seagrass below-ground environment to assess which features of that environment affect sediment metabolism. Dense, complex root mats may oxygenate sediments more and drive higher sediment metabolism rates, and the 3D structure of the roots may control what fauna can live there and by extension the metabolism-enhancing activities the fauna perform.

Specific goals of the project are to; (1) determine how the physical-chemical environment created by seagrass roots and rhizomes influences macrofaunal functional diversity, (2) assess how variation in the sediment physical-chemical environment at different locations in an expanding seagrass bed corresponds to variation in sediment metabolism, and (3) assess how differences in the sediment physical-chemical environment between seagrass taxa corresponds to variation in sediment metabolism. Physical structure will be characterized using CT imaging and geochemical patterns will be measured using planar optode imaging and targeted microprofiling.

The structural and geochemical data will be integrated together using multiphysics modeling techniques to simulate geochemical permeance into the sediment around seagrass roots. These model results will then be compared to measurements of sediment metabolism from in situ benthic chambers to draw conclusions about the influence of roots on total sediment oxygenation and faunal behavior.

The combination of non-destructive CT and planar optode imaging into a unified model will provide valuable information on the 3D structuring of seagrass sediments previously only described to a limited extent. The developed methodologies will also be of use to other researchers studying complexity in “opaque” systems, such as marshes, mangroves, and soils.

By describing variation in below-ground structure, this work will provide a mechanistic understanding of its processes that can then be connected to larger-scale patterns in seagrass ecosystems, generating valuable knowledge for seagrass conservation, restoration, and management, and informing on seagrass susceptibility to climate change.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.