Title

Seagrass soils as paleoenvironmental tools and biogeochemical sinks for management

Author Identifiers

Cristian Salinas

https://orcid.org/0000-0002-4716-5991

Date of Award

2022

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Science

First Advisor

Paul Lavery

Second Advisor

Oscar Serrano

Third Advisor

Maria Fernanda Adame

Fourth Advisor

Lourdes López-Merino

Abstract

Coastal vegetation plays an essential role in regulating climate change and water quality, especially seagrass meadows that cover up to 1.6 million km2 worldwide and have been identified as globally significant filters and sinks of biogeochemical elements, including organic carbon (Corg) and nitrogen (N). Over the past century, the accelerating intensity and duration of natural and anthropogenic disturbances have caused severe regime shifts in estuarine and other coastal ecosystem dynamics, causing seagrass losses and affecting their valuable ecosystem services. Although knowledge of long-term seagrass ecosystem dynamics is scarce, including baseline conditions prior to major disturbances, the limited available information has been crucial to assess the human and natural impacts on coastal ecosystems. For example, it has provided answers to key questions related to seagrass ecology, climate change and management, including the estimation of the ecological significance and the monetary value of the biogeochemical sinks associated with seagrass ecosystems. This thesis aims to use seagrasses biogeochemical sinks as archives to reconstruct and understand : 1) baseline conditions in Australian estuaries, and spatial variability in environmental change, including perturbations that triggered the loss of seagrass meadows; 2) the significance of seagrasses in global biogeochemical cycles, as well as the processes that support their potential to store carbon and nitrogen; and 3) the losses of biogeochemical sinks as a consequence of seagrass meadow losses through human-induced impacts and extreme weather events.

In this thesis, seagrass sedimentary archives encompassing the last few centuries have been studied to decipher centennial-scale environmental change in temperate estuaries, and the impacts of marine heatwaves and eutrophication on the Corg and N biogeochemical sinks associated to seagrass ecosystems. In the first data chapter of this thesis, the impact of land-use change on the ecological dynamics of temperate estuaries in Australia is reconstructed based on the analysis of multiple proxies in seagrass sedimentary archives encompassing the last 500 years. This palaeoecological study revealed the effect of land-use change following European settlement in the 1800s in Australia on the ecological condition of the estuaries, highlighting the deterioration of seagrass meadows following increased coastal development and agriculture activity after World War II.

The second data chapter examined the effect of seagrass loss due to eutrophication on seagrass soil Corg stocks and fluxes, and provides pioneering estimates of CO2 emissions following disturbance of seagrass ecosystems that can be used to support the development of seagrass blue carbon projects (conservation and restoration) to mitigate climate change. The results showed that seagrass loss alone does not necessarily drive erosion of soil Corg, but when combined with sufficient hydrodynamic energy at the sediment surface ( > 0.20 m/s in this case), significant losses occurred (88–95% of soil Corg stocks). The study provided first-order estimates of potential CO2 emissions from eutrophication-induced seagrass loss since the 1950s in Australia, with 161,150 hectares of seagrass habitat loss that likely resulted in the release of 11–21 Tg CO2 (equivalent to a 2% increase in annual CO2 emissions from land-use change). These data will be crucial to inform the implementation of seagrass blue carbon into the Australian climate change mitigation policy.

The third data chapter assessed soil N stocks and accumulation rates in Australian seagrass meadows, and provides pioneering estimates of soil N depletion following disturbance of seagrass meadows due to eutrophication and marine heatwaves, and identified the main drivers and potential ecological consequences of those losses. The results showed that Australian seagrasses capture 216–910 Gg N yr-1, equivalent to 96–105% of N runoff from Australian catchments. On the other hand, Australian-wide seagrass losses since the 1950s likely resulted in the loss of 435-720 Gg N from their soils, which likely enhanced eutrophication processes and resulted in adverse ecological consequences.

This thesis provides novel and key information on the role of seagrasses as biogeochemical sinks and sources. This information can inform management practices of estuarine and other coastal ecosystems and highlights the value of seagrass sedimentary archives for determining baseline cycles and to reconstruct the time-course of ecological change in response to natural and anthropogenic disturbances. This thesis also highlights the need to conserve and restore seagrass meadows due to their value as natural archives and biogeochemical sinks, demonstrating their potential as a Natural-based Solution for contributing to climate change mitigation.

Comments

Author also known as Cristian Salinas

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