Author Identifiers

Anna Lafratta

ORCID: 0000-0001-8414-2417

Date of Award

2020

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Science

First Advisor

Professor Paul Lavery

Second Advisor

Dr Oscar Serrano

Third Advisor

Professor Pere Masquè

Abstract

Since the Industrial Revolution, coastal pollution has increased and seagrasses, together with other vegetated coastal ecosystems (VCEs), have been identified as potentially important filters and sinks of pollutants, such as CO2 and heavy metals. VCEs can accumulate materials in their soils for millennia reducing their abundance in the surrounding environment. However, VCEs are among the most threatened ecosystems on Earth, putting at risk their ecological services. This dissertation aims to understand the role of VCEs, particularly seagrasses, as environmental archives and biogeochemical sinks. The research assessed: i) the capacity of Posidonia seagrass meadows to sequester metal pollution, through the reconstruction of metal fluxes recorded in seagrass sedimentary archives adjacent to the Port Pirie Pb-Zn smelter in South Australia (SA); and ii) the capacity of VCEs to contribute to climate change mitigation at a regional scale, through a Blue Carbon (BC) assessment for SA.

This research showed that seagrass soils in a metal polluted environment can be used to reconstruct the metal contamination history. The seagrass archives at Port Pirie revealed hot-spots of spatial and temporal contamination that reflected the operation of the smelter, its pollution management strategies, wind and currents, and the behaviour of the metals in the marine system. A seagrass area of 110 km 2 has sequestered ~20% of Pb, and ~50% of both Zn and Cd cumulative emissions from the smelter since 1999, highlighting its role in removing metal pollution from the broader environment.

Additionally, this work provided insights into the variability of Corg storage among VCEs in SA, an example of an arid region characterized by low productivity. Here the distribution of Corg in the VCEs’ soil reflected both abiotic (e.g. coastal settings, aridity, hydrodynamics) and biotic (habitat type) characteristics of the sampled locations. The mean (±SE) soil Corg stocks and accumulation rates (CAR) in SA for seagrasses, mangroves and tidal marshes were 6.3±0.4, 14±2 and 13±3 kg Corg m -2 and 15±2, 32±7, 30±10 g Corg m -2 -1 yr , respectively. Those estimates are lower than national and global averages, emphasizing the need for region-specific assessments to inform blue carbon (BC) policy and projects.

Finally, a seagrass case study showed that while seagrasses are generally very efficient at sequestering Corg, not all sites are suited to demonstrating this. At False Bay, a lack of net sediment accumulation and hydrodynamic-driven mixing prevented the application of radionuclide methods to determine CAR, thereby preventing to demonstrate ‘additionality’ using this method. This finding highlighted the importance of selecting suitable sites and methods for seagrass BC projects.

The findings demonstrate the important role that seagrasses play as biogeochemical sinks, in this case for metal pollution and atmospheric CO2, but potentially for a wide range of materials. The ability of seagrasses to sequester these materials over millennia also makes them useful as archives, revealing past environmental conditions including pollution histories. These features make seagrasses potentially useful management tools, to aid in the monitoring of coastal ecosystems and to understand past trajectories which can inform current and future management. The research also highlights the need to conserve VCEs to maintain their valuable role as biogeochemical sinks and to prevent the release of stored materials into the broader environment.

Available for download on Wednesday, July 06, 2022

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