Date of Award

2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Faculty of Health, Engineering and Science

First Advisor

Professor Paul S. Lavery

Second Advisor

Dr Oscar Serrano Gras

Abstract

Seagrass meadows are important ‘Blue Carbon’ sinks but many questions remain unaddressed in regards to the organic carbon (OC) sequestration capacity and processes leading to retention and persistence of OC in seagrass sediments. The research summarised in this dissertation examined 37 sediment cores from twelve Australian seagrass meadows (Posidonia australis and Halophila ovalis) in order to improve our understanding of OC storage and preservation in seagrass sediments. The research: quantified the OC storage in seagrass meadows and the reduction in stores after ecosystem degradation; the rates of OC accumulation; the roles of species composition and the depositional nature of the habitat as factors affecting OC storage; and, characterised the sedimentary organic matter (OM) accumulated over millennia using techniques not previously applied to seagrass sediments.

In Oyster Harbour, Western Australia, P. australis had been present over the past 6000 years, as evidenced from radiocarbon analysis of sedimentary matter. Both seagrass- and nonseagrass-derived OM contributed to high sedimentary organic stores (10.79-11.42 kg OC m- 2; 150 cm sediment depth). The persistence of sedimentary OM over millennial scales indicated that the carbon was well-preserved, thus showing a link between carbon storage and its preservation. By quantifying accumulation rates, and using historical accounts of the highest areal cover (6.1 to 6.7 km2) and recent losses in cover (by 2.8-3.1 km2) due to eutrophication, it was estimated that up to 11.17 Gg OC has been lost from shallow sediments (50 cm depth) following seagrass loss. This carbon was potentially remineralisable and may, therefore, have been liberated back to the atmospheric CO2 pool.

Nine Posidonia australis meadows were then investigated for the effect of the depositional environment on sedimentary OC stores. Based on hydrodynamic differences of meadows categorised as More Sheltered, Less Sheltered, and Exposed, the More Sheltered sites had OC stores 6-fold higher (4.57 ± 0.16 to 13.51 ± 0.53 kg OC m-2; 140 cm sediment depth) compared to Exposed meadows (2.24 ± 0.31 to 3.77 ± 0.85 kg OC m-2). The OC stores of Less Sheltered meadows were not significantly different to either of the other two categories. It was concluded that the depositional nature of a seagrass habitat can affect the OC stores, though the affects may be influenced by other site-specific characteristics.

The effect of species composition on OC stores and accumulation rates was subsequently investigated by comparing the stores in estuarine P. australis and H. ovalis meadows. Comparisons were based on stratigraphic- (OC stores over a set depth) and temporal-based (i.e. accumulation over a set period of time, and as accumulation rates) measures. Organic carbon stores were between 2- (P. australis: 10.81 ± 2.06 kg OC m-2, H. ovalis: 5.17 ± 2.16 kg OC m-2; 150 cm depth) and 11-fold (P. australis: 10.87 ± 2.86 kg OC m-2, H. ovalis: 0.97 ± 0.47 kg OC m-2; 2500 yr accumulation) different between meadows of the two species. While the OC stores were different between species, it was also apparent that environmental factors also contributed to the variability, with some H. ovalis meadows having stores comparable to some P. australis meadows. Thus, both the species and environmental factors needs to be considered for robust predictions of OC storage in seagrass meadows.

The final study reported here investigated the preservation of sedimentary OC in the P. australis meadow of Oyster Harbour. A range of biogeochemical variables (age, sediment grain size, anoxia, OM and OC contents, and _13C values) were characterised at increasing depth within a sediment core. Solid-state 13C nuclear magnetic resonance was applied to a seagrass core for the first time to characterise the biochemical constituents of the sedimentary OM. There was a 76-80% contribution of seagrass-derived organics (lignin, carbohydrate, and a black-carbon-like OM) into the sediment. The proportion of black-carbon-like material increased with age/depth, indicating that it underwent selective preservation. Carbohydrates decreased with depth/age and lignin showed no changes, indicating that they have undergone non-selective preservation. There was remarkable consistency in the biochemical makeup of the OM with depth, which accumulated over the past 1900 years, indicating a very high preservation potential within seagrass sediments.

Cumulatively, the research presented in this dissertation has highlighted the variability of OC stores in seagrass meadows and how OC may be preserved. The research has indicated that any attempts to estimate regional or global carbon stores must take into account both the species of seagrass that dominate the meadows and the type of depositional environment that the meadows occur in. It is also clear that Posidonia meadows in south-western Australia have the potential to store very large amount of Blue Carbon, comparable in some instances to the highest stores recorded globally, and to preserve these stores over millennia. Modelling future Blue Carbon stores requires an understanding of the fate of the stored carbon following disturbance. It is clear that this carbon can be lost from the meadow, but much of it appears to be in highly recalcitrant forms and it is unclear whether this material is available for subsequent re-mineralisation.

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