Year

2016

Degree Name

BEnvSci (Hons)

Department

School of Earth & Environmental Science

Advisor(s)

Kerrylee Rogers

Abstract

An increasing trend of mangrove expansion into saltmarsh communities has been observed in south-eastern Australia as part of a worldwide trend of woody vegetation encroachment on grasslands (Saintilan & Rogers 2013). Changes in coastal wetland vegetation communities as a result of sea-level rise are likely to alter ‘blue carbon’ sequestration rates, depending on sediment supply, primary productivity and the inherent ability of vegetation to adjust to regional hydrologic and geomorphic changes. The rate of sediment supply, both mineral and organic, many not meet the requirements to fill ‘accommodation space’ created by relative sea-level rise (Woodroffe et al. 2016). Changes in hydrological and sediment supply characteristics may be influenced by anthropogenic modifications to the estuary, including channel dredging, ocean breakwall construction, and land use change. This study investigated multi-decadal sedimentation rate changes and long-term vegetation community evolution at Ukerebagh Island, a coastal wetland within the sub-tropical Tweed River estuary on the NSW-Queensland border.

210Pb/137Cs analysis indicated that considerable acceleration of sedimentation rates has occurred throughout the latter part of the 20th century, in line with recorded sea level rise. However, any contributing effects of the operation of the Tweed River Sand Bypass, a sand pump assisting longshore drift, cannot be distinguished from sea-level rise related increases in sedimentation. Down-core variation in carbon stable isotope (δ13C) values were used to infer shifts between C3 (mangrove) and C4 (saltmarsh – Sporobolus virginicus) communities. Sediments in mangrove communities indicated the continuous influence of mangroves on soil carbon, while saltmarsh sediments displayed a marked shift between C3 and C4 signatures, suggesting the former presence of either mangroves or C3 saltmarsh species. Mixed cores exhibited an intermediate δ13C signature, indicating the joint contribution of mangrove and saltmarsh to soil organic carbon. Variations in δ13C correspond to time series analysis of vegetation distribution by Saintilan (1998) which indicated relatively recent mangrove expansion.

Observed changes in soil organic carbon content primarily related to decomposition over time. However, total carbon store values did not differ significantly between vegetation types, which may be related to differing accumulation time periods, or the methodological approach adopted. Additionally, new vegetation community mapping indicated that previously observed patterns of mudflat formation and landwards mangrove transgression are continuing, suggesting that while carbon sequestration levels may increase with projected sea-level rise, the long-term resilience and provision of ecosystem services by saltmarsh may be limited.

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