Jeffrey J. Kelleway

Carbon sequestration by Australian tidal marshes

Australia’s tidal marshes have suffered significant losses but their recently recognised importance in CO2 sequestration is creating opportunities for their protection and restoration. We compiled all available data on soil organic carbon (OC) storage in Australia’s tidal marshes (323 cores). OC stocks in the surface 1 m averaged 165.41 (SE 6.96) Mg OC ha−1 (range 14–963 Mg OC ha−1). The mean OC accumulation rate was 0.55 ± 0.02 Mg OC ha−1 yr−1. Geomorphology was the most important predictor of OC stocks, with fluvial sites having twice the stock of OC as seaward sites. Australia’s 1.4 million hectares of tidal marshes contain an estimated 212 million tonnes of OC in the surface 1 m, with a potential CO2-equivalent value of

Review of the ecosystem service implications of mangrove encroachment into salt marshes

USD7.19 billion. Annual sequestration is 0.75 Tg OC yr−1, with a CO2-equivalent value of

Life stage influences the resistance and resilience of black mangrove forests to winter climate extremes

USD28.02 million per annum. This study provides the most comprehensive estimates of tidal marsh blue carbon in Australia, and illustrates their importance in climate change mitigation and adaptation, acting as CO2 sinks and buffering the impacts of rising sea level. We outline potential further development of carbon offset schemes to restore the sequestration capacity and other ecosystem services provided by Australia tidal marshes.

Effect of acidification on elemental and isotopic compositions of sediment organic matter and macro-invertebrate muscle tissues in food web research.

Salt marsh and mangrove have been recognized as being among the most valuable ecosystem types globally in terms of their supply of ecosystem services and support for human livelihoods. These coastal ecosystems are also susceptible to the impacts of climate change and rising sea levels, with evidence of global shifts in the distribution of mangroves, including encroachment into salt marshes. The encroachment of woody mangrove shrubs and trees into herbaceous salt marshes may represent a substantial change in ecosystem structure, although resulting impacts on ecosystem functions and service provisions are largely unknown. In this review, we assess changes in ecosystem services associated with mangrove encroachment. While there is quantitative evidence to suggest that mangrove encroachment may enhance carbon storage and the capacity of a wetland to increase surface elevation in response to sea-level rise, for most services there has been no direct assessment of encroachment impact. On the basis of current understanding of ecosystem structure and function, we theorize that mangrove encroachment may increase nutrient storage and improve storm protection, but cause declines in habitat availability for fauna requiring open vegetation structure (such as migratory birds and foraging bats) as well as the recreational and cultural activities associated with this fauna (e.g., birdwatching and/or hunting). Changes to provisional services such as fisheries productivity and cultural services are likely to be site specific and dependent on the species involved. We discuss the need for explicit experimental testing of the effects of encroachment on ecosystem services in order to address key knowledge gaps, and present an overview of the options available to coastal resource managers during a time of environmental change.

Grazing kangaroos act as local recyclers of energy on semiarid floodplains

In subtropical coastal wetlands on multiple continents, climate change-induced reductions in the frequency and intensity of freezing temperatures are expected to lead to the expansion of woody plants (i.e., mangrove forests) at the expense of tidal grasslands (i.e., salt marshes). Since some ecosystem goods and services would be affected by mangrove range expansion, there is a need to better understand mangrove sensitivity to freezing temperatures as well as the implications of changing winter climate extremes for mangrove-salt marsh interactions. In this study, we investigated the following questions: (1) how does plant life stage (i.e., ontogeny) influence the resistance and resilience of black mangrove (Avicennia germinans) forests to freezing temperatures; and (2) how might differential life stage responses to freeze events affect the rate of mangrove expansion and salt marsh displacement due to climate change? To address these questions, we quantified freeze damage and recovery for different life stages (seedling, short tree, and tall tree) following extreme winter air temperature events that occurred near the northern range limit of A. germinans in North America. We found that life stage affects black mangrove forest resistance and resilience to winter climate extremes in a nonlinear fashion. Resistance to winter climate extremes was high for tall A. germinans trees and seedlings, but lowest for short trees. Resilience was highest for tall A. germinans trees. These results suggest the presence of positive feedbacks and indicate that climate-change induced decreases in the frequency and intensity of extreme minimum air temperatures could lead to a nonlinear increase in mangrove forest resistance and resilience. This feedback could accelerate future mangrove expansion and salt marsh loss at rates beyond what would be predicted from climate change alone. In general terms, our study highlights the importance of accounting for differential life stage responses and positive feedbacks when evaluating the ecological effects of changes in the frequency and magnitude of climate extremes.

Trophic structure of benthic resources and consumers varies across a regulated floodplain wetland

Stable isotope techniques in food web studies often focus on organic carbon in food sources which are subsequently assimilated in the tissue of consumer organisms through diet. The presence of non-dietary carbonates in bulk samples can affect their δ(13)C values, altering how their results are interpreted. Acidification of samples is a common practice to eliminate any inorganic carbon present prior to analysis. We examined the effects of pre-analysis acidification on two size fractions of sediment organic matter (SOM) from marine and freshwater wetlands and pure muscle tissue of a common freshwater invertebrate (Cherax destructor). The elemental content and isotopic ratios of carbon and nitrogen were compared between paired samples of acidified and control treatments. Our results showed that acidification does not affect the elemental or isotopic values of freshwater SOM. In the marine environment acidification depleted the δ(13)C and δ(15)N values of the fine fraction of saltmarsh and δ(15)N values of mangrove fine SOM. Whilst acidification did not change the elemental content of invertebrate muscle tissue, the δ(13)C and δ(15)N values were affected. We recommend to researchers considering using acidification techniques on material prepared for stable isotope analysis that a formal assessment of the effect of acidification on their particular sample type should be undertaken. Further detailed investigation to understand the impact of acidification on elemental and isotopic values of organic matter and muscular tissues is required.

Geochemical analyses reveal the importance of environmental history for blue carbon sequestration

On Australian semiarid floodplains, large herbivores such as kangaroos have a role in the cycling of energy (carbon) through the mechanism of feeding and defaecation of vegetative material. The degree to which kangaroos are vectors of energy within this system is not fully understood. This study describes the stable carbon isotope signature of floodplain plants and kangaroo scats at two close study sites. Kangaroos were found to deposit scats that mirrored the forage composition at each particular feeding site. Scats were 3.94‰ higher in δ13C values at the site where C4 grasses were available, indicating that this grass contributed ~25–30% of the diet of these kangaroos. The difference in diet due to the relative availability of C3 and C4 forage, detectable in the carbon stable isotope signature of scats, is used to demonstrate that kangaroos are recycling and redistributing energy locally, rather than transporting it more broadly across the floodplain.

Climate Change Impacts on the Coastal Wetlands of Australia

Riverine food webs are often laterally disconnected (i.e. between watercourses) in regulated floodplain wetlands for prolonged periods. We compared the trophic structure of benthic resources and consumers (crustaceans and fish) of the three watercourses in a regulated floodplain wetland (the Gwydir Wetlands, Australia) that shared the same source water but were laterally disconnected. The crustaceans Cherax destructor (yabby), Macrobrachium australiense (freshwater prawn), the exotic fish Cyprinus carpio (European carp) and Carassius auratus (goldfish) showed significantly different δ13C values among the watercourses, suggesting spatial differences in primary carbon sources. Trophic positions were estimated by using δ15N values of benthic organic matter as the base of the food web in each watercourse. The estimated trophic positions and gut contents showed differences in trophic positions and feeding behaviours of consumers between watercourses, in particular for Melanotaenia fluviatilis (Murray–Darling rainbowfish) and M. australiense. Our findings suggest that the observed spatial variation in trophic structure appears to be largely related to the spatial differences in the extent and type of riparian vegetation (i.e. allochthonous carbon source) across the floodplain that most likely constituted part of the benthic resources.

Blue carbon in coastal landscapes: a spatial framework for assessment of stocks and additionality

Coastal habitats including saltmarshes and mangrove forests can accumulate and store significant blue carbon stocks, which may persist for millennia. Despite this implied stability, the distribution and structure of intertidal-supratidal wetlands is known to respond to changes imposed by geomorphic evolution, climatic, sea level and anthropogenic influences. In this study, we reconstruct environmental histories and biogeochemical conditions in four wetlands of similar contemporary vegetation in SE Australia. The objective is to assess the importance of historic factors to contemporary organic carbon (C) stocks and accumulation rates. Results from the four cores – two collected from marine influenced saltmarshes (WAP-M and POR-M) and two from fluvial influenced saltmarshes (WAP-F and POR-F) – highlight different environmental histories and preservation conditions. High C stocks are associated with the presence of a mangrove phase below the contemporary saltmarsh sediments in the POR-M and POR-F cores. 13C NMR analyses show this historic mangrove root C to be remarkably stable in its molecular composition despite its age, consistent with its position in deep sediments. WAP-M and WAP-F cores did not contain mangrove root C, however, significant preservation of char C (up to 46% of C in some depths) in WAP-F reveals the importance of historic catchment processes to this site. Together, these results highlight the importance of integrating historic ecosystem and catchment factors into attempts to upscale C accounting to broader spatial scales.

Author Correction: global patterns in mangrove soil carbon stocks and losses

The Australian continent spans coastal wetland settings ranging from extensive mangrove forest and sabkha plains occupying in the tropical north, to the southern half of the continent, where high wave energy constrains wetlands within numerous barrier-fronted estuaries, drowned river valleys and coastal embayments. Only on the island of Tasmania are mangroves absent; elsewhere mangroves, Casuarina, Melaleuca and saltmarsh interact in ways illustrative of the effects of ongoing climate, tidal and sea-level change. Observations over several decades have suggested that recent anthropogenic climate change may already be impacting Australian coastal wetlands in important ways. A period of accelerating sea-level rise has been associated with saline intrusion, mangrove encroachment and Melaleuca dieback in the tropical north, punctuated by widespread mangrove mortality in drought periods. The consistent trend of mangrove encroachment and replacement of saltmarsh in the south, is associated with an “accretion deficit” in saltmarsh during contemporary sea-level rise. We review the ecological and cultural implications of these changes, including impacts on habitat provision for migratory birds, fisheries values, carbon sequestration and Indigenous cultural values. Current legislative and policy protections may not be sufficient to meet the increasingly dynamic impacts of climate change in altering wetland boundaries, composition and function.