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Maher, D 2011, 'Carbon cycling in South-East Australian estuaries', PhD thesis, Southern Cross University, Lismore, NSW.

Copyright D Maher 2011


Our conceptual understanding of estuarine carbon cycling is based predominantly on northern hemisphere temperate systems which differ from south-east Australian estuaries in many ways. Further, significant information gaps exist in our understanding of the estuarine carbon cycle, in particular the biogeochemical cycling of dissolved organic carbon (DOC) and air-water fluxes of CO2. The aim of this study was to measure the major biogeochemical fluxes of carbon within three estuaries of differing geomorphology over an annual cycle to develop a conceptual understanding of what drives estuarine metabolism, DOC cycling and air-water CO2 fluxes in these systems, and to compare and contrast the carbon cycle in these systems to other estuarine systems.

Benthic metabolism in each estuary was dominated by productivity and respiration in seagrass habitats, and integrated estuary-wide annual net benthic production in the three systems ranged from net heterotrophic in the river dominated estuary (-25 gC m-2 yr-1) to autotrophic in the two marine dominated systems (~100 gC m-2 yr-1). Artificial neural network (ANN) models were successfully implemented using a suite of readily measured parameters to model benthic metabolism, and may provide a suitable alternative to tradition linear regression models, particularly in seagrass habitats where relationships between metabolism and physico-chemical parameters are often non-linear. ANN models used to predict benthic metabolism changes associated with a modest 1 - 2°C increase in temperature indicate that net benthic production will increase by up to ~ 30% indicating a potential negative feedback mechanism to global climate change.

Benthic DOC fluxes exhibited seasonal and diel variation, with highest effluxes during summer from seagrass habitats (up to 50 mmolC m-2 d-1), and generally DOC uptake observed during the dark. Annually, the sediments within each estuary were a net source of DOC to the water column. DOC fluxes were driven by the interaction between autotrophic production (exudation) and heterotrophic (bacterial) consumption, and the quantity and source of organic matter loading to the sediments. A compilation of previous studies and the results from this study suggests that the benthic DOC flux from the coastal ocean may be up to four fold higher than previous estimates.

The composition of the water column dissolved organic matter pool was altered due to the benthic flux of DOC, with the shift in ratio of DOC to dissolved organic nitrogen (DON) indicating preferential uptake of “carbon rich” organic matter during the dark and release of carbon rich dissolved organic matter (DOM) during the light. The modeled δ13C value of the DOC taken up by the sediments during the dark indicated a terrestrial origin, and may be linked to flocculation of terrestrial derived DOM. During the light the modeled δ13C value of the effluxed DOC indicated that the source of DOC appeared to be early digenesis of higher order (non algae) plants based on the correlation between these values and the isotope value of sedimentary long chain fatty acids (LCFA).

Estuarine net ecosystem metabolism (NEM) was autotrophic and similar in magnitude in each estuary annually (~8 – 10 molC m-2 yr-1), however the contribution of different primary producers to this net production varied across the three estuaries. In the river-dominated system, NEM was solely supported by phytoplankton production, with the benthos being net heterotrophic annually. In the two marine dominated systems the contribution of phytoplankton varied between ~20 and 50% of total estuarine metabolism. Carbon burial was ~ four-fold higher in the two marine dominated systems (~4 mol C m-2 yr-1) than the river dominated estuary (~1 mol C m-2 yr-1). All three estuaries were a net sink of atmospheric CO2 annually ranging from 0.4 - 2 mol C m-2 yr-1. A compilation of data from this study and previous studies suggests that the estuarine air-water CO2 flux is coupled to NEM, and that previous global estimates based solely on heterotrophic systems may significantly over-estimate the magnitude of this efflux.

This study shows that the carbon cycle in these autotrophic, relatively pristine estuaries differs significantly from our conceptual understanding of estuarine carbon biogeochemistry which is based predominantly on heterotrophic and/or impacted systems. Therefore a different management approach is required to maintain ecosystem health in these benthic driven systems.