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Carbon and nitrogen cycling on intertidal mudflats in a temperate Australian estuary

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Cook, PLM (2002) Carbon and nitrogen cycling on intertidal mudflats in a temperate Australian estuary. PhD thesis, University of Tasmania.

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Abstract

The sources of organic matter, benthic metabolism (light/dark 02 and TCO2 fluxes),
benthic dissolved nitrogen fluxes, denitrification, nitrogen fixation and sediment
NH4+ production were studied on the upper and lower regions of two mudflats in
Huon Estuary, south east Tasmania over four seasons. One study site was located in
the upper euryhaline part of the estuary and the other study site was located in a
"marine" side arm of the estuary. The aim of the study was to develop a detailed
conceptual understanding of sedimentary nitrogen cycling processes in relation to the
activity of microphytobenthos (MPB) in this system.
The organic matter pool at both sites was generally dominated by that derived from
terrestrial sources. Organic matter derived from microphytobenthos generally only
comprised a small fraction of the organic matter pool. Compound-specific stable
isotope ratio analysis of bacterial and algal fatty acids suggested the algal-derived
fraction of organic matter was most likely the driver of bacterial respiration within
the sediment. As such, this fraction of organic matter had a high turnover rate, and
never built up to significant amounts. The MPB at both sites consisted of a mixed
community of diatoms, chlorophytes and cyanobacteria, the relative composition of
which varied with site, position on the mudflat and season. Rates of primary production by MPB were influenced by an exposure to wave energy
and an availability of light. Rates of primary production by MPB were significantly
greater on the upper mudflat than the lower mudflat at the site in the upper estuary.
It is proposed this arose as a consequence of light limitation across the inundation
gradient caused by high concentrations of coloured dissolved organic matter in the
water at this site. Benthic respiration at this site was controlled by temperature, as
well as organic matter input from MPB. At the site in the marine side arm of the
estuary, rates of primary production were not significant different between the upper
and lower mudflat and were significantly lower than at the site in the upper estuary.
A greater exposure to wave energy, as indicated by sediment grain size and aspect
was the most likely cause of the lower rates of primary production at this site. As a
consequence, both the upper and lower mudflats at the site in the upper estuary were autotrophic on an annual basis, while both the upper and lower mudflats at the site in
the marine side arm of the estuary were heterotrophic on an annual basis.
The balance between production and respiration was of fundamental importance in
determining whether the sediments were a net source or sink for dissolved inorganic
nitrogen, with autotrophic sediments showing a net uptake of nitrogen and
heterotrophic sediments showing a net release. Primary production also influenced
the exchange of gaseous nitrogen species. Rates of denitrification were generally
very low and negatively correlated with rates of primary production, while, rates of
N2 fixation were at times high and were positively correlated with primary
production. Dissolved nitrogen fluxes were dominated by dissolved organic nitrogen
(DON) where and when high rates of production (uptake of DON) and respiration
(release of DON) were observed. MPB also profoundly influenced the nitrogen cycle through the production of labile,
but high C:N ratio organic material. At times of high primary production, the
calculated demand for nitrogen - based on simple but widely used stoichiometric
models - was found to be well in excess of the measured uptake. Subsequent
measurements of N2 fixation using the acetylene reduction assay (calibrated using
15N-N2) showed that N2 fixation could, at times, account for the observed deficit in
nitrogen uptake. In general, however, N2 fixation could not account for the deficit in
dissolved nitrogen assimilation. It is suggested that a stoichiometric relationship
between carbon and nitrogen assimilation reflecting the C:N ratio of algal cells will
only occur during the initial development of the MPB biofilm. Once the MPB
biofilm has become established the majority of carbon assimilation is directed into
the production of extracellular organic carbon (EOC) such as extracellular polymeric
substances (EPS), rather than cell growth. It is proposed that the input of this labile,
but high C:N ratio organic material to the sediment drove bacterial respiration as well
as stimulating bacterial nitrogen reassimilation. As a consequence, the ratio of
TCO2:NI-14+produced within the sediment was generally in excess of 15 and in some
cases in excess of 60.

Item Type: Thesis (PhD)
Keywords: Biogeochemical cycles, Nitrogen cycle, Carbon cycle (Biogeochemistry)
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003.

Date Deposited: 25 Nov 2014 00:52
Last Modified: 28 Mar 2017 22:41
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