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

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.