Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary: I. Benthic metabolism

The light and dark inundated fluxes of O2and total CO2 (TCO2), as well as the concentrations of chlorophyll a and phaeopigments, were measured (ex situ) on the upper and lower portions of 2 intertidal mudflats—1 in the upper Huon Estuary (salinity 4 to 32) and 1 in a marine side-arm of the estuary (salinity 17 to 34)—over 4 seasons. Dark-exposed fluxes of O2 and CO2 were also measured on the upper and lower mudflats of both sites over 2 seasons. Exposed fluxes of O2 were generally not significantly different to the fluxes measured during inundation. Exposed fluxes of CO2 were generally 3 to 5 times lower than inundated fluxes of TCO2. At the more sheltered site in the upper estuary, significantly greater rates of primary production were measured on the upper mudflat. In contrast, the more marine site had lower rates of primary production, and no significant difference in rates of primary production were observed across the inundation gradient. It is proposed that a greater exposure to wave energy (as indicated by sediment grain size) at the marine site was the cause of the lower rates of primary production. Rates of TCO2 consumption in the light were generally greater than those of 2production. It is suggested that O2 effluxes are greatly reduced in the light as a consequence of the re-oxidation of sulphides within the sediments

Benthic fluxes of nitrogen in the tidal reaches of a turbid, high nitrate subtropical river

Benthic fluxes of dissolved inorganic nitrogen (NO3- and NH4+), dissolved organic nitrogen (DON), N2 (denitrification), O2 and TCO2 were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO3- were generally directed into the sediments at rates of up to -225 µmol N m-2 h-1. NH4+ was mostly taken up by the sediments at rates of up to -52 µmol N m-2 h-1, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into (-105 µmol m-2 h-1) and out of (39 µmol m-2 h-1) the sediments. Average N2 fluxes at all sampling sites were similar during summer (162 µmolN m-2 h-1) and winter (153 µmolN m-2 h-1). Denitrification was fed both by nitrification within the sediment and NO3- from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO3- fluxes were significantly related to TCO2 fluxes (p\u3c0:01), with a release of NO3- from the sediment only occurring at respiration rates below 1000 µmol C m-2 h-1. Rates of denitrification increased with respiration up to TCO2 fluxes of 1000 µmol C m-2 h-1. At sediment respiration rates above 1000 µmol C m-2 h-1, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO3- entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO3- denitrified dropped to about 6% of the total river NO3-load

Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary: II. Nitrogen cycling

Benthic fluxes of dissolved nitrogen, rates of denitrification, N2 fixation and NH4+ upward flux within the sediment (calculated from porewater profiles) were measured on the upper and lower mudflats at 2 study sites, 1 in the upper, river-dominated part of the estuary, and 1 in the lower, more marine part of the Huon Estuary, Tasmania, Australia. The calculated upward flux of NH4+ from within the sediment based on porewater profiles was generally in excess of measured benthic fluxes, suggesting that NH4+ was reassimilated at the sediment surface by microphytobenthos (MPB). The ratio of total CO2 (TCO2):NH4+ produced within the sediment was generally in excess of 15, and in some cases in excess of 60. Significant influxes and effluxes of dissolved organic nitrogen (DON) were measured where the activity of MPB was highest. At times, DON influxes and effluxes were well in excess of dissolved inorganic nitrogen (DIN) fluxes, highlighting the importance of measuring DON fluxes where the activity of MPB is high. Rates of denitrification were very low, and represented only a small loss of N from the sediment, most probably as a consequence of the activity of MPB. Estimates of nitrogen assimilation by MPB showed that N2 fixation was likely to be the major source of nitrogen during the summer at the study site in the upper estuary. There was also a high estimated C:N ratio (~20) of TCO2 and nitrogen assimilated at this site, suggesting that a significant proportion of primary production was exuded as dissolved organic carbon rather than cellular production

Pollution-tolerant invertebrates enhance greenhouse gas flux in urban wetlands.

One of the goals of urban ecology is to link community structure to ecosystem function in urban habitats. Pollution-tolerant wetland invertebrates have been shown to enhance greenhouse gas (GHG) flux in controlled laboratory experiments, suggesting that they may influence urban wetland roles as sources or sinks of GHG. However, it is unclear if their effects can be detected in highly variable conditions in a field setting. Here we use an extensive data set on carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O) flux in sediment cores (n = 103) collected from 10 urban wetlands in Melbourne, Australia during summer and winter in order to test for invertebrate enhancement of GHG flux. We detected significant multiplicative enhancement effects of temperature, sediment carbon content, and invertebrate density on CH4 and CO2 flux. Each doubling in density of oligochaete worms or large benthic invertebrates (oligochaete worms and midge larvae) corresponded to ~42% and ~15% increases in average CH4 and CO2 flux, respectively. However, despite exceptionally high densities, invertebrates did not appear to enhance N2 O flux. This was likely due to fairly high organic carbon content in sediments (range 2.1-12.6%), and relatively low nitrate availability (median 1.96 μmol/L NO3- -N), which highlights the context-dependent nature of community structural effects on ecosystem function. The invertebrates enhancing GHG flux in this study are ubiquitous, and frequently dominate faunal communities in impaired aquatic ecosystems. Therefore, invertebrate effects on CO2 and CH4 flux may be common in wetlands impacted by urbanization, and urban wetlands may make greater contributions to the total GHG budgets of cities if the negative impacts of urbanization on wetlands are left unchecked

The “salt wedge pump”: convection-driven pore-water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over-enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high-resolution radon (a natural groundwater and pore-water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced pore-water exchange at this area. We use the term “salt wedge pump” to describe convection-driven advective pore-water exchange at the sediment–water interface during the upstream propagation of the salt wedge. Radon-derived convection-driven pore-water exchange rates within the salt wedge were estimated at 2.8 cm d−1, a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore-water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes ∼ 10-fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5-fold higher than oxygen uptake related to advective pore-water exchange. Estimated fluxes, associated with the nonconservative DIC, δ13C-DIC, and ammonium behavior within the estuary support convective pore-water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems

Hypoxic events stimulate nitrogen recycling in a shallow salt-wedge estuary: the Yarra River estuary, Australia

The Yarra River estuary is a salt-wedge estuary prone to periods of stratification-induced anoxia and hypoxia (O2 \u3c 100 µmol L−1) during low-flow events. Nitrate reduction pathways were examined using the 15N isotope pairing technique in intact sediment cores, emulating in situ conditions, to evaluate the fate of NO during changing oxygen conditions. Water-column concentrations of dissolved inorganic carbon (DIC), O2, NH, and NOx (NO + NO) were also measured to examine any deviation from conservative behavior (denoted Δ) in response to oxygen variability within the estuary. The estuary was a source of NH in the anoxic bottom waters. Whole-system estimates using deviations from conservative behavior and core incubations were in good agreement and showed that NH was regenerated more efficiently relative to DIC under hypoxic conditions. For the whole system, mean ΔDIC : ΔNH ratios under oxic (85 ± 33) and hypoxic (20 ± 3) conditions were significantly different. The more-efficient NH regeneration during hypoxia was attributed to rapid mineralization rates and cessation of nitrification; dissimilatory nitrate reduction to ammonium (DNRA) was not a significant contributor. Unexpectedly, the denitrification : DNRA ratio was significantly higher under hypoxic conditions, with denitrification contributing 99.1% ± 0.3% of total nitrate reduction. DNRA rates were significantly higher during oxic conditions (123.5 ± 30.7 µmol m−2 h−1) when compared with rates during hypoxia (0.6 ± 0.1 µmol m−2 h−1). The increase in DNRA in the presence of oxygen was attributed to the alleviation of NO limitation during these conditions

Adapting urban water systems to a changing climate: lessons from the millennium drought in southeast Australia.

This paper explores how the Millennium Drought changed the way Melburnians source and use their water resources and discuss what these changes may portend for other large cities in water-scarce and climate-change-vulnerable regions of the world, in particular, the Southwest region of the United States

Taking the "waste" out of "wastewater" for human water security and ecosystem sustainability.

Humans create vast quantities of wastewater through inefficiencies and poor management of water systems. The wasting of water poses sustainability challenges, depletes energy reserves, and undermines human water security and ecosystem health. Here we review emerging approaches for reusing wastewater and minimizing its generation. These complementary options make the most of scarce freshwater resources, serve the varying water needs of both developed and developing countries, and confer a variety of environmental benefits. Their widespread adoption will require changing how freshwater is sourced, used, managed, and priced

Taking the "waste" out of "wastewater" for human water security and ecosystem sustainability.

Humans create vast quantities of wastewater through inefficiencies and poor management of water systems. The wasting of water poses sustainability challenges, depletes energy reserves, and undermines human water security and ecosystem health. Here we review emerging approaches for reusing wastewater and minimizing its generation. These complementary options make the most of scarce freshwater resources, serve the varying water needs of both developed and developing countries, and confer a variety of environmental benefits. Their widespread adoption will require changing how freshwater is sourced, used, managed, and priced