Bradley D Eyre

Causes of deoxygenation in the Richmond River estuary, Final report to Environmental Trust (2001/RD/60005)

Bradley D. Eyre — Mon, 04 May 2009 16:34:55 PDT

Reply to comment on our paper: Comparison of isotope pairing and N2: Ar methods for measuring sediment dentrification rates

Bradley D. Eyre — Wed, 15 Apr 2009 16:26:17 PDT

Dry season suspended sediment concentration and sedimentation in the Richmond River estuary, northern NSW, Australia

Shahadat Hossain — Wed, 15 Apr 2009 16:26:15 PDT

Dry season suspended sediment concentration and sedimentation in the Richmond River catchment were investigated during 2 hydrological years (1994-96). Longitudinal suspended sediment transects during low flow months showed the Richmond River estuary remained well mixed and maintained <30>mg/L suspended sediment concentration without any visible turbidity maximum zone all along the estuary. During the entire dry season, there is very little exchange of suspended sediment between the upper and lower estuaries because of very small inputs from the upper catchment. The estuary receives net sediment input from the continental shelf during the dry months under normal tidal circulation, and marker horizon core samples confirmed that most of these imported sediments were deposited in the lower estuary; during the 2 dry seasons, lower estuary sedimentation rate varied about 0.84 � 0.31 cm to 0.48 � 0.3 cm. Flushing times of the Richmond River estuary show that all point and non-point source inputs of sediments and pollutants into the estuary can be flushed out during one dry season.

Comparison of isotope pairing and N₂/Ar methods for measuring sediment dentrification - assumptions, modifications and implications

Bradley D. Eyre — Wed, 15 Apr 2009 16:26:13 PDT

Denitrification has been measured during the last few years using two different methods in particular: isotope pairing measured on a triple-collector isotopic ratio mass spectrometer and N2:Ar ratios measured on a membrane inlet mass spectrometer (MIMS). This study compares these two techniques in short-term batch experiments. Rates obtained using the original N2:Ar method were up to 3 to 4 times higher than rates obtained using the isotope pairing technique due to O2 reacting with the N2 during MIMS analysis. Oxygen combines with N2 within the mass spectrometer ion source forming NO+ which reduces the N2 concentration. The decrease in N2 is least at lower O2 concentrations and since oxygen is typically consumed during incubations of sediment cores, the result is often a pseudo-increase in N2 concentration being interpreted as denitrification activity. The magnitude of this oxygen effect may be instrument specific. The reaction of O2 with N2 and the subsequent decrease in N2 was only partly corrected using an O2 correction curve for the relationship between N2 and O2 concentrations. The O2 corrected N2:Ar denitrification rates were lower, but still did not match the isotope pairing rates and the variability between replicates was much higher. Using a copper reduction column heated to 600�C to remove all of the O2 from the sample before MIMS analysis resulted in comparable rates (slightly lower), and comparable variability between replicates, to the isotope pairing technique. The N2:Ar technique determines the net N2 production as the difference between N2 production by denitrification and N2 consumption by N-fixation, while N-fixation has little effect on the isotope pairing technique which determines a rate very close to the gross N2 production. When the two different techniques were applied on the same sediment, the small difference in rates obtained by the two methods seemed to reflect N-fixation as also supported from measurements of ethylene production in acetylene enriched sediment cores. The N2:Ar and isotope pairing techniques may be combined to provide simultaneous measurements of denitrification and N-fixation. Both techniques have several assumptions that must be met to achieve accurate rates; a number of tests are outlined that can be applied to demonstrate that these assumptions are being meet.

Influence of climate, geology and humans on spatial and temporal nutrient geochemistry in the subtropical Richmond River catchment, Australia

Lester J. McKee — Wed, 15 Apr 2009 16:26:11 PDT

Water quality was monitored on a spatial and temporal basis in the subtropical Richmond River catchment over two years. Nutrient concentrations varied seasonally in a complex manner with highest concentrations (maximum = 3110 �g N L- 1 and 572 �g P L - 1) associated with floods. However, median (444 �g N L - 1 and 55 �g P L - 1) concentrations were relatively low compared with other parts of the world. The forms of nitrogen and phosphorus in streams varied seasonally, with greater proportions of inorganic nitrogen and phosphorus during the wet season. Minimum nutrient concentrations were found 2--3 months after flood discharge. With the onset of the dry season, concentration increases were attributed to point sources and low river discharge. There were statistically significant relationships between geology and water quality and nutrient concentrations increased downstream and were significantly related to population density and dairy farming. In spite of varying geology and naturally higher phosphorus in soils and rocks in parts of the catchment, anthropogenic impacts had the greatest effects on water quality in the Richmond River catchment. Rainfall quality also appeared to be related both spatially and seasonally to human activity. Although the responses of the subtropical Richmond River catchment to changes in land use are similar to those of temperate systems of North America and Europe, the seasonal patterns appear to be more complex and perhaps typical of subtropical catchments dominated by agricultural land use.

Deoxygenation potential of the lower Richmond River floodplain, northern NSW, Australia

Bradley D. Eyre — Wed, 15 Apr 2009 16:26:09 PDT

Periodic deoxygenation events (DO < 1 mg/L) occur in the Richmond River Estuary on the east coast of Australia following flooding and these events may be accompanied by total fish mortality. This study describes the deoxygenation potential of different types of floodplain vegetation in the lower Richmond River catchment and provides a catchment scale estimate of the relative contribution of floodplain vegetation decomposition to deoxygenation of floodwaters. Of the major vegetation types on the floodplain slashed pasture was initially (first 5 to 7 h) the most oxygen demanding vegetation type after inundation (268 � mg O2 m-2 h-1), followed by dropped tea tree cuttings (195 � 18 mg O2 m-2 h-1) and harvested cane trash (110 � 8 mg O2 m-2 h-1). However, 10 h after inundation the oxygen consumption rates of slashed pasture (105 � 5 mg O2 m-2 h-1) and tea tree cuttings (59 � 7 mg O2 m-2 h-1) had decreased to a rate less than the harvested cane trash (110 � 8 mg O2 m-2 h-1). The oxygen demands of the different floodplain vegetation types when inundated were highly correlated with their nitrogen content (r2 = 0.77) and molar C:N ratio (r2 = 0.82) reflecting the dependence of oxygen demand of vegetation types on their labile carbon content. The floodplain of the lower Richmond River (as flooded in February 2001) has the potential to deoxygenate about 12.5 � 103 mL of saturated freshwater at 25�C per day which is sufficient to completely deoxygenate floodwater stored on the floodplain with 3 to 4 days. In addition, oxidation of Fe2+ mobilized during the decomposition of floodplain vegetation via iron reduction and discharged from groundwater and surface runoff in acid sulfate soil environments could account for about 10% of the deoxygenation of floodwater stored on the floodplain. Management options to reduce floodplain deoxygenation include removing cuttings from slashed pasture and transporting off-site, reducing slashed pasture windrow loads by using comb-type mowers, returning areas of the floodplain to wetlands to allow the establishment of inundation tolerant vegetation and retaining deoxygenated floodwaters in low lying areas of the floodplain to allow oxygen consumption process to be completed before releasing this water back to the estuary.

Quantification of dentrification in permeable sediments: insights from a two dimensional simulation analysis and experimental data

Perran LM Cook — Wed, 15 Apr 2009 16:26:07 PDT

Using a two-dimensional simulation analysis, we investigated the effects of sediment flushing on denitrification and the implications for two methods commonly used to measure denitrification in intact sediment cores: the N2:Ar-ratio method and isotope pairing technique (IPT). Our simulations of experimental chamber incubations showed that advective flushing of the sediment can significantly increase sediment denitrification driven by NO3- from the water column (up to a factor of 5), but that nitrification and coupled nitrification-denitrification is reduced under conditions of sediment flushing (up to a factor of 6). N2 fluxes across SWI may differ significantly from actual rates of denitrification for periods lasting from 1 up to more than 5 d after changes in parameters such as sediment flushing rate and water column NO3- concentrations. Simulations of the isotope pairing technique showed that the rate of labeled N2 production, after the addition of 15NO3- may take up to ~24 h to reach steady state, depending on NO3- concentrations in the water column and sediment flushing rate. Measurements of denitrification in sand using IPT confirmed that short term incubations (11 h) underestimated the actual denitrification. Furthermore, model simulations were able to give a good estimate of measured N2 fluxes across SWI at different flushing rates under non-steady state conditions, confirming the ability of the model to realistically simulate experimental situations.

Carbon, nitrogen and phosphorus budgets for a shallow sub-tropical coastal embayment (Moreton Bay, Australia)

Bradley D. Eyre — Wed, 15 Apr 2009 16:26:05 PDT

Average annual carbon, nitrogen, and phosphorus budgets were constructed for Moreton Bay. Primary production was the dominant source of carbon (by two orders of magnitude), N fixation was the dominant source of nitrogen, and point sources were the dominant source of phosphorus to the bay. About 41% of the nitrogen and 70% of the phosphorus entering Moreton Bay was exported to the ocean, and about 56% of the nitrogen was lost to denitrification. The high percentage loss of phosphorus to the ocean was directly related to the short residence time of the bay (46 d), which was consistent with other shallow coastal ecosystems. In contrast, the percentage loss of nitrogen to the ocean was low compared to other coastal systems due to the high percentage loss through denitrification associated with autotrophic sediments in the bay that enhance denitrification. Because most denitrification studies have been carried out using only dark incubations, the importance of denitrification to the nitrogen budgets of coastal systems in general may be underestimated. Carbon loss from Moreton Bay was dominated (by two orders of magnitude) by atmospheric exchange of CO2 associated with benthic and pelagic respiration. The distinct difference between Moreton Bay (subtropical) and temperate systems was the dominance of biological (microbial: N fixation and denitrification) over physical inputs and losses of nitrogen. High N fixation in turn fuels a positive annual mean net ecosystem metabolism (NEM) of 21 g C m-2 yr-1 and suggests that primary production in the bay is phosphorus limited at the whole ecosystem scale.

Suspended sediment dynamics in the sub-tropical microtidal Richmond River estuary, Australia

Shahadat Hossain — Wed, 15 Apr 2009 16:26:03 PDT

Suspended sediment transport and sedimentation dynamics in the sub-tropical Richmond River estuary were investigated between July 1994 and June 1996. During low flow months the estuary received net sediment inputs from the continental shelf; during high flow the estuary exported sediment to the continental shelf depending on the magnitude of floods. Flow-weighted sampling along with a one-dimensional unsteady flow hydrodynamic model of the estuary estimated that about fifty percent of the flood-borne suspended sediment was transported through the estuary to the continental shelf during two minor floods (with a 2-year return period), whereas, one hundred percent of the flood-borne sediment was transported to the continental shelf during a moderate flood (with a 5-year return period). Marker horizons confirmed a net accumulation of sediment in the Richmond River estuary at the salt-fresh water interface during all post flood recovery stages.

Nutrient cycling in the sub-tropical Brunswick estuary, northern NSW, Australia

Angus JP Ferguson — Wed, 15 Apr 2009 16:26:00 PDT

A combination of mixing plots, one-dimensional salt balance modelling, nutrient loading budgets, and benthic flux measurements were used to assess nutrient cycling pathways in the enriched sub-tropical Brunswick estuary during different freshwater flows. A simple model accounting for freshwater residence times and nutrient availability was found to be a good predictor of phytoplankton biomass along the estuary, and suggested that biomass accumulation may become nutrient-limited during low flows and that recycling within the water column is important during blooms. Dissolved inorganic nitrogen (DIN) cycling budgets were constructed for the estuary during different freshwater flows accounting for all major inputs (catchment, sewage, and urban) to the estuary. Internal cycling due to phytoplankton uptake (based on measured biomass) and sediment-water fluxes (based on measured rates in each estuarine reach) was considered. Four different nutrient cycling states were identified during the study. In high flow, freshwater residence times are less than 1 d, internal cycling processes are bypassed and virtually all dissolved, and most particulate, nutrients are delivered to the continental shelf. During the growth phase of a phytoplankton bloom enhanced recycling occurs as residence times increase sufficiently to allow biomass accumulation. Remineralization of phytoplankton detritus during this phase can supply up to 50% of phytoplankton DIN demands. In post-bloom conditions, DIN uptake by phytoplankton decreases in the autumn wet season when biomass doubling times begin to exceed residence times. OM supply to the sediments diminishes and the benthos becomes nutrient-limited, resulting in DIN uptake by the sediments. As flows decrease further in the dry season, there is tight recycling and phytoplankton blooms, and uptake by the sediments can account for the entire DIN loading to the estuary resulting in complete removal of DIN from the water column. The ocean is a potentially important source of DIN to the estuary at this time. The results of the DIN cycling budgets compared favorably with mixing plots of DIN at each time. The results suggest that a combination of different approaches may be useful in developing a more comprehensive understanding of nutrient cycling behavior and the effects of nutrient enrichment in estuaries.