Sub-tropical Australian Estuaries Research Articles

Saltmarsh grass supports fishery food webs in subtropical Australian estuaries

All fishery food webs are ultimately underpinned by organic matter produced by algae and plants, some of it supplied by primary producers at the fringes of fish habitats. This is no different in tropical and subtropical estuaries where secondary production by crustaceans and finfish may depend on coastal wetlands (e.g. mangroves, seagrass, saltmarshes) abutting channels. Coastal urbanisation is a major cause of wetland loss globally. Hypothetically, reduced wetland area may propagate to less fisheries production if wetland contributions to food webs are substantial – this is the prime question addressed here. We sampled key fisheries species in subtropical estuaries in reaches of moderate urbanisation (18–33% of shoreline hardened) and in reference locations dominated by wetlands, on the east coast of Australia. We used triple stable isotopes (C, N, S) to estimate (using mixing models) the trophic contributions of the key primary producers to regional fisheries species. Organic matter from wetland plants, particularly saltmarsh grass (Sporobolus virginicus), underpinned fishery food webs in most cases (with median contributions ranging from 18 to 88% for saltmarsh grass, followed by 6–70% for C3 wetland plants (e.g. mangroves, phragmites) and 3–36% by benthic algae). Moderate levels of urban transformation of estuarine margins did not change the dominance of wetland carbon for fish and crustaceans in settings without significant seagrass meadows. Given the demonstrated importance of saltmarshes, and other coastal wetlands, to regional fisheries production, conservation of these habitats is necessary in the face of multiple anthropogenic threats.

Seasonal Drivers of Carbon Dioxide Dynamics in a Hydrologically Modified Subtropical Tidal River and Estuary (Caboolture River, Australia)

AbstractEstuaries are dynamic hot spots for carbon cycling and atmospheric evasion. Here we assess the dynamics and drivers of CO2 in a hydrologically modified subtropical Australian estuary. Over 1 year, 10 high‐resolution spatial surveys of pCO2, radon, chromophoric dissolved organic matter, chlorophyll a, and physicochemical parameters were conducted from the estuary mouth to a weir located 25 km upstream. The riverine respiratory quotient revealed that processes besides water column respiration were driving high CO2 within the tidal river (salinity < 2) but not in the estuary (salinity > 2). Conservative mixing plots for pCO2 and 222Rn implied that groundwater may be a contributing source of CO2 during most surveys, but not during dry conditions. A multiple linear regression model explained 88% of the annual pCO2 variability, indicating that mixing, metabolism, temperature, and groundwater inputs were key drivers of CO2. Inputs from an upstream wastewater outfall potentially fuel observed seasonal algal blooms, resulting in the lowest daytime CO2 emissions periods. Postbloom surveys had the highest daytime CO2 emissions. The average tidal river CO2 atmospheric flux rate was 379 ± 53 mmol m−2 day−1. The average estuarine CO2 flux was 78 ± 17 mmol m−2 day−1, equating to 28 ± 6 mol m−2 yr−1. Although the tidal river surface area was ~10 times smaller than the estuary, about one third (35%) of the CO2 emissions were derived from the tidal river. Our results suggest that CO2 emissions along the tidal river‐estuary continuum are dynamic over small temporal and spatial scales and that a combination of hydrological and biological processes is a controlling factor of this variability.

Umbrellas can work under water: Using threatened species as indicator and management surrogates can improve coastal conservation

Species surrogates, the use of particular species to index habitat condition or to represent ecological assemblages are commonly identified in many ecosystems, but are less tested, and therefore less employed in estuaries. Estuaries provide important ecosystem goods (e.g. harvestable species) and services (e.g. carbon processing, coastal armouring), but require protection from multiple human activities, meaning that finding surrogates for estuarine condition or faunal assemblages is a significant knowledge gap. In this study, we test the efficacy of the threatened estuary ray Hemitrygon fluviorum, as a suitable indicator of ecosystem condition and management umbrella surrogate species for conservation prioritisation and monitoring purposes within estuaries. We surveyed fish assemblages and ray presence at ten sites within each of 22 estuaries in southeast Queensland, Australia, using one hour deployments of baited video arrays. We then tested for correlations between ray presence, a series of environmental variables considered important to ecosystem management within estuaries (i.e. testing rays as indicator species), and the co-occurring fish species (i.e. testing rays as umbrella species). Estuary rays function as both umbrella species and ecological indicators of habitat status in subtropical Australian estuaries. As umbrellas, ray occurrence concords with elevated species richness. As ecological indicators, ray distribution concords with habitats of good water quality (especially low turbidity) and more natural vegetation remaining in the catchment. These results highlight the potential for other threatened aquatic vertebrates that are both readily detectable and that are reliable proxies for ecosystems status to be become useful management tools in estuaries. The protection of such large, threatened species in coastal seascapes allows managers to address multiple targets for conservation, especially; (1) protecting species of conservation concern; (2) maintaining diversity; and (3) protecting optimal habitats by better placing reserves.

The impact of suspended oyster farming on nitrogen cycling and nitrous oxide production in a sub-tropical Australian estuary

In this study we quantified nitrate (NO3−) reduction (denitrification, anammox and DNRA) and N2O production in sediments and epibiont communities associated with Sydney Rock Oyster (Saccostrea glomerata) farming. In sediments beneath an active suspended oyster farm, DNRA accounted for 98% of NO3− reduction with rates of up to 169 ± 45 μmol N m−2 h−1. Much of this DNRA was fuelled by NO3− derived from nitrification. Reference sediments had significantly lower DNRA rates of 83.8 ± 28.2 μmol N m−2 h−1, however this constituted 96% of the sites total NO3− reduction. Fatty acid analysis showed that sediment organic matter was more labile in the oyster impacted sediments, facilitating subtle shifts in sediment oxygen demand which increased the Fe2+ availability with respect to the reference sediments. The difference in DNRA rate between the sites was attributed to autotrophic oxidation of soluble Fe2+ in sediments underlying the oyster cultures. DNRA was absent in the oyster shell epibiont communities and rates of anammox and denitrification were lower than in the sediments. Production of NH4+ from the oysters and their associated epibionts was larger than DNRA and reached a rate of 206.2 μmol N m−2 h−1. Nitrous oxide production rates were generally low compared to other aquaculture systems and the net flux of N2O for the combined oyster cultivation system (i.e. sediments plus epibionts) was negative, i.e. there was N2O consumption in the sediments beneath the oysters. Overall, subtropical suspended oyster farming systems favour inorganic N retention over N loss.

Hydrogen isotope response to changing salinity and rainfall in Australian mangroves.

Hydrogen isotope ratios ((2) H/(1) H, δ(2) H) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their δ(2) H values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid δ(2) H and leaf and xylem water δ(2) H and δ(18) O values from three mangrove species over 9.5 months in a subtropical Australian estuary. Net (2) H/(1) H fractionation between surface water and leaf lipids decreased by 0.5-1.0‰ ppt(-1) for n-alkanes and 0.4-0.8‰ ppt(-1) for isoprenoids. Xylem water was (2) H depleted relative to surface water, reflecting (2) H discrimination of 4-10‰ during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water (2) H enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water δ(2) H values cannot explain the salinity-dependent (2) H depletion in leaf lipids, nor the 30‰ range in leaf lipid δ(2) H values among species. Biochemical changes in direct response to salt stress, such as increased compatible solute production or preferential use of stored carbohydrates, and/or the timing of lipid production and subsequent turnover rates, are more likely causes.

A management focused investigation into phytoplankton blooms in a sub-tropical Australian estuary

Tidal flushing, primary production and phytoplankton and zooplankton communities were investigated to establish the processes leading to persistently high levels of phytoplankton (in excess of 15 to 60 μg l−1 chlorophyll a) in a particular reach of Berowra Estuary, a relatively large drowned river valley estuary in New South Wales, Australia. Residence times at the bloom site were sufficiently long to provide an opportunity for exponential growth of algae and to result in large accumulations of cells and high rates of primary production. Phytoplankton taxa over the study period were dominated by various diatoms, Chaetoceros, Pseudonitzschia, Thalassiosira and Skeletonema. Zooplankton biomass (wet weight of taxa between 90 and 900 μ m equivalent circular diameter), estimated using image analysis technology was significantly greater in the bloom area (200 mg m−3), than upstream (140 mg m−3) and downstream (120 mg m−3) sites. Numbers of individuals in the smallest zooplankton size classes were significantly related to chlorophyll a concentrations. The relationship decreased with increases in the particle size classes, showing an uncoupling between chlorophyll a concentrations and abundance of larger zooplankton. Residence times in the bloom reach were sufficient for secondary production (somatic and egg production) to occur in the zooplankton taxa present. Growth of phytoplankton under the conditions of nutrients, light and temperature exceeded losses due to zooplankton grazing and estuarine flushing, maintaining chlorophyll concentrations of more than 10 μ g·l−1.

Feeding by fish visiting inundated subtropical saltmarsh

Australian saltmarshes are inundated less frequently and for shorter periods than most northern hemisphere marshes, and when inundated provide transient fish a diverse prey assemblage. We determined the extent of feeding on saltmarsh by examining stomach contents of a common marsh transient, glassfish ( Ambassis jacksoniensis), in the Coombabah estuary in subtropical Queensland. We tested the hypotheses that fish caught after visiting the marsh (after (M)) would have similar quantities of food (stomach fullness index, SFI) but different prey composition (abundance, weight) both to fish collected before (Before) saltmarsh inundation and to fish that had not visited the marsh but were caught after marsh inundation (After (NM)). Sampling was done on multiple nights over 3 months in winter, when the marsh is inundated on spring tides at night only. SFI values of After (M) fish were significantly higher (SFI ≈ 12%) than those of Before and After (NM) fish (SFI ≈ 0–1%). After (M) fish also had very different prey composition, eating more crab zoea (> 100 zoea fish − 1 ) than Before fish (10) and After (NM) fish (0). After (M) fish showed a consistent pattern in zoea abundances among sampling nights, in all months, with lower zoea abundances on the first night that the marsh was inundated than on subsequent nights. This is attributed to the synchronized spawning of crabs resident on the marsh, releasing their zoea on the ebb of the second inundating tide of the month. Fish stomach contents did not differ before and after smaller high tides not inundating the marsh (SFI ≈ 0–1%). Experimental evidence showed that glassfish evacuate their stomach contents in about an hour under starvation conditions, further strengthening our contention that the stomach contents of After (M) fish represent prey ingested on the marsh. The demonstration of intensive feeding by fish visiting this marsh points to a potentially important role of saltmarsh in the trophodynamics of subtropical Australian estuaries.

Benthic nutrient fluxes in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Diurnal benthic fluxes of dissolved inorganic and organic nitrogen (DIN and DON) and dinitrogen gas (N2) were measured in euphotic sediments of 3 shallow sub-tropical Australian estuaries during 4 seasons. The estuaries included 2 impacted by sewage effluent (Brunswick and Simpsons estuaries) and 1 relatively pristine system (Sandon estuary). Sediments acted predomi- nantly as net sinks of DIN throughout the year, except in the nutrient-enriched upper reaches of the Brunswick estuary, where large effluxes of NH4 + occurred during the summer wet season. Distinct light/dark variations in NH4 + fluxes with reduced effluxes or reversal to uptakes occurred during the light in productive sediments. NO3 - was predominantly taken up by sediments at rates proportional to ambient concentrations in the water column. DON commonly comprised a major fraction of fluxes and was controlled primarily by heterotrophic processes in the upper to middle estuaries and autotrophic processes in the lower estuaries. Large deficits in the amount of remineralised DIN (assuming the breakdown of 'Redfield' algae) indicated that a significant amount of nitrogen is either denitrified or immobilised in biomass in the sediments. Benthic fluxes of N2 suggest that denitrifica- tion accounts for a relatively small fraction of the missing nitrogen and immobilisation in biomass (and flow up the food chain) is a potentially major pathway of nitrogen removal in these estuaries. Significant rates of benthic productivity also stimulated secondary heterotrophic production, promot- ing competition for nutrient resources in the sediments. The highest rates of DIN uptake coincided with maximum metabolic rates in sediments where the diurnal p/r (gross productivity/respiration) was between 0.5 and 1, suggesting a peak in competition at this metabolic state. Denitrification rates were lowest at p/r 0.5 to 1 and strongly related to uptake of DIN from the water column, suggesting that competition may cause NO3 - limitation in these euphotic sediments. Dissimilatory NO3 - reduc- tion to ammonium may become relatively more important as water column oxygen saturation drops below 40%, a condition that occurs regularly in the upper Brunswick estuary. NH4 + was only effluxed in net heterotrophic sediments (p/r 1). A conceptual model for benthic nutrient cycling in shallow sub-tropical estuaries is proposed whereby benthic productivity favours to the immobilisation of nitrogen in biomass at the expense of denitrification and recycling back to the water column.

Organic matter and benthic metabolism in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Sediment organic matter (OM), chloropigments and benthic metabolism (oxygen, carbon dioxide and alkalinity fluxes) were investigated along 3 sub-tropical Australian estuaries during different seasons. Significant rates of benthic productivity occurred throughout the year in the study estuaries, especially in the lower reaches where sediments were commonly net autotrophic. Benthic microalgae (BMA) contributed up to 20% of the total carbon content in the lower estuary sands, while organic matter was dominated by phytodetritus and allochthonous material in the middle to upper estuaries. Sediment oxygen demand (SOD) incubations were carried out on surface sediments to investigate labile carbon contents. SOD normalised to sediment carbon content (SOD mol C -1 ) confirmed a much higher proportion of labile OM in the lower estuary sediments due to a predominance of living BMA, while the OM pool in the upper and middle estuaries was relatively more degraded. SOD mol C -1 correlated well with both the C:N and the chlorophyll:pheophytin ratios of surface sediments, providing 3 independent measures of sediment OM quality. Comparisons between SOD and benthic community respiration rates (O 2 fluxes) measured in sediment core incubations suggested that oxygen demand due to the oxidation of reduced sulphur compounds formed a significant fraction of the O 2 flux, and that fluxes may be enhanced by bioturbation. O 2 and TCO 2 respiration rates were overall poorly correlated and suggested that benthic metabolism is characterised by periods of anaerobic mineralisation (TCO 2 :O 2 > 2) during periods of high OM supply, followed by aerobic respiration and reoxidation (TCO 2 :O 2 1 to 2) as OM supply waned. At many sites throughout the study TCO 2 :O 2 flux ratios were less than 1 and coupled with alkalinity uptakes, indicating that either sulphide oxidation or nitrification may be significant influences on the benthic fluxes. These processes cause a net drawdown on O 2 effluxes that can lead to significant underestimations of benthic productivity. As such, the use of CO 2 and O 2 fluxes yield distinctly different measures of benthic metabolism.

Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 205:61-83 (2000) - doi:10.3354/meps205061 Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries Bradley D. Eyre* Centre for Coastal Management, Southern Cross University, PO Box 157, Lismore, New South Wales 2480, Australia *E-mail: beyre@scu.edu.au ABSTRACT: Nutrient transformation and phytoplankton growth were examined in 9 river-dominated sub-tropical east Australian estuaries in 1996 using modified mixing diagrams. The sampling program was rapid and strictly controlled, using 2 or 3 boats simultaneously, so that all 9 estuaries were usually sampled within 4 to 5 d of each other. River samples were collected at the head of each estuary on a flow-weighted basis, and variations in river concentrations and flushing times were used to calculate conservative mixing lines. This was to avoid Œapparent¹ non-linear distributions in the mixing diagrams associated with river-source variations on a time scale less or equal to the flushing time of the estuary. A number of general patterns of biogeochemical behaviour were observed across most, or all, of the estuaries. The 4 northern, and most likely the 5 southern, estuaries flushed fresh to the mouth during a flood in May, allowing most of the flood-borne material to escape from the system. Phytoplankton appear to exert the dominant control on nutrient transformation in the 9 estuaries, with non-biological processes only playing a minor role, if any. Eight of the 9 estuaries are potentially nutrient-limited, with nutrient concentrations falling below the upper half-saturation constants required for phytoplankton growth. The estuaries became potentially more P-limited, and less N-limited, as the wastewater loading to each system increased. During most sampling runs, maximum and mean concentrations of phytoplankton biomass (chlorophyll a) in the Tweed, Brunswick, Bellinger, Nambucca, Macleay and Hastings estuaries were significantly correlated with the wastewater DIN (dissolved inorganic nitrogen) loading index (daily wastewater load per m3 of estuary volume multiplied by the flushing time of the estuary in days). In contrast, the diffuse DIN loading index appeared more important for controlling phytoplankton biomass (chlorophyll a) in the 3 estuaries (Richmond, Clarence and Manning) that received a low wastewater DIN load. Sub-tropical Australian estuaries are characterised by a high degree of variability in nutrient delivery and phytoplankton growth. The timing and magnitude of hydrological factors appears to be the major feature that determines the differences in the temporal patterns of phytoplankton growth between sub-tropical and temperate regions. Nutrient-loading and phytoplankton growth in the 9 estuaries appears to be in phase, suggesting that stored and recycled nutrients may play a smaller role in maintaining phytoplankton growth in these systems compared to the larger partially mixed temperate systems. Management efforts in the 9 estuaries should be first directed towards reducing the wastewater DIN loading index to <1, followed by management strategies focused on controlling diffuse loadings. There may, however, be a trade-off associated with reducing the wastewater loading index with a proportional reduction in fisheries production. KEY WORDS: Sub-tropical · Estuary · Comparison · Mixing plot · Nutrients · Phytoplankton growth Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 205. Online publication date: October 19, 2000 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2000 Inter-Research.

Water quality changes in an episodically flushed sub-tropical Australian estuary: A 50 year perspective

Abstract Recent and historical data sets from the Richmond River estuary (New South Wales, Australia) were examined for possible impacts of changing land use patterns in the catchment and estuary floodplain on water quality over the last 50 years. Floodplain management practices, including further draining and disturbance of acid sulphate soils appear to have had no appreciable effect on the processes that control the degree of oxygen saturation in the estuary. Following runoff events phosphate and nitrate concentrations at a given salinity are respectively 2.5 and 3.0 times higher than 50 years ago, due to leaching from agricultural areas where fertilisers are applied. However, these concentrations quickly decrease after the runoff event due to rapid flushing of the system. Land use changes in the last 50 years appear to have had no impact on dry or low-flow phosphate and nitrate concentrations in the estuary, which are dominantly controlled by internal processes due to long water residence times. Particulate and bottom sediment phosphorus buffering apparently maintain dry season phosphate concentrations in the estuarine water column between about 0.30 and 0.80 μM. Nitrate levels are quite low (

A classification of tropical and subtropical Australian estuaries

AbstractThe ability to use large databases to classify estuaries on the basis of any number of characteristics may be valuable in planning a national system of estuarine reserves in Australia which will conserve the range of variability within and between regions.A recently completed inventory of Australian estuaries and marine embayments has provided quantitative data on a large number of estuaries from which comparisons are made with existing classifications based on qualitative descriptions.Area measurements are analysed for mangroves, salt marsh/clay pan, seagrass, intertidal flats and open water from 571 tropical and subtropical estuaries north from Carnarvon on the west coast around to Coffs Harbour on the east.The relative proportions of mangroves and salt marsh within estuaries show strong relationships with average annual rainfall although there are no consistent trends in the absolute area of each wetland type.The estuaries are classified into three distinct groups on the basis of rainfall and the dominant wetland type.Greater tide range coincides with a greater area per estuary of all wetland types. As with rainfall, the estuaries fall into three categories of wetland proportions.