Diurnal benthic fluxes of dissolved inorganic and organic nitrogen (DIN and DON) and dinitrogen gas (N 2 ) 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 predominantly as net sinks of DIN throughout the year, except in the nutrient-enriched upper reaches of the Brunswick estuary, where large effluxes of NH 4 + occurred during the summer wet season. Distinct light/dark variations in NH 4 + fluxes with reduced effluxes or reversal to uptakes occurred during the light in productive sediments. NO 3 -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 N 2 suggest that denitrification 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, promoting 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 NO 3 -limitation in these euphotic sediments. Dissimilatory NO 3 -reduction 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. NH 4 + was only effluxed in net heterotrophic sediments (p/r < 1), while fluxes tended towards zero in net autotrophic 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.

Nitrogen that has been recycled in the benthos supports high rates of primary and secondary production in estuaries. However, little is known about the effect of future climate on benthic nitrogen recycling and assimilation. An ex situ core incubation was used to assess the impact of combinations of warming (8°C range) and ocean acidification (OA) (i.e. increased pCO2 and decreased pH) on ammonium (NH4+) and nitrate/nitrite (NOx) fluxes and 15N-nitrate assimilation in shallow unvegetated estuarine sediments. Dissolved inorganic nitrogen (DIN = NH4+ + NOx) fluxes were significantly affected by the interaction of warming and OA, highlighting the importance of considering combined stressor treatments when investigating ecosystem responses to future climates. Warming alone increased DIN efflux from the sediments. At current mean ambient temperatures (23°C) and below (Δ-3°C), OA significantly increased DIN effluxes, but there was little to no effect of OA on DIN fluxes at warmer temperatures (Δ+3°C and Δ+5°C). OA reduced the 15N assimilation/retention of the sediments across all temperatures, suggesting that nitrogen retention in bacterial biomass was reduced, despite OA also increasing primary productivity. As such, under the projected future climate of ~3°C warming and doubling of pCO2 (~1000 µatm), unvegetated estuarine sediments are likely to have a more rapid turnover of DIN driven by greater microphytobenthos production and recycling.

Anthropogenically driven alterations to coastal sediments and their benthic macroinvertebrate communities impair ecosystem function. However, this paradigm is yet to be tested in ecosystems that typically harbour underdeveloped communities lacking larger bioturbating species. Here, we investigated the effects of sediment condition and macroinvertebrate communities on benthic metabolism, nutrient exchange and denitrification (N2 production), and assessed the relative importance of taxon richness, abundance, biomass and community bioturbation potential in influencing these processes in 2 regions of the highly modified, microtidal Peel-Harvey Estuary in temperate Western Australia. Sediment condition influenced benthic metabolism more than the macroinvertebrate community, whereas the reverse was true for nutrient exchange. Denitrification was driven by sediment condition and the community in the upper and lower estuary, respectively, highlighting the change in controls of this nitrogen-removal process within estuaries. Overall, benthic macroinvertebrates had little to no effect on many ecosystem processes, exhibiting the limited functional role played by these chronically stressed biota in this estuary. There was also no interaction between sediment condition and the community, suggesting a functional decoupling between these 2 ecosystem components. Where significant macroinvertebrate effects were detected, community biomass was the most frequently selected predictor, demonstrating its fundamental role in ecosystem function. This study reveals pressing implications of what might be expected when benthic environments become particularly degraded and the highly limited potential of the resultant benthic macroinvertebrate communities to provide key ecosystem services such as nutrient processing.

Understanding the influence of macroinvertebrates on ecosystem function often relies on experimental defaunation with methods that remove fauna through minimal sample disturbance. Defaunation is challenging and can lead to confounding effects and/or loss of empirical information when unsuccessful. We evaluated the ability of a deoxygenation treatment to remove macroinvertebrates from sediment cores collected in 2 regions of a microtidal estuary. Only 1 of 16 cores was fully defaunated following 3 deoxygenation cycles. To counteract confounding effects of partial defaunation, we quantified the biomass remaining in each core and used these data as a covariate in statistical models. The unremoved biomass had, in some cases, significant effects on alkalinity fluxes, with positive linear relationships evident, and net phosphate fluxes. The community in the upper estuary that regularly experiences hypoxia exhibited stronger sediment emergence responses (82-100%). The remaining fauna were spread equally among annelids, molluscs and arthropods in abundance, although arthropods dominated the biomass. In contrast, fewer macroinvertebrates emerged from sediments from the lower estuary (47-89%), with most of the remaining biomass and abundance being annelids and molluscs. These findings suggest that estuarine taxa have divergent responses to hypoxia and that regional communities are variably prone to eradication of sensitive taxa. Our study shows how the use of defaunation by deoxygenation can create systematic bias, particularly when comparing areas with disparate in situ oxygen regimes, and provides a way to quantitively account for partial defaunation without sacrificing statistical power or using overly destructive methods.