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The influence of natural populations of the sub-surface deposit-feeding amphipod Victoriopisa australiensis on sediment biogeochemistry was assessed by randomly collecting 21 sediment cores in a zone of Coombabah Lake, southern Moreton Bay, Australia, where the benthic infauna was dominated by this species. Cores were incubated sequentially to determine sediment-water column fluxes of oxygen, dissolved inorganic carbon and inorganic N species, followed by incubations to determine rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) using the isotope pairing technique. Finally, each core was sieved in order to determine the population and biomass of amphipods present. Whilst all measures of overall benthic metabolism (sediment oxygen demand, and effluxes of inorganic carbon and nitrogen) showed increased with amphipod density, with rates being stimulated 70-220% at the highest categorised density range of 2,500-3,500 ind m(-2), only the correlation with dissolved inorganic carbon was statistically significant. In contrast, there were no discernable trends between amphipod densities and any of the N-cycle processes with the slopes of all correlations being very close to zero. These results highlight the differences in mesocosm simulations of fauna effects, which primarily relate to shifts in rates of organic matter turnover, compared to natural sediments where fauna effects relate more to induced changes in rates of organic matter deposition. Therefore, while mesocosms represent a powerful tool to investigate the mechanisms by which fauna influences microbial metabolism in the sediment, only studies of natural sediments can determine to what extent these mechanisms function in situ. Refereed/Peer-reviewed

Dunn, R.J.K.; Lemckert, C.J.; Teasdale, P.R., and Welsh, D.T., 2013. Macroinfauna dynamics and sediment parameters of a subtropical estuarine lake—Coombabah Lake (Southern Moreton Bay, Australia). Journal of Coastal Research, 29(6A), 156–167. Coconut Creek (Florida), ISSN 0749-0208. The distribution, composition, density, and biomass of benthic macrofauna within estuarine environments typically exhibit significant variations attributable to heterogeneity in and interactions between physical, biological, and chemical processes. The spatial and temporal dynamics of benthic macroinfauna assemblages and physicochemical sediment parameters within the intertidal mudflats of a subtropical estuarine lake (Coombabah Lake, Southern Moreton Bay) were studied at four sites from August 2006 to April 2007. No significant seasonal changes were observed at any site for all physical sediment parameters. The northern sample sites were characterised by fine- to medium-grained to moderately to poorly sorted sediments and the southern sample sites by fine-grained to moderately well to well-sorted. A total of 1029 individuals representing species from three orders, including deposit feeding and filter feeding macroinfaunal groups, were collected. The highest combined species densities occurred in the fine-grained southern sites, with the greatest combined species density occurring at Site 4 during winter. Amphipods (Victoriopisa australiensis) and polychaete worms (Simplisetia aequisetis) dominated the lake-wide faunal community with V. australiensis, representing 49% of the total retrieved macroinfauna. Significant correlations between mean macroinfauna densities, biomassDW, sediment parameters, and seasonal maximum monthly temperatures were identified during the study. Seasonal trends in combined site densities were observed at each of the lake sites, with the highest combined density occurring during winter. Spatial and temporal variations might also be partially explained by the predation pressures of fish and migratory wading birds within the lake, with the seasonal presence of migratory wading birds coinciding with the minimum observed macroinfauna densities at each sample site.

Benthic oxygen and nutrient fluxes, and rates of nitrate reduction, were determined seasonally under light and dark conditions at four sites within a sub-tropical coastal lagoon (Coombabah Lake, Australia). Sediments at all sites were strongly heterotrophic acting as strong oxygen sinks and sources of dissolved inorganic nitrogen (DIN) in all seasons during both light and dark incubations. Sediment oxygen demand (SOD) and DIN effluxes were greatest during summer, but showed only a relatively small degree of seasonal variation. In contrast, there was a strong spatial trend in SOD and DIN effluxes, which were consistently greater at the sites with fine grained compared to the coarser sediments. Microphytobenthos (MPB) directly influenced SOD and DIN effluxes, with lower SOD and DIN effluxes measured during all light incubations. Strong correlations were found between sediment chlorophylla content and light-dark shifts in oxygen and ammonium fluxes (Delta O-2 and Delta NH4+), and between Delta O-2 and Delta NH4+. Rates of total nitrate reduction were relatively low ranging from 3 to 26 mu mol N m(-2) h(-1) and exhibited only minor seasonal variations. Dissimilatory nitrate reduction to ammonium (DNRA) was the dominant pathway for nitrate reduction, accounting for on average, 65 and 68% of total nitrate reduction during light and dark incubations. respectively. Nitrification was the dominant source of nitrate fuelling nitrate reduction processes, accounting for approximately 90% of total nitrate supply. In contrast to typical MPB colonised sediments, rates of nitrification and, as a consequence, nitrate reduction rates were consistently stimulated in the light, indicating that MPB primarily influenced these processes through photosynthetic oxygen evolution rather than through competition for inorganic N-species. Refereed/Peer-reviewed

A previous study has demonstrated that in sandy sediment the marine yabby (Trypaea australiensis) stimulated benthic metabolism, nitrogen regeneration and nitrification, but did not stimulate denitrification, as the intense bioturbation of the yabbies eliminated anoxic microzones amenable to denitrification. It was hypothesised that organic matter additions would alleviate this effect as the buried particles would provide anoxic microniches for denitrifiers. To test this hypothesis a 55-day microcosm (75 cm x 36 cm diameter) experiment, comprising four treatments: sandy sediment (S), sediment + yabbies (S + Y), sediment + A. marina litter (S + OM) and sediment + yabbies + A. marina litter (S + Y + OM), was conducted. Trypaea australiensis significantly stimulated benthic metabolism, nitrogen regeneration, nitrification and nitrate reduction in the presence and the absence of litter additions. In contrast, the effects of litter additions alone were more subtle, developed gradually and were only significant for sediment oxygen demand. However, there was a significant interaction between yabbies and litter with rates of total nitrate reduction and denitrification being significantly greater in the S + Y + OM than all other treatments, presumably due to the decaying buried litter providing anoxic micro-niches suitable to nitrate reduction. In addition, both T. australiensis and litter significantly decreased rates of DNRA and its contribution to nitrate reduction. Refereed/Peer-reviewed