Description: The distribution of diazotrophs and the magnitude of N2 fixation along with the input of new N through this process remains poorly constrained globally, but particularly in the Southern Pacific Ocean. Here we present a high-resolution dataset (every 0.5° latitude) describing the different N-cycling pathways which control the fixation and sequestration of carbon in the surface waters along a 7000 km transect in the South Pacific Ocean. Key oceanographic features along the P15S GO-SHIP transect from the Antarctic ice edge to the equator, included crossing of the subtropical front (STF), from the sub-Antarctic waters towards the oligotrophic tropics, and the equatorial upwelling region. We show how the natural isotopic abundance of particulate organic matter relate to different biogeochemical transformations in the N-cycle across four oceanic provinces. At all stations we measured N2 fixation rates. In the cold and nutrient rich waters of the Southern Ocean we found measurable N2 fixation rates (〉0.2 nmol L-1.d-1), which increased after the subtropical front, and remained at about 15 nmol L-1.d-1 until the equator. In the subtropical and tropical waters, the input of new nitrogen through N2 fixation could fuel on average 13±19% of the net primary productivity. Our data analysis showed that nitrifying organisms, from bacterial and archaeal genera such as Nitrospina, Nitrospinaceae, Nitrosopumilus, Nitrosopelagicus and Nitrosoarchaeum, prevailed in the Southern Ocean and within the STF. Nitrification rates ranged up to 60 nmol L-1.d-1 within the mixed layer depth in the Southern Ocean. Our results suggest that nitrification above the pycnoclines is an important component of the N cycle in the Southern Ocean. Our data has given us a better understanding of how the different N-cycling pathways relate to primary productivity in the South Pacific Ocean, as well as insights into the potential geographical N pathway shifts in light of the rapidly changing climate.

Description: Dissolved organic/inorganic carbon and oxygen fluxes from whole sediment core incubations subject to temperature and ocean acidification manipulations. Estuaries make a disproportionately large contribution of dissolved organic carbon (DOC) to the global carbon cycle, but it is unknown how this will change under a future climate. As such, the response of DOC fluxes from microbially dominated unvegetated sediments to individual and combined future climate stressors of warming (from Δ-3 °C to Δ+5 °C on ambient mean temperatures) and ocean acidification (OA, ~2 times the current partial pressure of CO2, pCO2) was investigated ex situ. Warming alone increased sediment heterotrophy, resulting in a proportional increase in sediment DOC uptake, with sediments becoming net sinks of DOC (3.5 to 8.8 mmol-C m-2 d-1) at warmer temperatures (Δ+3 °C and Δ+5 °C, respectively). This temperature response changed under OA conditions, with sediments becoming more autotrophic and a greater sink of DOC (1 to 4 times greater than under current-pCO2). This response was attributed to the stimulation of heterotrophic bacteria with the autochthonous production of labile organic matter by microphytobenthos. Extrapolating these results to the global area of unvegetated subtidal estuarine sediments, the future climate of warming (Δ+3 °C) and OA may decrease the estuarine export of DOC by ~80 % (~150 Tg-C yr-1) and have a disproportionately large impact on the global DOC budget.

Description: Ocean acidification (OA) is expected to reduce the net ecosystem calcification (NEC) rates and overall accretion of coral reef ecosystems. However, despite the fact that sediments are the most abundant form of calcium carbonate (CaCO3) in coral reef ecosystems and their dissolution may be more sensitive to OA than biogenic calcification, the impacts of OA induced sediment dissolution on coral reef NEC rates and CaCO3 accretion are poorly constrained. Carbon dioxide addition and light attenuation experiments were performed at Heron Island, Australia in an attempt to tease apart the influence of OA and organic metabolism (e.g. respiratory CO2 production) on CaCO3 dissolution. Overall, CaCO3 dissolution rates were an order of magnitude more sensitive to elevated CO2 and decreasing seawater aragonite saturation state (Omega Ar; 300-420% increase in dissolution per unit decrease in Omega Ar) than published reductions in biologically mediated calcification due to OA. Light attenuation experiments led to a 70% reduction in net primary production (NPP), which subsequently induced an increase in daytime (115%) and net diel (375%) CaCO3 dissolution rates. High CO2 and low light acted in synergy to drive a 575% increase in net diel dissolution rates. Importantly, disruptions to the balance of photosynthesis and respiration (P/R) had a significant effect on daytime CaCO3 dissolution, while average water column ?Ar was the main driver of nighttime dissolution rates. A simple model of platform-integrated dissolution rates was developed demonstrating that seasonal changes in photosynthetically active radiation (PAR) can have an important effect on platform integrated CaCO3 sediment dissolution rates. The considerable response of CaCO3 sediment dissolution to elevated CO2 means that much of the response of coral reef communities and ecosystems to OA could be due to increases in CaCO3 sediment and framework dissolution, and not decreases in biogenic calcification.