Role of carbonate burial in Blue Carbon budgets

Calcium carbonates (CaCO 3 ) often accumulate in mangrove and seagrass sediments. As CaCO 3 production emits CO 2 , there is concern that this may partially offset the role of Blue Carbon ecosystems as CO 2 sinks through the burial of organic carbon (C org ). A global collection of data on inorganic carbon burial rates (C inorg , 12% of CaCO 3 mass) revealed global rates of 0.8 TgC inorg yr −1 and 15–62 TgC inorg yr −1 in mangrove and seagrass ecosystems, respectively. In seagrass, CaCO 3 burial may correspond to an offset of 30% of the net CO 2 sequestration. However, a mass balance assessment highlights that the C inorg burial is mainly supported by inputs from adjacent ecosystems rather than by local calcification, and that Blue Carbon ecosystems are sites of net CaCO 3 dissolution. Hence, CaCO 3 burial in Blue Carbon ecosystems contribute to seabed elevation and therefore buffers sea-level rise, without undermining their role as CO 2 sinks. © 2019, The Author(s)

Nitrogen incorporation and retention by bacteria, algae, and fauna in a subtropical, intertidal sediment: An in situ 15N-labeling study

We performed a 15N-labeling study to investigate nitrogen incorporation and retention by the benthic microbial community (bacteria and benthic microalgae) and fauna in the intertidal sediment of the subtropical Australian Brunswick Estuary. The main experiment involved an in situ 15N pulse–chase experiment. After injection of 15NH4+ into the sediment, 15N was traced into bulk sediment, total hydrolyzable amino acids (THAAs, representing bulk proteinaceous biomass), the bacterial biomarker D-alanine, and fauna over a 30- d period. Additional experiments included short-term (24 h) incubations of sediment cores injected with different 15N-labeled substrates (NH4+, NO3-, urea, and an amino acid mixture) and sediment core incubations for analysis of benthic fluxes of O2, dissolved inorganic carbon, NH4+, NOx-, dissolved organic nitrogen, and N2. 15N was rapidly incorporated and strongly retained in microbial biomass (THAAs) during the 30-d period in situ, indicating efficient recycling of 15N by the benthic microbial community. Analysis of 15N in D-alanine revealed a major bacterial contribution (50–100%) to total microbial 15N incorporation and retention. 15N was also incorporated into fauna via grazing on 15N-labeled microbial biomass, but this was a negligible fraction (<1%) of total 15N in the sediment. Altogether, results show that efficient recycling of nitrogen by the benthic microbial community can be an important mechanism for nitrogen retention in the sediment and an important pathway supporting benthic microbial production.

A rapid protocol for assessing sediment condition in eutrophic estuaries

The enrichment of sediments with nutrients and organic matter (eutrophication) is a key anthropogenic stressor of estuaries worldwide, impacting their sediment condition, ecology and ecosystem service provision. A key challenge for estuary managers and scientists is how to effectively quantify and monitor these changes in ecological condition in a timely and cost-effective manner. We developed a Rapid Assessment Protocol (RAP) for characterizing sediment condition based on the qualitative characteristics of sediment colour, odour and texture. We evaluated its utility for assessing sediment condition, and particularly the degree and effects of sediment enrichment (as quantified by complementary measurements of total C, organic C and total N) across 97 sites throughout a eutrophic microtidal estuary. RAP results were strongly and significantly correlated with the degree of sediment enrichment, with RAP scores correctly identifying the assigned enrichment class (low, medium, high) of 83.5% of sites. More enriched sediments exhibited poorer condition, manifested as significantly lower RAP scores for sediment colour, texture and odour, particularly (but not only) where enrichment coincided with elevated mud content. The RAP was particularly successful (<12% misclassification) at identifying sites with low levels of enrichment, indicating its promise as a first-pass survey approach for identifying potential reference or control sites to support impact assessments. RAP approaches based on qualitative sediment characteristics can provide a useful proxy for the degree and impacts of inorganic and organic enrichment, with potentially broad applicability for supporting timely, cost-effective assessment and monitoring of sediment condition in estuaries worldwide

Novel use of cavity ring-down spectroscopy to investigate aquatic carbon cycling from microbial to ecosystem scales

Development of cavity ring-down spectroscopy (CRDS) has enabled real-time monitoring of carbon stable isotope ratios of carbon dioxide and methane in air. Here we demonstrate that CRDS can be adapted to assess aquatic carbon cycling processes from microbial to ecosystem scales. We first measured in situ isotopologue concentrations of dissolved CO2 (12CO2 and 13CO2) and CH4 (12CH4 and 13CH4) with CRDS via a closed loop gas equilibration device during a survey along an estuary and during a 40 h time series in a mangrove creek (ecosystem scale). A similar system was also connected to an in situ benthic chamber in a seagrass bed (community scale). Finally, a pulse-chase isotope enrichment experiment was conducted by measuring real-time release of 13CO2 after addition of 13C enriched phytoplankton to exposed intertidal sediments (microbial scale). Miller-Tans plots revealed complex transformation pathways and distinct isotopic source values of CO2 and CH4. Calculations of δ13C-DIC based on CRDS measured δ13C-CO2 and published fractionation factors were in excellent agreement with measured δ13C-DIC using isotope ratio mass spectroscopy (IRMS). The portable CRDS instrumentation used here can obtain real-time, high precision, continuous greenhouse gas data in lakes, rivers, estuaries and marine waters with less effort than conventional laboratory-based techniques

Quantification of denitrification in permeable sediments: Insights from a two-dimensional simulation analysis and experimental data

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.

Author Correction: The future of Blue Carbon science (Nature Communications, (2019), 10, 1, (3998), 10.1038/s41467-019-11693-w)

10.1038/s41467-019-13126-0Nature Communications101514