Nitrous oxide is a powerful, long‐lived greenhouse gas, but we know little about the role of estuarine areas in the global N₂O budget. This review summarizes 56 studies of N₂O fluxes and associated ...biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N₂O production occurs as a result of sediment denitrification, although the water column contributes N₂O through nitrification in suspended particles. The most important factors controlling N₂O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N₂O flux to the atmosphere (range 0.15–0.91; median 0.31; Tg N₂O‐N yr⁻¹). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N₂O emissions from estuaries. We estimate that a doubling of current median NO₃ ⁻ concentrations would increase the global estuary water–air N₂O flux by about 0.45 Tg N₂O‐N yr⁻¹ or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N₂O emissions of 0.002 Tg N₂O‐N yr⁻¹. In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N₂O‐N yr⁻¹. Decreased oxygen concentrations may inhibit production of N₂O by nitrification; however, sediment denitrification and the associated ratio of N₂O:N₂ is expected to increase.
Advective flows rapidly transport water, solutes, and particles into and out of permeable sand beds and significantly affects the biogeochemistry of coastal environments. In this paper, we reviewed ...the drivers of porewater and groundwater advection in permeable shelf sediments in an attempt to bridge gaps among different disciplines studying similar problems. We identified the following driving forces: (1) terrestrial hydraulic gradients, (2) seasonal changes in the aquifer level on land moving the location of the subterranean estuary, (3) wave setup and tidal pumping, (4) water level differences across permeable barriers, (5) flow- and topography-induced pressure gradients, (6) wave pumping; (7) ripple and other bed form migration, (8) fluid shear, (9) density-driven convection, (10) bioirrigation and bioturbation, (11) gas bubble upwelling, and (12) sediment compaction. While these drivers occur over spatial scales ranging from mm to km, and temporal scales ranging from seconds to years, their ultimate biogeochemical implications are very similar (i.e., they are often a source of new or recycled nutrients to seawater and transform organic carbon into inorganic carbon). Drivers 2–12 result in no net water input into the ocean. Taking all these mechanisms into account, we conservatively estimate that a volume equivalent to that of the entire ocean is filtered by permeable sediments at time scales of about 3000 years. Quantifying the relative contribution of these drivers is essential to understand the contribution of sediments to the global cycles of matter.
Ocean acidification refers to the lowering of the ocean's pH due to the uptake of anthropogenic CO
from the atmosphere. Coral reef calcification is expected to decrease as the oceans become more ...acidic. Dissolving calcium carbonate (CaCO
) sands could greatly exacerbate reef loss associated with reduced calcification but is presently poorly constrained. Here we show that CaCO
dissolution in reef sediments across five globally distributed sites is negatively correlated with the aragonite saturation state (Ω
) of overlying seawater and that CaCO
sediment dissolution is 10-fold more sensitive to ocean acidification than coral calcification. Consequently, reef sediments globally will transition from net precipitation to net dissolution when seawater Ω
reaches 2.92 ± 0.16 (expected circa 2050 CE). Notably, some reefs are already experiencing net sediment dissolution.
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 (ΩAr; 300–420% increase in dissolution per unit decrease in ΩAr) 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 ΩAr 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.
•Benthic metabolism has a strong influence on CaCO3 dissolution in permeable coral reef sediments.•Changes in sediment P/R could act to exacerbate or alleviate sediment dissolution under elevated seawater CO2.•Empirical relationships developed here demonstrate how sediment dissolution changes under different light regimes.•Dissolution of sediments could push coral reefs from net precipitating to net dissolving by the year 2100.
Groundwater discharge could be a major, but as yet poorly constrained, source of carbon dioxide to lakes, wetlands, rivers, estuaries, and coastal waters. We demonstrate how coupled radon (222Rn, a ...natural groundwater tracer) and pCO2 measurements in water can be easily performed using commercially available gas analysers. Portable, automated radon and pCO2 gas analysers were connected in series and a closed air loop was established with gas equilibration devices (GED). We experimentally assessed the advantages and disadvantages of six GED. Response times shorter than 30 min for 222Rn and 5 min for pCO2 were achieved. Field trials revealed significant positive correlations between 222Rn and pCO2 in estuarine waterways and in a mangrove tidal creek, implying that submarine groundwater discharge was a source of CO2 to surface water. The described system can provide high resolution, high precision concentrations of both radon and pCO2 with nearly no additional effort compared to measuring only one of these gases. Coupling automated 222Rn and pCO2 measurements can provide new insights into how groundwater seepage contributes to aquatic carbon budgets.
Estuaries have high rates of primary production and respiration and can be hotspots for carbon dioxide and methane enriched submarine groundwater discharge. Here, we report high resolution pCO2, CH4, ...δ13C–CO2, δ13C–CH4 and radon (222Rn, a natural groundwater tracer) observations along North Creek estuary, Australia (S28°48′, E153°34′) during four spatial surveys over a diurnal cycle in January 2013 (summer). There were distinct tidal and diurnal differences in estuarine pCO2 and CH4, which lead to tidal differences of 3.6 fold and 5 fold in the estimated CO2 and CH4 diffusive water to air fluxes respectively, and up to a 2.4 fold difference in diurnal flux estimates of CH4. Carbon stable isotopes revealed tidal and diurnal differences in the source δ13C value of CO2 and CH4, and minor CH4 oxidation within the estuary. The CO2 outgassing rates based on the spatial surveys were different than the outgassing derived from three fixed time series stations along the estuary. There was agreement between the methods in the lower and upper estuary where pCO2 had a relatively low range over the study (~600μatm and 3000μatm respectively). However, in the mangrove surrounded mid estuary where pCO2 ranged from ~1450 to 11,000μatm over a tidal cycle, fluxes estimated by the survey method were ~30% of the time series estimates. This study highlights the importance of considering tidal and diurnal variability when estimating the flux of CO2 and CH4 from estuaries, and discusses how a combination of diurnal (productivity/respiration) and tidal (groundwater/mixing) processes may drive surface water pCO2 and CH4 over short-term time scales.
•We measured CO2, CH4, δ13C-CO2, δ13C-CH4 and 222Rn along an estuary.•Concentrations and isotopes varied over tidal and diel cycles along the estuary.•Concurrent estimates of water to air fluxes varied between surveys and time series.•Tidal and diel variability of CO2 and CH4 fluxes in estuaries can be significant.
Nitrogen (N) loss from different benthic habitats via net denitrification and burial was quantified, and first-order N budgets were constructed, for three geomorphically distinct shallow warm ...temperate South-East Australian barrier estuaries. Seagrass communities were the most important benthic habitats for N loss via net denitrification due to a combination of their area and high denitrification rates. Similarly, the largest N loss via burial occurred in the seagrass communities in the Hastings River Estuary and Wallis Lake, but in contrast, the largest annual loss of N via burial in the Camden Haven occurred in the subtidal muds due to their large area. N inputs to the river-dominated Hasting River Estuary were dominated by diffuse sources from the catchment. Budget deficits in Camden Haven and Wallis Lake suggest that the largest input of N may have been from the ocean, although missing N-fixation and/or groundwater cannot be excluded. Export to the ocean was the largest loss of N in the Hasting River Estuary followed by net denitrification and then burial. Net denitrification was the largest loss of N in the Camden Haven and Wallis Lake followed by burial. As the systems mature (evolve) the burial of N per m², the loss of N via denitrification per m² and the % of the total N load that is removed as fish per m², all decrease. Overall N loss via denitrification for a given residence time may be higher in shallow and oligotrophic coastal systems with extensive seagrass habitats than deeper temperate systems.
N2 flux rates (net denitrification) were measured over a diel cycle, seasonally, in 12 benthic habitats across three warm temperate Australian coastal systems. Dark N2‐N fluxes were strongly ...controlled by sediment oxygen demand (SOD) across the 3 estuaries, 4 seasons, and 12 benthic habitats (r2 = 0.743; p < 0.001; n = 142; slope = 0.0170). However, some of the slopes differed significantly between seasons and among estuaries and habitats, and all of the slopes were correlated with the δ13C values and C:N ratios of sediment organic matter. Ternary mixing diagrams with the contribution of algal, seagrass, and terrestrial/mangrove material to sediment organic matter showed that habitats, seasons, and estuaries dominated by a mixture of seagrass and algal material had the lowest slopes, and slopes increase as habitats, seasons, and estuaries have an increasing contribution from terrestrial/ mangrove material. Overall, the slopes of dark N2 fluxes versus SOD were low compared to previous studies, most likely due to either, or a combination of, the C:N ratio of the organic matter, the mixture of C:N ratios making up the organic matter, the structure of the organic matter, and/or the SOD rates. This study demonstrated that it is not only the quantity but also the type (quality), and maybe the mixture, of organic matter that is an important control on denitrification. As such, rapid global changes to detrital sources to coastal systems due to losses of mangrove, seagrasses, and saltmarshes, and associated increases in algae and macrophytes, are also expected to impact system level losses of nitrogen via denitrification.
Key Points
OM effect on denitrification in coastal systems mostly focused on quantity
Quantity and quality of OM is an important control on denitrification
Global changes to detrital sources may impact N loss via denitrification
Attributing nitrogen export to specific land use within heterogeneous catchments remains a challenge due to the spatio‐temporal variability in conditions influencing the mobilization and fate of ...nitrogen species. This study demonstrates that the measurement of dual stable isotopes of nitrate, taken along with routine tributary measurement of nitrogen in nitrate (NO3−‐N) and ammonium (NH4+‐N), aids in apportioning sources of the overall nitrogen load during wet periods. An inverse modeling technique was developed to estimate the land use‐specific export rates of NO3−‐N and NH4+‐N from the Caboolture River Catchment in Queensland, Australia. Measurements of nitrogen in streamflow at 51 locations during six sampling campaigns (May 2012 to April 2013) were made along with catchment geospatial data that was used to disaggregate sub‐catchments into six land use fractions. A hydrological model was applied to compute the runoff from each fraction and water routing through the stream network. This data was used within a nitrogen mixing model with inclusion δ15NNO3 and δ18ONO3. The land uses specific export rate was computed inversely as the posterior of a Bayesian interference applied to the model. During higher rainfall periods when export rates were highest, the main land use exporting nitrogen was wetland (110 g/ha/day NO3−‐N, 27 g/ha/day NH4+‐N) resulted from mineralization and nitrification of organic N, followed by urban (16 g/ha/day NO3−‐N, 2.3 g/ha/day NH4+‐N). The advantage of using the dual isotopes in conjunction with the nitrogen concentration data was demonstrated by reduced uncertainty in the computed rates during the higher rainfall periods, relative to calculations without δ15NNO3 and δ18ONO3.
Plain Language Summary
Attributing nitrogen export to specific land use types within heterogeneous catchments is difficult. This paper demonstrates the use of an inverse modeling technique, dissolved inorganic nitrogen concentration data, a dual stable isotope measurement data, and geospatial data to estimate the land use‐specific export rates of dissolved inorganic nitrogen from six land use types in the Caboolture River Catchment in Queensland, Australia. The results show that during higher rainfall periods when export rates were highest, the main source of nitrogen was found to be wetland (110 g/ha/day NO3−‐N and 27 g/ha/day NH4+‐N), followed by urban (16 g/ha/day NO3‐N and 2.3 g/ha/day NH4+‐N).
Key Points
An inverse modeling technique was developed to estimate the land use‐specific export rate of nitrogen using isotopes signatures
The model was embedded within a Bayesian framework to assess the predictive uncertainty with and without inclusion of dual isotopes of nitrate
During wet periods, when nitrogen export rates were highest, the predictive uncertainty was reduced when the model included dual isotopes of nitrate
PDF
