Seasonal, daily and diel N<sub>2</sub> effluxes in permeable carbonate sediments

Abstract

Abstract. Benthic metabolism and inorganic nitrogen and N2 flux rates (denitrification) were measured in permeable carbonate sands from Heron Island (Great Barrier Reef). Some of the N2 flux rates were among the highest measured in sediments. All benthic fluxes showed a significant difference between seasons with higher rates in summer and late summer. There was no distinct response of the benthic system to mass coral spawning. Instead, changes in benthic fluxes over 12 days in summer appear to be driven by tidal changes in water depth and associated changes in phytosynthetically active radiation reaching the sediments. Dark N2 fluxes were strongly correlated to benthic oxygen consumption across all sites and seasons (r2 = 0.63; p < 0.005; slope = 0.035). However, there were seasonal differences with a steeper slope in summer than winter, reflecting either more efficient coupling between respiration and nitrification–denitrification at higher temperatures or different sources of organic matter. Adding data from published studies on carbonate sands revealed two slopes in the dark N2 flux versus benthic oxygen consumption relationship. The lower slope (0.035) was most likely due to high carbon : nitrogen (C : N) organic matter from coral reefs, and associated assimilation of nitrogen by heterotrophic bacteria including enhanced heterotrophic N-fixation, but competition by benthic microalgae or inefficient coupling between respiration and nitrification–denitrification cannot be excluded. The steeper slope (0.089) was most likely due to respiration being driven by low C : N phytodetritus. If the different slopes were driven by the sources of organic matter, then global estimates of continental shelf denitrification are probably about right. In contrast, global estimates of continental shelf denitrification may be over-estimated if the low slope was due to inefficient coupling between respiration and nitrification–denitrification and also due to reduced N2 effluxes in the light associated with competition by benthic microalgae for nitrogen and N-fixation.

The contribution of denitrification and burial to the nitrogen budgets of three geomorphically distinct Australian estuaries: Importance of seagrass habitats

“…Denitrification, burial and nitrogen budgets the light were excluded from a number of sites with high rates of benthic production due to the formation of bubbles in the incubation cores and chambers (Eyre et al 2013a), which reduces the N 2 /Ar ratio giving a pseudo-rate of N-fixation (see Discussion in . As such, there may have been unmeasured benthic N-fixation that could account for some of the N deficit (Eyre et al 2013b). Additional unmeasured sources could be groundwater inputs of N and pelagic N-fixation.…”

Seasonal variability of calcium carbonate precipitation and dissolution in shallow coral reef sediments

“…These lagoons and cays provide habitat for a variety of reef organisms, including invertebrates, fish, seagrass, macroalgae, and benthic microalgae (BMA). Additionally, reef sediments can influence whole ecosystem biogeochemical cycles through photosynthesis and respiration (Glud et al ; Werner et al ) calcification and CaCO 3 dissolution (Andersson et al ; Cyronak et al ), nitrogen fixation and denitrification (Capone et al ; Eyre et al , ), and production of dimethylsulfide (Deschaseaux et al ), among other processes.…”

Does denitrification occur within porous carbonate sand grains?

“…Using a simulation model of a flow field within a ripple, it has previously been shown that, in the absence of any intra-granular porosity effect, potential denitrification rates in the range of those measured here scale up to only ∼ 5 µmol m −2 h −1 ; however under the same conditions with intra-granular porosity, rates increased by an order of magnitude to 50 µmol m −2 h −1 owing to intense coupled nitrification denitrification within the sediment grains near the oxic-anoxic interface (Kessler et al, 2014). Previous chamber measurements for sites 1 and 2 typically show denitrification rates on the order of 60 µmol m −2 h −1 , and no significant difference was observed between the two sites (Eyre et al, 2013b). It is therefore possible that intra-granular porosity can explain some of the discrepancy between the chamber and modelled results, particularly at site 1.…”

“…Model simulations have shown that there may be "wash-out" of nitrogen accumulated within porewaters which can enhance measured nitrogen fluxes (Cook et al, 2006), possibly explaining the higher chamber rates. However, strong relationships between respiration and denitrification in permeable carbonate sands measured using chambers suggest the denitrification rates are reliable (Eyre et al, 2013b).…”

“…In systems with high bottom-water nitrate concentrations, high rates of denitrification have been reported (Gao et al, 2012;Sokoll et al, 2016). Previous studies have also reported high rates of denitrification in carbonate sands (Eyre et al, , 2013b, which is surprising given the low nitrate concentrations in the over-lying water and their highly oligotrophic nature. One possible explanation for high rates of denitrification is the coupling of nitrification and denitrification within microniches associated with sand grains (Jahnke, 1985;Rao et al, 2007;Santos et al, 2012).…”