Seasonal, daily and diel N2 effluxes in permeable carbonate sediments

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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 (r 2 = 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 N 2 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 compen t tition by benthic microalgae for nitrogen and N-fixation. © Author(s) 2013.

Figures

  • Table 1. Summary of benthic chamber sampling campaigns.
  • Table 2. Results from linear mixed model analysis of the effect of season and sediment type on benthic fluxes.
  • Table 3. Results from paired t test to determine differences in dark and light fluxes.
  • Table 4. Denitrification rates in carbonate sediments.
  • Fig. 1. Benthic dark O2 consumption (respiration), light O2 fluxes (net primary production (NPP), gross primary production (GPP) and GPP/respiration (p/r) (mean ±SE where replicated; see Table 1).
  • Table 5. Slope of the relationship between benthic oxygen respiration (µmol O2 m−2 h−1) and dark N2 efflux (µmol N2-N m−2 h−1). Regressions were forced through zero as there would be no N2 efflux when respiration equals zero and to allow comparisons with previous studies. Outliers were removed from all regressions (see Discussion).
  • Fig. 2. Benthic dark, light, and net ammonium (NH+4 ) fluxes (mean ±SE where replicated; see Table 1).
  • Fig. 3. Benthic dark, light, and net nitrate (NO−3 ) fluxes (mean ±SE where replicated; see Table 1).

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APA

Eyre, B. D., Santos, I. R., & Maher, D. T. (2013). Seasonal, daily and diel N2 effluxes in permeable carbonate sediments. Biogeosciences, 10(4), 2601–2615. https://doi.org/10.5194/bg-10-2601-2013

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