Seasonal, daily and diel N2 effluxes in permeable carbonate sediments

Benthic metabolism and inorganic nitrogen and N-2 flux rates (denitrification) were measured in permeable carbonate sands from Heron Island (Great Barrier Reef). Some of the N-2 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 N-2 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 N-2 effluxes in the light associated with competition by benthic microalgae for nitrogen and N-fixation.