Seasonal, daily and diel N2 effluxes in permeable carbonate sediments

[摘要] Benthic metabolism and inorganic nitrogen and N₂ flux rates(denitrification) were measured in permeable carbonate sands from HeronIsland (Great Barrier Reef). Some of the N₂ flux rates were among thehighest measured in sediments. All benthic fluxes showed a significantdifference between seasons with higher rates in summer and late summer. Therewas no distinct response of the benthic system to mass coral spawning.Instead, changes in benthic fluxes over 12 days in summer appear to be drivenby tidal changes in water depth and associated changes in phytosyntheticallyactive radiation reaching the sediments. Dark N₂ fluxes were stronglycorrelated to benthic oxygen consumption across all sites and seasons (r²= 0.63; p < 0.005; slope = 0.035). However, there were seasonaldifferences with a steeper slope in summer than winter, reflecting eithermore efficient coupling between respiration andnitrification–denitrification at higher temperatures or different sources oforganic matter. Adding data from published studies on carbonate sandsrevealed two slopes in the dark N₂ flux versus benthic oxygenconsumption relationship. The lower slope (0.035) was most likely due to highcarbon : nitrogen (C : N) organic matter from coral reefs, and associatedassimilation of nitrogen by heterotrophic bacteria including enhancedheterotrophic N-fixation, but competition by benthic microalgae orinefficient coupling between respiration and nitrification–denitrificationcannot be excluded. The steeper slope (0.089) was most likely due torespiration being driven by low C : N phytodetritus. If the differentslopes were driven by the sources of organic matter, then global estimates ofcontinental shelf denitrification are probably about right. In contrast,global estimates of continental shelf denitrification may be over-estimatedif the low slope was due to inefficient coupling between respiration andnitrification–denitrification and also due to reduced N₂ effluxes inthe light associated with competition by benthic microalgae for nitrogen andN-fixation.

[学科分类] 地球化学与岩石