Seasonal, daily and diel N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; effluxes in permeable carbonate sediments

B D Eyre,D T Maher,I R Santos

doi:10.5194/bg-10-2601-2013

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.

Highlights

Hydrology and Denitrification permaneEntlay rtrhemSovyesstneitmrogen from an ecosystem which can by converting be lost to fithxeedaStnmictoriosepghneencreteo. sTdhineitirmogpeonrtagnacse, of denitrification, because it may control system production by removing nitrogen that would otherwise be available for primary producers, is demonstrated by the numerous denitrification studies in mOocstetyapnes Sofceiceonsycsteems (Seitzinger et al, 2006)
Gao et al (2012) argue that global rates of continental shelf denitrification may be under-estimated due to high rates of denitrification measured in permeable quartz sands
There have been few measurements of denitrification in carbonate sediments; these have all been in the dark in coral reef sediments, and most of these have been ex situ (Table 4)

Summary

Introduction

Hydrology and Denitrification permaneEntlay rtrhemSovyesstneitmrogen from an ecosystem which can by converting be lost to fithxeedaStnmictoriosepghneencreteo. sTdhineitirmogpeonrtagnacse, of denitrification, because it may control system production by removing nitrogen that would otherwise be available for primary producers, is demonstrated by the numerous denitrification studies in mOocstetyapnes Sofceiceonsycsteems (Seitzinger et al, 2006). Continental shelves account for a third to half of global denitrification (Middelburg et al, 1996; Seitzinger et al, 2006). These estimates are based mostly on measurements and calculations from muddy sediments, and recent modelling of permeable quartz sands suggest that global rates of continentaSl oshleildf dEenaitrritfihcation are overestimated by a factor of 2 to 7 (Cardenas et al, 2008). Gao et al (2012) argue that global rates of continental shelf denitrification may be under-estimated due to high rates of denitrification measured in permeable quartz sands.

Methods

Results

Conclusion