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Oxygen diffusion limitation and ammonium production within sinking diatom aggregates under hypoxic and anoxic conditions

Journal article
Authors Helle Ploug
Johanna Bergkvist
Published in Marine Chemistry
Volume 176
Pages 142-149
ISSN 0304-4203
Publication year 2015
Published at Department of marine sciences
Department of Biological and Environmental Sciences
Pages 142-149
Language en
Keywords Diatom aggregates, Mass transfer, Microsensors, Fluorometry, Modeling, marine snow, pelagic environment, benthic microalgae, respiration rates, upwelling system, ballast minerals, carbon turnover, organic-matter, surface waters, sea-water, Chemistry, Oceanography
Subject categories Oceanography


Sinking diatom aggregates are important components of vertical elemental fluxes and represent 'hotspots' of microbial-driven remineralization in the water column. A combined analytical approach of microsensors and fluorometry was used to measure oxygen (02) and ammonium (NH4+) concentrations in sinking diatom aggregates as a function of the ambient 02 concentrations. Diffusive fluxes of 02 and NH4+ within and around sinking aggregates were subsequently analyzed using a diffusion reaction model. Diffusion limitation of 02 within the diatom aggregates occurred when ambient 02 concentrations decreased beneath 100 mu M. At ambient 02 concentrations of 20 mu M, the measured flux of 02 to aggregates was equivalent to 33% of fluxes when 02 concentrations were in equilibrium with the atmosphere and approximately 50% of the aggregate volume was anoxic. When the diatom aggregates were maintained under hypoxic conditions, was produced in a ratio of 8.9 mol O-2 consumed:1 mol NH4+ produced which is within the expected range during ammonification. The average POC-specific respiration rate and the average PON-specific NH4+ production rate under hypoxic conditions were 0.065 d(-1) and 0.052 d(-1), respectively. Under anoxic conditions, the NH4+ release was 18% of that measured under hypoxic conditions. Our empirical and modeled data revealed diatom aggregates to be microenvironments of elevated NI-It concentrations ranging from 1 to 8 mu M and therefore to be potential sources of NH4+ in the oxygen minimum zones in the ocean. (C) 2015 Elsevier B.V. All rights reserved.

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