Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

Oxygen minimum zones (OMZs) in the ocean are of key importance for pelagic fixed-nitrogen loss (N-loss) through microbial denitrification and anaerobic ammonium oxidation (anammox). Recent studies document that zooplankton is surprisingly abundant in and around OMZs and that the microbial community...

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Bibliographic Details
Published in:Frontiers in Marine Science Vol. 4
Main Authors: Stief, Peter, Lundgaard, Ann Sofie B., Morales-Ramírez, Álvaro, Thamdrup, Bo, Glud, Ronnie N.
Format: Journal Article
Language:English
Published: Lausanne Frontiers Research Foundation 23-05-2017
Frontiers Media S.A
Subjects:
Abundance
Ammonia-oxidizing bacteria
Ammonium
Ammonium compounds
Anaerobic microorganisms
animal-microbe interactions
Anoxia
Aquatic crustaceans
Bacteria
Biogeochemistry
Bottom water
Carcasses
copepods
Denitrification
Ecosystems
Hypoxia
Incubation period
Invertebrates
Metabolism
Microorganisms
Mollusks
Nitrate reduction
Nitrogen
nitrogen cycle
Nitrogen isotopes
Nitrous oxide
ostracods
Oxidation
Oxygen
oxygen minimum zone
Plankton
Sediments
Sinking
Zooplankton
Denmark
Golfo Dulce
Costa Rica
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Summary:Oxygen minimum zones (OMZs) in the ocean are of key importance for pelagic fixed-nitrogen loss (N-loss) through microbial denitrification and anaerobic ammonium oxidation (anammox). Recent studies document that zooplankton is surprisingly abundant in and around OMZs and that the microbial community associated with carcasses of a large copepod species mediates denitrification. Here, we investigate the complex N-cycling associated with sinking zooplankton carcasses exposed to the steep O2 gradient in a coastal OMZ (Golfo Dulce, Costa Rica). 15N-stable-isotope enrichment experiments revealed that the carcasses of abundant copepods and ostracods provide anoxic microbial hotspots in the pelagic zone by hosting intense anaerobic N-cycle activities even in the presence of ambient O2. Carcass-associated anaerobic N-cycling was clearly dominated by dissimilatory nitrate reduction to ammonium (DNRA) at up to 30.8 nmol NH4+ individual-1 d-1, followed by denitrification (up to 10.8 nmol N2-N individual-1 d-1), anammox (up to 1.6 nmol N2-N individual-1 d-1), and N2O production (up to 1.2 nmol N2O-N individual-1 d-1). In contrast, anaerobic N-cycling mediated by free-living bacteria proceeded mainly through anammox and denitrification in the anoxic bottom water, which underpins the distinctive microbial metabolism associated with zooplankton carcasses. Pelagic N-loss is potentially enhanced by zooplankton carcasses both directly through N2 and N2O production, and indirectly through NH4+ production that may fuel free-living anammox bacteria. We estimate that in the hypoxic water layer of Golfo Dulce, carcass-associated N2 and N2O production enhance N-loss as much as 1.4-fold at a relative carcass abundance of 36%. In the anoxic bottom water, however, N-loss is likely enhanced only marginally due to high ambient rates and low zooplankton abundance. Thus, zooplankton carcasses may enhance N-loss mainly at the hypoxic boundaries of OMZs which are usually more extensive in open-ocean than in coastal settings. Notably, these contributions by zooplankton carcasses to pelagic N-loss remain undetected by conventional, incubation-based rate measurements.
ISSN:2296-7745
2296-7745
DOI:10.3389/fmars.2017.00152