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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.70  supl.1 Concepción oct. 2006

http://dx.doi.org/10.4067/S0717-65382006000300013 

Suplemento Gayana 70: 62-67, 2006


Biogeochemistry of the OMZ of Chile

Biogeoquímica de la ZMO frente a Chile


Silvio Pantoja

Department of Oceanography & Center for Oceanographic Research in the Eastern South Pacific (FONDAP-COPAS), University of Concepcion, P.O. Box 160-C, Concepción, Chile, spantoja@udec.cl


ABSTRACT

Most microbial reactions are in one way or another associated to decomposition of organic matter, and occurs under two general conditions: in the presence and in the absence of oxygen. Here, we will examine current information on the biogeochemical reactions in the area of the oxygen minimum zone off Chile. The aim of this presentation is to examine mechanisms of processing of organic matter under upwelling and O2 depletion.

Keywords: Microbial reaction, decomposition, sinking, DOC.


RESUMEN

La mayoría de las reacciones microbianas están, de una u otra manera, asociadas a la descomposición de materia orgánica, la que ocurre bajo dos condiciones generales: ante la presencia y ante la ausencia de oxígeno. En este trabajo, examinaremos información actual respecto de las reacciones biogeoquímicas en el área de mínima de oxígeno frente a Chile. El objetivo de esta presentación es examinar los mecanismos de procesamiento de materia orgánica en áreas de surgencia y bajos niveles de O2.

Palabras claves: Reacción microbiana, descomposición, sedimentación, COD.


Most of the organic matter is produced by autotrophs in the photic zone of the world ocean. Primary organic matter is biosynthesized from inorganic nutrients by photosynthetic plankton, using light as the major energy source. Biological processes such as grazing, excretion, cell lysis and enzymatic hydrolysis of cellular material transform particulate organic matter (POM) into dissolved organic matter (DOM). Much of DOM becomes refractory but a significant fraction fuels the microbial loop. Whereas conventional wisdom was that most POM was grazed by zooplankton, it is now becoming clear that the flux of organic matter through the microbial loop may process a significant

fraction of the oceanic primary production (Azam 1998). Organic matter produced in the photic zone, mainly by photosynthetic organisms, is degraded to inorganic compounds (mineralization). Degradation provides a mechanism to release nutrients from the organic matrix through its transformation into the inorganic form. A very small fraction of primary production becomes part of the sinking flux, by which POM is transferred from the upper ocean to deep-sea sediments. In sediments, production and degradation continue, leaving only a small fraction of the organic matter produced by photosynthesis to be preserved in the sedimentary record (ca. 0.1%, Hedges 1992).

Production and sinking of organic matter (Fig. 1)




Figure 1. Primary production rates are in the order of 1 Kg C m-2 y-1 (Daneri et al. 2000).

We have shown that microbial degradation of total particulate protein is not attenuated by the presence of the suboxic layer between 30 and 300 m in the water column off northern Chile, consistent with a model of degradation of particulate protein controlled by extracellular enzymatic hydrolysis and is not dependent on O2 availability (Pantoja et al. 2004).

An important effect in degradation and transport may be due to the distribution of metazoa in the water column, which may be affected by the occurrence of the oxygen minimum zone. González et al. (2000) found that the oxygen minimum zone (OMZ) off northern Chile constituted a barrier for the intrusion of most zooplankton with the notable exception of euphausiids, capable of performing diel vertical migrations (Gonzalez et al. 2000). This in turn may accelerate or retard organic matter sinking (Fig. 2).



Figure 2. In a field experiment off Antofagasta, northern Chile, higher abundance of flux retarding than accelerating zooplankton was observed in the coastal zone, coincident with lower tan expected sinking fluxes and rain rates of organic matter. On the contrary, higher abundance of accelerating organisms than retarding in the oceanic zone associates with higher than expected organic matter fluxes.



Figure 3. Dissolved organic carbon (DOC) concentrations are similar to what found in other areas lacking upwelling (Cuevas et al. in prep.)

 

One possibility to explain observations of Fig. 3, is degradation. Exceptionally high degradation rates of dissolved organic carbon (DOC; 1.1-21.6 µM h-1) were estimated in amended and unamended incubation incubations with both oxic and suboxic water samples from the coastal upwelling ecosystem in the Humboldt Current System (HCS) off Chile. These rates, directly measured by determining DOC changes in incubation experiments, when combined with bacterial carbon demands estimated from a comprehensive data set from several cruises in upwelling centers in the HCS, strongly suggest that the microbial community in the photic layer is capable of rapidly processing freshly synthesized organic matter. This conclusion is supported by the examination of in situ rates of integrated community respiration that indicates that over 80% of the carbon fixed by primary production (Fig. 4) is respired in the photic zone in what appears to be a highly coupled primary and bacterial secondary production ecosystem (Daneri et al. submitted).




Figure 4. Primary production (PP) versus bacterial carbon demand (BCD) from cruises SECTORIAL (Antofagasta), JGOFS (Coquimbo), COPAS Time Series (Concepción), and MIRC (Concepción).


We found similar microbial degradation rates of labile dissolved organic matter in oxic and suboxic waters off northern Chile. Rates of peptide hydrolysis and amino acid uptake in unconcentrated water samples below levels of ambient concentrations were not significantly attenuated under oxygen-depleted conditions.

Microbial rates of extracellular peptide hydrolysis using fluorescent peptide substrates (LYA-peptides) and amino acid uptake indicate that peptide hydrolysis occurs at similar or faster rates than amino acid uptake in the water column, and that the hydrolysis of peptides is not a rate-limiting step for the complete remineralization of labile macromolecules (Fig. 5). Low-O2 waters process about 10 tons of peptide carbon per h, double the amount processed in surface oxygenated water. In the oxygen minimum zone, we suggest that the C balance may be affected by the low lability of the dissolved organic matter when this is upwelled to the surface. An important fraction of dissolved organic matter is processed in the oxygen minimum layer, a prominent feature of the coastal ocean in the highly productive Humboldt Current System.



Figure 5. Vertical profiles of O2 concentration, bacteria abundance, bacterial secondary production, and rates of peptide hydrolysis and amino acid uptake off Antofagasta in northern Chile (Pantoja et al. in review).


N2O. Cornejo et al. (2006) elucidated the temporal variability of coastal production of N2O off Central Chile, observing that sea-to-air fluxes of N2O approximately followed the upwelling dynamic throughout the year (see Fig. 5 in Cornejo et al. 2006). They suggested that, according to negative correlations with O2 and positive with NO3-, that aerobic ammonium oxidation sustain their rates of N2O production.

CO2. High fCO2 values are observed in the Antofagasta, Coquimbo and Concepción upwelling centers (Fig. 6), suggesting that while northern and central Chile are characterized by CO2 rich/mixed upwelled waters which cause a CO2 flux from the ocean to the atmosphere, the fjord region is characterized by stratified/CO2 poor waters causing a CO2 flux from the atmosphere to the ocean.


Figure 6. Left panel, main direction of the winds along the Chilean coast. Central panels, distribution of salinity and fCO2 along the coast and within the fiords area. Dashed line depict the approximate value of the atmospheric fCO2. Right panel, net influx/outflow of CO2 (Torres et al. in prep).


N stable isotope distribution

The Rayleigh fractionation model predicts that higher d15N of organic nitrogen during its synthesis by phytoplankton occurs concurrent to NO3- drawdown. This relationship was not observed in the Chilean coast where we noted enrichment of surface sediment associated to high nitrate concentration (Fig. 7). The expected relationship (seen by others in some upwelling areas) considers higher N-isotope fractionation at nitrate-replete conditions (closer to the coast compare to offshore) under the influence of nitrate uptake by phytoplankton in surface waters. We will discuss transformation of PON during its transit to surface sediments or alteration of the nitrate source signal in surface waters that mask the fertilization signature. Anaerobic ammonium oxidation (detected in northern Chile, Thamdrup et al. 2006) and denitrification could be responsible for altering source d15N.




Figure 7. Relationship between d15Nss and natural logarithm of nitrate concentration in surface water (average 0-10 m) in transects in northern and central Chile

 

ACKNOWLEDGEMENTS

Thanks to J. Soto, R. Castro, G. Daneri, R. Torres, and H.E. Gonzalez for sharing unpublished data and providing useful insights to the subject. This research was partially funded by FONDECYT Grant 1040503.


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