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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/S0717-65382006000300007 

Suplemento Gayana 70: 26-28, 2006



Downward fluxes of particulate organic matter in coastal and oceanic areas off Chile: The role of the OMZ and functional groups of the plankton




Flujo vertical de materia orgánica particulada en áreas costeras y oceánicas de Chile: El rol de la ZMO y los grupos funcionales del plancton

H E. González1, 2, G. Daneri1, 3, E. Menschel1, 2, M. Marchant1, 4, S. Pantoja1, 4, D. Hebbeln5, C. Barria1, 2, R. Vera1, 2 & J. Iriarte6

1. Center for Oceanographic Research in the Eastern South Pacific (FONDAP-COPAS), University of Concepcion, Concepción, Chile.

2. Institute of Marine Biology, Austral University of Chile, Valdivia, Chile, hgonzale@uach.cl

3. Patagonian Ecosystems Research Center, Coyhaique, Chile.

4. Department of Oceanography, University of Concepción, Concepción, Chile.

5. Geowissenschaften, RCOM, University of Bremen, Germany,

6. Institute of Aquaculture, Austral University of Chile, Puerto Montt, Chile.


ABSTRACT

Vertical fluxes of particles have been estimated during the last decade in coastal and oceanic systems off central-southern Chile. Free-drifting, cylindrical sediment traps have been deployed below the mixing layer from coastal areas, while automatic, cone-shaped sediment traps have been anchored in deep oceanic areas. Vertical fluxes toward 2300 m depth off Coquimbo were dominated by calcium carbonate (~70 mg m-2 d-1) instead of particulate organic carbon (POC ~7 mg m-2 d-1), where foraminifers (Neogloboquadrina pachyderma) and faecal pellets of Euphausia mucronata and Oikopleura spp, played important roles as vehicles for carbonate and carbon, respectively. In coastal upwelling systems, (i. e. Concepción), the dominant vertical flux was POC, mainly driven by euphausiids faecal strings and diatoms of the genera Thalassiosira, Chaetoceros and Skeletonema. Bacterial degradation of exported POC towards deeper layer of the ocean seems not to be significantly affected by the oxic versus hypoxic levels of dissolved oxygen characteristics of different water masses.

Keywords: Particulate organic matter, fluxes, oxygen minimum zone, zooplankton functional groups, sediment traps, central Chile


RESUMEN

Flujos verticales de partículas han sido estimados durante la última década en sistemas costeros y oceánicos frente a Chile centro-sur. Trampas cilíndricas derivando con la corriente han sido usadas bajo la capa de mezcla de sistemas costeros, mientras que trampas automáticas cónicas han sido ancladas en sistemas oceánicos profundos. Los flujos hacia 2300 m de profundidad frente a Coquimbo estuvieron dominados por carbonado de calcio (~70 mg m-2 d-1) más que carbono orgánico particulado (POC ~7 mg m-2 d-1), donde foraminiferos (Neogloboquadrina pachyderma) y pellet fecales de Euphausia mucronata y Oikopleura spp, juegan un papel muy importante como vehículos de carbonato y carbono, respectivamente. En sistemas de surgencia costera (i.e., Concepción), el flujo vertical dominante fue POC, principalmente gobernado por pellets fecales de eufáusidos y diatomeas de los géneros Thalassiosira, Chaetoceros y Skeletonema. La degradación bacteriana de material orgánico exportado hacia zonas más profundas del océano no aparece significativamente afectada por los niveles característicos de concentración de oxígeno disuelto óxicos versus hipóxicos en las diferentes masas de agua.

Palabras Claves: Materia orgánica particulada, zona de mínimo oxígeno, zooplancton functional groups, trampas de sedimento, central Chile.


INTRODUCTION

The Humboldt Current System (HCS) was one of the least Eastern Boundary Current (EBS) to be studied from an oceanographical and climatological point of view (Strub et al. 1998; Morales & Lange 2004). With an extension about 40° latitude and with its continuous coastal upwelling, the HCS is the largest high productive EBC region. Here, the high phytoplankton concentrations induced by upwelling along the South American coast support one of the richest fisheries in the world (Alheit & Bernal 1993).

The intense upwelling and the subsequent high biological productivity result in extremely high up- and downward fluxes of carbon, and, therefore, this region plays an important role in the global carbon cycle. Slight variations in the dynamics of this system may have considerable effects on both the ocean-atmosphere exchange of CO2 (Houghton et al. 1990), and the global carbon budget. Thus, the budget of CO2 in the HCS depends on the interplay between physical processes (CO2 source due to upwelling) and biological processes (CO2 sink due to primary production).


METHODS

A fixed time series station (St. T) located over the continental shelf of Concepción (maximum depth 90 m) was sampled (vertical flux of POC, CTD, dissolved O2 and Chl-a) monthly from August 2002 to May 2005. Chlorophyll-a (Parsons, Maita & Lalli 1984), POC (Bodungen, Wunsch & Fürderer 1991) and plankton abundance and composition (Utermöhl 1958) were measured according to standard procedures. The vertical flux of particulates was estimated by using surface-tethered, cylindrical sediment traps for periods ranging from 12 hours to one day. Aliquots of the sedimented material were used for microzooplankton (tintinnids, dinoflagellates) and faecal material counting and sizing with standard microscopy methods (Utermöhl 1958) and scanning electron microscopy.

Vertical fluxes of biogenic elements (carbon, carbonate) and their main sources (faecal material, foraminifers, and coccolithophorids) were studied during 1993_1995 with a sediment trap deployed at a depth of 2300m in the oceanic area off Coquimbo, Chile (30ºS).


RESULTS AND DISCUSSION

Vertical fluxes of POC and carbonate along the HCS off Chile

Off Coquimbo, the average flux of particulate organic carbon (POC) at 2300 m depth was 7 mg m-2 d-1. Faeces from euphausiids and appendicularians contributed the most to total faecal carbon flux, being the euphausiid Euphausia mucronata and the appendicularian Oikopleura sp. key species in the POC and carbonate export to the deep-sea in form of faecal material. Average carbonate fluxes of foraminifers and coccosphere+coccolith were 56 and 7 mg m-2 d-1, respectively, or 73% and 11% of the total carbonate flux. The sinking flux of CaCO3 was significantly higher than that of POC throughout the year, averaging 90% of the total carbon and suggesting deep-water delivery of biogenic CaCO3 to be the main pathway for removing carbon from the upper-ocean biosphere in the study area (González et al. 2004).

Off Concepcion, at St. T, the average POC export production (below 50m depth) was 16.6% (SD=17%; range 2-67%; n=16). The biological mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepcion depend upon key groups of phytoplankton (Thalassiosira spp. and Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively.

The role of OMZ in modifying vertical flux of POC

The fate of sinking proteinaceous material in the OMZ off Iquique (21ºS), northern Chile, was studied by deploying sediment traps at 30 m (base of the oxygenated layer) and 300 m (bottom of the O2-depleted layer) during a 3-day experiment. Most of photosynthetically-produced protein (82%) degraded in the top 30 m, an additional 15% decays between 30 and 300 m, within the sub-oxic zone, and ca. 1% reaches surface sediments at 1200 m depth (Pantoja et al. 2004). These authors 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 not dependent on O2 availability.

Off Concepción, (~36ºS) similar results showed that most of the photosynthetic fixed carbon (84%) degraded in the upper 100 m water column. An additional of 4% and 7% decay between 100-300 m (OMZ) and 300-600 m (AAIW) depth layers, suggesting that microbial degradation within hypoxic and oxic deep water masses are not significantly different (Fig. 1).



Figure 1: Vertical carbon flux (mg m-2 d-1) at 100, 300 and 600 m depth, and POC mineralization associated to O2-rich Subtropical Waters (STW), O2-poor Equatorial Subsurface Waters (ESSW) and O2-rich Antarctic Intermediate Waters (AAIW).


ACKNOWLEDGMENTS

Financial support to conduct several of the studies reported was obtained from the Center for Oceanographic Research in the Eastern South-Pacific (COPAS Grant 15010007) and FONDECYT Grants 1000419 and 1000366.


REFERENCES

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