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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/S0717-65382006000300005 

Suplemento Gayana 70: 19-21, 2006

 

Zooplankton interactions with the oxygen minimum zone in the eastern south pacific

 

Interacciones del zooplancton con la zona de mínimo oxigeno en el Pacífico sur oriental



Rubén Escribano

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


ABSTRACT

Zooplankton in the coastal upwelling zone off the eastern South Pacific must cope with a shallow oxygen minimum zone. In this work, I summarize available information on the zooplankton species that interact with the OMZ in the upwelling region off northern Chile. Data are available from the MinOx cruise carried out in March 2000 and the ZOMEI cruise perfomed in October 2005, both onboard the R/V Vidal Gormaz of the Chilean navy. Nineteen species of copepods and 10 of euphausiids were analyzed in their vertical distribution and migrating behavior. Several abundante species appeared constrained by the OMZ and restricted to the upper 50m layer. However, other abundant species inhabit temporarily or permanently the OMZ contributing substantially to the vertical flux of C produced in the photic zone through active vertical migration. Population dynamics and life cycle strategies, as well as metabolic adaptations of the species, may not only modulate their contributions to vertical fluxes of C, but also to the maintenance and persistence of the oxygen-depleted water mass.

Keywords: Carbon flux, copepods, euphausiids, vertical migration, zooplankton


RESUMEN

El zooplancton de la zona de surgencia costera en el Pacífico Sur Oriental se encuentra afecto a una zona de mínimo de oxígeno muy somera (ZMO). En el presente trabajo, se resume la información disponible de las especies del zooplancton que interactuan con la ZMO en la región de surgencia del norte de Chile. Los datos provienen desde los cruceros MinOx y ZOMEI realizados en marzo del 2000 y octubre del 2005, respectivamente a bordo del R/V Vidal Gormáz de la Armada de Chile. Cerca de 19 especies de copépodos y 10 de eufáusidos se analizaron en su distribución vertical y conducta migratoria. Varias especies abundantes se encuentran restringidas por la ZMO a la capa superior de 50 m. Sin embargo, otras especies abundantes habitan temporal o permanentemente la ZOM, contribuyendo sustancialmente al flujo vertical de C producido en la zona fótica a través de la migración vertical activa. La dinámica poblacional y las estrategias de los ciclos de vida, así como las adaptaciones metabólicas de las distintas especies, pueden no solo modular la contribución de ellas a los flujos verticales de C, sino que además a la mantención y persistencia de la masa de agua deficiente en oxígeno.

Palabras Claves: Flujo de carbono, copépodos, eufáusidos, migración vertical, zooplancton.


INTRODUCTION

The zooplankton in the coastal upwelling zone of the eastern South Pacific may play a significant role by promoting carbon flux into the sub-surface oxygen minimum zone (OMZ). Many species aggregate and feed on freshly produced phytoplankton C within the upper photic zone, but thereafter some of them actively migrate into the deeper oxycline or even into the OMZ carrying with them organic matter, which can be released at depth through several physiological processes. Other species are able to permanently inhabit the OMZ system sustaining their populations on sinking organic matter or through complex food webs. Presently, we know very little about these zooplanktonic communities in the eastern South Pacific region. In this work, I summarize information on dominant species of zooplankton inhabiting the upwelling area off northern Chile and interacting with the OMZ system. About 20 species of copepods and 10 species of euphausiids are found to be distributed between the surface and 800 m depth (Table 1). Some abundant species remain in the upper well-oxygenated layer (<40 m), with no evidence of diel vertical migration (DVM) and apparently restricted by the shallow oxycline. Other species, however, appear closely associated with the OMZ and related to specific ranges of oxygen levels (Fig. 1).

 

Table 1. Dominant zooplankton species found at coastal waters off northern in the
area under influence of the OMZ system (from Escribano & Hidalgo, submitted).



The
habitat used by each species, estimated as the mean-weighed depth of abundance (DWA), was examined for day and night conditions through the entire water column (Fig. 2). Among the abundant species, the large-sized copepod Eucalanus inermis is found living below the oxycline and performing DVM into the OMZ (Hidalgo et al. 2005), whereas the very abundant Euphausia mucronata perform DVM extensively between surface waters and the core of the OMZ (200 m), or even crossing it. Ontogenetic migration was also evident in some species of Eucalanus and in E. mucronata.



Figure 1. Vertical distribution of zooplankton species in the coastal upwelling zone off northern Chile in association with the vertical distribution of dissolved oxygen and temperature. Each point represents a single species in day (a) and night (b) conditions. Position of each species is the mean weighed depth of abundance (number m-3).



Figure 2. Dominant habitat and daily movements of zooplankton species in and out of the main core of the OMZ (shaded area) during March 2000. Day (white dots) and night (black dots) positions represent each one of the species and the arrow the extension of their vertical movement. These positions were estimated from mean weighed depths of abundance (from Escribano & Hidalgo, submitted).

The analysis of enzimatic activity of bulk zooplankton and for some dominant species from different depth strata reveals strong activity of both aerobic (ETS) and anaerobic metabolic (LDH activity) pathways within the OMZ system, suggesting that zooplankton metabolism can play a significant role for maintaining hypoxic conditions from the oxycline and below.

Estimates of species biomass showed a substantial (>75% of total zooplankton biomass) daily exchange of C between the photic layer and the OMZ. E. inermis can actively exchange more than 10 mg C m-3 between the base of the thermocline (30-60 m) and the deeper OMZ layer (60-200 m), whereas E. mucronata contributes with nearly 1 g C m-3 to the daily exchange of C between the upper 200 m and the deeper (>200 m) zone. This movement of zooplankton in and out of the OMZ, mainly derived from the migrating behavior of E. mucronata, implies the existence of a very efficient mechanism to incorporate freshly produced C into the OMZ system. In summary, from the viewpoint of zooplankton, the OMZ system exhibits a very unique community structure strongly interacting with the system itself. Population dynamics and life cycle strategies, as well as metabolic adaptations, may not only substantially contribute to C flux, but also to maintenance and persistence of the oxygen-depleted water mass.


ACKNOWLEDGEMENTS

Studies on the structure and functioning of the OMZ have been supported by the Chilean National Comission for Science and Technology (CONICYT) through the FONDAP Humboldt Program and the COPAS Center. I am grateful to O. Ulloa for inviting us to the MinOx cruise. I thank C. Krautz for all the sampling work and D. Fernández and Víctor Flores helped with sample analyses. P. Hidalgo significantly contributed to data analysis and discussion.

REFERENCES

Escribano, R. & P. Hidalgo. Zooplankton associated with the oxygen minimum zone system in the northern upwelling region of Chile during March 2000. Deep Sea Research, Part II, The Oceanography of the eastern South Pacific (submitted).         [ Links ]

Hidalgo, P., R. Escribano & C.E. Morales. 2005. Ontogenetic vertical distribution and diel migration of the copepod Eucalanus inermis in the oxygen minimum zone off northern Chile. Journal of Plankton Research 27:519-529         [ Links ]

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