C.J. CARRILLO and D.M. KARL, Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822
Accurate estimation of carbon dioxide (CO2) fluxes, coupled with an understanding of the processes that control these fluxes, is necessary to predict future CO2 concentrations in the southern oceans. The chemical, physical, and biological controls on in situ CO2 concentrations cause habitat variability both temporally and spatially. Open ocean areas presently have high nutrient concentrations but low standing stocks of phytoplankton and low rates of primary production. In sharp contrast to the high-nutrient, low-productivity open ocean areas, coastal regions of Antarctica exposed to the annual advance and retreat of sea ice, sustain seasonal phytoplankton blooms with high rates of primary production (Smith and Nelson 1985; Holm-Hansen et al. 1989). Consequently, coastal and ice-edge regions of Antarctica could potentially remove atmospheric CO2, but these local sinks may be offset by equally large sources of CO2 during winter periods of net heterotrophy or as a result of the upwelling of CO2-enriched waters. The seasonal advance of the ice in the fall and retreat in the spring may also affect the flux of CO2 in the ice-dominated Arctic Ocean and southern oceans. Quantifying these fluxes will require sampling in the dissimilar ecosystems that make up the southern oceans. The Palmer Long-Term Ecological Research (LTER) Program was established in 1990 to study the physical determinants on the antarctic marine ecosystem. The central tenet of the Palmer LTER program is that the annual advance and retreat of sea ice is a major physical determinant of spatial and temporal changes in the structure and function of the antarctic marine ecosystem, from total annual primary production to breeding successes in seabirds (Smith et al. 1995.
During the 1995-1996 and 1996-1997 austral summer LTER field seasons, an automated underway CO2 measurement system was deployed on the R/V Polar Duke . During each field season, spatial surveys of surface water CO2 concentrations were conducted in coastal and open ocean ecosystems over a 3-month period from mid-December to mid-February. These surveys included four transects across the Drake Passage, five to eight surveys of Arthur Harbor near the U.S. research base at Palmer Station, and a survey of the LTER grid area located off the Antarctic Peninsula (figure 1). The survey of the LTER grid area included transects into coastal areas of Marguerite Bay and Crystal Sound. Overall, 11,679 surface -water and 7,702 atmospheric CO2 concentrations were measured over the 2-year period (table).
The underway CO2 system analyzes surface seawater from the ship's bow intake located approximately 5 meters below the surface and atmospheric air obtained from a line at the top of the bridge. Surface seawater concentrations are determined by continuously pumping water through a counter-flow rotating-disk equilibrator (Schink et al. 1970). A fixed volume of recirculated air is equilibrated with water flowing through the equilibrator by the motion of rotating disks. Equilibrated air is then analyzed for CO2 concentration with a LICOR 6262 infared CO2 analyzer. The LICOR is calibrated every 3 hours with a set of standard gases. The system is automated using a PC computer and LabVIEW® software. Equilibrator temperature is measured with an Omega RTD, and system pressure is measured with a Setra pressure transducer. Between calibrations, equilibrator and atmospheric samples are measured every 5 minutes. During the 1996 field season, an Orion pH electrode and an Endeco pulsed oxygen electrode were added to the system. Other underway measurements include salinity, temperature, fluorescence, light, and meteorological parameters.
Initial analysis of three LTER grid lines from the 1995-1996 and 1996-1997 field seasons shows surface seawater CO2 concentrations range from 100 microatmospheres to 380 microatmospheres compared to a mean atmospheric CO2c oncentration of 360±3 microatmospheres. Typically, areas of surface ocean supersaturation (surface-water CO2 concentration is greater than atmospheric CO2 concentration) were found at the oceanic edge of the outer shelf, implying upwelling as a potential source for these CO2-enriched waters. Areas of undersaturation (surface-water CO2 concentration is less than atmospheric CO2 concentration) were encountered in coastal waters and were associated with increases in chlorophyll and oxygen concentrations implying a biological source. For example, undersaturations of 260 and 160 microatmospheres are found approximately 50 kilometers from shore on the 200 and 600 grid lines (figure 2). CO2 concentrations typically increased with increasing distance from shore. In comparison, CO2 concentrations remained relatively constant along the 300 line (located between the 200 and 600 line) from 50 to 200 kilometers from shore. The coastal areas on the 200 and 600 line are near the mouths of large submarine canyons that may sustain large phytoplankton blooms by an enhanced macro- and micronutrient supply. Further analysis is needed to test the numerous ecological predictions of this hypothesis.
We thank Capt. Karl Sanden, the crew of the R/V Polar Duke , and the staff of Antarctic Support Associates for assistance. This work was supported by National Science Foundation grant OPP 96-32763 to R.C. Smith through a subcontract to D.M. Karl.
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