RACER: Dissolved oxygenand nitrate dynamics

during the 1989austral spring bloom

DAVID M. KARL and DALE V. HEBEL

School of Ocean and Earth Science and TechnologyUniversity of Hawaii

Honolulu, Hawaii 96822

Measurements of gross, net, and new (i.e., "export" sensuEppley 1989) plankton community production and respirationare fundamental to our understanding of carbon and energyilow in antarctic coastal ecosystems. All of these important rate'.rocesses affect the ambient pools and turnover rates of dis-scived inorganic carbon (DIC), oxygen (02), nitrate (NO3), andpIosphate (PO4), and, for "average" marine plankton, do soin predictable stoichiometries (Redfield, Ketchum, and Rich-ards 1963):

106 CO2 + 122 H2O +16 HNO3 + H3PO4 PhotosynthesisRespiration

(CH20)1 (NH3 ) 16 (H3PO4 ) + 138 02

Historically, photosynthetic rates in antarctic ecosystems havebeen derived using incubation methods, including both car-bon-14 and oxygen light-dark bottle methods for estimates of"gross" and "net" primary production (see production defi-nitions in Bender et al. 1987), nitrogen-15 bottle incubationsfor estimates of new production (Olson 1980) and moored orfree-floating sediment traps for estimates of export production(Karl, Tilbrook, and Tien in press).

In regions of the ocean where temporal variability is highor where there exist strong spatial gradients in the magnitudeof plankton rate processes, such as the Antarctic Peninsula, itis often difficult to obtain accurate regional or seasonal esti-mates of production from in vitro incubation methods. As acomplementary indirect approach, one can quantify the sea-sonal depletion in either DIC (Weiss, Ostlund, and Craig 1979;Codispoti et al. 1982; Codispoti, Friederich, and Hood 1986)or in dissolved inorganic nutrients (Jennings, Gordon, andNelson 1984; Simon 1986; Sambrotto et al. 1986) or seasonalincrease in oxygen content (this study) of an identifiable watermass. A major advantage in the water-mass approach is thatit provides a time-and-space average of primary and secondaryproduction processes that is less likely to be influenced bysampling bias.

During the Research on Antarctic Coastal Ecosystem Rates(RACER) 1989-1990 field program, we had an opportunity tocombine measurements of seasonal dissolved inorganic car-bon, dissolved organic carbon, oxygen, nitrate, dissolved or-ganic nitrogen, and phosphate dynamics and seasonal sediment-trap measurements (Karl and Asper, Antarctic Journal, this is-sue) with direct estimates of autotrophic (0. Holm-Hansenprogram component), microheterotrophic (D. Karl programcomponent), and macrozooplankton (M. Huntley and E. Brin-ton components) biomass and rate processes. We now presenta portion of that database which focuses primarily on the dis-solved oxygen and nitrate dynamics during the 1989 springbloom.

Water samples for dissolved oxygen were collected duringboth the fast grid (regional, quasi-synoptic) surveys and atstation A (see Huntley et al., Antarctic Journal, this issue for amore comprehensive discussion of sampling strategy and geo-graphical locations). The fast grid samples provide an assess-ment of the time and space variability in oxygen concentrationsassociated with the onset of the spring bloom. They also allowfor a comparison with estimates of plankton biomass (ATP,chlorophyll a, LPS) and productivity obtained by RACER col-leagues. At each fast grid station, a 5-meter water sample wasobtained using the conductivity-temperature-depth (CTD)-ro-sette and subsamples for oxygen were drawn into precalibratediodine flasks. The samples were immediately fixed and theoxygen content determined at sea using a semiautomated ti-tration system with a visual starch endpoint detection. ThisWinkler procedure, with Carpenter modifications (Carpenter1965), is capable of a precision of approximately 0.1 percent.At station A, we collected water samples from 0 to 200 metersand made simultaneous measurements of dissolved inorganicand dissolved organic nutrients (carbon, nitrogen, and phos-phorus) using standard autoanalyzer and ultraviolet oxidationtechniques (Smith et al. 1987).

The distribution of dissolved oxygen with water depth atstation A revealed a systematic temporal increase in the nearsurface water concentrations (figure 1) which coincided with

02, pmol 1.1

2503003504004500 - -I

Figure 1. Oxygen (0 2) concentrations versus depth for samplescollected at station A in the northern Gerlache Strait during the1989 RACER field program. The increase in the dissolved oxygencontent of the water is a reflection of the amount of net ecosystemproduction which has occurred during this portion of the springbloom. (m denotes meter. tLmol 1' denotes micromoles per liter.)

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the initiation of the spring phytoplankton bloom (also see Tienet al., Antarctic Journal, this issue). Between 31 October and 7November 1989, there was a net accumulation of 933 millimolesof oxygen per square meter in the upper 0-20 meters of thewater column (figure 1). Using a theoretical photosyntheticquotient of 1.3 (i.e., 1.3 moles oxygen evolved per mole ofcarbon dioxide reduced; see equation above), the oxygen ac-cumulation data indicate a sustained net ecosystem productionof approximately 1.2 grams of carbon per square meter perday. During this same period, we observed a net increase inmicrobial (<202 micrometers) biomass (ATP) that was equiv-alent to approximately 0.8 grams of carbon per square meterper day (Tien et al., Antarctic Journal, this issue). This differencebetween the estimate of net ecosystem carbon production andnet microbial biomass accumulation suggests that either theproduction of dissolved organic carbon, the production of mac-roplankton (>202 micrometer) biomass or the loss of reducedcarbon due to sedimentation may be important processes inour study area. None of these processes was included in ourmicrobial biomass inventories. Our final data analysis, whichwill include direct estimates of DOC accumulation, grazingand particle flux, should help resolve this interesting discrep-

ancy. Identical calculations (based on the integrated 0-20 me-ters of oxygen flux data) for the period of the entire RACERfield program (30 October to 24 November) suggest a meannet ecosystem productivity of approximately 0.9 grams of car-bon per square meter per day.

During the development of the spring bloom at station A,we also observed a net uptake of nitrate which coincided withoxygen production (figure 2). The absolute magnitude of theslope of the oxygen vs. nitrate regression analysis for thesewater samples was 8.5, a value that is nearly identical to thetheoretical slope of 8.6 for nitrate-based photosynthetic pro-duction (see equation). From these data we conclude that ni-trate was a major source of nitrogen for the phytoplanktonbloom and that ammonium and dissolved organic nitrogenproduction and subsequent utilization by phytoplankton musthave been negligible during the period of our observations.However, estimations of nitrate and ammonium uptake basedon the results of in situ incubations using nitrogen-15 tracersindicate that ammonium was an important substrate (L. Tupasand 0. Holm-Hansen personal communication). These ap-parent contradictory results will, we hope, be resolved whenthe entire RACER database is available for interpretation. Pos-

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NO3 , pmol I1Figure 2. Plot of dissolved oxygen (0 2) concentration vs. dissolved nitrate concentration for water samples collected over the 0-200-meterdepth range at station A in the northern portion of the Gerlache Strait during the RACER field program (31 October 1989 to 19 November1989). The linear regression analysis is: 0 2 (in micromoles per liter, tmol l') = -8.507 nitrate (NO3) (in micromoles per liter) + 533; n =40; r2 = 0.813.

150 ANTARCTIC JOURNAL

sible explanations for this discrepancy include: an importantrole of bacterial nitrification or the importation of ammoniuminto the surface waters by migrating zooplankton.

Finally, we hypothesize that the processes which we havejust described for water samples collected at station A may becharacteristic of antarctic coastal habitats in general. Clearly,a regional investigation of the near-surface water (5 meter)dissolved oxygen concentrations over our limited RACER studyarea has identified substantial areas of net ecosystem produc-tion during November 1989, with oxygen concentrations inexcess of 400 micromoles per liter (figure 3). In spite of thisnow well-documented seasonal productivity pulse, however,we know little about either the short-term or long-term fate ofthis new ecosystem production.

We thank the officers and crew of the RIV Polar Duke, ITTsupport personnel, our RACER program colleagues, and es-pecially the University of Hawaii participants (L. Asato, D.Bird, A. Brittain, R. Letelier, and C. Tien) for their help withsample collection and analysis and R. Letelier for helpful com-ments. This research was supported, in part, by National Sci-ence Foundation grant DPP 88-18899, awarded to David M.Karl. Contribution number 2413 of the School of Ocean andEarth Science and Technology of the University of Hawaii.

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Figure 3. Regional distribution of surface water (5 meters) dissolvedoxygen (in micromoles per liter) content over the RACER studyarea for the sampling period (16-19 November 1989).

References

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