Mitchell, B.G., D. Menzies, and 0. Holm-Hansen. 1987. RACER: op-tical oceanography of the western Bransfield Strait. Antarctic Journal of

the U.S., 22(5).Sievers, H.A. 1982. Description of the physical oceanographic condi-

tions, in support of the study on the distribution and behavior of krill.

Institute Antartico Chileno, Scientific Series, 28, 73-122.Stein, M. 1983. The distribution of water masses in the South Shetland

Islands area during FIBEX. In T. Nernoto and T. Matsuda (Eds.),Proceedings of the Biomass Colloquium 1982. (Memoir of the NationalInstitute for Polar Research, 27, 16-23.)

RACER: Optical oceanography of thewestern Bransfield Strait

B.C. MITCHELL, D. MENZIES,and 0. HOLM-HANSEN

Polar Research ProgramScripps Institution of OceanographyUniversity of California at San Diego

La Jolla, California 92093

Studies in polar waters suggest that light is the primary limit-ing factor for phytoplankton growth (Holm-Hansen et al. 1977;Platt et al. 1982; Sakshaug and Holm-Hansen 1986; Tilzer, vonBodungen, and Smetacek 1985). The optical environment,which appears to be especially critical for antarctic phy-toplankton, has rarely been studied in any detail. There havebeen no intensive optical measurements made in a variety ofantarctic oceanic environments using modern in situ opticalinstrumentation. The use of a Secchi disk to estimate lightattenuation, as employed by the great majority of biologicalprograms concerned with phytoplankton distribution and ac-tivity in antarctic waters, cannot provide the investigator withthe quantitative radiometric data which are needed to helpanswer basic questions regarding phytoplankton metabolicrates in the upper 100 meters of the water column.

During the Research on Antarctic Coastal Ecosystem Ratesprogram (RACER) 1986-1987 field year, detailed studies wereconducted on the spectral light flux throughout the upper 200meters of the water column and of the optical properties of theparticulate materials. In situ analyses included profiles of down-welling spectral irradiance (7 channels centered at 683, 630, 560,520, 488, 441, and 410 nanometers), upwelling spectral radiance(five channels centered at 683, 560, 520, 488, and 441nanometers), photosynthetically available radiation (2-pi,400-700 nanometers), chlorophyll a fluorescence, beam at-tenuation, and solar-induced chlorophyll a fluorescence. Par-ticulate analyses included determination of spectral absorptioncoefficients, spectral fluorescence excitation, and pigment ana-lyses by conventional fluorescence measurements as well as byhigh-performance liquid chromatography procedures.

An in situ optical profiling capability was custom designedand interfaced to a General Oceanics Rosette with either 5- or10-liter Niskin sample bottles. The optical-physical packageconsisted of a Biospherical Instruments reflectance spec-troradiometer (MER 1012-F), in situ fluorometer (Sea Tech),transmissometer (Sea Tech), and temperature and conductivityprobes (Sea-Bird Electronics). Eighteen channels were multi-

plexed and digitized by the MER unit and communicated to thesurface as a frequency signal via a standard single conductoroceanographic cable. Sampling rates were set so that approx-imately five samples per meter were acquired while profiling at30 meters per minute. The data density in the vertical domain isthus comparable to traditional conductivity-temperature-depthdata. All data from these 18 channels were automatically record-ed in our shipboard computer and also displayed in real time ona video screen. The water sample bottles could thus be closedeither at predetermined standard depths, or at depths chosenon the basis of the profile data displayed on the screen. Thecomputer also recorded incident photosynthetically availableradiation (400-700 nanometers) from a 2-pi, gimbal-mounteddeck cell which was located in a shade-free area above thehelicopter deck. Whenever a water sampling bottle was closed,all data from the 18 channels plus the deck cell were automat-ically printed out. The light energy sensed by the submersible 2-pi photocell was recorded both in absolute energy units and alsoas the ratio of the light intensity relative to that of the deck cell.

During the RACER cruises, stations in the Cerlache Strait (sta-tion 43) and Drake Passage (station 20) represented contrastingexamples of coastal high stability and open ocean low stabilityregimes, respectively. Figure 1 shows the profiles of sigma-t,temperature, beam attenuation, and downwelling irradiance at488 nanometers for these two stations during December 1986. Itis evident that station 20 had a deep mixed layer (40 meters), lowbiomass, and deep penetration of irradiance. By contrast, sta-tion 43 had a shallow mixed layer (10 meters), high biomass,and "complete" attenuation of irradiance within the shallowmixed layer due to absorption by the high concentrations ofphytoplankton pigments. Spectral diffuse attenuation coeffi-cients and beam attenuation coefficients observed during max-imal bloom conditions were among the highest observed foroceanic waters where the dominant particulates are of biogenicorigin. The high attenuation coefficients result in very low meanlight levels within the mixed layer. Populations below the mixedlayer under such circumstances have inadequate light for pho-tosynthesis.

The downwelling spectral irradiance for five depths at station20 in January, 1987, is illustrated in figure 2. Several importantfeatures should be noted. First, spectral narrowing of the irra-diance occurs relatively deep in the water column. This is ofsignificance with respect to the light quality available for photo-synthetic organisms. Since this was a clear, low-biomass station,the peak irradiance at depth is around 490 nanometers (Jerlov1968). The second feature to note is the apparently higher irra-diance below 20 meters for the 683-nanometer channel as com-pared to the 630-nanometer channel. Such an observationwould not occur in pure seawater since attenuation coefficientsincrease rapidly at longer wavelengths. The phenomenon isattributed to natural fluorescence of absorbed solar radiation

140 ANTARCTIC JOURNAL

0

-10

-20

E-40

II-0LU0

-70

Temp°C — 2—10 I2Temp°C — 2—I0I2

0

-10

-20

-40

-70

Sigma-t 26 27 28 SigmQ — t 26 27 28A I p A

In (Ed 488)-2—I0I23 In (Ed 488)-2—I0I24 4 4A44 4 4

C(m) 0I234C(m)0I234

Figure 1. Upper water column characteristics in Gerlache Strait (station 43; 500 meters depth) and Drake Passage (station 20; 4,000 metersdepth). Ed is downwelling irradiance at 488 nanometers in units of microwatts per square centimeter per second; C is beam attenuation; Sigma-tis water density; T is temperature in Celsius; m is meter; m 'is per meter.

and is discussed in more detail in Mitchell et al. (AntarcticJournal, this issue).

Over 300 profiles from 0 to 200 meters depth were obtainedwith our 19-channel profiling system during the RACER cruisesbetween December and April. We thus have a large data setdocumenting the spectroradiometric light flux in the water col-umn in the various water masses described by Amos (Antarctic

3

0Lda.

—1

—2

—3

—4

400

440460520560600640680

Wavelength (n,n)

0 1,n+ 20m0 40m0 60mx 80

Figure 2. Downwelling spectral irradiance (in microwatts per squarecentimeter per second) at 5 depths for station 20 in the DrakePassage. Ed is downwelling irradiance; m is meter; nm is nanometer.

Journal, this issue). These profiles were obtained under varyingconditions of sea state, sun angle, and sky conditions. Whenthese data are analyzed, together with the beam attenuationdata and the spectral absorption coefficients of the particulatematerial, we will have a much better insight into the lightregime experienced by antarctic phytoplankton throughout thewater column. This knowledge of the optical characteristics ofthe upper mixed layer of antarctic waters, regarding both ab-sorption and scattering of light, will also be of much interest andimportance in regard to refining bio-optical algorithms used tocalculate phytoplankton biomass from airborne remote sensorsin high-latitude environments.

Shipboard personnel included B.C. Mitchell (6 December to 8February) and 0. Holm-Hansen and D. Menzies (8 February to6 April 1987). This research was supported by National ScienceFoundation grant DPP 85-19908.

References

Amos, A.F. 1987. RACER: Physical oceanography of the westernBransfield Strait. Antarctic Journal of the U.S., 22(5).

Holm-Hansen, 0., S.Z. El-Sayed, C. Franceschinni, and R. Cuhel.1977. Primary production and the factors controlling phytoplanktongrowth in the Southern Ocean. In G.A. Llano (Ed.), Adaptations withinAntarctic ecosystems. Houston, Tex.: Gulf Publishing Co.

Jerlov, N.G. 1968. Optical oceanography. New York, New York: ElsevierOceanography.

Mitchell, B.C., C. Stallings, 0. Holm-Hansen, and D.A. Kiefer. 1987.RACER: Optical prediction of photobiological properties. AntarcticJournal of the U.S., 22(5).

1987 REVIEW 141

Platt, T., W.G. Harrison, B. Irwin, E.P. Home, and C.L. Gallegos. 1982.Photosynthesis and photoadaptation of marine phytoplankton in theArctic. Deep-Sea Research, 29, 1159-1170.

Sakshaug, E., and 0. Holm-Hansen. 1986. Photoadaptation in Antarcticphytoplankton: Variations in growth rate, chemical composition andP versus I curves. Journal of Plankton Research, 8, 459-473.

Tilzer, M.M., B. von Bodungen, and V. Smetacek. 1985. Light-depend-ence of phytoplankton photosynthesis in the Antarctic Ocean: Im-plications for regulating productivity. in W.R. Siegfried, P.R. Condy,and R.M. Laws (Eds.), Antarctic nutrient cycles and food webs. Berlin:Springer-Verlag.

RACER: Temporal and spatialdistribution of phytoplankton biomass

and primary production

0. HOLM-HANSEN, R. LETELIER,and B.C. MITCHELL

Polar Research ProgramScripps Institution of OceanographyUniversity of California at San Diego

La Jolla, California 92093

The RACER program (Research on Antarctic Coastal Eco-system Rates) is an interdisciplinary study which seeks to un-derstand the dynamic mechanisms controlling the planktonicecosystem structure of the Antarctic Peninsula coastal environ-ment (see Huntley et al., Antarctic Journal, this issue). The pho-tobiology component of this program is concerned with tem-poral and spatial dynamics of phytoplankton populations,especially in relation to physical mixing processes and the en-vironmental factors influencing growth rates.

At each "fast" grid station our physical-optical-biological pro-filing unit was deployed from 0 to 200 meters to give continuousrecording of depth, temperature, conductivity, 2-'rr irradiance(400-700 nanometers), seven channels downwelling irradiance,five channels upwelling radiance, beam attenuation (trans-missometer), and chlorophyll a fluorescence (fluorometer withpulsed blue activating light). Water samples were obtained in5.0-liter, polyvinyl chloride Niskin bottles at eight standarddepths during the "up" cast of the profiling unit, and the waterwas used for determination of chlorophyll a, particulate organiccarbon and nitrogen, inorganic nutrient concentrations, andfloristic examination. Surface water samples were also obtainedby "bucket" in conjunction with the microbiology program andused to measure chlorophyll a in nanoplankton (less than 20micrometers in diameter) and microplankton (more than 20micrometers). During the "slow" grid stations the above mea-surements were supplemented with studies concerned withphotoadaptational status of the phytoplankton, as well as within situ primary productivity measurements using radiocarbonincorporation. During the in situ incubations (12-24 hours) nu-merous profiles of light attenuation in the upper 100 meters ofthe water column were obtained with our profiling unit so thattotal light flux at various depths could be calculated and used inquantum efficiency calculations.

The reults of our time series of fast-grid surface chlorophylldistributions are presented in figure 1. Concentrations of chlo-

rophyll a were extremely variable, ranging over two orders ofmagnitude from 0.2 to 20 milligrams per cubic meter. The deepwaters to the northwest of the South Shetland Islands were lowin chlorophyll a (less than 0.5 milligrams per cubic meter).Highest levels of phytoplankton pigments were found in pro-tected embayments and in the vicinity of islands as predicted bythe central hypotheses of RACER. (See Huntley et al., AntarcticJournal, this issue.) Furthermore, a pronounced seasonal trendis evident with the peak occurring in December and decreasingprogressively from January through March.

Preliminary examination of approximately 300 profiles ofchlorophyll a concentration and of water density (-t) as record-ed in the upper 200 meters of the water column by our profilingunit showed, first, that chlorophyll a is fairly uniformly dis-tributed throughout the entire upper mixed layer and, second,that concentrations decrease rapidly below the pycnocline. Ex-tracted chlorophyll a values from standard depths confirmedthese observations, and are in agreement with similar con-clusions first reported by Dustan, Olson, and Holm-Hansen(1979). Distribution maps of chlorophyll a in surface waters(figure 1) will thus show the same general patterns as mapsshowing spatial distribution of integrated chlorophyll a for theupper 200 meters.

Hart (1942) first documented the rapid increase in phy-toplankton biomass in antarctic waters during austral spring,followed by a rapid decline. Our time series from December1986 to March 1987 of integrated water column production atfive stations in the RACER grid is presented in figure 2a. Thepronounced seasonality of these data emphasizes the impor-tance of high-resolution sampling through a seasonal cycle.Peak bloom conditions were already evident in mid-December.This is considerably earlier than the phytoplankton peak re-ported near the South Orkney Islands by Home et al. (1969).

During RACER we found that within our 25,000-square-kilo-meter study area, surface pigments and integrated productionvaried over a range equal to previous reports for the world'soceans. This is summarized in figure 2b where the data fromRACER are superimposed upon a summary plot of integratedproduction as a function of surface phytoplankton pigmentscompiled by Eppley etal. (1985). Understanding the underlyingmechanism of such dynamic range within antarctic coastal eco-systems is one of the goals of RACER. The rate of primary produc-tion will be a function of both the phytoplankton standing stockand phytoplankton specific growth rates. Of the major factorswhich affect phytoplankton growth rates, available data indi-cate that light is the most important. One focus of RACER is thathigh productivity in the area of the Antarctic Peninsula iscaused by high mean light levels experienced by the phy-toplankton due to relatively shallow and stable upper watermixed layers.

142 ANTARCTIC JOURNAL