Antarctic plankton and the marinegeochemistry of radium-226, barium,

and germanium

GABRIEL A. VARGO,KENT A. FANNING, and

LINDA M. BELL

University of South FloridaDepartment of Marine ScienceSt. Petersburg, Florida 33701

Vertical distributions of radium-226, barium, and germaniumin the world's oceans show surface depletions and deep-waterenrichments similar to the major nutrients required by phy-toplankton. Linear correlations between silica concentrationsand radium-226, barium, and germanium concentrations(Broecker, Goddard, and Sarmiento 1976; Chan et al. 1976; Ku,Huh, and Chen 1980; and Froelich and Andreae 1981) suggestthat diatoms may be very important in the removal of theseelements from surface water. Our preliminary data and thework of Shannon and Cherry (1971) show that radium-226 isconcentrated several thousandfold above water column levelsby diatom-dominated phytoplankton in temperate and sub-tropical regions. Because there is no known biochemical re-quirement for radium, barium, or germanium, it is likely thatthese elements are incorporated into frustules during silicifica-tion. Siliceous radiolarian tests may also contain radium-226,barium, and germanium.

Sedimentation and burial of biogenic silica could thereforeconstitute a major removal mechanism of radium-226, barium,and germanium from the ocean. The largest marine deposit ofsiliceous ooze lies between the antarctic convergence and diver-gence, covers 1.9 x 10 11 square centimeters, and receives 60-80percent of the biogenic silica deposited on the sea floor (De-Master 1981). The calculations in table 1 demonstrate that theincorporation of radium-226, barium, and germanium by ant-arctic phytoplankton could be important. However, there arelarge ranges in the amounts potentially affected, primarily dueto uncertainties in the incorporation ratios radium-226 to sil-icon, barium to silicon, and germanium to silicon.

The circumnavigation of the antarctic continent by the USCGCPolar Star during the austral summer of 1982-83 provided anexcellent opportunity to investigate the role of antarcticplankton in the biogeochemistry of radium-226, barium, andgermanium. Our primary objectives were (1) to determine thedistribution and standing stock of dissolved and particulateradium-226, barium, germanium, and silicon in four size frac-tions of plankton near the ice edge and (2) to estimate growthrates of the dominant diatoms in the same region.

Plankton samples were obtained directly from one of theship's sea chests located 8 meters below the water line. Hydro-static pressure fed seawater directly into a series of three spe-cially designed, nested Nitex(R) plankton nets (202 micrometers,73 micrometers, and 53 micrometers) seated above a reservoir.The water was then pressure-filtered using diaphragm pumpsthrough a 5 micrometers, 293 millimeters tortuous-path filter(Nuclepore')).

The filter system was operated along 15 portions of the cruisetrack between the Ross Sea and the Weddell Sea (figure).Planktonic size fractions from each filter run were frozen for

Table 1. Impact of the production, dissolution, and deposition of biogenic silica in the Antarctic Ocean

Input to the entire ocean

SourceAmounta

7 x 10'3.2 x 1012b

10 >< 109d2.5-6.1 x 109d

230-2403-112.1 x 107h

5.1 x 107h

Element orisotope

SiliconRiver

HydrothermalRiverBariumHydrothermalSedimentRadium-226HydrothermalRiverGermaniumHydrothermal

a In moles per year.b From DeMaster (1981).

Amount affected by antarctic biogenic silica

Uptake into frustules inNet accumulation inReleased from frustules backsurface watersasiliceous sedimentsainto water column"

194-36.8 x 1012c 4.2 x 1 012 15.2-32.6 x 1012

1-40.5 x lOge

0.2-4.6 X 109e

0.8-35.9 x 10

3•9_74 0.8-80

3.1-660

14.2-26.9 X 107h 3.1 x 10'

11.1-23.8 x 10

Area of antarctic siliceous sediments taken as area of uptake into frustules = 1.9 x 10 17 square centimeter from DeMaster (1981); rates of uptake andaccumulation per unit area from Nelson and Gordon (1982).

d From Edmond et al. (1979).Using the silicon uptake or accumulation rates calculated just above and a range of Ba/Si ratios of 0.5-11 x 10 mole/mole from Dehairs, Chesselet, andJedwab (1980), Martin and Knauer (1973), and Li et al. (1973).Based on estimates of Broecker, Goddard, and Sarmiento (1976).Based on Ra/Si ratios in South Atlantic phytoplankton from Li et al. (1973) and Krishnaswami, Lal, and Somayajulu (1976): 0.2-2 x 1012 mole/mole.Ge/Si ratio of 7.3 x 10 mole/mole from Froelich and Andreae (personal communication).Krishnaswami and Turekian (1982).

174 ANTARCTIC JOURNAL

0'

SOUTH AOCI

60°E

60°S

900E

INDIAN

OCEAN

120°E

LEGENDICE SHELFICE EDGE

- CRUISE TRACK

i F FILTER RUNS

S NET TOWS

0 200km

0200

STATUS MILES

60°W

600S

900W

SOUTH

PACIFIC

OCEAN

120°W

SOUTH PACIFIC OCEAN

1500W 150°E

180°

Cruise track and transect locations of filtering runs for the 1982-1983 antarctic trip of the USCGC Polar Star

later analysis. The incorporation ratios (radium-226 to silicon,barium to silicon, and germanium to silicon) will be determinedin the size fractions. Ratios of dissolved radium-226, barium,and germanium to silicon will also be measured in filteredseawater collected throughout each run for comparison withratios in the plankton. Efficiencies will be calculated as thepercentage of chlorophyll a, particulate silica, and particulateorganic carbon retained on the nets and filters. Subsamples ofeach size fraction were preserved for a qualitative assessment ofspecies. Net tows (0.5 meters, 80 micrometers) were taken tocomplement the filter runs.

An average of 10,700 liters were filtered during each run.Input chlorophyll a concentrations ranged from 0.1 to 2.7 micro-grams per liter. Maximum phytoplankton biomass was found

on run 13 in the southern Weddell Sea (figure) and consistedprimarily of the ice-associated diatoms Fragilariopsis andNitzschia. Preliminary retention efficiencies from chlorophyllmeasurements aboard ship ranged from 68 to 94 percent andwere highest near ice or when diatoms predominated. Zoo-plankton biomass appeared to be low, but krill were capturedduring several runs.

Diffusion cultures of natural populations were incubated indialysis tubing (12,000 molecular weight cut-off) on a rotatingwheel in a plexiglass tank to estimate phytoplankton growthrates (Sakshaug and Jensen 1978) and the incorporation rates ofradium-226, barium, germanium, and silicon. Light levels weremaintained at 15 percent and 30 percent of incidence irradianceusing neutral density screens. Seawater in the tank was re-

1983 REVIEW 175

placed periodically with surface water. Growth rates were esti-mated from cell counts (Sedgewick-Rafter cell) and chlorophyll(Holm-Hansen and Riemann 1978). Incorporation rates of radi-um-226, barium, and germanium will be calculated from cel-lular carbon estimates (Eppley, Reid, and Strickland 1970), par-ticulate silica and carbon-to-silicon ratios from the four sizefractions of the filter runs, and the element-to-silicon ratios inthe size fractions. A separate large-volume experiment wasconducted to measure incorporation rates using a communitycomposed of several Chaetoceros, Rhizosolenia, and Fragilariopsisspecies. Growth rates (table 2) from several experiments werelow compared to those of phytoplankton in temperate water(Furnas 1983; Vargo 1979).

Table 2. Growth rates, as divisions per day (1092), for selected spe-cies or groups of species based on shipboard counts; (unless other-wise noted all data are from dialysis encapsulation of natural

populations)

ExperimentSpecies or groupDatesDivisionsnumber per day

R03Fragi/ariopsis spp. 2/7-2/160.37

2/16-2/230.61Rhizosolenia hebetata2/16-2/230.25

Total population 2/7-2/160.08

2/16-2/230.16Chactoceros criophilum2/11-2/140.38

2/14-2/180.31

R04Fragilariopsis spp. 2/19-2/220.42

R05Fragilariopsis spp.dialysis tube 2/16-2/190.64

2/19-2/220.21

2/16-2/220.43Polycarbonate 2/16-2/190.51

bottle 2/19-2/220.41

2/16-2/220.46

Species of the ice diatom Fragilariopsis were the most commonphytoplankton present during the cruise. Fragilariopsis spp.have a circumpolar distribution and division rates of approx-imately 0.5 day- 1 . Though additional research is required, ourinitial data suggest that ice flora, and particularly Fragilariopsisspp., may represent a major contribution to antarctic carbonproduction and to the geochemical cycling of radium-226, bar-ium, and germanium in the ocean.

Field personnel (17 January to 10 March 1983) includedGabriel A. Vargo, Kent A. Fanning, and Linda M. Bell.

This research was supported by National Science Foundationgrant contract DPP 82-14213.

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