supply oxygen and pump water out to expel waste products, we regarded the 
biopumping model as more appropriate. In the model of McCaffrey at al (8), it 
was assumed that organisms pumped water across the sediment-water interface 
at a certain “biopumping rate” (with units of volume of water per unit surface 
area per unit time). The biopumping rate was determined experimentally by 
bringing box cores of Jamestown North sediment into the laboratory, spiking 
the supernatant solution with ^^Na, and measuring the decrease in the 
supernatant “~Na concentration with time. The experiments gave a 
biopumping rate of 0.7±0.3 cm^ cm'“ day'^. The biopumping flux is then 
taken as the product of the biopumping rate and the difference between pore 
water and bottom water concentrations. 
Model diffusive and advective fluxes for summer Jamestown North 
sediments are given in Table 2-3. Surprisingly, both fluxes are of the same 
magnitude. 
RESULTS OF BENTHIC FLUX MEASUREMENTS 
In order to test our predictions that nutrient and manganese fluxes have the 
values calculated from the model outlined in the previous section, and to make 
direct measurements of copper and nickel fluxes, we have measured benthic 
fluxes in the field using the “bell jar” instruments developed and extensively 
deployed by Hale (7) and Nixon et al (9). In these experiments, PVC pipe 
halves with closed ends PVC flanges around the base are placed on the 
sediment. At the start of the experiment, a sample is withdrawn from the 
Table 2-3. 
Diffusive 
Flux 
Advective flux 
calculated from 
biopumping model* 
Measured 
fluxes: 
x ± 1 <7 (n) 
H^SiO^ 
0.3 
0.3 
1.2±0.2(7) 
nh 3 
0.19 
0.07 
0.27±0.08(15) 
III 
o 
CL 
0.018 
0.02 
0.07±0.02(9) 
£C0 2 
0.9 
0.6 
2 ( 1 ) 
.. ++ 
Mn 
0.02 
0.01 
0.049±0.018(14) 
3 -2 -1 
* Assuming a biopumping rate of 0.7 cm cm day 
17 
