chamber from a valve at one end; a second sample is withdrawn at the end of 
the experiment. An ambient bottom water sample is also taken, and a dark 
bottle is filled with water at the start of the experiment and sampled at the 
end. The samples are analyzed for metals and nutrients after filtration. Benthic 
fluxes are calculated from the change in the concentration of a constituent in a 
chamber, the mean height of the chamber (chamber volume/enclosed sediment 
surface area), and the length of the experiment. Dark bottle “fluxes” are 
calculated in the same manner as benthic fluxes, taking the dark bottle 
concentration at the end of the experiment as the final concentration, and the 
ambient bottom water concentration as the initial concentration. The dark 
bottle results are important for metals; because if they are equal to zero within 
analytical uncertainties, they indicate that metal analyses are not seriously 
affected by sample contamination during collection. 
Metal concentrations were measured with the techniques used for pore 
waters. The ambient water column metal concentrations at Jamestown North 
are about 10 ppb (0.2 juM) for Mn and Fe, 3 ppb (0.06 nM) for Ni, 2 ppb (0.04 
juM) for Cu, and 0.1 ppb (.001 juM) for Cd. Precisions in individual metal 
analyses are ±10%; hence uncertainties in fluxes are 40 juM m'“ day'^ for Mn 
and Fe, 8 juM m‘- day*^ for Cu and Ni, and 2 /iM m'~ day’* for Cd. Flistograms 
of dark bottle results at Jamestown (Figure 2-3) are roughly as predicted from 
these errors. 
Results of experimentally determined fluxes of Mn ++ and nutrients at 
Jamestown North are given in Table 2-3, along with model fluxes for these 
chemicals presented earlier. The model fluxes agree quite well with the 
measured values, indicating that the system is well characterized. 
A histogram of Jamestown North metal fluxes is shown in Figure 2-4 , and 
averages are tabulated in Table 2-4. Manganese fluxes are similar to the model 
values. Pore water iron concentrations are similar to those of manganese, and 
comparable fluxes are predicted. Observed iron fluxes are about an order of 
magnitude lower than manganese (and predicted) fluxes; this is ascribed to 
rapid oxidation of iron in the supernate following diffusion out of sediments. 
Nickel, copper, and cadmium fluxes are predicted to be negligible (see 
discussion of pore water values, above) and in fact measured fluxes are 
generally equal to zero within the analytical uncertainty. 
From the concentration of constituents in the Bay, the average height of the 
water column (taken as 10 m) and the fluxes, we can calculate doubling times 
of metals in the Bay with respect to benthic fluxes. These values are given in 
Table 2-4. Upper limits on Cu, Ni and Cd doubling times were calculated taking 
the flux as less than or equal to the sum of the mean flux and one standard 
deviation. Doubling times are to be compared with residence times of water in 
Narragansett Bay of about one month. The results show that, in the 
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