Table 2-4. Benthic Fluxes Measures at the Jamestown 
North Study Site and Estimate Doubling Times 
for Cu, Ni, Mn and Fe in Narragansett Bay 
Mean flux and Concentration of Time for benthic flux 
standard deviation dissolved metal in to double water column 
(jug cm' 2 day* 1 (1) Narragansett Bay (ppb) concentration (days) 
Cd 
Cu 
Ni 
Mn 
Fe 
NOTES: 
(1) Based on twelve determinations. 
(2) Calculated excluding one anomalously high value believed to reflect 
contamination. 
-0.0029±0.0043 
0.10 
>71 
-0.009±0.044 
2.0 
>57 
-0.035±0.064 
3.0 
>100 
2 .1±0.8 (2) 
10 
5 
0.17±0.23 
summertime, release of manganese from sediments is a major source of 
manganese in Narragansett Bay, but release of nickel, copper and cadmium are 
probably not significant. 
TRACE METAL BUDGETS IN NARRAGANSETT BAY: 
Mn AND Cu AS EXAMPLES 
The preceeding discussion suggests that benthic fluxes will be a source of 
Mn, but not Cu, to the waters of Narragansett Bay. The distribution of these 
metals in the Bay provides a check on these conclusions, as will be seen from 
the following discussion. 
Graham et al (6) did a mass balance for dissolved and particulate manganese 
in Narragansett Bay in the Summer of 1973. Their results for the distribution 
of dissolved manganese in the main part of the Bay are shown in Figure 2-5. It 
is readily seen that dissolved manganese is not a single-valued function of 
salinity: at a given salinity, surface waters have far lower manganese 
concentrations than deep waters. The manganese concentration of surface 
samples is far less than that expected from a simple conservative mixing model 
in which the deep water concentration is approximately equal to the 
conservative concentration. Graham et al (6) interpreted these results as 
indicating that manganese is scavenged (presumably by oxidation and 
precipitation) throughout the waters of the estuary, and bottom waters are 
enriched relative to surface waters by the benthic flux of manganese. 
21 
