Biogeochemical control of the summer distribution and speciation of arsenic in the Tamar estuary

Measurements of the distribution of dissolved inorganic and methylated arsenic in the Tamar estuary (south-west England) over a number of summers have illustrated the consistent non-conservative behaviour of inorganic arsenic and extensive bio-utilization. The concentration difference between the estuary freshwater endmember, which contains between 2·7 and 8·8 μg of dissolved arsenic per litre and the seawater endmember ( c 1 μg 1 −1 of dissolved arsenic) is not reflected in the distribution of dissolved arsenic in the estuary, with dissolved inorganic arsenic being particularly elevated in the higher salinity (tidal mudflat) region. At times, the dissolved inorganic arsenic levels in this area remain remarkably constant with respect to salinity. Short-term laboratory mixing of the river and seawater endmembers (unfiltered and filtered) shows no evidence of either salinity-induced release of arsenic from the suspended sediments or the removal of arsenic from the dissolved phase. The distribution of dissolved inorganic arsenic in the estuary therefore appears to be determined by a combination of secondary inputs arising from old mining drainage, and advective transport of arsenic-enriched sediment interstitial waters into the water column. Bio-utilization of the element during the warmer months results in the release of dissolved monomethylarsenic and dimethylarsenic and these have reached maximum concentrations of 0·46 and 1·27 μg As l −1, respectively, in the estuary mouth. The distribution of these species was not correlated to the distribution of chlorophyll- a (phytoplankton biomass). A highly productive community of the freshwater diatom Cyclotella atomus in the brackish water end of the estuary had little impact on arsenic speciation, whereas extensive arsenic biomethylation was observed in the seaward end of estuary. This highlights differences in metabolism between marine and freshwater phytoplankton living at similar arsenic and phosphate concentrations, and suggests that biomethylation in the Tamar estuary is governed by marine biological processes.