| Benthic (i.e. sediment-associated) diatom assemblages distributed along a gradient of riverine to marine conditions in Weeks Bay, Alabama were tested for their tolerance of arsenate, a toxic species transported to estuaries via riverine flow. Natural assemblages from riverine, estuarine, and marine sites showed no significant response to arsenate delivered via leachate devices when sediment chlorophyll a concentration was the response variable. However, generic richness and diversity were reduced at the marine site suggesting sensitivity to arsenate. Nitzschia quadrangula, a nearly ubiquitous member of estuarine and marine benthic diatom assemblages, was cultured in the laboratory and found to be tolerant of arsenate, even increasing its growth relative to controls at intermediate concentrations of arsenate when the concentration of phosphate was low. A final experiment crossed arsenate with phosphate concentrations but employed in situ primary production of the diatom assemblages as the response variable. Assemblages from riverine sites showed no decrease in primary production in the presence of arsenate whereas primary production at the marine site significantly decreased. Sites where phosphate was limiting to primary production in some cases showed phosphate protection from arsenate toxicity. Overall the results showed that the riverine sites, and to a lesser extent the estuarine site, were tolerant of arsenate due to their historical exposure. However, the lack of exposure of diatom assemblages at the marine site to arsenate was reflected in their sensitivity to arsenate. |
