Sediments are a major factor influencing the fate and concentration of some toxicants. The use of sediment microbial activity in water quality assessment could enhance our ability to understand the impact of many toxicants. Sediments from six diverse lake sites were spiked with arsenic species, and effects on microbial activity were assessed with a test battery measuring dehydrogenase (TTC), acridine orange direct counts (AODC), formazan crystal production (INT), phosphatase, proteolysis, total direct counts (DC), direct viable counts (DVC), and total plate counts (TPC). Individual tests were modified for use in sediments. Oxygen, sediment age and volume, incubation time, diluent type, nutrient amendment, and agitation were studied as major variables.

Phosphatase and TTC activity were higher in microaerobic sediment microcosms. DVC accurately detected microbial activity in water but not in sediments. INT assays revealed that sediments contain higher percentages of dormant bacteria than overlying waters. Arsenic generally failed to suppress enzyme activity in nutrient-stimulated sediments. TTC, phosphatase, and INT activity showed variable responses to arsenic additions, depending on test conditions. Proteolysis was suppressed by arsenite under aerobic conditions after one month. Sediment bacterial isolates were not sensitive to arsenate, but they were very susceptible to arsenite. Important test modifications enabling accurate assessment with these assays include both sediment and nutrient concentrations, diluent type, and incubation condition.

The activity test sequence described here demonstrates the diversity of microbial community response to model toxicants. The use of several microbial activity tests best defines the degree of stimulation or inhibition in sediment toxicology. The diverse and complex nature of typical sediments requires careful pretesting before accurate activity assays can be run.