Donna D'angelo Morrall

N uptake as a function of concentration in streams

Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3− and NH4+ at constant addition rates in the spring and summer of 1998. NH4+ uptake was also measured with 15N-NH4+ tracer additions and short-term unlabeled NH4+ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH4+ uptake rate per unit area (Ut) was positively correlated to nutrient concentration across all sites (r2 = 0.41, log–log relationship). Relationships between concentration and Ut were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, Ut was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient Ut. Linear extrapolation of Ut assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient Ut by ∼3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 μmol/L. The absolute concentration at which Ut was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH4+ at ambient concentrations in all streams were higher than would be expected, assuming Ut does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, Ut is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), Ut may not saturate, even at high concentrations.

Acute and chronic aquatic toxicity structure-activity relationships for alcohol ethoxylates

Acute and chronic toxicity tests using the cladoceran Daphnia magna were conducted on several alcohol ethoxylate surfactants. Exposure and homologue distributions were confirmed using specific analytical methods. These data were used to test currently available acute structure-activity relationships (SARs) and to develop a new chronic SAR to extrapolate test data to effluent or receiving water mixtures. Existing acute SARs adequately predicted the toxicity of these materials with an r(2) of 0.96 based on alkyl chain length and number of ethoxylates and 0.98 based on logK(ow). The additional chronic toxicity data allowed for development of a chronic SAR based on logK(ow) and produced an r(2) of 0.93. Slopes of new and published acute, chronic, and mesocosm SARs based on logK(ow) ranged from -0.61 to -0.87. These new data and refined SARs will assist in the toxicity prediction of mixtures in the environment.