Stream chloride (Cl-) concentrations are increasing with watershed urbanization because of road deicing, wastewater discharge, and saline groundwater intrusions. Excess Cl- is detrimental to many freshwater animals, yet in many urban watersheds, Cl- concentrations exceed the USEPA recommended aquatic life criteria thresholds. The implications this has for stream biofilm communities are unknown. Specifically, impacts of salinity on early stages of biofilm community development and whether development of Cl- tolerant-populations results in reduced biofilm functionality are understudied.
Flumes lined with un-glazed ceramic tiles were constructed to observe Cl- stress impacts on biofilm succession. Water from the Cuyahoga River (northeast Ohio) served as the source community. Treatments were natural Cl- concentrations (control) and the 95th (mid) and 99th (high) percentile of Cl- concentrations observed at the collection site. Biofilms were examined after 1, 3, 5, 7, and 9 days for DNA extraction, which was used in 16S rDNA sequencing and quantification of genes associated with nitrogen cycling (amoA and nosZ) via quantitative PCR. At each sampling time, extracellular enzyme activity and nutrient concentrations were determined to assess carbon, nitrogen, and phosphorus acquisition patterns during community succession.
Results/Conclusions
Based on the initial analyses, several significant trends emerged among treatments. For instance, within the control treatment, activity ratios of carbon acquisition enzymes (b-glucosidase:b-xylosidase) were significantly (P<0.05) correlated with sampling time (r=0.57), indicating a shift in type of carbon available. Within the high Cl- treatment, phosphorus acquisition enzyme activity (phosphatase) significantly increased with time (P<0.05, r=0.63), while activity ratios of carbon:phosphorus (b-glucosidase:phosphatase, b-xylosidase:phosphatase) and nitrogen:phosphorus (leucine aminopeptidase:phosphatase) acquiring enzymes exhibited significant downward trends (P<0.05; r=-0.71, r=-0.76, and r=-0.64, respectively)—suggesting a potentially altered phosphorus dynamic occurred within the high Cl- treatments.
In conclusion, this study highlights the complexity of biofilm succession under Cl- stress. These data suggest that functional redundancy may preserve biofilm functionality under the increasing salinization of freshwater environments. However, under extreme duress, phosphorus dynamics may ultimately affect the ability of the biofilm community to maintain its functional role within the ecosystem.