Abstract
Recharge of aquifers by urban stormwater may trigger significant ecological changes that can be detrimental to the biodiversity and functioning of groundwater ecosystems. Here, the effects of aquifer recharge (AR) on three levels of parameters were investigated: dissolved organic carbon (DOC) quantity and quality, global biofilm characteristics, and diversity changes of bacterial communities. As DOC enrichment by AR can be mitigated by vadose zone (VZ) thickness, three AR sites with thin VZ (< 3m) and three sites with thick VZ (> 10m) were selected. For each AR site, clay beads were incubated over a 10-day-long rainy period through wells in recharged and non-recharged groundwaters. Total proteins, dehydrogenase, and hydrolytic activities were monitored from clay beads to assess biofilm development. Bacterial richness on beads was estimated by 16S rRNA-based metabarcoding. AR was found to significantly increase DOC and biodegradable DOC (BDOC) concentrations, biofilm development, and bacterial richness especially in sites with thin VZ. VZ thickness was inversely related to microbial growth indicators and bacterial richness in groundwater, through a control of DOC availability. The proportion of Bacteroidetes 16S rRNA gene reads was higher in recharged groundwater than in non-recharged groundwater, suggesting that this phylum could be used as an indicator of DOC enrichment associated with AR. Quantitative PCR assays for Bacteroides DNA confirmed these trends and showed an enrichment of this bacterial group in DOC-rich aquifer waters. The positive linear relationships between BDOC concentrations and biofilm variables highlighted a strong C-limitation of groundwater impacting bacterial species sorting and activity.
Highlights
PCR amplified according to Layton et al (2006) using the Brilliant II SYBR Green low ROX qPCR
Number of cycles needed to have a significant signal was compared with wells where only plasmid harboring int2 was added to the qPCR mix
One would have expected that AR would have produced higher DOC concentrations than those expected from the model of Pabich et al (2001) because of the stronger hydrological connectivity between the surface and the aquifer in these AR sites (Foulquier et al 2011)
Summary
Jena, Germany) based on thermocatalytic oxidation (850°C) of organic carbon and infrared detection of CO2, after removal of dissolved inorganic C with HCl. Biodegradable DOC (BDOC) was determined by the method of Servais et al (1987, 1989) following Mermillod-Blondin et al (2015). DOC concentrations were measured from filtered (0.2 μm) water samples at the start and the end of the incubation period to determine the initial DOC concentration and the remaining DOC concentration after 30 days of incubation (representing the refractory dissolved organic carbon, RDOC), respectively. PCR amplified according to Layton et al (2006) using the Brilliant II SYBR Green low ROX qPCR
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