Dissolved organic carbon transformations and microbial community response to variations in recharge waters in a shallow carbonate aquifer

Abstract

In carbonate aquifers, dissolved organic carbon from the surface drives heterotrophic metabolism, generating CO 2 in the subsurface. Although this has been a proposed mechanism for enhanced dissolution at the water table, respiration rates and their controlling factors have not been widely evaluated. This study investigates the composition and concentration of dissolved organic carbon (DOC) reaching the water table from different recharge pathways on a subtropical carbonate island using a combination of DOC concentration measurements, fluorescence and absorption characterisation. In addition, direct measurements of the microbial response to the differing water types were made. Interactions of rainfall with the vegetation, via throughfall and stemflow, increase the concentration of DOC. The highest DOC concentrations are associated with stemflow, overland recharge and dissolution hole waters which interact with bark lignin and exhibit strong terrestrial-derived characteristics. The groundwater samples exhibit the lowest concentrations of DOC and are comprised of refractory humic-like organic matter. The heterotrophic response seems to be controlled by the concentration of DOC in the sample. The terrestrially sourced humic-like matter in the stemflow and dissolution hole samples was highly labile, thus increasing the amount of biologically produced CO 2 to drive dissolution. Based on the calculated respiration rates, microbial activity could enhance carbonate dissolution, increasing porosity generation by a maximum of 1 % kyr -1 at the top of the freshwater lens.

Microbial Processing and Production of Aquatic Fluorescent Organic Matter in a Model Freshwater System

“…This further supports previous studies that reported Peak B to be a labile and microbially-derived compound [34,35]. This work also provides further evidence that Peak T is derived from an active bacterial population, via bacterial metabolic processes, as has been previously suggested in recent literature [6,36,37]. This is highlighted by the accelerated development of Peak T fluorescence when increasing the incubation temperature, from 20 • C to 30 • C (Figure 3).…”

“…The omnipresence of fluorescence Peaks C and M within the environmental samples ( Figure 2), which are identified as humic and terrestrial in nature, supports the notion that the majority of the AFOM found in environmental samples (components 2 and 4, Table 1) is inherently recalcitrant in nature [25,43,44]. This is in line with the general understanding that component 4, Peak C, represents high molecular weight non-bioavailable AFOM, currently considered to be of allochthonous origin [37], whereas component 2, Peak M, is biodegraded material. However, some variation in the fluorescence intensity for these components was observed over time, suggesting that the production and processing of these AFOM components may be occurring in situ [6].…”

Spatial and temporal dynamics of bacterioplankton community composition in a subtropical dammed karst river of southwestern China

“…Karst rivers are commonly regulated by damming, yet the influence of these dams on changes in hydrological series of water discharge is negative or positive (Miao, Ni, Borthwick, & Yang, ). Although the diversity and dynamics of microbes in karst springs (Farnleitner et al., ; Ohad et al., ; Savio et al., ), unsaturated and saturated karst aquifers (Cooper et al., ; Gray & Engel, ; Johnson et al., ; Lin et al., ; Menning et al., ), and water pools (Shabarova et al., ) as well as in groundwater‐surface water exchange systems (Li, Song, et al., ) have been discussed in the literature, much less attention has been paid to the structure of bacterioplankton communities in dammed karst rivers. In addition, previous studies on bacterioplankton communities in the canyon‐shaped and meso‐eutrophic Rimov Reservoir (Simek et al., ), the dammed Ebro River (Ruiz‐González, Proia, Ferrera, Gasol, & Sabater, ), and the rivers controlled by the Three Gorges Dam (Huang et al., ; Li, Lu, et al., ; Yan et al., ) did not include the seasonal variation or depth dynamics in bacterioplankton.…”

The in situ bacterial production of fluorescent organic matter; an investigation at a species level

“…The compounds associated with terrestrially derived FDOM are known to be stable higher molecular weight aromatic compounds, generally considered non-labile (Cooper et al, 2016). However, recent work concerning the marine environment has suggested that humic-like FDOM could be a consequence of bacterial metabolism (Guillemette and del Giorgio, 2012;Kramer and Herndl, 2004;Romera-Castillo et al, 2011;Shimotori et al, 2012).…”