Abstract
A critical component of assessing the impacts of climate change on watershed ecosystems involves understanding the role that dissolved organic matter (DOM) plays in driving whole ecosystem metabolism. The hyporheic zone—a biogeochemical control point where ground water and river water mix—is characterized by high DOM turnover and microbial activity and is responsible for a large fraction of lotic respiration. Yet, the dynamic nature of this ecotone provides a challenging but important environment to parse out different DOM influences on watershed function and net carbon and nutrient fluxes. We used high-resolution Fourier-transform ion cyclotron resonance mass spectrometry to provide a detailed molecular characterization of DOM and its transformation pathways in the Columbia river watershed. Samples were collected from ground water (adjacent unconfined aquifer underlying the Hanford 300 Area), Columbia river water, and its hyporheic zone. The hyporheic zone was sampled at five locations to capture spatial heterogeneity within the hyporheic zone. Our results revealed that abiotic transformation pathways (e.g., carboxylation), potentially driven by abiotic factors such as sunlight, in both the ground water and river water are likely influencing DOM availability to the hyporheic zone, which could then be coupled with biotic processes for enhanced microbial activity. The ground water profile revealed high rates of N and S transformations via abiotic reactions. The river profile showed enhanced abiotic photodegradation of lignin-like molecules that subsequently entered the hyporheic zone as low molecular weight, more degraded compounds. While the compounds in river water were in part bio-unavailable, some were further shown to increase rates of microbial respiration. Together, river water and ground water enhance microbial activity within the hyporheic zone, regardless of river stage, as shown by elevated putative amino-acid transformations and the abundance of amino-sugar and protein-like compounds. This enhanced microbial activity is further dependent on the composition of ground water and river water inputs. Our results further suggest that abiotic controls on DOM should be incorporated into predictive modeling for understanding watershed dynamics, especially as climate variability and land use could affect light exposure and changes to ground water essential elements, both shown to impact the Columbia river hyporheic zone.
Highlights
Climate changes such as rising temperatures, more frequent extreme storms and changes in season precipitation rates are expected to have a profound impact on biogeochemical cycling within hydrological systems
Samples were collected from four ground water wells (GW), Columbia river water (RW), and its hyporheic zone (HZ) (Figure 1; Supplementary Table 1)
We do recognize that river level fluctuations play an important role in hyporheic zone dynamics, further molecular characterization of river level influences on dissolved organic matter (DOM) changes should be explored in future work
Summary
Climate changes such as rising temperatures, more frequent extreme storms and changes in season precipitation rates are expected to have a profound impact on biogeochemical cycling within hydrological systems (i.e., rivers, lakes, and streams). DOM quality and composition are influenced by new plant and soil inputs into water systems, biotic, and abiotic processing in these systems and overall hydrological transportation controls (Watts et al, 2001), the majority of which may be altered in the future by a changing climate (Ritson et al, 2014). Perturbations to the rates and types of DOM introduced into watersheds from climate variability and land use can impact the extent of abiotic and biotic degradation, subsequently influencing CO2 emissions to the atmosphere. In terms of the hyporheic zone, the biogeochemically active zone in hydrological systems, the degree of DOM degradability and composition of ground and river water inputs can strongly influence microbial hyporheic communities and lotic respiration as a whole. Parsing DOM differences between these regions (ground water, river water, hyporheic zone) is critical to understanding biogeochemical cycling and net fluxes through integrated river systems
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