Terrestrial-aquatic Continuum Research Articles

Concentrations and quality of DOC along the terrestrial–aquatic continuum in a boreal forested catchment

C cycling and dissolved organic C (DOC) inputs to boreal aquatic systems probably will change substantially with climate change. DOC concentrations already are increasing in surface waters. Terrestrial C is a major source of C to boreal freshwater ecosystems, but the interface between these 2 ecosystems, the riparian zone, has not been studied often. To improve our understanding of the importance of terrestrial inputs of DOC to aquatic systems from surrounding forests, we followed the changes of DOC concentration along a continuum of precipitation, throughfall, soil water, ground water, lake, and brook water in a pristine, boreal, forested headwater catchment and developed a lake C balance based on terrestrial and lacustrine C fluxes. We also examined DOC quality changes using the ratio of absorbance at 465 and 665 nm (E4/E6). DOC concentrations increased from 2.4 mg/L in precipitation to 132.3 mg/L in soil water as water passed through the terrestrial ecosystem. DOC concentrations in the riparian zone were correlated with DOC concentrations in the adjacent outflowing brook but not in the headwater lake. E4/E6 ratios indicated that the DOC in precipitation and throughfall was dominated by higher molecular weight compounds and that the DOC in soil and ground water was dominated by lower molecular weight compounds. The input of terrestrial DOC to the aquatic ecosystem was estimated to be 5 to 13 g C m−2 y−1, which is small compared with the C fluxes between atmosphere and vegetation, but can significantly decrease the net ecosystem exchange of an old-growth forest catchment. Terrestrial DOC was a major source of C in the lake, rendering it heterotrophic. The DOC export (3 g DOC m−2 y−1) made up almost 70% of total C export.

For improvement in understanding eco-hydrological processes in mire

Various anthropogenic stressors have caused mire degradation in subarctic northern Japan, such as drying and the invasion of an alder-dominant shrub forest. The Japanese government recently started a nature conservation project to restore a formerly meandering river in order to prevent invasive forest species and to rehabilitate the original mire ecosystem. In this study, the process-based National Integrated Catchment-based Eco-hydrology (NICE) model is further developed to improve accuracy in evaluation of nonlinear interactions and feedback of hydro-geomorphology and vegetation dynamics in the ecosystem. The simulated results clarify the impact of groundwater level change, sediment deposition, and nutrient availability on the complex alder invasion after refining the model to include newly-developed down-scaling and feedback processes. The model also shows the interaction between groundwater and inundated flow and its effect on vegetation change, which sheds light on two conflicting conceptualizations of peatland hydrology. This integrated system links the hydrological approach to the ecological one implicitly in the model, and throws some light on the refinement of tipping-point early warning systems for sustainable development and the improvement in boundless biogeochemical cycles along terrestrial-aquatic continuum for global environmental change.

Hydrologic sources of carbon cycling uncertainty throughout the terrestrial–aquatic continuum

AbstractLinking hydrologic interactions with global carbon cycling will reduce the uncertainty associated with scaling‐up empirical studies and facilitate the incorporation of terrestrial–aquatic linkages within global and regional change models. Much of the uncertainty in estimates of carbon fluxes associated with precipitation and hydrologic transport results from the extensive spatial and temporal heterogeneity in both intrinsic functioning and anthropogenic modification of hydrological cycles. To better understand this variation we developed a landscape ecological approach to coupled hydrologic–carbon cycling that merges local mechanisms with multiple‐scale spatial heterogeneity. This spatially explicit framework is applied to examine variability in hydrologic influences on carbon cycling along a continental scale water availability gradient with an explicit consideration of human sources of variability. Hydrologic variation is an important component of the uncertainty in carbon cycling; accounting for this variation will improve understanding of current conditions and projections of future ecosystem responses to global change.