Biogeochemical reaction and transport within hydrologic landscapes: crossing disciplinary and ecosystem boundaries

Abstract

Delivery of materials from catchments to coasts constitutes a significant flux within many global elemental cycles. However, large uncertainties bracket estimates of land-sea fluxes, due to limited understanding of interactions among material retention, transport, and transformation within hydrologic landscapes. Freshwater ecosystems facilitate biogeochemical reaction by bringing reactants together in complex physico-chemical environments. Further, they comprise the transport network by which materials move from catchments to coasts. Whereas there have been significant gains in understanding and quantifying hydrologic transport (HT) and biogeochemical reaction (BR) within specific types of freshwater ecosystems (e.g., nutrient spiraling in streams), disparate methodologies and approaches among ecosystems hinder synthesis efforts across the hydrologic landscape. Our goal is to increase the potential for synthesis of HT and BR across traditional ecosystem boundaries. We review the methods and metrics for quantifying HT and BR for the major ecosystems within hydrologic landscapes: lakes, rivers, wetlands, and groundwater. We then identify the research challenges that currently limit integration of HTBR across hydrologic landscapes and discuss the potential for a common set of metrics and approaches to represent HT and BR across multiple freshwater ecosystems. We advocate an approach that ties distribution functions of water residence time explicitly with retention efficiency of materials and nutrients. Such an approach reduces the impact of ecosystem-specific complexities that confound scaling exercises, avoids the assumption of steady-state, and provides a means for direct comparison of material dynamics across the hydrologic landscape.