Hydrological and Biogeochemical Controls on the Fate of Dissolved Organic Matter in Large Drainage Networks: The Pulse-Shunt Concept

Abstract:

Dissolved organic matter (DOM) is central to the ecology and chemistry of inland waters as an energy and nutrient source, transporter of heavy metals and other pollutants, and a control on light attenuation. In this research, we examine the manner in which hydrologic variation interacts with biogeochemical processes to affect the utilization and concentrations of DOM throughout streams and rivers of a large drainage basin. The Pulse-Shunt Concept (PSC) forms the framework for our analysis. The PSC is based, in part, on field observations of pulsed inputs of terrestrial DOM to streams of headwater catchments during high-discharge events that are associated with rainfall and snowmelt. The pulse is followed by the shunt, which occurs as rising flows rapidly transmit terrestrially derived DOM from headwater streams to larger streams and rivers of the drainage network. Owing to the reduction in channel residence times, the shunt perturbs the downstream gradient in DOM lability and compositional diversity established under base-flow conditions, leading to the riverine export of biochemically reactive DOM that has retained its terrestrial signature. We explore the fate of terrestrial DOM subsidies in context of the PSC by developing a simple model that routes water and DOM through an idealized 200,000 km² basin. This basin is drained by a dendritic channel network comprised of 30,000 first- through seventh-order streams that ultimately feed a 500-km long, eighth-order river. Published scaling laws are used to specify the numbers, lengths, and connectivities of the stream segments, while the routing algorithm describes the precipitation-induced delivery of terrestrial DOM to headwater reaches and the down-network changes in DOM concentrations resulting from rate-limited decomposition and mixing of waters from different reaches. Results of model simulations conducted to date underscore the dominant contribution of low-frequency precipitation events to the annual subsidy of terrestrial DOM to the drainage network. The results also suggest that most of this DOM subsidy is utilized in large rivers, far from where it enters the drainage network.