River network saturation concept: factors influencing the balance of biogeochemical supply and demand of river networks

Abstract

River networks modify material transfer from land to ocean. Understanding the factors regulating this function for different gaseous, dissolved, and particulate constituents is critical to quantify the local and global effects of climate and land use change. We propose the River Network Saturation (RNS) concept as a generalization of how river network regulation of material fluxes declines with increasing flows due to imbalances between supply and demand at network scales. River networks have a tendency to become saturated (supply) demand) under higher flow conditions because supplies increase faster than sink processes. However, the flow thresholds under which saturation occurs depends on a variety of factors, including the inherent process rate for a given constituent and the abundance of lentic waters such as lakes, ponds, reservoirs, and fluvial wetlands within the river network. As supply increases, saturation at network scales is initially limited by previously unmet demand in downstream aquatic ecosystems. The RNS concept describes a general tendency of river network function that can be

Relating spatial patterns of stream metabolism to distributions of juveniles salmonids at the river network scale

“…These spatial nutrient patterns along with the NDS results suggest that during summer low‐flow periods, in‐stream demand for nutrients in UGR and CC exceeds supply and therefore exerts control on nutrient concentrations (Wollheim et al. ).…”

“…During periods of low flow, nutrient demand is expected to be highest and nutrient supply lowest, increasing the potential for biotic controls on nutrient concentrations (Wollheim et al. ). Rates of production during these low‐flow periods may increase demand and further drive spatial patterns (Finlay et al.…”

“…During periods of low flow, nutrient demand is expected to be highest and nutrient supply lowest, increasing the potential for biotic controls on nutrient concentrations (Wollheim et al. ). We therefore expected limiting nutrient concentrations to decrease with watershed area due to biological uptake and depletion of nutrients as has been observed in other study systems during low‐flow conditions (Finlay et al.…”

LINX I and II: Lessons Learned and Emerging Questions

“…Saturation also occurs at the network scale whereby rates of nutrient supply outpace demand and rates of removal. This "network-scale saturation" (Wollheim et al, 2018) is evident in declining effectiveness of N removal mechanisms in headwaters, and subsequent transport of large quantities of NO − 3 downstream (Hall et al, 2009;Mulholland et al, 2009). As NO − 3 concentrations increase in small streams, the resulting increase in NO − 3 export shifts the site of most N removal downstream to larger rivers.…”

“…This indicates that N inputs and associated removal processes can be highly separated in space and time creating a mosaic of N processing rates within and across river networks. It should be noted that saturation of N removal can still occur in higher-order rivers resulting in little to no removal/processing at the network scale (Wollheim et al, 2018). While headwaters prove to be a highly effective component of the river network (Bernot and Dodds, 2005), ultimately it is the whole stream network that drives N removal based on landscape-scale inputs and heterogeneity in the demand for N by in-stream biota at the network-scale.…”