Nutrient Retention and the Problem of Hydrologic Disconnection in Streams and Wetlands

Abstract

Some aquatic systems have disproportionately high nutrient processing rates, and may be important to nutrient retention within river networks. However, the contribution of such biogeochemical hot spots also depends on water residence time and hydrologic connections within the system. We examined the balance of these factors in a comparative study of nitrate (NO 3 -) uptake across stream and flowthrough wetland reaches of northern Wisconsin, USA. The experimental design compared NO 3 -uptake at different levels: the ecosystem level, for reaches (n = 9) consisting of morphologically contrasting subreaches (SLOW, low mean water velocity; REF, reference, or higher mean water velocity); the sub-ecosystem level, for subreaches consisting of morphologically contrasting zones (TS, transient storage zone; MC, main channel zone). SLOW subreaches had 45% lower ecosystem-level uptake rate (K, t -1 ) on average, indicating reduced uptake efficiency in flow-through wetlands relative to streams. The four largest K values (total n = 24) also occurred in REF subreaches. TS:MC uptake rate varied (range 0.1-6.0), but MC zones consistently accounted for most NO 3 -uptake by the ecosystem. In turn, TS influence was limited by a tradeoff between TS zone uptake rate and the strength of TS-MC hydrologic connection (a or F med ). Additional modeling of published hydrologic parameter sets showed that strong MC dominance of uptake (>75% of total uptake), at the scale of solute release methods (meters to kilometers, hours to days), is common among streams and rivers. Our results emphasize that aquatic nutrient retention is the outcome of a balance involving nutrient uptake efficiency, water residence time, and the strength of hydrologic connections between nutrient sources and sinks. This balance restricts the influence of hydrologically disconnected biota on nutrient transport, and could apply to diverse ecosystem types and sizes.

Nitrogen retention in the main channel and two transient storage zones during nutrient addition experiments

“…Uptake rates in the HTS zones were higher than the MC and STS zones in every Truckee River subreach, in agreement with some previous findings (Stewart et al ; Argerich et al 2011 c ). STS uptake rates alternated between being larger and smaller than that of the MC in these subreaches, which is also similar to a previous study (Powers et al ). Confidence in the MC uptake rates was greater than confidence in the STS and HTS uptake rates (Table ) showing that overall nitrate uptake was most sensitive to the MC uptake rate.…”

“…Differences in connectivity, residence time, and processing capabilities between the two TS zones led to the large differences in the overall nutrient uptake in the two TS zones. Similar to our results, a previous study observed higher uptake rates in TS zones with higher residence times and less connectivity (Powers et al ). For STS zones, biological uptake is controlled by how many solute molecules are exposed to the biologic communities within the storage zone because they are in most cases very well mixed and connected to the MC, have uptake rates similar to or less than the MC, and have small residence times.…”

Hydrologic connectivity as a framework for understanding biogeochemical flux through watersheds and along fluvial networks

“…In many small streams of the Rocky Mountains U.S., land and water management has led to stream incision and disconnection from valley floodplains (Wohl and Beckman, 2014). Disconnecting rivers and floodplains can lead to a loss of retention capacity and can have strong implications for the down-network transport of water, sediment, organic material, and nutrients (Alexander et al, 2009;Powers et al, 2012;Wohl and Beckman, 2014;Wollheim et al, 2008b).…”

Nutrient Retention in Restored Streams and Rivers: A Global Review and Synthesis

“…The commonality between all of these stream restoration practices is that they reconnect surface and groundwater, foster connectivity between N sources and C-rich soils, and increase retention time in order to promote N retention. Limited hydrologic connectivity can reduce the effectiveness of floodplains and other transient storage zones as nutrient sinks [103]. From our database of hydrologic stream restoration studies, we learned that the effect of all of the typologies was an overall retention of nutrients.…”