A round-trip ticket: the importance of release processes for in-stream nutrient spiraling

Abstract

Most nutrient-spiraling studies have focused on estimates of gross uptake (U gross ), which show that streams take up dissolved inorganic nutrients very efficiently. However, studies based on estimates of net uptake (U net ) emphasize that streams tend to be at biogeochemical steady state (i.e., U net ≈ 0), at least on a time scale of hours. These findings suggest that streams can be highly reactive ecosystems but remain at short-term biogeochemical steady state if U gross is counterbalanced by release (R), a process that remains widely unexplored. Here, we propose a novel approach to infer R by comparing U net and U gross estimated from ambient and plateau concentrations obtained from standard short-term nutrient additions along a reach. We used this approach to examine the temporal variation of R and its balance with U gross in 2 streams with contrasting hydrological regime (i.e., perennial vs intermittent) during 2 years. We focused on the spiraling metrics of NH 4 + and soluble reactive P (SRP), essential sources of N and P in stream ecosystems. R differed substantially between the 2 streams. The perennial stream had a higher proportion of dates with R > 0 and a 2× higher mean R than the intermittent stream for both nutrients. Despite these differences, the magnitude of R and U gross tended to be similar for both nutrients within each stream, which lead to U net ≈ 0 in most cases. A notable exception occurred for SRP in the intermittent stream, where R tended to be higher than U gross during most of the winter period, probably because of desorption of P from stream sediments. Together, our findings shed light on the contribution of release processes to the dynamics of nutrient spiraling and support the idea that streams can be active ecosystems with high spiraling fluxes while simultaneously approaching short-term biogeochemical steady-state.

Decoupling of dissolved organic matter patterns between stream and riparian groundwater in a headwater forested catchment

“…These discrepancies could be partially explained by the fact that some of these manipulative experiments used monomeric carbohydrates that are easily bioavailable. Moreover, and as previously reported for nutrients, differences between estimates of in-stream gross and net uptake suggest that DOM consumption and production likely occur simultaneously within 25 the stream, and that the former is counterbalanced to some extend by the latter (von Schiller et al, 2015). Supporting this idea, median values of UDOC were >100 fold lower than DOC consumption inferred from measurements of ecosystem respiration calculated from diel cycles of dissolved oxygen concentrations in the same study stream (Lupon et al, 2016b).…”

Contribution of Hydrologic Opportunity and Biogeochemical Reactivity to the Variability of Nutrient Retention in River Networks

“…Nonetheless, nutrient retention in streams is usually estimated in the space domain from spiraling metrics using combined short‐term additions of a conservative and a reactive solute (Stream Solute Workshop, ; Webster & Valett, ). Noteworthy, this method provides information about gross nutrient uptake, and thus, it can be considered as an estimate of transitory nutrient retention because a fraction of the nutrients retained will be ultimately released to the water column (von Schiller et al, ). Basically, the nutrient spiraling approach allows estimating nutrient uptake rate per unit stream length ( k w , m −1 ), by solving the first‐order equation: where C is nutrient concentration (e.g., NO 3 − , NH 4 + , and SRP) and Con is the ambient conservative tracer concentration (e.g., chloride) at the top of the reach (top) and at the sampling sites located x m downstream during plateau conditions reached by the solute addition.…”

“…Nonetheless, nutrient retention in streams is usually estimated in the space domain from spiraling metrics using combined short-term additions of a conservative and a reactive solute (Stream Solute Workshop, 1990;Webster & Valett, 2006). Noteworthy, this method provides information about gross nutrient uptake, and thus, it can be considered as an estimate of transitory nutrient retention because a fraction of the nutrients retained will be ultimately released to the water column (von Schiller et al, 2015). Basically, the nutrient spiraling approach allows estimating nutrient uptake rate per unit stream length (k w , m À1 ), by solving the first-order equation:…”

Spatial and temporal characterization of nutrient net uptake in a vegetated urban stream: Stream bank features leading to net uptake hotspots

“…With a tight and strategically localized sampling methodology, frequent DIN net uptake was detected on a rooted bank and was higher along two subreaches where the root mat was more abundant, suggesting that in this section gross uptake surpassed release processes more consistently (von Schiller et al, ). The lack in FRP net uptake suggested that gross uptake and release processes for this nutrient are consistently in biochemical steady state.…”

“…Roberts and Mulholland () therefore suggested that to understand the controls in‐stream features retention processes have on nutrient fluxes and associated export, net uptake was a more useful metric. However, net uptake studies focussing on its triggers and controls under different hydrological, chemical, and biological conditions have remained far less studied, despite increasing interest over its impact on nutrients fluxes (Bernal, Lupon, Ribot, Sabater, & Martí, ; Bernal, von Schiller, Martí, & Sabater, ; Lupon, Martí, Sabater, & Bernal, ; von Schiller, Bernal, & Martí, ; von Schiller, Bernal, Sabater, & Marti, ).…”