Critical zone structure controls concentration‐discharge relationships and solute generation in forested tropical montane watersheds

Abstract

Concentration‐discharge (C‐Q) relationships are poorly known for tropical watersheds, even though the tropics contribute a disproportionate amount of solutes to the global ocean. The Luquillo Mountains in Puerto Rico offer an ideal environment to examine C‐Q relationships across a heterogeneous tropical landscape. We use 10–30 years of weekly stream chemistry data across 10 watersheds to examine C‐Q relationships for weathering products (SiO2(aq), Ca2+, Mg2+, and Na+) and biologically controlled solutes (dissolved organic carbon [DOC], dissolved organic nitrogen [DON], NH4+, NO3–, PO43–, K+, and SO42–). We analyze C‐Q relationships using power law equations and a solute production model and use principal component analysis to test hypotheses regarding how the structure of the critical zone controls solute generation. Volcaniclastic watersheds had higher concentrations of weathering solutes and smaller tributaries were approximately threefold more efficient at generating these solutes than larger rivers. Lithology and vegetation explained a significant amount of variation in the theoretical maximum concentrations of weathering solutes (r2 = 0.43–0.48) and in the C‐Q relationships of PO43– (r2 = 0.63) and SiO2(aq) (r2 = 0.47). However, the direction and magnitude of these relationships varied. Across watersheds, various forms of N and P displayed variable C‐Q relationships, while DOC was consistently enriched with increasing discharge. Results suggest that PO43– may be a useful indicator of watershed function. Relationships between C‐Q and landscape characteristics indicate the extent to which the structure and function of the Critical zone controls watershed solute fluxes.

DOC Transport and Export in a Dynamic Tropical Catchment

“…In contrast to most studies from forested catchments showing predominantly preevent water contributions to stormflow (Klaus & McDonnell, 2013), the high correlation between large rainfall events and runoff generation with high event water contributions (ranging from 42.4±0.3% up to 98.2±0.3% of the total discharge) indicated that surface and shallow lateral fluxes with more limited subsurface mixing governed DOC transport to the stream (Mei et al, 2014;Shih et al, 2018;Suhett et al, 2007;Wymore et al, 2017) with a relatively quick response time after the rainfall maxima. The latter coupled with moderate to steep slopes Journal of Geophysical Research: Biogeosciences ( Figure 1a) favored rapid DOC leaching from the canopy and understory providing fresh litter material (Figures 1c and 1d) as well as DOC from the rich organic soil surface toward Quebrada Grande.…”

Multiyear Trends in Solute Concentrations and Fluxes From a Suburban Watershed: Evaluating Effects of 100‐Year Flood Events

“…Many studies have observed enrichment behavior for DOC with increases in river discharge (e.g., Herndon et al, ; Hinton et al, ; Hornberger et al, ; Koenig et al, ; Shanley et al, ; Wymore et al, ), and similarly in lakes, wet years have been associated with increased DOC concentrations (Strock et al, ). Despite these well‐documented relationships between DOC and discharge at other sites we did not observe a response of DOC concentration to discharge in this 15‐year record of weekly observations.…”

Concentration‐Discharge Relations in the Critical Zone: Implications for Resolving Critical Zone Structure, Function, and Evolution

“…Power law equations of the form C = aQ – k enable synoptic comparisons of C‐Q relations across multiple systems where the slope k can be related to potential CZ structural controls. For example, Wymore et al () used 10–30 years of weekly stream chemistry and discharge data across 10 watersheds in Luquillo CZO (Puerto Rico, USA) to show that vegetation, lithology and basin order exert strong controls over the power law exponent for both weathering products (Si, Ca, Mg, Na, and SO 4 ) and biologically controlled solutes (DOC, PO 4 , and inorganic N). In a study of C‐Q relations in headwater streams of the Sierra Nevada, Hunsaker and Johnson () observed chemostatic behavior for bedrock‐derived solutes and nonchemostatic behavior for biologically active solutes, with the former showing slight dilution and the latter, deriving from soil solutions, showing increased concentration during storm events.…”