Stormflows Drive Stream Carbon Concentration, Speciation, and Dissolved Organic Matter Composition in Coastal Temperate Rainforest Watersheds

Abstract

Stream water carbon concentrations can be highly dynamic on the time scales of both individual storm events and seasonal hydroclimatic shifts. We collected stream water daily over a 6-day storm from three headwater subcatchments of varying landcover (poor fen, forested wetland, and upland forest) and the catchment outlet to evaluate how precipitation events impact the concentration and speciation of carbon (organic vs. inorganic) as well as the composition of dissolved organic matter (DOM) exported laterally from coastal temperate rainforest catchments. Dissolved and particulate organic carbon concentrations increased during the storm at all sites, while dissolved inorganic carbon concentrations were diluted during peak flows. These results highlight the importance of quantifying all forms of lateral carbon export when evaluating the role of storms in catchment-scale carbon cycling. Isotopic hydrograph separation using stream water δ 18 O showed that percent new water was significantly related to carbon concentration and form providing a clear link between stream water sources (i.e., recent event water) and soil carbon source areas that become connected to surface water during storms. Furthermore, ultrahighresolution mass spectrometry showed that stream water DOM exported from the upland forest contained the greatest molecular diversity of the three landscape types and had the largest changes in composition over the storm suggesting that the wetland-dominated subcatchments were less compositionally diverse with regard to soil DOM pools active during the storm. Overall, this study provides insight into hydrobiogeochemical drivers that control lateral carbon export from forested catchments in a region where an increasing fraction of precipitation is falling as rain. Plain Language Summary Streams transport large amounts of terrestrially derived carbon to the ocean, especially during large rainstorms. We collected water samples daily over a 6-day storm from small drainage areas of varying landcover to see how the concentration and type of carbon changed over the course of a storm. Our results show that the amount and type of carbon in the stream changed dramatically during the storm and originated from different areas of the landscape. The flow of water through the soil also changed during the storm and was related to the type and amount of carbon entering the stream. Storm events not only impact carbon entering the stream but also may impact its transfer to coastal marine ecosystems. Climate in the study region is projected to become warmer and wetter in coming decades. These shifts in climate could lead to more carbon export during storms, especially during winter because of more precipitation falling as rain rather than snow.

Trees in the Stream: Determining Patterns of Terrestrial Dissolved Organic Matter Contributions to the Northeast Pacific Coastal Temperate Rainforest

“…Precipitation occurred throughout the year in rain events typical of the region (Figure S1 in the Supporting Information ; Fellman et al., 2020; Pierre et al., 2020). October was the wettest month and March the driest (35.1 v. 11.0 cm water equivalent, respectively).…”

“…In our previous research, ∼90% of source‐specific marker formulae disappeared during a 14‐day bioincubation, demonstrating high bioavailability under nutrient replete conditions (Behnke et al., 2022). Those 14‐day incubations likely exceeded the residence time of water and DOM in sub‐catchment in Aangooxa Yé, which are thought to be on the order of hours to a couple days (though exact residence times for the system are not known; Fellman, Hood, Edwards, & D'Amore, 2009; Fellman et al., 2020). The appearance of all the bioavailable source‐specific formulae in this data set in soil pore waters and tributaries, and the mainstem suggests that: (a) residence times in the watershed are enough shorter than the 14‐day bioincubations that microbes did not have enough time to process DOM along the terrestrial‐stream continuum; (b) nutrient availability in situ was too low to allow microbial degradation; and/or 3) stationary bottle conditions in the incubation do not accurately mimic dynamic watershed processing.…”

“…A SonTek FlowTracker ADV was used to determine discharge measurements across a wide range of flow conditions during the sampling period (May–October) of 2018. The stage‐discharge relationship was used to calculate streamflow during the sampling period when stage‐discharge relationships were actively assessed (Fellman et al., 2020). Water table in each sub‐catchment was tracked using stilling wells in each outlet stream (near the “head” sampling locations).…”

Hydrological Controls on Dissolved Organic Matter Composition throughout the Aquatic Continuum of the Watershed of Selin Co, the Largest Lake on the Tibetan Plateau

“…To better understand the influence of the ongoing warming and wetting on downstream DOM dynamics, it is fundamental to elucidate the extent of the hydrological control of DOM dynamics throughout the aquatic continuum. 11 Specifically, water isotope values (δ 2 H-H 2 O and δ 18 O-H 2 O) are effective proxies for water sources and hydrological conditions 24 with enriched δ 2 H-H 2 O and δ 18 O-H 2 O due to evaporation in lakes with a relatively long residence time and a high ratio of lake area/catchment area. 23,25 In this study, we investigated variability in DOM quality and quantity from headwaters and downstream rivers and lakes along the watershed of Selin Co (Figure 1) by integrating measurements of DOC concentration, optical properties (absorbance and fluorescence), and molecular composition using ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).…”

“…To better understand the influence of the ongoing warming and wetting on downstream DOM dynamics, it is fundamental to elucidate the extent of the hydrological control of DOM dynamics throughout the aquatic continuum . Specifically, water isotope values (δ 2 H-H 2 O and δ 18 O-H 2 O) are effective proxies for water sources and hydrological conditions with enriched δ 2 H-H 2 O and δ 18 O-H 2 O due to evaporation in lakes with a relatively long residence time and a high ratio of lake area/catchment area. , …”