Spatial and temporal heterogeneity of methane ebullition in lowland headwater streams and the impact on sampling design

Abstract

Headwater streams are known sources of methane (CH 4 ) to the atmosphere, but their contribution to global scale budgets remains poorly constrained. While efforts have been made to better understand diffusive fluxes of CH 4 in streams, much less attention has been paid to ebullitive fluxes. We examine the temporal and spatial heterogeneity of CH 4 ebullition from four lowland headwater streams in the temperate northeastern United States over a 2-yr period. Ebullition was observed in all monitored streams with an overall mean rate of 1.00 AE 0.23 mmol CH 4 m À2 d À1 , ranging from 0.01 to 1.79 to mmol CH 4 m À2 d À1 across streams. At biweekly timescales, rates of ebullition tended to increase with temperature. We observed a high degree of spatial heterogeneity in CH 4 ebullition within and across streams. Yet, catchment land use was not a simple predictor of this heterogeneity, and instead patches scale variability weakly explained by water depth and sediment organic matter content and quality. Overall, our results support the prevalence of CH 4 ebullition from streams and high levels of variability characteristic of this process. Our findings also highlight the need for robust temporal and spatial sampling of ebullition in lotic ecosystems to account for this high level of heterogeneity, where multiple sampling locations and times are necessary to accurately represent the mean rate of flux in a stream. The heterogeneity observed likely indicates a complex set of drivers affect CH 4 ebullition from streams which must be considered when upscaling site measurements to larger spatial scales.

Dominance of Diffusive Methane Emissions From Lowland Headwater Streams Promotes Oxidation and Isotopic Enrichment

“…Calculated rates of gas exchange were similar to limited direct measurements in each of the streams (Supplementary Figure S1) using short-term continuous injections of a volatile gas tracer (Hall and Ulseth, 2020). Median diffusive CH 4 flux was 5.7 Robison et al (2021) during the same period of record (Table 2; Figure 2D). Across all four streams, the median diffusive CH 4 flux (5.7 ± 2.3 mmol CH 4 m −2 d −1 ) was greater than the median ebullitive CH 4 flux (0.6 ± 0.1 mmol CH 4 m −2 d −1 ).…”

“…Watersheds were selected to contrast CH 4 dynamics in the two predominant land use conditions in this region (developed versus forested). These streams have been the focus of previous studies that examined nutrient and carbon cycling and the impact of land use (e.g., Wollheim et al, 2005;Wollheim et al, 2015;Wollheim et al, 2017), and were all monitored for CH 4 ebullition from May through October 2019 (Robison et al, 2021). The watersheds of two streams, Sawmill Brook (D1) and College Brook (D2), are characterized by a relatively developed, suburban landscape, while the other two, Cart Creek (F1) and Dube Brook (F2), are predominantly forestcovered.…”

“…Stream chemistry is measured as part of the Plum Island Ecosystems LTER project (Morse and Wollheim, 2014;Wollheim et al, 2015). Stream reach characteristics were examined in a concomitant study (Robison et al, 2021). Mean reach depth at baseflow ranged from 7.2 cm at D2 to 14.5 cm at F1, and mean reach width at baseflow ranged from 1.94 m at D2 to 2.83 m at D1.…”

“…Ebullitive CH 4 fluxes were measured during the same time frame as diffusive flux by Robison et al (2021). Briefly, stationary bubble traps were deployed in triplicate at three to four locations in each stream.…”

“…Sediment cores were collected the first 2 weeks of July 2019 and processed in the field. Cores were collected at patches in the stream near each set of bubble traps as described in Robison et al (2021). Four patches at each stream were sampled, except D2 where only three patches were sampled.…”