Comparing CO2 and CH4 fluxes in prevailing saltmarsh zones in the southern Gulf of Saint Lawrence

Abstract

Tidal saltmarshes are unique coastal ecosystems that have historically provided important ecosystem services, including efficient carbon sequestration. In the northwestern North Atlantic, salt water adapted vegetation species organize saltmarshes into vegetation zones distributed along a tidal gradient. In the southern Gulf of St. Lawrence, saltmarshes’ primary low marsh vegetation zone is dominated by Sporobolus alterniflorus. Recently, significant community structure change has been observed in several southern Gulf of St. Lawrence saltmarshes. Particularly, an expansion of saltmarsh area bare of dominant vegetation has been observed. I have called these areas dead zones. Given that vegetation is a controlling factor in the flux of greenhouse gasses into and out of the soil environment, I compared the fluxes of carbon dioxide (CO2) and methane (CH4) in S. alterniflorus zones and dead zones. Simulating daytime (light) and night-time (dark) conditions, I collected 43 dead zone soil gas flux measurements between June 16 and September 25, 2023, and 24 S. alterniflorus zone soil gas flux measurements, between August 17 and September 25, 2023, using a soil gas chamber and a mobile gas concentration analyzer at five different saltmarshes. Following the analysis of my measurements, I found that the mean dark CO2 fluxes of late growing season S. alterniflorus zones (𝑥̅ = 963.3 ± 626.2 mg1m-2hr-1) was significantly greater (Fstat = 7.161, P-value = 0.001250) than that of dead zones (𝑥̅ = 200.1 ± 259.9 mg1m-2hr-1), but I found no significant difference between the mean light and daily net CO2 fluxes of the two groups (𝑥̅ = -168.7 ± 710.7 mg1m-2hr-1, Fstat = 1.474, P-value = 0.2466; 𝑥̅ = 89.03 ± 382.7 mg1m-2hr-1, Fstat = 0.3313, P-value = 0.8028; respectively). I also found no significant differences between the mean light, dark or net daily CH4 fluxes of late growing season S. alterniflorus zones and dead zones (𝑥̅ = 0.02832 ± 0.4635 mg1m-2hr-1, Fstat = 0.06789, P-value = 0.9765; 𝑥̅ = 0.009973 ± 0.2602 mg1m-2hr-1, Fstat = 2.033, P-value = 0.1349; 𝑥̅ = 0.01434 ± 0.2858 mg1m-2hr-1, Fstat = 0.4226, P-value = 0.7385; respectively). I did however observe seasonal normal-resembling magnitude curves of the light and dark CO2 fluxes, and to lesser extent, light and dark CH4 fluxes. Both light and dark CO2 fluxes trended to be positive at the end of August and in September, after S. alterniflorus had senesced. As climate change warms autumns in Nova Scotia, this period of S. alterniflorus and dead zones being net sources of CO2 may lengthen and intensify. My work characterizes dead zone CO2 and CH4 fluxes over the majority of one growing season. The greenhouse gas fluxes of dead zones should continue to be studied, along with how they change the community structure ecosystem function of saltmarshes.