Understanding hydrological and ecological controls on methane flux in
Lake Erie estuarine mineral-soil marsh
Abstract
Methane emissions from freshwater, mineral-soil wetlands represent an
important portion of the global greenhouse gas budgets. We use long-term
observations in Old Woman Creek (OWC), an estuarine wetland at the coast
of Lake Erie. OWC is characterized by a fluctuating water level
controlled by a natural sand barrier. OWC water levels are high when the
barrier is closed. When it breaks, OWC is directly connected to the
lake. Long term water level rise of Lake Erie provides a trend of water
level at OWC. These changes to hydrology drive changes to the ecology.
In OWC the dominant eco-hydrological patch types are mudflats, cattails
(Typha), floating-leaf vegetation (Lotus, water lily), and open water.
The seasonal and long term changes to water level lead to dynamic
changes in the patch type composition, and as OWC gets deeper, mudflats
and cattails give way to open water and floating-leaf vegetation. We
developed an approach to classify the eco-hydrological patch type from
remote sensing images. We used seasonal time series of NDVI from HLS (a
composite dataset of Sentinel and Landsat). These time series were
classified to patch types according to their similarity to the seasonal
NDVI profiles of pixels identified and ground-truthed as pure pixels of
a specific patch type. We then used the DG-SWEM high resolution
hydrodynamic model to simulate the flow velocity throughout the wetland.
Combining the eco-hydrological patch locations and the high-resolution
flow simulation allowed for the calculation of an effective
patch-type-level residence time. We found differences in residence times
between the different patch types. We measured the relations between
methane flux and CO2 uptake at the whole wetland scale,
the vegetation patch scale, and directly from leaves. We found different
methane-CO2 relations among the floating leaved and
emergent species. Phenological transitions throughout the growing season
continued to make an important effect only in Typha. Our observations
represent a valuable foundation towards a more robust models of methane
fluxes in wetlands at the resolution of within-wetland vegetation patch
type, and resolving the effects of seasonal and within-season vegetation
phenology in ecosystem-scale models.