Methane emissions from drowned river mouth estuaries of Lake Michigan are linked to lotic-lentic exchanges

Freshwater estuaries play an important role in the carbon cycle via primary production and the emission of methane (CH4) and carbon dioxide (CO2). There are more than 100 drowned river mouth (DRM) freshwater estuaries in the Laurentian Great Lakes Basin, characterized by mixing of distinct water masses from tributary inflow and intrusions from the Great Lakes, creating unique hydraulic and biological transition zones. These DRMs exist across a latitudinal gradient of land-use and range from oligotrophic to hypereutrophic. We hypothesized that CH4 and CO2 emissions across DRMs would mirror the eutrophication gradient, and within DRMs the emissions would vary with bidirectional river and Great Lake inputs. We measured CH4 and CO2 at three Lake Michigan DRMs ranging from mesotrophic (Muskegon and White lakes) to hypereutrophic (Lake Macatawa) by deploying continuous CH4 samplers in littoral zones and sampling CH4 and CO2 monthly in three pelagic zones (river mouth, deepest point, and Lake Michigan channel). Within-DRM variability was present in pelagic zone diffusive emissions (26.2 and 13.0 μmol CH4 m-2 hr-1 at the river and channel, respectively). Littoral zone emissions were an order of magnitude greater, with medians of 0.15 - 0.34 mmol CH4 m-2 hr-1 across DRMs. Differences in pelagic CH4 fluxes across trophic status were minor, and not apparent in littoral zone fluxes. However, river mouth CH4 fluxes were greater than the channel and deepest point at multiple times, regardless of estuary, reflecting the influence of physical transport mechanisms and distinct chemical and biological zones in these lotic-lentic linked systems.