The role of short-term and long-term water level and wave variability in coastal carbon budgets.

We investigated soil organic carbon dynamics at three freshwater coastal sites in the Laurentian Great Lakes using a simple carbon budget box model. Long-term carbon budgets (1939-2018) were developed using aerial photography and then compared to short-term carbon export (2018-2019) developed using drone data. This study puts forth a refined coastal carbon budget model that advances previous model iterations by: (1) examining spatial variability in carbon budgets, (2) including a temporally dynamic carbon inventory term, and (3) updating the erosional term. Half of the initial carbon stock of the combined sites was lost in the 80-year study period, which is severely imbalanced with the age of those coastal habitats (400-2000 cal years BP). Major periods of carbon loss corresponded to periods of elevated water level. Short-term loss of carbon during 2018-2019 corresponded to northeasterly extreme wave events during a period of above-average water level.

Author Correction: Modeling organic carbon loss from a rapidly eroding freshwater coastal wetland.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Modeling organic carbon loss from a rapidly eroding freshwater coastal wetland.

Shoreline erosion can transition freshwater coastal wetlands from carbon sinks to carbon sources. No studies have explored the impacts of coastal geomorphic processes on freshwater wetland carbon budgets. To do so, we modified a saltmarsh carbon budget model for application in freshwater coastal wetlands. We validated the model with data from a shoreline wetland in the Laurentian Great Lakes. The model generates the carbon budget by differencing carbon export and carbon storage. The inputs for carbon storage are the carbon inventory and maximum wetland age. Inputs for carbon export include erosion rates and overwash extent. The model demonstrates that the wetland examined in this study transitioned to a source of carbon during periods of erosion. In fact, the net carbon export between 2015 and 2018 was 10% of the wetland's original carbon stock. This study indicates that geomorphic change can dictate whether and how freshwater coastal wetlands serve as sources or sinks for terrestrial carbon, and that carbon stocks can fluctuate on a geologically rapid timescale. We recommend that such geomorphic processes be considered when developing carbon budgets for these marginal environments. Furthermore, the carbon budget model refined in this study can be used to prioritize wetlands in land management and conservation efforts.