Shifting Sources and Fates of Carbon With Increasing Hydrologic Presses and Pulses in Coastal Wetlands.

Coastal ecosystems are rapidly shifting due to changes in hydrologic presses (e.g., sea‐level rise) and pulses (e.g., seasonal hydrology, disturbances, and restoration of degraded wetlands). Changing water levels and sources are master variables in coastal wetlands that can alter carbon concentrations, sources, processing, and export. Yet, how long‐term increases in water levels from marine and freshwater sources influence dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition is uncertain. We quantified how long‐term changes in water levels are affecting DOC concentration (2001–2021) and DOM composition (2011–2021) differently across the Florida Everglades. DOC concentrations decreased with high water depths in peat marshes and increased with high water levels in marl marshes and across mangroves, and these relationships were reproduced in freshwater peat marshes and shrub mangroves. In the highly productive riverine mangroves, cross‐wavelet analysis highlighted variable relationships between DOC and water level were largely modulated by hurricane disturbances. By comparing relationships between water level and DOC concentrations with carbon sources from DOM fluorescence indices, we found that changing water sources between the dry and wet season shift DOM from algal to detrital sources in freshwater marshes, from detrital marsh to detrital mangrove sources in the brackish water ecotone, and from detrital mangrove to algal marine sources in downstream mangroves. As climate change and anthropogenic drivers continue to alter water levels in coastal wetlands, integrating spatial and temporal measurements of DOC concentrations and DOM compositions is essential to better constrain the transformation and export of carbon across these coastal ecosystems. Plain Language Summary: Water inputs to coastal ecosystems are changing in response to both sea‐level rise and water management. Changes in the amount and source of water can have different effects on organic carbon cycling across coastal ecosystems. We analyzed 20 years of data from the Florida Everglades to understand how changing water levels control carbon cycling. Our results show that higher water levels dilute carbon concentrations, and slow the release of carbon from peat soils, while also contributing to the transport of organic carbon downstream. We additionally found evidence of seasonal shifts in the composition of organic carbon, with higher algal contributions in the dry season for freshwater sites and higher marine contributions during the dry season for sites with marine influence. Our findings highlight that inputs, of both fresh and marine water, are transporting carbon within and between coastal wetlands, but only freshwater inputs prevent the release of carbon from peat stores. This research provides valuable insights into how inputs of water from different sources control the production, release, and movement of carbon throughout coastal wetland ecosystems. These insights are needed to preserve the value of organic carbon stored in coastal ecosystems, as the water within them changes. Key Points: Dissolved organic carbon concentrations decrease with water level in peat marshes but increase with level in mangroves and marl marshesDetrital carbon is mobilized by both fresh and marine hydrologic pulses, but only stored during freshwater hydrologic pulsesFreshwater restoration is increasing humic, colored dissolved organic matter entering the Everglades [ABSTRACT FROM AUTHOR]