Accounting for winter: freeze-thaw controls on salt marsh-creek water and carbon exchange

Salt marshes are among the most carbon-dense ecosystems on Earth, yet their net climate benefit depends on carbon exchanges across the atmosphere-soil-water continuum, including lateral export to adjacent coastal waters. Most mechanistic understanding of lateral exchanges is derived from warm-season observations, leaving uncertainties about how cold-season conditions regulate soil-water connectivity and associated solute and carbon transport. We address this gap using year-round, high-frequency measurements of soil temperature, groundwater and surface-water elevations, and tidal creek discharge across multiple New England salt marshes (Gouldsboro, northern Maine; Wells, southern Maine; and Chatham, Cape Cod, Massachusetts). Soil temperatures decreased with latitude, and sustained freezing occurred at both Maine sites from December through mid-March. Within marshes, freezing was strongly elevation-dependent: creek beds remained unfrozen due to persistent exposure to relatively warm, saline seawater, whereas higher-elevation platforms that were inundated less frequently froze to depths of 25-30 cm. Despite frozen ground, we observed minimal seasonal changes in water-table fluctuations. However, reduced hydraulic conductivity during winter suggests diminished but ongoing water and solute exchange between marsh sediments and tidal creeks. Together, these observations indicate that cold-season freeze-thaw alters marsh-creek exchange but does not eliminate lateral water and solute export to tidal channels. Incorporating cold-season controls on marsh-creek exchange and lateral export into marsh carbon assessments is essential for closing year-round carbon budgets and evaluating blue carbon under changing winter conditions and inundation regimes.