Christopher L. Osburn

The increasing frequency of extreme weather events, including tropical cyclones, is altering the delivery and quality of organic matter (OM) exported from landscapes to estuaries. However, the mechanisms regulating these shifts, especially during multi‐year climatic oscillations, remain poorly understood. We used a new fluorescence–based mixing model to estimate dissolved OM (DOM) sources to the Neuse River Estuary, during a dry period (2010–2012) and a wet period (2015–2019), both punctuated by extreme precipitation events. Wet periods were characterized by positive anomalies in dissolved organic carbon (DOC) and a ₃₅₀ —a proxy for colored DOM (CDOM)—relative to dry period median values. During the wet period, extreme rainfall events collapsed the longitudinal gradient of wetland‐derived DOC between the tidal freshwater reach and the estuary mouth, resulting in spatial homogenization of source contributions. These events coincided with DOC and a ₃₅₀ values that significantly exceeded dry‐year medians, indicating hydrologically driven expansion of land–ocean connectivity and downstream replacement of background river–ocean mixtures of DOM with wetland‐derived carbon. We propose the Carbon Source and Weather (C‐SAW) paradigm, in which wetter conditions poise coastal watersheds to release pulses of DOM stored in the coastal landscape following extreme rainfall. These events overwhelm long‐residence time estuaries with distinctive DOM from wetland soils, sediments, and water, and potentially lead to dramatic changes to the estuarine carbon cycle. C‐SAW integrates elements of the Flood Pulse and Pulse‐Shunt Concepts, providing a unifying framework linking watershed carbon storage, hydrologic connectivity, and estuarine carbon cycling.

Aquatic monitoring of estuarine and coastal waters remains limited due to steep maintenance requirements for in situ hardware stemming from rapid biofouling and harsh environmental conditions. Monitoring sites will also ideally span a mix of shoreline and open water locations, and the need to travel to remote locations can strain resources and limit the geographic footprint of coastal monitoring programs. To overcome these limitations, we explored opportunities to partner with aquatic industry professionals who work on the water and may be able to incorporate routine site visits for sensor maintenance as part of their day-to-day routines. We specifically focused on collaborating with oyster growers to collect continuous salinity and water level observations on their farms, as part of a community-enabled monitoring program we named “See Salt.” In this short communication, we present the See Salt program as a case study and provide recommendations for others who may be interested in establishing a similar community-supported coastal monitoring program. Our recommendations include the following: (1) establish multi-method evaluation strategies to gain insights on program improvement, (2) invest in hardware that can go long durations without recalibration and cleaning, and (3) provide participants with the equipment and training needed to maintain sensors and validate measurements on their own. While we focus on our experiences monitoring salinity and water levels with oyster growers, the recommendations are not specific or unique to these variables or this partner population; our findings are broadly applicable to aquatic monitoring performed in partnership with non-academic professionals.

Flocculation of riverine dissolved organic matter (DOM) in estuaries is crucial for transforming and removing terrestrial carbon inputs across the land-to-ocean aquatic continuum. We measured variations in chromophoric DOM (CDOM) absorption and fluorescence of riverine DOM through mixing experiments conducted across various seasons and environments, identifying patterns in salt-induced flocculation. Our observations show a systematic reduction in CDOM absorption in the 250–450 nm range at salinity 2, with a sharper decrease at higher wavelengths. Flocculation led to decreased relative fluorescence intensity below emission wavelength of 360 nm and an increased intensity at higher emission wavelengths across the excitation spectrum measured (250–450 nm). We introduce a new metric, red shift ratio , a fluorescence-based metric calculated as the ratio of emission intensity at 300–350 nm to that at 360–500 nm, at excitation wavelengths between 250 and 300 nm, for detecting flocculation-induced changes in CDOM across estuarine systems. The observed sensitivity of CDOM to flocculation in low salinities challenges its use as a conservative tracer in coastal gradients, suggesting that recalibrations are required for remote sensing algorithms and carbon flux estimations across land-sea continuum, particularly in systems with similar characteristics.

Carbonate-associated organic matter (CAOM) is the organic matter associated with carbonate minerals, and a survey of carbonate-rich surface sediments suggests that it is incorporated at a consistent amount scaling with the internal surface area of the carbonate grains. As the carbonate sediment is sensitive to changes in saturation state due to benthic biogeochemical processing, we predicted that CAOM could exhibit interesting biogeochemical cycling, based on its potential to bridge particulate and dissolved pools of organic matter. Here, we report on a study in a seagrass meadow in central Florida Bay, USA. We utilize a combination of inorganic stable isotope (C, S, O) and high resolution mass spectrometry (21T FT ICR-MS) techniques to explore the carbon and sulfur cycles here, with a particular emphasis on dissolved organic matter (DOM) characterization. CAOM is examined similar to standard solid phase extraction (SPE-DOM) methods, after first washing carbonate sediment and dissolving it incompletely under a mild hydrochloric acid treatment. The δ34S and δ18O of sulfate, as well as the δ13C of dissolved inorganic carbon (DIC), suggest that the promotion of sulfide oxidation in the seagrass rhizosphere drives rapid carbonate dissolution and re-precipitation cycles. Sulfide oxidation, as well as elevated sulfide concentration, promotes sulfurization of CAOM, which is more sulfurized than porewater and surface water, as 42% of CAOM formulas vs 28% of surface water are sulfurized. Furthermore, a substantial quantity of molecular formulas present in the overlaying surface waters (90% of formulas, 97% by relative abundance) are also present in CAOM. Despite the CAOM sample containing nearly twice the number of formulas compared to surface water, due in part to its higher dissolved organic carbon concentration, these shared formulas make up 75% of the abundance of CAOM formulas. We argue that repeated coupled sulfur and inorganic carbon cycles, intensified by seagrasses, leads to increased sulfurization and release of CAOM, affecting DOM quality in the broader aquatic system. We estimate that approximately 9% of the particulate organic carbon (POC) stored in the sediments of this site are CAOM. Our results suggest that CAOM here is a form of “dissolvable” organic carbon which cycles much more rapidly than POC more broadly.

The particulate organic matter buried in carbonate-rich seagrass ecosystems is an important blue carbon reservoir. While carbonate sediments are affected by alkalinity produced or consumed in seagrass-mediated biogeochemical processes, little is known about the corresponding impact on organic matter. A portion of particulate organic matter is carbonate-associated organic matter. Here, we explore its biogeochemistry in a carbonate seagrass meadow in central Florida Bay, USA. We couple inorganic stable isotope analyses (δ³⁴S, δ¹⁸O) with a molecular characterization of dissolved and carbonate associated organic matter (21 tesla Fourier-transform ion cyclotron resonance mass spectrometry). We find that carbonate-associated molecular formulas are highly sulfurized compared to surface water dissolved organic matter, with multiple sulfurization pathways at play. Furthermore, 97% of the formula abundance of surface water dissolved organic matter is shared with carbonate-associated organic matter, indicating connectivity between these two pools. We estimate that 9.2% of the particulate organic matter is carbonate-associated, and readily exchangeable with the broader aquatic system as the sediment dissolves and reprecipitates.

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.