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Global sea-level rise is transforming coastal ecosystems, especially freshwater wetlands, in part due to increased episodic or chronic saltwater exposure, leading to shifts in biogeochemistry, plant- and microbial communities, as well as ecological services. Yet, it is still difficult to predict how soil microbial communities respond to the saltwater exposure because of poorly understood microbial sensitivity within complex wetland soil microbial communities, as well as the high spatial and temporal heterogeneity of wetland soils and saltwater exposure. To address this, we first conducted a two-year survey of microbial community structure and bottom water chemistry in submerged surface soils from 14 wetland sites across the Florida Everglades. We identified ecosystem-specific microbial biomarker taxa primarily associated with variation in salinity. Bacterial, archaeal and fungal community composition differed between freshwater, mangrove, and marine seagrass meadow sites, irrespective of soil type or season. Especially, methanogens, putative denitrifying methanotrophs and sulfate reducers shifted in relative abundance and/or composition between wetland types. Methanogens and putative denitrifying methanotrophs declined in relative abundance from freshwater to marine wetlands, whereas sulfate reducers showed the opposite trend. A four-year experimental simulation of saltwater intrusion in a pristine freshwater site and a previously saltwater-impacted site corroborated the highest sensitivity and relative increase of sulfate reducers, as well as taxon-specific sensitivity of methanogens, in response to continuously pulsing of saltwater treatment. Collectively, these results suggest that besides increased salinity, saltwater-mediated increased sulfate availability leads to displacement of methanogens by sulfate reducers even at low or temporal salt exposure. These changes of microbial composition could affect organic matter degradation pathways in coastal freshwater wetlands exposed to sea-level rise, with potential consequences, such as loss of stored soil organic carbon. 

To determine whether mangrove soil accretion can keep up with increasing rates of sea level rise, we modeled the theoretical, steady-state (i.e., excluding hurricane impacts) limits to vertical soil accretion in riverine mangrove forests on the southwest coast of Florida, USA. We measured dry bulk density (BD) and loss on ignition (LOI) from mangrove soils collected over a period of 12 years along an estuarine transect of the Shark River. The plotted relationship between BD and LOI was fit to an idealized mixing model equation that provided estimates of organic and inorganic packing densities in the soils. We used these estimates in combination with measures of root production and mineral deposition to calculate their combined contribution to steady-state, vertical soil accretion. On average, the modeled rates of accretion (0.9 to 2.4 mm year−1) were lower than other measured rates of soil accretion at these sites and far less than a recent estimate of sea level rise in south Florida (7.7 mm year−1). To date, however, no evidence of mangrove “drowning” has been observed in this region of the Everglades, indicating that assumptions of the linear accretion model are invalid and/or other contributions to soil accretion (e.g., additional sources of organic matter; feedbacks between physical sedimentation processes and biological responses to short-term environmental change) make up the accretion deficit. This exercise highlights the potential positive impacts of hurricanes on non-steady-state soil accretion that contribute to the persistence of neotropical mangroves in regions of high disturbance frequency such as the Gulf of Mexico and the Caribbean region. 

Hurricanes are recurring high-energy disturbances in coastal regions that change community structure and function of mangrove wetlands. However, most of the studies assessing hurricane impacts on mangroves have focused on negative effects without considering the positive influence of hurricane-induced sediment deposition and associated nutrient fertilization on mangrove productivity and resilience. Here, we quantified how Hurricane Irma influenced soil nutrient pools, vertical accretion, and plant phosphorus (P) uptake after its passage across the Florida Coastal Everglades in September 2017. Vertical accretion from Irma’s deposits was 6.7 to 14.4 times greater than the long-term (100 y) annual accretion rate (0.27 ± 0.04 cm y⁻¹). Storm deposits extended up to 10-km inland from the Gulf of Mexico. Total P (TP) inputs were highest at the mouth of estuaries, with P concentration double that of underlying surface (top 10 cm) soils (0.19 ± 0.02 mg cm⁻³). This P deposition contributed 49 to 98% to the soil nutrient pool. As a result, all mangrove species showed a significant increase in litter foliar TP and soil porewater inorganic P concentrations in early 2018, 3 mo after Irma’s impact, thus underscoring the interspecies differences in nutrient uptake. Mean TP loading rates were five times greater in southwestern (94 ± 13 kg ha⁻¹d⁻¹) mangrove-dominated estuaries compared to the southeastern region, highlighting the positive role of hurricanes as a natural fertilization mechanism influencing forest productivity. P-rich, mineral sediments deposited by hurricanes create legacies that facilitate rapid forest recovery, stimulation of peat soil development, and resilience to sea-level rise.