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Water column nutrient concentrations and autotrophy in oligotrophic ecosystems are sensitive to eutrophication and other long-term environmental changes and disturbances. Disturbance can be defined as an event or process that changes the structure and response of an ecosystem to other environmental drivers. The role disturbance plays in regulating ecosystem functions is challenging because the effect of the disturbance can vary in magnitude, duration, and extent spatially and temporally. We measured changes in total nitrogen (TN), dissolved inorganic nutrient (DIN), total phosphorus (TP), soluble reactive phosphorus (SRP), total organic carbon (TOC), and chlorophyll-a (Chl-a) concentrations throughout the Everglades, Florida Bay, and the Florida Keys. This region has been subjected to a variety of natural and anthropogenic disturbances including tropical storms, fires, eutrophication, and rapid increases in water levels from sea-level rise and freshwater restoration. We hypothesized that the rate of change in water quality would be greatest in the coastal ecotone where disturbance frequencies and marine P concentrations are highest, and in freshwater marshes closest to hydrologic changes from restoration. We applied trend analyses on multi-decadal data (1996–2019) collected from 461 locations distributed from inland freshwater Everglades (ridge and slough) to outer marine reefs along the Florida Keys, USA. Total Organic Carbon decreased throughout the study area and was the only parameter with a systematic trend throughout the study area. All other parameters had spatially heterogeneous patterns in long-term trends. Results indicate more variability across a large spatial and temporal extent associated with changes in biogeochemical indicators and water quality conditions. Chemical and biological changes in oligotrophic ecosystems are important indicators of environmental change, and our regional ridge-to-reef assessment revealed ecosystem-specific responses to both long-term environmental changes and disturbance legacies. 

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. 

Phytoplankton assembly dynamics in lakes are highly sensitive to variability in climate drivers and resulting physicochemical changes in lake water columns. As climate change increases the frequency of major precipitation events and droughts, many lakes experience increased inputs of colored dissolved organic carbon (CDOC) and nutrients. How these CDOC-related changes in resources, transparency, and thermal stability affect phytoplankton assemblages, succession, and resilience is understudied, particularly in subtropical lakes. Here, we used time series, multivariate, and trait-based functional redundancy analyses to elucidate the roles of phytoplankton in ecosystem resilience and determine potential drivers of assemblage shifts in a subtropical monomictic lake with fluctuating CDOC inputs (Lake Annie, Highlands County, Florida, USA). We found that phytoplankton assemblages and successional patterns differed between two dark-water states (late 2005–mid-2007, late 2012–2019) bracketing a clear-water state (mid-2007–late 2012), caused by shifting CDOC and nutrient concentrations associated with oscillating groundwater levels. Diatoms (Bacillariophyta), which were dominant during the two dark-water states, nearly disappeared and were replaced by synurophytes during the clear-water state. Assemblages had greater interannual consistency in the dark-water states, while mean functional redundancy decreased in the clear-water state. Seasonal phytoplankton successional changes were also more pronounced and synchronized with seasonal hydrologic shifts in the dark-water states. Multiyear assemblage shifts occurred more quickly in clear-to-dark than dark-to-clear state transitions, suggesting phytoplankton in dark-water states may be more resistant to state transitions or even contribute to dark-water state resilience via feedback loops. 

Planktonic microbial communities mediate many vital biogeochemical processes in wetland ecosystems, yet compared to other aquatic ecosystems, like oceans, lakes, rivers or estuaries, they remain relatively underexplored. Our study site, the Florida Everglades (USA)—a vast iconic wetland consisting of a slow-moving system of shallow rivers connecting freshwater marshes with coastal mangrove forests and seagrass meadows—is a highly threatened model ecosystem for studying salinity and nutrient gradients, as well as the effects of sea level rise and saltwater intrusion. This study provides the first high-resolution phylogenetic profiles of planktonic bacterial and eukaryotic microbial communities (using 16S and 18S rRNA gene amplicons) together with nutrient concentrations and environmental parameters at 14 sites along two transects covering two distinctly different drainages: the peat-based Shark River Slough (SRS) and marl-based Taylor Slough/Panhandle (TS/Ph). Both bacterial as well as eukaryotic community structures varied significantly along the salinity gradient. Although freshwater communities were relatively similar in both transects, bacterioplankton community composition at the ecotone (where freshwater and marine water mix) differed significantly. The most abundant taxa in the freshwater marshes include heterotrophic Polynucleobacter sp. and potentially phagotrophic cryptomonads of the genus Chilomonas, both of which could be key players in the transfer of detritus-based biomass to higher trophic levels. 

Abstract Mangroves are the most blue-carbon rich coastal wetlands contributing to the reduction of atmospheric CO₂through photosynthesis (sequestration) and high soil organic carbon (C) storage. Globally, mangroves are increasingly impacted by human and natural disturbances under climate warming, including pervasive pulsing tropical cyclones. However, there is limited information assessing cyclone’s functional role in regulating wetlands carbon cycling from annual to decadal scales. Here we show how cyclones with a wide range of integrated kinetic energy (IKE) impact C fluxes in the Everglades, a neotropical region with high cyclone landing frequency. Using long-term mangrove Net Primary Productivity (Litterfall, NPP_L) data (2001–2018), we estimated cyclone-induced litterfall particulate organic C (litter-POC) export from mangroves to estuarine waters. Our analysis revealed that this lateral litter-POC flux (71–205 g C m⁻² year⁻¹)—currently unaccounted in global C budgets—is similar to C burial rates (69–157 g C m⁻² year⁻¹) and dissolved inorganic carbon (DIC, 61–229 g C m⁻² year⁻¹) export. We proposed a statistical model (PULITER) between IKE-based pulse index and NPP_Lto determine cyclone’s impact on mangrove role as C sink or source. Including the cyclone’s functional role in regulating mangrove C fluxes is critical to developing local and regional climate change mitigation plans. 

Abstract Degradation of wetland ecosystems results from loss of hydrologic connectivity, nutrient enrichment, and altered fire regimes, among other factors. It is uncertain how drivers of wetland ecosystem processes and wetland vegetation communities interact in reversing the ecological trajectory from degraded to restored conditions. We analyzed biogeochemical and vegetation data collected in wetlands of the Florida Everglades at the start of (2015) and during (2018 and 2021) the initial stages of rehydration. Our objectives were to analyze the allocation of carbon and nutrients among ecosystem compartments and correlated trajectories of vegetation community change following rehydration, to identify the drivers of change, including fire, and analyze macrophyte species‐specific responses to drivers. We expected to see changes in vegetation toward more hydric communities that would differ based on wetland baseline conditions and the magnitude of the hydrologic change. During the study period, both length of inundation and surface water depth increased throughout wetlands in the region, and four fires occurred, which affected 51% of the sampling locations. We observed biogeochemical shifts in the wetland landscape, driven by both hydrology and fire. Total phosphorus concentrations in soil and flocculent detrital material decreased, while soil carbon:phosphorus and nitrogen:phosphorus mass ratios increased at sites further away from water management infrastructure. Transitions in vegetation communities were driven by an increase in hydroperiods and by the distinct changes in nutrient concentrations or soil stoichiometric ratios in each subregion. The abundance of macrophyte species typical of short‐hydroperiod prairies strongly decreased, while dominant long‐hydroperiod species, such asEleocharis cellulosa, expanded. Fire facilitated the expansion of thickly vegetated plumes of invasiveTyphaat sites close to the water inflow sources. Overall, restored hydrology shifted vegetation community composition toward higher abundance of long‐hydroperiod species within six years. In contrast, removal of invasive vegetation controlled by soil phosphorus concentrations will likely require long‐term and interactive restoration strategies.