Eutrophication and chronic harmful algal blooms (HABs) are a challenge for ecosystem managers and restoration planners. Drivers of HABs include nutrient availability, temperature patterns, atmospheric conditions, rainfall-runoff relationships, and lake hydrodynamics. In South Florida, water is managed by water control plans that leverage restoration and water management infrastructure to control water levels in Lake Okeechobee and downstream systems. This study evaluated factors that contribute to algal blooms within Lake Okeechobee, assessed the long-term trends in algal biomass, and developed a modeling tool to evaluate lake algal bloom risk in the context of restoration and water management planning. For this study, Lake Okeechobee was divided into five distinct ecological zones based on physical (i.e., bathymetric), chemical (i.e., nutrient concentrations), and ecological (i.e., littoral, shallow, and open water zones) characteristics. Long-term changes in chlorophyll-a concentrations were interrelated with lake stage, volume, residence time, nitrogen, phosphorus, and temperature. Algal biomass, as indicated by concentrations of chlorophyll-a and phycocyanin, was significantly influenced by stage elevation, season, and location within the lake. Given the spatially unique characteristics of the lake and the potential drivers of algal blooms, two separate models were developed to evaluate scenarios. The first was an updated and expanded stage-based algal bloom indicator model used in prior restoration planning efforts. This model demonstrated the sensitivity of average summer chlorophyll-a concentration and bloom frequency across the lake, with littoral south, littoral west, and nearshore zones being the most responsive to changes in stage. The second model was a hierarchical model that used hydrodynamic and biogeochemical variables to predict chlorophyll-a concentrations across the lake. This model enhanced the understanding of summer chlorophyll-a concentrations across ecological zones. Moreover, these models both demonstrated how changes in water management regimes and restoration infrastructure can improve ecological conditions and significantly shift algal bloom potential for the lake. These models are valuable tools for understanding algal bloom potential and can be incorporated as a performance measure to evaluate future restoration planning efforts.

The Everglades Agricultural Area (EAA) in South Florida (USA) is a recognized source of total phosphorus (TP) that has impacted downstream oligotrophic Everglades marshes. Treatment wetlands, called stormwater treatment areas (STAs), were constructed and subsequently expanded to remediate EAA-derived TP, ideally yielding long-term outflow concentrations to the Everglades Protection Area (EvPA) of 13 µg/L TP or less. To date, TP-remediation has been insufficient relative to TP loads discharged from some EAA basins. We assessed 20 years of EAA basin-level and farm-level TP concentration and outflow data with the goal of understanding trends over time and identifying TP hot spots. Using monitoring data from water year (WY) 2000 through WY 2019, TP discharged from EAA farms averaged 74.68 ± 38.87 µg/L and was as high as 269.38 µg/L. We identified spatial and temporal variations in TP concentration, farm outflow, TP load, and TP flow-weighted mean concentration from the EAA farms. EAA basins TP concentration showed the presence of a decreasing trend between WY 2000 and WY 2012 and increasing trends for the more recent period WY 2010 to WY 2019. Using a nine-parameter Analytic Hierarchy Process (AHP), we observed that 31

Hydrologic restoration at the watershed scale is a complex process that can influence other processes and conditions within the watershed and downstream systems. Given these interrelated watershed scale processes, tools like models are essential to assess the potential effectiveness of sub-watershed to regional remediation strategies for ecological restoration. The goal of Everglades restoration is to restore the quality, quantity, and timing of water to one of the most ecologically unique and complex systems. This study aimed to evaluate changes to water quality associated with hydrologic restoration efforts within the Feeder Canal Basin as identified by the Western Everglades Restoration Project (WERP) by demonstrating, through simulations the potential effectiveness of implementing water quality remediation measures such as Best Management Practices (BMPs) and Stormwater Treatment Areas (STAs) within the WERP area. Additionally, to understand potential phosphorus sources and pre-develpoement conditions within the basin, a pre-development baseline was developed. Using the Watershed Assessment Model (WAM) various model alternatives including the WERP tentatively selected plan (TSP) were developed for a 22 year period of simulation. The implementation of both moderate and aggressive BMPs significantly reduced total phosphorus concentrations within the canal system at various levels indicating an improvement to near-field conditions. Meanwhile, not until the moderated BMPs and operation of an STA do water quality improvement become realized further downstream. The combined strategy of BMPs and STA demonstrates the integrated nature of nutrient remediation with both near and far-field benefits to the system with hydrologic restoration. This integrated strategy of water quality and hydrologic restoration can lead to a more sustainable and resilient restored ecosystem.

Emergent marsh and open water have been identified as alternate stable states in tidal marshes with large, relative differences in hydrogeomorphic conditions. In the Florida coastal Everglades, concern has been raised regarding the loss of non-tidal, coastal peat marsh via dieback of emergent vegetation and peat collapse. To aid in the identification of alternate stable states, our objective was to characterize the variability of hydrogeomorphic and biologic conditions using a field survey and long-term monitoring of hydrologic and geomorphic conditions across a range of vegetated (emergent, submerged) and unvegetated (open water) communities, which we refer to as “ecosystem states,” in a non-tidal, brackish peat marsh of the coastal Everglades. Results show (1) linear relationships among field-surveyed geomorphic, hydrologic, and biologic variables, with a 35-cm mean difference in soil surface elevation between emergent and open water states, (2) an overall decline in soil elevation in the submerged state that was related to cumulative dry days, and (3) a 2× increase in porewater salinity during the dry season in the emergent state that was also related to the number of dry days. Coupled with findings from previous experiments, we propose a conceptual model that describes how seasonal hydrologic variability may lead to ecosystem state transitions between emergent and open water alternate states. Since vegetative states are only moderately salt tolerant, as sea-level rise pushes the saltwater front inland, the importance of continued progress on Everglades restoration projects, with an aim to increase the volume of freshwater being delivered to coastal wetlands, is the primary management intervention available to mitigate salinization and slow ecosystem state shifts in non-tidal, brackish peat marshes.

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.

Karenia brevis blooms occur nearly annually along the southwest coast of Florida, and effective mitigation of ecological, public health, and economic impacts requires reliable real-time forecasting. We present two boosted random forest models that predict the weekly maximum K. brevis abundance category across the Greater Charlotte Harbor estuaries over one-week and four-week forecast horizons. The feature set was restricted to data available in near-real time, consistent with adoption of the models as decision-support tools. Features include current and lagged K. brevis abundance statistics, Loop Current position, sea surface temperature, sea level, and riverine discharges and nitrogen concentrations. During cross-validation, the one-week and four-week forecasts exhibited 73 % and 84 % accuracy, respectively, during the 2010-2023 study period. In addition, we assessed the models' reliability in forecasting the onset of 10 bloom events on time or in advance; the one-week and fourweek models anticipated the onset eight times and five times, respectively.

Florida Bay is a large, subtropical estuary whose salinity varies from yearly and seasonal changes in rainfall and freshwater inflows. Water management changes during the 20th century led to a long-term reduction in inflows that increased mean salinity, and the frequency and severity of hypersalinity. Climate change may exacerbate salinity conditions in Florida Bay; however, future salinity conditions have not been adequately evaluated. Here, we employed a Multilayer Feedforward Artificial Neural Network model to develop baseline salinity models for nearshore and offshore sites. Then, we examined the impacts of climate change on salinity using forecasted changes in various input variables under two climate change scenarios, representative concentration pathways (RCP) 4.5 and 8.5. Salinity could rise by 30% and 70% under the RCP4.5 and RCP8.5 forecasts, respectively. Climate change affected nearshore salinity significantly more, which rapidly fluctuated between mesohaline (5 to 18 PSU) and metahaline (40 to 55 PSU) to hypersaline conditions (>55 PSU). Offshore salinities ranged between euhaline (30 to 40 PSU) to metahaline (40 to 55 PSU) conditions. Our study suggests that increased freshwater flow would help maintain suitable estuarine conditions in Florida Bay during climate change, while our novel modeling approach can guide further Everglades restoration efforts.

Coastal wetlands are globally important stores of carbon (C). However, accelerated sea-level rise (SLR), increased saltwater intrusion, and modified freshwater discharge can contribute to the collapse of peat marshes, converting coastal peatlands into open water. Applying results from multiple experiments from sawgrass (Cladium jamaicense)-dominated freshwater and brackish water marshes in the Florida Coastal Everglades, we developed a system-level mechanistic peat elevation model (EvPEM). We applied the model to simulate net ecosystem C balance (NECB) and peat elevation in response to elevated salinity under inundation and drought exposure. Using a mass C balance approach, we estimated net gain in C and corresponding export of aquatic fluxes ( F AQ $$ {F}_{\mathrm{AQ}} $$ ) in the freshwater marsh under ambient conditions (NECB = 1119 ± 229 gC m-2  year-1 ; FAQ  = 317 ± 186 gC m-2  year-1 ). In contrast, the brackish water marsh exhibited substantial peat loss and aquatic C export with ambient (NECB = -366 ± 15 gC m-2  year-1 ; FAQ  = 311 ± 30 gC m-2  year-1 ) and elevated salinity (NECB = -594 ± 94 gC m-2  year-1 ; FAQ  = 729 ± 142 gC m-2  year-1 ) under extended exposed conditions. Further, mass balance suggests a considerable decline in soil C and corresponding elevation loss with elevated salinity and seasonal dry-down. Applying EvPEM, we developed critical marsh net primary productivity (NPP) thresholds as a function of salinity to simulate accumulating, steady-state, and collapsing peat elevations. The optimization showed that ~150-1070 gC m-2  year-1 NPP could support a stable peat elevation (elevation change ≈ SLR), with the corresponding salinity ranging from 1 to 20 ppt under increasing inundation levels. The C budgeting and modeling illustrate the impacts of saltwater intrusion, inundation, and seasonal dry-down and reduce uncertainties in understanding the fate of coastal peat wetlands with SLR and freshwater restoration. The modeling results provide management targets for hydrologic restoration based on the ecological conditions needed to reduce the vulnerability of the Everglades' peat marshes to collapse. The approach can be extended to other coastal peatlands to quantify C loss and improve understanding of the influence of the biological controls on wetland C storage changes for coastal management.

Coastal areas are increasingly vulnerable to the effects of sea level rise, and the coastal Everglades is no exception. The Comprehensive Everglades Restoration Plan (CERP) was launched in 2000 to restore the natural flow regime to the freshwater marshes of the Everglades. However, sea level rise has been affecting coastal habitats with increasing water levels and residence times of both salinity and marine nutrients. Here, we combined empirical data of water levels and sea levels with modeled CERP restoration scenarios. Water levels and fresh-to-marine head differences (FMHDs) were used as primary indicators of the vulnerability of the Everglades coastal areas to sea level rise and evaluate the relative benefits of restoration to these habitats. Four model scenarios were considered: a simulation of existing conditions baseline (ECB), full CERP implementation (CERP0), Central Everglades Planning Project with the Everglades Agricultural Area Reservoir (CEPPP), and a simulation of the natural/pre-drainage system scenario (NSM). Results demonstrated the differential vulnerability of coastal areas to sea level rise. The CEPPP restoration scenario achieved at least 60% and 30% of the full CERP freshwater and FMHD benefits in Shark River Slough, the largest freshwater flow-way in ENP, and Taylor Slough, the smaller flow-way in southeastern ENP, respectively. Hence, integration of FMHD metric in restoration efforts is essential to ensure restoration benefits extend beyond the freshwater marshes and improve coastal habitats.

Soil phosphorus (P) built up due to past management practices, legacy P, in the Lake Okeechobee Watershed (LOW) in south-central Florida, U.S.A., is often discussed as the root cause of lake eutrophication. Improvement of the lake’s water quality requires the identification of critical P sources and quantifying their contributions. We performed a global sensitivity analysis of the Watershed Assessment Model (WAM), a common evaluation tool in LOW environmental planning, using the Morris method. A pre-calibrated WAM setup (Baseline) of the LOW sub-watershed, Taylor Creek Nubbin Slough (TCNS), was used as a test case. Eight scenarios were formulated to estimate the contributions of various P sources. The Morris analysis indicated that total phosphorus (TP) loads were highly sensitive to legacy P in improved pastures, the major land use covering 46.2% of TCNS. The scenario modeling revealed that legacy P, inorganic fertilizers, and other sources contribute 63%, 10%, and 32%, respectively, to the Baseline TP load of 111.3 metric tons/y to the lake. Improved pastures, dairies, citrus, and field crops are the top TP load contributors. Our results have important implications for water quality improvement plans in the LOW and highlighted the need for accurate spatial mapping of legacy P and incorporation of such information in modeling efforts for watersheds demonstrating legacy P problems.

Percentile-Range Indexed Mapping and Evaluation (PRIME) is a new tool to visualize and quantifying spatio-temporal dynamics of long-term datasets. PRIME is based on categorical partitioning of magnitude based on user defined indices assigned to ranges of percentile and mapping subsets of data at selected percentiles of long-term data. Indices can reflect attributes such as water management decisions, tolerable range of water quality to a species, ecological risk, response to and recovery from disturbance, and values of ecosystem services. PRIME provides visual and robust datascapes and flexibility to evaluate variability in space and time for long-term environmental assessment. Here, we demonstrate the utility of PRIME using 16 years of hydrologic and salinity data from 14 sites representing three unique hydrological systems in the Florida Coastal Everglades (FCE). The resulting PRIME datascapes reveal interaction between water management and sea-level rise to drive salinity levels in the FCE.

The Everglades is a large, complex, and highly managed ecosystem, and its natural hydrologic properties, water quality, soils, flora, and fauna have been significantly altered by the Central and Southern Florida Project. The multi-billion-dollar Comprehensive Everglades Restoration Plan (CERP) was authorized in 2000 to restore and protect the Everglades ecosystem. However, the progress in implementing CERP projects has been slow because of their size, complexity, and long-term nature. In 2016, a review of CERP recommended updating it, and in 2018, a mid-course assessment was recommended to incorporate new scientific knowledge. This study began a reassessment of the CERP by evaluating the relative importance of a range of factors, including operations, water storage, removal of barriers to water flow, and seepage control. The purpose was to identify those elements most affecting hydrologic and ecological responses in different regions of the Everglades. The South Florida Water Management Model, a regional hydrologic model, was used as a primary tool to simulate the restoration scenarios over a 36-year period (1965-2000). All scenarios provided substantial improvements to the hydrologic system when compared to the existing conditions. However, restoration benefits of these scenarios were unequally distributed across the Everglades landscape. The scenario that expanded the surface storage in the Everglades Agricultural Area (EAA) to2.5 billion m3reduced flood control discharges to the northern estuaries by 66% relative to the existing conditions, which was 8% higher than CERP. However, the scenario with CERP projects significantly increased flows to Everglades National Park. The findings of this study are relevant to Everglades restoration especially in light of recent and emerging science on storage feasibility and climate change. (c) 2020 American Society of Civil Engineers.

Economic analyses incorporating ecological and hydrological benefits are useful in guiding and justifying large and complex environmental restoration programs. The Greater Everglades region in Florida, USA is a large area of freshwater and estuarine wetlands adjacent to dense human populations undergoing an extensive restoration effort. Six Everglades restoration scenarios were modeled involving combinations of decompartmentalization (removal of canals and levees), extent of seepage control, and volume of water storage (296-3084 mil-lion m(3))-all core components of the Comprehensive Everglades Restoration Plan. Under these scenarios, we simulated changes in ecological indicators across the Everglades, including Everglade snail kites and apple snails, wading birds, American alligators, and spotted seatrout. We also evaluated reductions of harmful discharges from Lake Okeechobee to the St. Lucie and Caloosahatchee estuaries in each scenario. Results were structured as options in a willingness-to-pay survey to aid in quantifying the benefits of restoration. Overall, we found Everglades restoration would increase ecosystem service benefits by $1.18 billion to $1.53 billion annually. A moderate amount of water storage improved ecosystem service benefits for Everglade snail kites and apple snails, American alligators, and wading birds, but additional storage led to equal or reduced benefits. For discharge reduction and spotted seatrout, ecosystem service benefits increased in proportion to the volume of water storage. Moderate water storage (1850 million m(3)) led to the greatest total ecosystem service benefits for the entire region, while the highest water storage volume led to the greatest benefits for specific Everglades regions like Florida Bay, St. Lucie, and Caloosahatchee estuaries. These results demonstrate the importance of economic valuation in estimating the potential ecosystem service benefits from large-scale environmental restoration and as a decision-support tool for prioritizing projects.

AbstractLong‐term ecological research can resolve effects of disturbance on ecosystem dynamics by capturing the scale of disturbance and interactions with environmental changes. To quantify how disturbances interact with long‐term directional changes (sea‐level rise, freshwater restoration), we studied 17 yr of monthly dissolved organic carbon (DOC), total nitrogen (TN), and phosphorus (TP) concentrations and bacterioplankton productivity across freshwater‐to‐marine estuary gradients exposed to multiple disturbance events (e.g., droughts, fire, hurricanes, and low‐temperature anomalies) and long‐term increases in water levels. By studying two neighboring drainages that differ in hydrologic connectivity, we additionally tested how disturbance legacies are shaped by hydrologic connectivity. We predicted that disturbance events would interact with long‐term increases in water levels in freshwater and marine ecosystems to increase spatiotemporal similarity (i.e., synchrony) of organic matter, nutrients, and microbial activities. Wetlands along the larger, deeper, and tidally influenced Shark River Slough (SRS) drainage had higher and more variable DOC, TN, and TP concentrations than wetlands along the smaller, shallower, tidally restricted Taylor River Slough/Panhandle (TS/Ph) drainage. Along SRS, DOC concentrations declined with proximity to coast, and increased in magnitude and variability following drought and flooding in 2015 and a hurricane in 2017. Along TS/Ph, DOC concentrations varied by site (higher in marine than freshwater wetlands) but not year. In both drainages, increases in TN from upstream freshwater marshes occurred following fire in 2008 and droughts in 2010 and 2015, whereas downstream increases in TP occurred with coastal storm surge from hurricanes in 2005 and 2017. Decreases in DOC:TN and DOC:TP were explained by increased TN and TP. Increases in bacterioplankton productivity occurred throughout both drainages following low‐temperature events (2010 and 2011) and a hurricane (2017). Long‐term TN and TP concentrations and bacterioplankton productivity were correlated (r > 0.5) across a range of sampling distances (1–50 km), indicating spatiotemporal synchrony. DOC concentrations were not synchronized across space or time. Our study advances disturbance ecology theory by illustrating how disturbance events interact with long‐term environmental changes and hydrologic connectivity to determine the magnitude and extent of disturbance legacies. Understanding disturbance legacies will enhance prediction and enable more effective management of rapidly changing ecosystems.

Coastal freshwater and brackish wetlands are exposed to pulses of saltwater during times of reduced freshwater flows (i.e., dry seasons, droughts), periodic storm surges, and increased tidal extent associated with rising seas. The effects of saltwater pulses on belowground processing rates of detrital organic matter as mediated by microbial activities are uncertain. Our objectives were to quantify whether and how pulses of saltwater (i) change soil porewater physicochemistry, (ii) change soil microbial extracellular enzyme activities, and (iii) change root litter breakdown over time in freshwater and brackish marshes. From 2014 to 2016, we simulated saltwater intrusion as monthly in situ pulsed additions of artificial seawater in experimental dosing chambers (1.4 m diameter) within freshwater and brackish marshes of Everglades National Park. At monthly intervals, we collected soil porewater chemistry, and measured microbial extracellular enzymes, elemental stoichiometric ratios, and breakdown rates (k) of incubated (0–30 cm depth) root litter and compared these responses over time. Saltwater pulses increased sulfate and nitrogen concentrations in porewater at the freshwater site. However, saltwater pulses generally decreased porewater constituents (e.g., dissolved organic carbon, dissolved nitrogen and phosphorus species) at the brackish site. One saltwater pulse increased root litter k by 1.25-fold in the brackish marsh. However, long-term (740 days) k in brackish wetlands, and both short- and long-term k in freshwater wetlands, were not affected by 24 monthly pulses of saltwater. Enzyme activities fluctuated with time and did not respond to multiple saltwater pulses. Our results suggest that detrital organic matter stocks and associated soil microbial activities are relatively resistant to single and multiple (n = 24) pulses of saltwater.

Context The greater Everglades region in Florida (USA) is an area of wetlands that has been altered and reduced to 50% of its original area and faces multiple threats. Spatial landscape analysis can help guide a large and complex ecosystem restoration process, involving billions of dollars and multiple groups of stakeholders. Objectives To guide Everglades restoration efforts, we evaluated ecological performance of different hydrologic restoration scenarios using a novel technique, the structural similarity index (SSIM), which quantitatively compares similarity between pairs of gridded maps in terms of mean, variance, and covariance. Methods Using the SSIM, we evaluated system-wide performance of apple snails, American alligators, Great egrets, and long- and short-hydroperiod vegetation types under multiple restoration scenarios that varied in water management strategies, amounts of water storage, removal of levees and canals (decompartmentalization), and seepage control barriers. We then compared species and habitat responses under each restoration scenario to a target scenario simulating the historical, natural system. Results The SSIM approach provides a reliable means of scenario comparison, accounting for both the local magnitude and spatial structure of the underlying data. Our results demonstrated that decompartmentalization benefits the indicator species. In general, scenarios with increased water storage were closer to the target scenario. Conclusions This spatial comparison technique is useful for evaluating restoration efforts at multiple spatial scales, ranging from the entire ecosystem down to individual compartments or sub-compartments. The results can be used to inform management and restoration efforts and to guide policy for the greater Everglades area.

Coastal wetlands are exposed to high-energy storms that influence plant and soil structure. To understand how nutrient availability interacts with storm-induced plant stress, we tested how defoliation interacts with nutrient enrichment to affect carbon (C) and nutrient (nitrogen, N; phosphorus, P) cycling and storage within soils and plants. In outdoor experimental mesocosms, we defoliated red mangrove saplings (Rhizophora mangle), added 30g of inorganic P to peat soils, and quantified plant [elemental stoichiometry (C:N, C:P, N:P), leaf count, and above- and below- ground biomass] and soil responses [C:N, C:P, N:P, litter breakdown rate (k), soil CO2 efflux] during a 42-d recovery period. Mangroves rapidly regrew all removed leaves and recovered nearly 30% of leaf biomass. Mangrove biomass %P increased by 50% with added P; however, soil stoichiometry remained unchanged. Defoliation reduced Soil CO2 efflux by 40% and root litter k by 30%. Phosphorus was quickly incorporated into mangrove biomass and stimulated nighttime soil CO2 efflux. This work highlights the importance of testing interactions of nutrient availability and plant stress on plant and soil biogeochemical cycling and suggests that plants quickly incorporate available nutrients into biomass and defoliation can lead to reduced soil C losses.

Increasing rates of sea-level rise (SLR) threaten to submerge coastal wetlands unless they increase soil elevation at similar pace, often by storing soil organic carbon (OC). Coastal wetlands face increasing salinity, marine-derived nutrients, and inundation depths from increasing rates of SLR. To quantify the effects of SLR on soil OC stocks and fluxes and elevation change, we conducted two mesocosm experiments using the foundation species sawgrass (Cladium jamaicense) and organic soils from freshwater and brackish Florida Everglades marshes for 1 year. In freshwater mesocosms, we compared ambient and elevated salinity (fresh, 9 ppt) and phosphorus (ambient, + 1 g P m−2 year−1) treatments with a 2 × 2 factorial design. Salinity addition reduced root biomass (48%), driving 2.8 ± 0.3 cm year−1 of elevation loss, while soil elevation was maintained in freshwater conditions. Added P increased root productivity (134%) but also increased breakdown rates (k) of roots (31%) and leaves (42%) with no effect on root biomass or soil elevation. In brackish mesocosms, we compared ambient and elevated salinity (10, 19 ppt) and inundated and exposed conditions (water level 5-cm below and 4-cm above soil). Elevated salinity decreased root productivity (70%) and root biomass (37%) and increased k in litter (33%) and surface roots (11%), whereas inundation decreased subsurface root k (10%). All brackish marshes lost elevation at similar rates (0.6 ± 0.2 cm year−1). In conclusion, saltwater intrusion in freshwater and brackish wetlands may reduce net OC storage and increase vulnerability to SLR despite inundation or marine P supplies.

Periphyton plays key ecological roles in karstic, freshwater wetlands and is extremely sensitive to environmental change making it a powerful tool to detect saltwater intrusion into these vulnerable and valuable ecosystems. We conducted field mesocosm experiments in the Florida Everglades, USA to test the effects of saltwater intrusion on periphyton metabolism, nutrient content, and diatom species composition, and how these responses differ between mats from a freshwater versus a brackish marsh. Pulsed saltwater intrusion was simulated by dosing treatment chambers monthly with a brine solution for 15 months; control chambers were simultaneously dosed with site water. Periphyton from the freshwater marsh responded to a 1-ppt increase in surface water salinity with reduced productivity and decreased concentrations of total carbon, nitrogen, and phosphorus. These functional responses were accompanied by significant shifts in periphytic diatom assemblages. Periphyton mats at the brackish marsh were more functionally resilient to the saltwater treatment (~ 2 ppt above ambient), but nonetheless experienced significant shifts in diatom composition. These findings suggest that freshwater periphyton is negatively affected by small, short-term increases in salinity and that periphytic diatom assemblages, particularly at the brackish marsh, are a better metric of salinity increases compared with periphyton functional metrics due to functional redundancy. This research provides new and valuable information regarding periphyton dynamics in response to changing water sources in the southern Everglades that will allow us to extend the use of periphyton, and their diatom assemblages, as tools for environmental assessments related to saltwater intrusion.