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  1. Abstract Aim and Questions

    Sea‐level rise has been responsible for extensive vegetation changes in coastal areas worldwide. The intent of our study was to analyze vegetation dynamics of a South Florida coastal watershed within an explicit spatiotemporal framework that might aid in projecting the landscape's future response to restoration efforts. We also asked whether recent transgression by mangroves and other halophytes has resulted in reduced plant diversity at local or subregional scales.

    Location

    Florida’'s Southeast Saline Everglades, USA.

    Methods

    We selected 26 locations, representing a transition zone between sawgrass marsh and mangrove swamp, that was last sampled floristically in 1995. Within this transition zone, leading‐ and trailing‐edge subzones were defined based on plant composition in 1995. Fifty‐two site × time combinations were classified and then ordinated to examine vegetation–environment relationships using 2016 environmental data. We calculated alpha‐diversity using Hill numbers or Shannon–Weiner index species equivalents and compared these across the two surveys. We used a multiplicative diversity partition to determine beta‐diversity from landscape‐scale (gamma) diversity in the entire dataset or in each subzone.

    Results

    Mangrove and mangrove associates became more important in both subzones: through colonization and establishment in the leading edge, and through population growth combined with the decline of freshwater species in the trailing edge. Alpha‐diversity increased significantly in the leading edge and decreased nominally in the trailing edge, while beta‐diversity declined slightly in both subzones as well as across the study area.

    Conclusions

    Recent halophyte encroachment in the Southeast Saline Everglades continues a trend evident for almost a century. While salinity is an important environmental driver, species’ responses suggest that restoration efforts based on supplementing freshwater delivery will not reverse a trend that depends on multiple interacting factors. Sea‐level‐rise‐driven taxonomic homogenization in coastal wetland communities develops slowly, lagging niche‐based changes in community structure and composition.

     
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    Free, publicly-accessible full text available May 1, 2025
  2. Free, publicly-accessible full text available December 1, 2024
  3. null (Ed.)
    Northeast Shark River Slough (NESS), lying at the northeastern perimeter of Everglades National Park (ENP), Florida, USA, has been subjected to years of hydrologic modifications. Construction of the Tamiami Trail (US 41) in 1928 connected the east and west coasts of SE Florida and essentially created a hydrological barrier to southern sheet flow into ENP. Recently, a series of bridges were constructed to elevate a portion of Tamiami Trail, allow more water to flow under the bridges, and attempt to restore the ecological balance in the NESS and ENP. This project was conducted to determine aspects of soil physiochemistry and microbial dynamics in the NESS. We evaluated microbial respiration and enzyme assays as indicators of nutrient dynamics in NESS soils. Soil cores were collected from sites at certain distances from the inflow (near canal, NC (0–150 m); midway, M (150–600 m); and far from canal, FC (600–1200 m)). Soil slurries were incubated and assayed for CO2 emission and β-glucoside (MUFC) or phosphatase (MUFP) activity in concert with physicochemical analysis. Significantly higher TP contents at NC (2.45 times) and M (1.52 times) sites than FC sites indicated an uneven P distribution downstream from the source canal. The highest soil organic matter content (84%) contents were observed at M sites, which was due to higher vegetation biomass observed at those sites. Consequently, CO2 efflux was greater at M sites (average 2.72 µmoles g dw−1 h−1) than the other two sites. We also found that amendments of glucose increased CO2 efflux from all soils, whereas the addition of phosphorus did not. The results indicate that microbial respiration downstream of inflows in the NESS is not limited by P, but more so by the availability of labile C. 
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  4. null (Ed.)
    This study determines the relationships between water flow and water quality in three types of channels in southern Florida, USA: Shark River Slough, Peace River, and Hillsboro Canal. Peace River most resembles a natural channel with floodplain connectivity, sinuosity, and uninhibited flow. Shark River Slough has a natural, shallow channel with sheet flow, while the Hillsboro Canal is the most modified channel due to dredging, straightening, and regulated flow. Hydrologic indices for each channel were estimated to characterize flow regimes and flow variability, while concentration–discharge (C–Q) relationships were determined to quantify the impact of flow regime on water quality. The greatest variability in flow occurred at the Hillsboro Canal, followed by Peace River and Shark River Slough. Connectivity to floodplains and long durations of low and high flow pulses at Peace River and Shark River Slough contributed to the dilution of water quality constituent concentrations at higher flows. Conversely, the channelized characteristics of the Hillsboro Canal resulted in an enrichment of constituents, especially during high flows. This study suggests that C–Q relationships can be used in canal discharge management to prevent water quality degradation of sensitive downstream wetland and aquatic ecosystems. 
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  5. null (Ed.)
    Naturally formed forest patches known as tree islands are found within lower-statured wetland matrices throughout the world, where they contrast sharply with the surrounding vegetation. In some coastal wetlands they are embedded in former freshwater marshes that are currently exposed to saltwater intrusion and mangrove encroachment associated with accelerating sea-level rise. In this study we resurveyed tree composition and determined environmental conditions in tree islands of the coastal Florida Everglades that had been examined two decades earlier. We asked whether tree islands in this coastal transition zone were differentiated geomorphologically as well as compositionally, and whether favorable geomorphology enabled coastal forest type(s) to maintain their compositional integrity against rising seas. Patterns of variation in geomorphology and soils among forest types were evident, but were dwarfed by differences between forest and adjacent wetlands. Tree island surfaces were elevated by 12–44 cm, and 210Pb analyses indicated that their current rates of vertical accretion were more rapid than those of surrounding ecosystems. Tree island soils were deeper and more phosphorus-rich than in the adjoining matrix. Salinity decreased interiorward in both tree island and marsh, but porewater was fresher in forest than marsh in Mixed Swamp Forest, midway along the coastal gradient where tropical hardwoods were most abundant. Little decrease in the abundance of tropical hardwood species nor increase in halophytes was observed during the study period. Our data suggest that geomorphological differences between organic tree island and marl marsh, perhaps driven by groundwater upwelling through more transmissive tree island soils, contributed to the forests’ compositional stability, though this stasis may be short-lived despite management efforts. 
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  6. The size and distribution of Phytoplankton populations are indicators of the ecological status of a water body. The chlorophyll-a (Chl-a) concentration is estimated as a proxy for the distribution of phytoplankton biomass. Remote sensing is the only practical method for the synoptic assessment of Chl-a at large spatial and temporal scales. Long-term records of ocean color data from the MODIS Aqua Sensor have proven inadequate to assess Chl-a due to the lack of a robust ocean color algorithm. Chl-a estimation in shallow and coastal water bodies has been a challenge and existing operational algorithms are only suitable for deeper water bodies. In this study, the Ocean Color 3M (OC3M) derived Chl-a concentrations were compared with observed data to assess the performance of the OC3M algorithm. Subsequently, a regression analysis between in situ Chl-a and remote sensing reflectance was performed to obtain a green-red band algorithm for coastal (case 2) water. The OC3M algorithm yielded an accurate estimate of Chl-a for deep ocean (case 1) water (RMSE = 0.007, r2 = 0.518, p < 0.001), but failed to perform well in the coastal (case 2) water of Chesapeake Bay (RMSE = 23.217, r2 = 0.009, p = 0.356). The algorithm developed in this study predicted Chl-a more accurately in Chesapeake Bay (RMSE = 4.924, r2 = 0.444, p < 0.001) than the OC3M algorithm. The study indicates a maximum band ratio formulation using green and red bands could improve the satellite estimation of Chl-a in coastal waters. 
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  7. Wetland restoration requires managing long‐term changes in hydroperiod and ecosystem functions. We quantified relationships among spatiotemporal variability in wetland hydrology and total phosphorus (TP) and its stoichiometric relationships with total organic carbon (TOC:TP) and total carbon (TC:TP) and total nitrogen (TN:TP) in water, flocculent organic matter (floc), periphyton, sawgrass (Cladium jamaicense), and soil during early phases of freshwater wetland restoration—water year (WY) 2016 (1 May, 2015 to 30 April, 2016) to WY 2019—in Everglades National Park (ENP, Homestead, FL, U.S.A.). Wetland hydroperiod increased by 87 days, following restoration actions and rainfall events that increased median stage in the upstream source canal. Concentrations of TP were highest and most variable at sites closest (<1 km) to canal inputs and upstream wetland sources of legacy P. Surface water TOC:TP and TN:TP ratios were highest in wetlands >1 km downstream of the canal in wet season 2015 with spatial variability reflecting disturbances including droughts, fires, and freeze events. The TP concentrations of flocculent soil surface particles, periphyton, sawgrass, and consolidated soil declined, and TC:TP and TN:TP ratios increased (except soil) logarithmically with downstream distance from the canal. We measured abrupt increases in periphyton (wet season 2018) and sawgrass TP (wet season 2015 and 2018) at sites <1 km from the canal, likely reflecting legacy TP loading. Our results suggest restoration efforts that increase freshwater inflow and hydroperiod will likely change patterns of nutrient concentrations among water and organic matter compartments of wetlands as a function of nutrient legacies.

     
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