Hydrologic processes on tree islands in the Everglades (Florida, USA): tracking the effects of tree establishment and growth

The hydrodynamics of tree islands during the growth of newly planted trees has been found to be influenced by both vegetation biomass and geologic conditions. From July 2007 through June 2009, groundwater and surface-water levels were monitored on eight recently planted tree islands at the Loxahatchee Impoundment Landscape Assessment (LILA) facility in Boynton Beach, Florida, USA. Over the 2-year study, stand development coincided with the development of a water-table depression in the center of each of the islands that was bounded by a hydraulic divide along the edges. The water-table depression was greater in islands composed of limestone as compared to those composed of peat. The findings of this study suggest that groundwater evapotranspiration by trees on tree islands creates complex hydrologic interactions between the shallow groundwater in tree islands and the surrounding surface water and groundwater bodies.

The role of recharge and evapotranspiration as hydraulic drivers of ion concentrations in shallow groundwater on Everglades tree islands, Florida (USA)

Recently, evapotranspiration has been hypothesized to promote the secondary formation of calcium carbonate year-round on tree islands in the Everglades by influencing groundwater ions concentrations. However, the role of recharge and evapotranspiration as drivers of shallow groundwater ion accumulation has not been investigated. The goal of this study is to develop a hydrologic model that predicts the chloride concentrations of shallow tree island groundwater and to determine the influence of overlying biomass and underlying geologic material on these concentrations. Groundwater and surface water levels and chloride concentrations were monitored on eight constructed tree islands at the Loxahatchee Impoundment Landscape Assessment (LILA) from 2007 to 2010. The tree islands at LILA were constructed predominately of peat, or of peat and limestone, and were planted with saplings of native tree species in 2006 and 2007. The model predicted low shallow groundwater chloride concentrations when inputs of regional groundwater and evapotranspiration-to-recharge rates were elevated, while low evapotranspiration-to-recharge rates resulted in a substantial increase of the chloride concentrations of the shallow groundwater. Modeling results indicated that evapotranspiration typically exceeded recharge on the older tree islands and those with a limestone lithology, which resulted in greater inputs of regional groundwater. A sensitivity analysis indicated the shallow groundwater chloride concentrations were most sensitive to alterations in specific yield during the wet season and hydraulic conductivity in the dry season. In conclusion, the inputs of rainfall, underlying hydrologic properties of tree islands sediments and forest structure may explain the variation in ion concentration seen across Everglades tree islands.

Aquatic fauna as indicators for Everglades restoration: Applying dynamic targets in assessments

A major goal of the Comprehensive Everglades Restoration Plan (CERP) is to recover historical (pre-drainage) wading bird rookeries and reverse marked decreases in wading bird nesting success in Everglades National Park. To assess efforts to restore wading birds, a trophic hypothesis was developed that proposes seasonal concentrations of small-fish and crustaceans (i.e., wading bird prey) were a key factor to historical wading bird success. Drainage of the Everglades has diminished these seasonal concentrations, leading to a decline in wading bird nesting and displacing them from their historical nesting locations. The trophic hypothesis predicts that restoring historical hydrological patterns to pre-drainage conditions will recover the timing and location of seasonally concentrated prey, ultimately restoring wading bird nesting and foraging to the southern Everglades. We identified a set of indicators using small-fish and crustaceans that can be predicted from hydrological targets and used to assess management success in regaining suitable wading bird foraging habitat. Small-fish and crustaceans are key components of the Everglades food web and are sensitive to hydrological management, track hydrological history with little time lag, and can be studied at the landscape scale. The seasonal hydrological variation of the Everglades that creates prey concentrations presents a challenge to interpreting monitoring data. To account for the variable hydrology of the Everglades in our assessment, we developed dynamic hydrological targets that respond to changes in prevailing regional rainfall. We also derived statistical relationships between density and hydrological drivers for species representing four different life-history responses to drought. Finally, we use these statistical relationships and hydrological targets to set restoration targets for prey density. We also describe a report-card methodology to communicate the results of model-based assessments for communication to a broad audience.

Influence of teleost abundance on the distribution and abundance of sharks in Florida Bay, USA

Understanding the factors that influence the distribution and abundance of predators, including sharks, is important for predicting the impacts of human changes to the environment. Such studies are particularly important in Florida Bay, USA where there are planned large-scale changes to patterns of freshwater input from the Everglades ecosystem. Studies of many marine predators suggest that links between predator and prey habitat use may vary with spatial scale, but there have been few studies of the role of prey distribution in shaping habitat use and abundance of sharks. We used longline catches of sharks and trawls for potential teleost prey to determine the influence of teleost abundance on shark abundance at the scale of regions and habitats in Florida Bay. We found that shark catch per unit effort (CPUE) was not linked to CPUE ofteleosts at the scale of sampling sites, but shark CPUE was positively correlated with the mean CPUE for teleosts within a region. Although there does not appear to be a strong match between the abundance of teleosts and sharks at small spatial scales, regional shark abundance is likely driven, at least partially, by the availability of prey. Management strategies that influence teleost abundance will have cascading effects to higher trophic levels in Florida Bay.

Possible climate change impacts on the hydrological and vegetative character of Everglades National Park, Florida

The increasing threat of global climate change is predicted to have immense influences on ecosystems worldwide, but could be particularly severe to vulnerable wetland environments such as the Everglades. This work investigates the impact global climate change could have on the hydrologic and vegetative makeup of Everglades National Park (ENP) under forecasted emissions scenarios. Using a simple stochastic model of aboveground water levels driven by a fluctuating rainfall input, we link across ENP a location's mean depth and percent time of inundation to the predicted changes in precipitation from climate change. Changes in the hydrologic makeup of ENP are then related to changes in vegetation community composition through the use of relationships developed between two publically available datasets. Results show that under increasing emissions scenarios mean annual precipitation was forecasted to decrease across ENP leading to a marked hydrologic change across the region. Namely, areas were predicted to be shallower in average depth of standing water and inundated less of the time. These hydrologic changes in turn lead to a shift in ENP's vegetative makeup, with xeric vegetative communities becoming more numerous and hydric vegetative communities becoming scarcer. Noticeably, the most widespread of vegetative communities, sawgrass, decreases in abundance under increasing emissions scenarios. These results are an important indicator of the effects climate change may have on the Everglades region and raise important management implications for those seeking to restore this area to its historical hydrologic and vegetative condition.

Hydrological drivers of wetland vegetation community distribution within Everglades National Park, Florida

The influence of hydrological dynamics on vegetation distribution and the structuring of wetland environments is of growing interest as wetlands are modified by human action and the increasing threat from climate change. Hydrological properties have long been considered a driving force in structuring wetland communities. We link hydrological dynamics with vegetation distribution across Everglades National Park (ENP) using two publicly available datasets to study the probability structure of the frequency, duration, and depth of inundation events along with their relationship to vegetation distribution. This study is among the first to show hydrologic structuring of vegetation communities at wide spatial and temporal scales, as results indicate that the percentage of time a location is inundated and its mean depth are the principal structuring variables to which individual communities respond. For example, sawgrass, the most abundant vegetation type within the ENP, is found across a wide range of time inundated percentages and mean depths. Meanwhile, other communities like pine savanna or red mangrove scrub are more restricted in their distribution and found disproportionately at particular depths and inundations. These results, along with the probabilistic structure of hydropatterns, potentially allow for the evaluation of climate change impacts on wetland vegetation community structure and distribution.

Relationships Between Hydrology and Soils Describe Vegetation Patterns in Seasonally Flooded Tree Islands of the Southern Everglades, Florida

Tree island ecosystems are important and distinct features of Florida Everglades wetlands. We described the inter-relationships among abiotic factors describing seasonally flooded tree islands and characterized plant–soil relationships in tree islands occurring in a relatively unimpacted area of the Everglades. We used Principal Components Analysis (PCA) to reduce our multi-factor dataset, quantified forest structure and vegetation nutrient dynamics, and related these vegetation parameters to PCA summary variables using linear regression analyses. We found that, of the 21 abiotic parameters used to characterize the ecosystem structure of seasonally flooded tree islands, 13 parameters were significantly correlated with four principal components, and they described 78% of the variance among the study islands. Most variation was described by factors related to soil oxidation and hydrology, exemplifying the sensitivity of tree island structure to hydrologic conditions. PCA summary variables describing tree island structure were related to variability in Chrysobalanus icaco (L.) canopy cover, Ilex cassine (L.) and Salix caroliniana (Michx.) canopy cover, Myrica cerifera (L.) plot frequency, litter turnover, % phosphorus resorption of co-dominant species, and nitrogen nutrient-use efficiency. This study supported findings that vegetation characteristics can be sensitive indicators of variability in tree island ecosystem structure. This study produced valuable, information which was used to recommend ecological targets (i.e. restoration performance measures) for seasonally flooded tree islands in more impacted regions of the Everglades landscape.

Biogeochemical Contributions of Tree Islands to Everglades Wetland Landscape Nitrogen Cycling During Seasonal Inundation

In the Florida Everglades, tree islands are conspicuous heterogeneous elements in the herbaceous wetland landscape. We characterized the biogeochemical role of a seasonally flooded tree island during wet season inundation, specifically examining hydrologically mediated flows of nitrogen (N) and N retention by the tree island. We estimated ecosystem N standing stocks and fluxes, soil and litter N transformation rates, and hydrologic fluxes of N to quantify the net ecosystem N mass flux. Results showed that hydrologic sources of N were dominated by surface water loads of nitrate (NO3) and ammonium (NH4). Nitrate immobilization by soils and surficial leaf litter was an important sink for surface water dissolved inorganic N (DIN). We estimated that the net annual DIN retention by a seasonally flooded tree island was 20.5 ± 5.0 g m−2 during wet season inundation. Based on the estimated tree island surface water DIN loading rate, a seasonally flooded tree island retained 76% of imported DIN. As such, seasonally flooded tree islands have the potential to retain 55% of DIN entering the marsh landscape via upstream canal overland flow in the wet season. By increasing reactive surface area and DOC availability, we suggest that tree islands promote convergence of elements that enhance DIN retention. Tree islands of this region are thus important components of landscape-scale restoration efforts that seek to reduce sources of anthropogenic DIN to downstream estuaries.

Biogeochemical Processes on Tree Islands in the Greater Everglades: Initiating a New Paradigm

Scientists’ understanding of the role of tree islands in the Everglades has evolved from a plant community of minor biogeochemical importance to a plant community recognized as the driving force for localized phosphorus accumulation within the landscape. Results from this review suggest that tree transpiration, nutrient infiltration from the soil surface, and groundwater flow create a soil zone of confluence where nutrients and salts accumulate under the head of a tree island during dry periods. Results also suggest accumulated salts and nutrients are flushed downstream by regional water flows during wet periods. That trees modulate their environment to create biogeochemical hot spots and strong nutrient gradients is a significant ecological paradigm shift in the understanding of the biogeochemical processes in the Everglades. In terms of island sustainability, this new paradigm suggests the need for distinct dry-wet cycles as well as a hydrologic regime that supports tree survival. Restoration of historic tree islands needs further investigation but the creation of functional tree islands is promising.

Maintaining tree islands in the Florida Everglades: nutrient redistribution is the key

The Florida Everglades is an oligotrophic wetland system with tree islands as one of its most prominent landscape features. Total soil phosphorus concentrations on tree islands can be 6 to 100 times greater than phosphorus levels in the surrounding marshes and sloughs, making tree islands nutrient hotspots. Several mechanisms are believed to redistribute phosphorus to tree islands: subsurface water flows generated by evapotranspiration of trees, higher deposition rates of dry fallout, deposition of guano by birds and other animals, groundwater upwelling, and bedrock mineralization by tree exudates. A conceptual model is proposed, in which the focused redistribution of limiting nutrients, especially phosphorus, onto tree islands controls their maintenance and expansion. Because of increased primary production and peat accretion rates, the redistribution of phosphorus can result in an increase in both tree island elevation and size. Human changes to hydrology have greatly decreased the number and size of tree islands in parts of the Everglades. The proposed model suggests that the preservation of existing tree islands, and ultimately of the Everglades landscape, requires the maintenance of these phosphorus redistribution mechanisms.

Historic primary producer communities linked to water quality and hydrologic changes in the northern Everglades

The northern Everglades Water Conservation Areas have experienced recent ecological shifts in primary producer community structure involving marl periphyton mats and dense Typha-dominated macrophyte stands. Multiple investigations have identified phosphorus (P) as a driver of primary producer community structure, but effects of water impoundment beginning in the 1950s and changes in water hardness [e.g., (CaCO3)] have also been identified as a concern. In an effort to understand pre-1950, primary producer community structure and identify community shifts since 1950, we measured pigment proxies on three sediment cores collected in Water Conservation Area-2A (WCA-2A) along a phosphorus enrichment gradient. Photosynthetic pigments, sediment total phosphorus content (TP), organic matter, total organic carbon and nitrogen were used to infer historic primary producer communities and changes in water quality and hydrology regulating those communities. Excess 210Pb was used to establish historic dates for the sediment cores. Results indicate the northern area of WCA-2A increased marl deposition and increased algal abundance ca. 1920. This increase in (presumably) calcareous periphyton before intensive agriculture and impoundment suggest canal-derived calcium inputs and to some extent early drainage effects played a role in initiating this community shift. The northern area community then shifted to Typha dominance around 1965. The areas to the south in WCA-2A experienced increased marl deposition and algal abundance around or just prior to 1950s impoundment, the precise timing limited by core age resolution. Continued increases in algal abundance were evident after 1950, coinciding with impoundment and deepening of canals draining into WCA-2A, both likely increasing water mineral and nutrient concentrations. The intermediate site developed a Typha-dominated community ca. 1995 while the southern-most core site WCA-2A has yet to develop Typha dominance. Numerous studies link sediment TP >650 mg P/kg to marsh habitat degradation into Typha-dominance. The northern and intermediate cores where Typha is currently support this previous research by showing a distinct shift in the sediment record to Typha dominance corresponding to sediment TP between 600 and 700 mg P/kg. These temporal and spatial differences are consistent with modern evidence showing water-column gradients in mineral inputs (including Ca, carbonates, and phosphorus) altering primary producer community structure in WCA-2A, but also suggest hydroperiod has an effect on the mechanisms regulating periphyton development and Typha dominance.

Response of diatom assemblages to 130 years of environmental change in Florida Bay (USA)

Coastal ecosystems around the world are constantly changing in response to interacting shifts in climate and land and water use by expanding human populations. The development of agricultural and urban areas in South Florida significantly modified its hydrologic regime and influenced rates of environmental change in wetlands and adjacent estuaries. This study describes changes in diatom species composition through time from four sediment cores collected across Florida Bay, for the purposes of detecting periods of major shifts in assemblage structure and identifying major drivers of those changes. We examined the magnitude of diatom assemblage change in consecutive 2-cm samples of the 210Pb-dated cores, producing a record of the past ~130 years. Average assemblage dissimilarity among successive core samples was ~30%, while larger inter-sample and persistent differences suggest perturbations or directional shifts. The earliest significant compositional changes occurred in the late 1800s at Russell Bank, Bob Allen Bank and Ninemile Bank in the central and southwestern Bay, and in the early 1900s at Trout Cove in the northeast. These changes coincided with the initial westward redirection of water from Lake Okeechobee between 1881 and 1894, construction of several canals between 1910 and 1915, and building the Florida Overseas Railroad between 1906 and 1916. Later significant assemblage restructurings occurred in the northeastern and central Bay in the late 1950s, early 1960s and early 1970s, and in the southwestern Bay in the 1980s. These changes coincide with climate cycles driving increased hurricane frequency in the 1960s, followed by a prolonged dry period in the 1970s to late 1980s that exacerbated the effects of drainage operations in the Everglades interior. Changes in the diatom assemblage structure at Trout Cove and Ninemile Bank in the 1980s correspond to documented eutrophication and a large seagrass die-off. A gradual decrease in the abundance of freshwater to brackish water taxa in the cores over ~130 years implies that freshwater deliveries to Florida Bay were much greater prior to major developments on the mainland. Salinity, which was quantitatively reconstructed at these sites, had the greatest effect on diatom communities in Florida Bay, but other factors—often short-lived, natural and anthropogenic in nature—also played important roles in that process. Studying the changes in subfossil diatom communities over time revealed important environmental information that would have been undetected if reconstructing only one water quality variable.

Distribution of Diatoms and Development of Diatom-Based Models for Inferring Salinity and Nutrient Concentrations in Florida Bay and Adjacent Coastal Wetlands of South Florida (USA)

The composition and distribution of diatom algae inhabiting estuaries and coasts of the subtropical Americas are poorly documented, especially relative to the central role diatoms play in coastal food webs and to their potential utility as sentinels of environmental change in these threatened ecosystems. Here, we document the distribution of diatoms among the diverse habitat types and long environmental gradients represented by the shallow topographic relief of the South Florida, USA, coastline. A total of 592 species were encountered from 38 freshwater, mangrove, and marine locations in the Everglades wetland and Florida Bay during two seasonal collections, with the highest diversity occurring at sites of high salinity and low water column organic carbon concentration (WTOC). Freshwater, mangrove, and estuarine assemblages were compositionally distinct, but seasonal differences were only detected in mangrove and estuarine sites where solute concentration differed greatly between wet and dry seasons. Epiphytic, planktonic, and sediment assemblages were compositionally similar, implying a high degree of mixing along the shallow, tidal, and storm-prone coast. The relationships between diatom taxa and salinity, water total phosphorus (WTP), water total nitrogen (WTN), and WTOC concentrations were determined and incorporated into weighted averaging partial least squares regression models. Salinity was the most influential variable, resulting in a highly predictive model (r apparent 2  = 0.97, r jackknife 2  = 0.95) that can be used in the future to infer changes in coastal freshwater delivery or sea-level rise in South Florida and compositionally similar environments. Models predicting WTN (r apparent 2  = 0.75, r jackknife 2  = 0.46), WTP (r apparent 2  = 0.75, r jackknife 2  = 0.49), and WTOC (r apparent 2  = 0.79, r jackknife 2  = 0.57) were also strong, suggesting that diatoms can provide reliable inferences of changes in solute delivery to the coastal ecosystem.

Eastern oysters (Crassostrea virginica) as an indicator for restoration of Everglades Ecosystems

The Comprehensive Everglades Restoration Plan (CERP) attempts to restore hydrology in the Northern and Southern Estuaries of Florida. Reefs of the Eastern oyster Crassostrea virginica are a dominant feature of the estuaries along the Southwest Florida coast. Oysters are benthic, sessile, filter-feeding organisms that provide ecosystem services by filtering the water column and providing food, shelter and habitat for associated organisms. As such, the species is an excellent sentinel organism for examining the impacts of restoration on estuarine ecosystems. The implementation of CERP attempts to improve: the hydrology and spatial and structural characteristics of oyster reefs, the recruitment and survivorship of C. virginica, and the reef-associated communities of organisms. This project links biological responses and environmental conditions relative to hydrological changes as a means of assessing positive or negative trends in oyster responses and population trends. Using oyster responses, we have developed a communication tool (i.e., Stoplight Report Card) based on CERP performance measures that can distinguish between responses to restoration and natural patterns. The Stoplight Report Card system is a communication tool that uses Monitoring and Assessment Program (MAP) performance measures to grade an estuary's response to changes brought about by anthropogenic input or restoration activities. The Stoplight Report Card consists of both a suitability index score for each organism metric as well as a trend score (− decreasing trend, +/− no change in trend, and + increasing trend). Based on these two measures, a component score (e.g., living density) is calculated by averaging the suitability index score and the trend score. The final index score is obtained by taking the geometric score of each component, which is then translated into a stoplight color for success (green), caution (yellow), or failure (red). Based on the data available for oyster populations and the responses of oysters in the Caloosahatchee Estuary, the system is currently at stage “caution.” This communication tool instantly conveys the status of the indicator and the suitability, while trend curves provide information on progress towards reaching a target. Furthermore, the tool has the advantage of being able to be applied regionally, by species, and collectively, in concert with other species, system-wide.

Indirect and direct controls of macroinvertebrates and small fish by abiotic factors and trophic interactions in the Florida Everglades

1. The roles of nutrients, disturbance and predation in regulating consumer densities have long been of interest, but their indirect effects have rarely been quantified in wetland ecosystems. The Florida Everglades contains gradients of hydrological disturbance (marsh drying) and nutrient enrichment (phosphorus), often correlated with densities of macroinvertebrate infauna (macroinvertebrates inhabiting periphyton), small fish and larger invertebrates, such as snails, grass shrimp, insects and crayfish. However, most causal relationships have yet to be quantified. 2.  We sampled periphyton (content and community structure) and consumer (small omnivores, carnivores and herbivores, and infaunal macroinvertebrates inhabiting periphyton) density at 28 sites spanning a range of hydrological and nutrient conditions and compared our data to seven a priori structural equation models. 3.  The best model included bottom-up and top-down effects among trophic groups and supported top-down control of infauna by omnivores and predators that cascaded to periphyton biomass. The next best model included bottom-up paths only and allowed direct effects of periphyton on omnivore density. Both models suggested a positive relationship between small herbivores and small omnivores, indicating that predation was unable to limit herbivore numbers. Total effects of time following flooding were negative for all three consumer groups even when both preferred models suggested positive direct effects for some groups. Total effects of nutrient levels (phosphorus) were positive for consumers and generally larger than those of hydrological disturbance and were mediated by changes in periphyton content. 4.  Our findings provide quantitative support for indirect effects of nutrient enrichment on consumers, and the importance of both algal community structure and periphyton biomass to Everglades food webs. Evidence for top-down control of infauna by omnivores was noted, though without substantially greater support than a competing bottom-up-only model.