The interactive effects of predators, resources, and disturbance on freshwater snail populations from the Everglades

The origins of population dynamics depend on interplay between abiotic and biotic factors; the relative importance of each changing across space and time. Predation is a central feature of ecological communities that removes individuals (consumption) and alters prey traits (non-consumptive). Resource quality mitigates non-consumptive predator effects by stimulating growth and reproduction. Disturbance resets predator-prey interactions by removing both. I integrate experiments, time-series analysis, and performance trials to examine the relative importance of these on the population dynamics of a snail species by studying a variety of their traits. A review of ninety-three published articles revealed that snail abundance was much less in the Everglades and similar ecosystems compared to all other freshwater ecosystems considered. Separating consumptive from non-consumptive (cues) predator effects at different phosphorous levels with an experiment determined that phosphorous stimulated, but predator cues inhibited snail growth (34% vs. 23%), activity (38% vs. 53%), and reproductive effort (99% vs. 90%) compared to controls. Cues induced taller shells and smaller openings and moved to refugia where they reduced periphyton by 8%. Consumptive predator effects were minor in comparison. In a reciprocal transplant cage experiment along a predator cue and phosphorous gradient created by a canal, snails grew 10% faster and produced 37% more eggs far from the canal (fewer cues) when fed phosphorous-enriched periphyton from near the canal. Time-series analysis at four sites and predator performance trials reveal that phosphorous-enriched regions support larger snail populations, seasonal drying removes snails at all sites, crayfish negatively affect populations in enriched regions, and molluscivorous fish consume snails in the wet season. Combining these studies reveals interplay between resources, predators, and seasonality that limit snail populations in the Everglades and lead to their low abundance compared to other freshwater ecosystems. Resource quality is emerging as the critical factor because improving resources profoundly improved growth and reproduction; seasonal drying and predation become important at times and places. This work contributes to the general understanding in ecology of the relative importance of different factors that structure populations and provides evidence that bolsters monitoring efforts to assess the Comprehensive Everglades Restoration Plan that show phosphorous enrichment is a major driver of ecosystem change.

Water resources assessment and geographic information system (GIS)-based stormwater runoff estimates for artificial recharge of freshwater aquifers in New Providence, Bahamas

The Bahamas is a small island nation that is dealing with the problem of freshwater shortage. All of the country’s freshwater is contained in shallow lens aquifers that are recharged solely by rainfall. The country has been struggling to meet the water demands by employing a combination of over-pumping of aquifers, transport of water by barge between islands, and desalination of sea water. In recent decades, new development on New Providence, where the capital city of Nassau is located, has created a large area of impervious surfaces and thereby a substantial amount of runoff with the result that several of the aquifers are not being recharged. A geodatabase was assembled to assess and estimate the quantity of runoff from these impervious surfaces and potential recharge locations were identified using a combination of Geographic Information Systems (GIS) and remote sensing. This study showed that runoff from impervious surfaces in New Providence represents a large freshwater resource that could potentially be used to recharge the lens aquifers on New Providence.

Rapid responses of vegetation to hydrological changes in Taylor Slough, Everglades National Park, Florida, USA

We analyzed the dynamics of freshwater marsh vegetation of Taylor Slough in eastern Everglades National Park for the 1979 to 2003 period, focusing on cover of individual plant species and on cover and composition of marsh communities in areas potentially influenced by a canal pump station (‘‘S332’’) and its successor station (‘‘S332D’’). Vegetation change analysis incorporated the hydrologic record at these sites for three intervals: pre-S332 (1961–1980), S332 (1980–1999), post-S332 (1999–2002). During S332 and post-S332 intervals, water level in Taylor Slough was affected by operations of S332 and S332D. To relate vegetation change to plot-level hydrological conditions in Taylor Slough, we developed a weighted averaging regression and calibration model (WA) using data from the marl prairies of Everglades National Park and Big Cypress National Preserve. We examined vegetation pattern along five transects. Transects 1–3 were established in 1979 south of the water delivery structures, and were influenced by their operations. Transects 4 and 5 were established in 1997, the latter west of these structures and possibly under their influence. Transect 4 was established in the northern drainage basin of Taylor Slough, beyond the likely zones of influence of S332 and S332D. The composition of all three southern transects changed similarly after 1979. Where muhly grass (Muhlenbergia capillaris var. filipes) was once dominant, sawgrass (Cladium jamaicense), replaced it, while where sawgrass initially predominated, hydric species such as spikerush (Eleocharis cellulosa Torr.) overtook it. Most of the changes in species dominance in Transects 1–3 occurred after 1992, were mostly in place by 1995–1996, and continued through 1999, indicating how rapidly vegetation in seasonal Everglades marshes can respond to hydrological modifications. During the post-S332 period, these long-term trends began reversing. In the two northern transects, total cover and dominance of both muhly grass and sawgrass increased from 1997 to 2003. Thus, during the 1990’s, vegetation composition south of S332 became more like that of long hydroperiod marshes, but afterward it partially returned to its 1979 condition, i.e., a community characteristic of less prolonged flooding. In contrast, the vegetation change along the two northern transects since 1997 showed little relationship to hydrologic status.

A molecular and stable isotopic approach to investigate the importance of algal and detrital energy pathways in a freshwater marsh

The relative importance of algal and detrital energy pathways remains a central question in wetlands ecology. We used bulk stable isotope analysis and fatty acid composition to investigate the relative contributions of periphyton (algae) and floc (detritus) in a freshwater wetland with the goal of determining the inputs of these resource pools to lower trophic-level consumers. All animal samples revealed fatty acid markers indicative of both microbial (detrital) and algal origins, though the relative contributions varied among species. Vascular plant markers were in low abundance in most consumers. Detritivory is important for chironomids and amphipods, as demonstrated by the enhanced bacterial fatty acids present in both consumers, while algal resources, in the form of periphyton, likely support ephemeropteran larvae. Invertebrates such as amphipods and grass shrimp appear to be important resources for small omnivorous fish, while Poecilia latipinna appear to strongly use periphyton and Ephemeroptera larvae as food sources. Both P. latipinna and Lepomis spp. assimilated small amounts of vascular plant debris, possibly due to unintentional ingestion of floc while foraging for invertebrates and insect larvae. Physid snails, Haitia spp., were characterized by considerably different fatty acid compositions than other taxa examined, and likely play a unique role in Everglades’ food webs.

Responses of sawgrass and spikerush to variation in hydrologic drivers and salinity in Southern Everglades marshes

Aboveground net primary production (ANPP) by the dominant macrophyte and plant community composition are related to the changing hydrologic environment and to salinity in the southern Everglades, FL, USA. We present a new non-destructive ANPP technique that is applicable to any continuously growing herbaceous system. Data from 16 sites, collected from 1998 to 2004, were used to investigate how hydrology and salinity controlled sawgrass (Cladium jamaicense Crantz.) ANPP. Sawgrass live biomass showed little seasonal variation and annual means ranged from 89 to 639 gdw m)2. Mortality rates were 20–35% of live biomass per 2 month sampling interval, for biomass turnover rates of 1.3–2.5 per year. Production by C. jamaicense was manifest primarily as biomass turnover, not as biomass accumulation. Rates typically ranged from 300 to 750 gdw m)2 year)1, but exceeded 1000 gdw m)2 year)1 at one site and were as high as 750 gdw m)2 year)1 at estuarine ecotone sites. Production was negatively related to mean annual water depth, hydroperiod, and to a variable combining the two (depth-days). As water depths and hydroperiods increased in our southern Everglades study area, sawgrass ANPP declined. Because a primary restoration goal is to increase water depths and hydroperiods for some regions of the Everglades, we investigated how the plant community responded to this decline in sawgrass ANPP. Spikerush (Eleocharis sp.) was the next most prominent component of this community at our sites, and 39% of the variability in sawgrass ANPP was explained by a negative relationship with mean annual water depth, hydroperiod, and Eleocharis sp. density the following year. Sawgrass ANPP at estuarine ecotone sites responded negatively to salinity, and rates of production were slow to recover after high salinity years. Our results suggest that ecologists, managers, and the public should not necessarily interpret a decline in sawgrass that may result from hydrologic restoration as a negative phenomenon.

A synthesis of long-term research by the Florida Coastal Everglades LTER Program

This paper synthesizes research conducted during the first 5–6 years of the Florida Coastal Everglades Long-Term Ecological Research Program (FCE LTER). My objectives are to review our research to date, and to present a new central theme and conceptual approach for future research. Our research has focused on understanding how dissolved organic matter (DOM) from upstream oligotrophic marshes interacted with a marine source of the limiting nutrient, phosphorus (P), to control productivity in the oligohaline estuarine ecotone. We have been working along freshwater to marine transects in two drainage basins located in Everglades National Park (ENP). The Shark River Slough transect (SRS) has a direct connection to the Gulf of Mexico, providing this estuarine ecotone with a source of marine P. The oligohaline ecotone along our southern Everglades transect (TS/Ph), however, is separated from this marine P source by the Florida Bay estuary. We originally hypothesized an ecosystem productivity peak in the SRS ecotone, driven by the interaction of marine P and Everglades DOM, but no such productivity peak in the TS/Ph ecotone because of this lack of marine P. Our research to date has tended to show the opposite pattern, however, with many ecosystem components showing enhanced productivity in the TS/Ph ecotone, but not in the SRS ecotone. Water column P concentrations followed a similar pattern, with unexpectedly high P in the TS/Ph ecotone during the dry season. Our organic geochemical research has shown that Everglades DOM is more refractory than originally hypothesized. We have also begun to understand the importance of detrital organic matter production and transport to ecotone dynamics and as the base of aquatic food webs. Our future research will build on this substantial body of knowledge about these oligotrophic estuaries. We will direct our efforts more strongly on biophysical dynamics in the oligohaline ecotone regions. Specifically, we will be focusing on inputs to these regions from four primary water sources: freshwater Everglades runoff, net precipitation, marine inputs, and groundwater. We are hypothesizing that dry season groundwater inputs of P will be particularly important to TS/Ph ecotone dynamics because of longer water residence times in this area. Our organic geochemical, biogeochemical, and ecosystem energetics work will focus more strongly on the importance of detrital organics and will take advantage of a key Everglades Restoration project, scheduled for 2008 or 2009, that will increase freshwater inputs to our SRS transect only. Finally, we will also begin to investigate the human dimensions of restoration, and of a growing population in south Florida that will become increasingly dependent on the Everglades for critical ecosystem services (including fresh water) even as its growth presents challenges to Everglades sustainability.

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m−2 month−1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.

Factors affecting the concentration and flux of materials in two southern Everglades mangrove wetlands

Concentrations and fluxes of C, N, and P were measured in dwarf and fringe mangrove wetlands along the Taylor River, Florida, USA from 1996 to 1998. Data from these studies revealed considerable spatial and temporal variability. Concentrations of C, N, and P in the dwarf wetland showed seasonal trends, while water source was better at explaining concentrations in the fringe wetland. The total and dissolved organic carbon (TOC and DOC), total nitrogen (TN), and total phosphorus (TP) content of both wetlands was higher during the wet season or when water was flowing to the south (Everglades source). Concentrations of nitrate plus nitrite (NOx –), ammonium (NH4 +), and soluble reactive phosphorus (SRP) in the fringe wetland were all highest during the dry season or northerly flow (bay source). Nutrient concentrations most effectively explained patterns of flux in both wetlands. Increased wetland uptake of a given constituent was usually a function of its availability in the water column. However, the release of NOx – from the dwarf wetland was related to the NH4 + concentration, suggesting a nitrification signal. Nitrogen flux in the dwarf wetland was also related to surface water salinity and temperature. Our findings indicate that freshwater Everglades marshes are an important source of dissolved organic matter to these wetlands, while Florida Bay may be a source of dissolved inorganic nutrients. Our data also suggest that temperature, salinity, and nutrient concentrations (as driven by season and water source) influence patterns of materials flux in this mangrove wetland. Applying long-term water quality data to the relationships we extracted from these flux data, we estimated that TN and TP were imported by the dwarf wetland 87 ± 10 and 48 ± 17% of the year, respectively. With Everglades restoration, modifications in freshwater delivery may have considerable effects on the exchanges of nutrients and organic matter in these transitional mangrove wetlands.

A conceptual model of ecological interactions in the mangrove estuaries of the Florida Everglades

A brackish water ecotone of coastal bays and lakes, mangrove forests, salt marshes, tidal creeks, and upland hammocks separates Florida Bay, Biscayne Bay, and the Gulf of Mexico from the freshwater Everglades. The Everglades mangrove estuaries are characterized by salinity gradients that vary spatially with topography and vary seasonally and inter-annually with rainfall, tide, and freshwater flow from the Everglades. Because of their location at the lower end of the Everglades drainage basin, Everglades mangrove estuaries have been affected by upstream water management practices that have altered the freshwater heads and flows and that affect salinity gradients. Additionally, interannual variation in precipitation patterns, particularly those caused to El Nin˜o events, control freshwater inputs and salinity dynamics in these estuaries. Two major external drivers on this system are water management activities and global climate change. These drivers lead to two major ecosystem stressors: reduced freshwater flow volume and duration, and sea-level rise. Major ecological attributes include mangrove forest production, soil accretion, and resilience; coastal lake submerged aquatic vegetation; resident mangrove fish populations; wood stork (Mycteria americana) and roseate spoonbill (Platelea ajaja) nesting colonies; and estuarine crocodilian populations. Causal linkages between stressors and attributes include coastal transgression, hydroperiods, salinity gradients, and the ‘‘white zone’’ freshwater/estuarine interface. The functional estuary and its ecological attributes, as influenced by sea level and freshwater flow, must be viewed as spatially dynamic, with a possible near-term balancing of transgression but ultimately a long-term continuation of inland movement. Regardless of the spatio-temporal timing of this transgression, a salinity gradient supportive of ecologically functional Everglades mangrove estuaries will be required to maintain the integrity of the South Florida ecosystem.

Spatial and temporal patterns of aboveground net primary productivity (ANPP) along two freshwater-estuarine transects in the Florida Coastal Everglades

We present here a 4-year dataset (2001–2004) on the spatial and temporal patterns of aboveground net primary production (ANPP) by dominant primary producers (sawgrass, periphyton, mangroves, and seagrasses) along two transects in the oligotrophic Florida Everglades coastal landscape. The 17 sites of the Florida Coastal Everglades Long Term Ecological Research (FCE LTER) program are located along fresh-estuarine gradients in Shark River Slough (SRS) and Taylor River/C-111/Florida Bay (TS/Ph) basins that drain the western and southern Everglades, respectively. Within the SRS basin, sawgrass and periphyton ANPP did not differ significantly among sites but mangrove ANPP was highest at the site nearest the Gulf of Mexico. In the southern Everglades transect, there was a productivity peak in sawgrass and periphyton at the upper estuarine ecotone within Taylor River but no trends were observed in the C-111 Basin for either primary producer. Over the 4 years, average sawgrass ANPP in both basins ranged from 255 to 606 g m−2 year−1. Average periphyton productivity at SRS and TS/Ph was 17–68 g C m−2 year−1 and 342–10371 g C m−2 year−1, respectively. Mangrove productivity ranged from 340 g m−2 year−1 at Taylor River to 2208 g m−2 year−1 at the lower estuarine Shark River site. Average Thalassia testudinum productivity ranged from 91 to 396 g m−2 year−1 and was 4-fold greater at the site nearest the Gulf of Mexico than in eastern Florida Bay. There were no differences in periphyton productivity at Florida Bay. Interannual comparisons revealed no significant differences within each primary producer at either SRS or TS/Ph with the exception of sawgrass at SRS and the C−111 Basin. Future research will address difficulties in assessing and comparing ANPP of different primary producers along gradients as well as the significance of belowground production to the total productivity of this ecosystem.

Projected Reorganization of Florida Bay Seagrass Communities in Response to the Increased Freshwater Inflow of Everglades Restoration

Historic changes in water-use management in the Florida Everglades have caused the quantity of freshwater inflow to Florida Bay to decline by approximately 60% while altering its timing and spatial distribution. Two consequences have been (1) increased salinity throughout the bay, including occurrences of hypersalinity, coupled with a decrease in salinity variability, and (2) change in benthic habitat structure. Restoration goals have been proposed to return the salinity climates (salinity and its variability) of Florida Bay to more estuarine conditions through changes in upstream water management, thereby returning seagrass species cover to a more historic state. To assess the potential for meeting those goals, we used two modeling approaches and long-term monitoring data. First, we applied the hydrological mass balance model FATHOM to predict salinity climate changes in sub-basins throughout the bay in response to a broad range of freshwater inflow from the Everglades. Second, because seagrass species exhibit different sensitivities to salinity climates, we used the FATHOM-modeled salinity climates as input to a statistical discriminant function model that associates eight seagrass community types with water quality variables including salinity, salinity variability, total organic carbon, total phosphorus, nitrate, and ammonium, as well as sediment depth and light reaching the benthos. Salinity climates in the western sub-basins bordering the Gulf of Mexico were insensitive to even the largest (5-fold) modeled increases in freshwater inflow. However, the north, northeastern, and eastern sub-basins were highly sensitive to freshwater inflow and responded to comparatively small increases with decreased salinity and increased salinity variability. The discriminant function model predicted increased occurrences ofHalodule wrightii communities and decreased occurrences of Thalassia testudinum communities in response to the more estuarine salinity climates. The shift in community composition represents a return to the historically observed state and suggests that restoration goals for Florida Bay can be achieved through restoration of freshwater inflow from the Everglades.

Occurrence and distribution of novel botryococcene hydrocarbons in freshwater wetlands of the Florida Everglades

A high abundance of isoprenoid hydrocarbons, the botryococcenes, with carbon numbers from 32 to 34 were detected in the Florida Everglades freshwater wetlands. These compounds were present in varying amounts up to 106 μg/gdw in periphyton, 278 μg/gdw in floc, and 46 μg/gdw in soils. Their structures were determined based on comparison to standards, interpretation of their mass spectra and those of their hydrogenation products, and comparison of Kovats indexes to those reported in the literature. A total of 26 cyclic and acyclic botryococcenes with 8 skeletons were identified, including those with fewer degrees of unsaturation, which are proposed as early diagenetic derivatives from the natural products. This is the first report that botryococcenes occur in the Everglades freshwater wetlands. Their potential biogenetic sources from green algae and cyanobacteria were examined, but neither contained botryococcenes. Thus, the source implication of botryococcenes in this ecosystem needs further study.

Across-scale patterning of plant-soil-water interactions surrounding tree islands in Southern Everglades landscapes

The freshwater Everglades is a complex system containing thousands of tree islands embedded within a marsh-grassland matrix. The tree island-marsh mosaic is shaped and maintained by hydrologic, edaphic and biological mechanisms that interact across multiple scales. Preserving tree islands requires a more integrated understanding of how scale-dependent phenomena interact in the larger freshwater system. The hierarchical patch dynamics paradigm provides a conceptual framework for exploring multi-scale interactions within complex systems. We used a three-tiered approach to examine the spatial variability and patterning of nutrients in relation to site parameters within and between two hydrologically defined Everglades landscapes: the freshwater Marl Prairie and the Ridge and Slough. Results were scale-dependent and complexly interrelated. Total carbon and nitrogen patterning were correlated with organic matter accumulation, driven by hydrologic conditions at the system scale. Total and bioavailable phosphorus were most strongly related to woody plant patterning within landscapes, and were found to be 3 to 11 times more concentrated in tree island soils compared to surrounding marshes. Below canopy resource islands in the slough were elongated in a downstream direction, indicating soil resource directional drift. Combined multi-scale results suggest that hydrology plays a significant role in landscape patterning and also the development and maintenance of tree islands. Once developed, tree islands appear to exert influence over the spatial distribution of nutrients, which can reciprocally affect other ecological processes.

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g−1 vs. 107 μg g−1, respectively) and soil TP (284 μg g−1 vs. 145 μg g−1). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m−2). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly

Interactions between nutrient availability and hydroperiod shape macroinvertebrate communities in Florida Everglades marshes

Hydroperiod and nutrient status are known to influence aquatic communities in wetlands, but their joint effects are not well explored. I sampled floating periphyton mat and flocculent detritus (floc) infaunal communities using 6-cm diameter cores at short- and long-hydroperiod and constantly inundated sites across a range of phosphorus (P) availability (total phosphorus in soil, floc and periphyton). Differences in community structure between periphyton and floc microhabitats were greater than any variation attributable to hydroperiod, P availability, or other spatial factors. Multivariate analyses indicated community structure of benthic-floc infauna was driven by hydroperiod, although crowding (no. g−1 AFDM) of individual taxa showed no consistent responses to hydroperiod or P availability. In contrast, community structure of periphyton mat infauna was driven by P availability, while densities of mat infauna (no. m−2) were most influenced by hydroperiod (+correlations). Crowding of mat infauna increased significantly with P availability in short-hydroperiod marshes, but was constant across the P gradient in long-hydroperiod marshes. Increased abundance of floating-periphyton mat infauna with P availability at short-hydroperiod sites may result from a release from predation by small fish. Community structure and density were not different between long-hydroperiod and constantly inundated sites. These results have implications for the use of macroinvertebrates as indicators of water quality in wetlands and suggest the substrate sampled can influence interpretation of ecological responses observed in these communities.