Boundary Effects on Benthic Microbial Phosphorus Concentrations and Diatom Beta Diversity in a Hydrologically-modified, Nutrient-limited Wetland

Water flow and flooding duration in wetlands influence the structure and productivity of microbial communities partly through their influence on nutrient loading. The effect of flow-regulated nutrient loads is especially relevant for microbial communities in nutrient-poor settings, where delivery controls nutrient uptake rates and the intensity of microbial interactions. We examined the effect of hydrologic history and proximity to water sources on nutrient enrichment of benthic microbial assemblages (periphyton) and on their diatom species composition, along the artificial boundaries of Taylor Slough, a historically phosphorus-depleted drainage of the Florida Everglades. Concentrations of phosphorus in periphyton declined from the wetland boundary near inflow structures to 100-m interior, with spatial and temporal variability in rates dependent on proximity to and magnitude of water flow. Phosphorus availability influenced the beta diversity of diatom assemblages, with higher values near inflow structures where resources were greatest, while interior sites and reference transects contained assemblages with constant composition of taxa considered endemic to the Everglades. This research shows how hydrologic restoration may have unintended consequences when incoming water quality is not regulated, including a replacement of distinctive microbial assemblages by ubiquitous, cosmopolitan ones.

Edward Castaneda retrieving a soil core to assess sedimentation and nutrient accumulation rates in a dwarf mangrove forest, Taylor Slough

This material is based upon work supported by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Cooperative Agreements #DBI-0620409 and #DEB-9910514. This image is made available for non-commercial or educational use only.

Water source utilization and foliar nutrient status differs between upland and flooded plant communities in wetland tree islands

Tree islands in the Everglades wetlands are centers of biodiversity and targets of restoration, yet little is known about the pattern of water source utilization by the constituent woody plant communities: upland hammocks and flooded swamp forests. Two potential water sources exist: (1) entrapped rainwater in the vadose zone of the organic soil (referred to as upland soil water), that becomes enriched in phosphorus, and (2) phosphorus-poor groundwater/surface water (referred to as regional water). Using natural stable isotope abundance as a tracer, we observed that hammock plants used upland soil water in the wet season and shifted to regional water uptake in the dry season, while swamp forest plants used regional water throughout the year. Consistent with the previously observed phosphorus concentrations of the two water sources, hammock plants had a greater annual mean foliar phosphorus concentration over swamp forest plants, thereby supporting the idea that tree island hammocks are islands of high phosphorus concentrations in the oligotrophic Everglades. Foliar nitrogen levels in swamp forest plants were higher than those of hammock plants. Linking water sources with foliar nutrient concentrations can indicate nutrient sources and periods of nutrient uptake, thereby linking hydrology with the nutrient regimes of different plant communities in wetland ecosystems. Our results are consistent with the hypotheses that (1) over long periods, upland tree island communities incrementally increase their nutrient concentration by incorporating marsh nutrients through transpiration seasonally, and (2) small differences in micro-topography in a wetland ecosystem can lead to large differences in water and nutrient cycles.

Effect of leaf-cutting ant nests on plant growth in an oligotrophic Amazon rain forest

This study examined whether high nutrient concentrations associated with leaf-cutting ant nests influence plant growth and plant water relations in Amazon rain forests. Three nests of Atta cephalotes were selected along with 31 Amaioua guianensis and Protium sp. trees that were grouped into trees near and distant (>10 m) from nests. A 15N leaf-labelling experiment confirmed that trees located near nests accessed nutrients from nests. Trees near nests exhibited higher relative growth rates (based on stem diameter increases) on average compared with trees further away; however this was significant for A. guianensis (near nest 0.224 y−1 and far from nest 0.036 y−1) but not so for Protium sp. (0.146 y−1 and 0.114 y−1 respectively). Water relations were similarly species-specific; for A. guianensis, near-nest individuals showed significantly higher sap flow rates (16 vs. 5 cm h−1), higher predawn/midday water potentials (−0.66 vs. −0.98 MPa) and lower foliar δ13C than trees further away indicating greater water uptake in proximity to the nests while the Protium sp. showed no significant difference except for carbon isotopes. This study thus shows that plant response to high nutrient concentrations in an oligotrophic ecosystem varies with species. Lower seedling abundance and species richness on nests as compared with further away suggests that while adult plants access subterranean nutrient pools, the nest surfaces themselves do not encourage plant establishment and growth.

Accumulation and fate of mercury in an Everglades aquatic food web

This project examined the pathways of mercury (Hg) bioaccumulation and its relation to trophic position and hydroperiod in the Everglades. I described fish-diet differences across habitats and seasons by analyzing stomach contents of 4,000 fishes of 32 native and introduced species. Major foods included periphyton, detritus/algal conglomerate, small invertebrates, aquatic insects, decapods, and fishes. Florida gar, largemouth bass, pike killifish, and bowfin were at the top of the piscine food web. Using prey volumes, I quantitatively classified the fishes into trophic groups of herbivores, omnivores, and carnivores. Stable-isotope analysis of fishes and invertebrates gave an independent and similar assessment of trophic placement. Trophic patterns were similar to those from tropical communities. I tested for correlations of trophic position and total mercury. Over 4,000 fish, 620 invertebrate, and 46 plant samples were analyzed for mercury with an atomic-fluorescence spectrometer. Mercury varied within and among taxa. Invertebrates ranged from 25–200 ng g −1 ww. Small-bodied fishes varied from 78–>400 ng g −1 ww. Large predatory fishes were highest, reaching a maximum of 1,515 ng−1 ww. Hg concentrations in both fishes and invertebrates were positively correlated with trophic position. I examined the effects of season and hydroperiod on mercury in wild and caged mosquitofish at three pairs of marshes. Nine monthly collections of wild mosquitofish were analyzed. Hydroperiod-within-site significantly affected concentrations but it interacted with sampling period. To control for wild-fish dispersal, and to measure in situ uptake and growth, I placed captive-reared, neonate mosquitofish with mercury levels from 7–14 ng g−1 ww into field cages in the six study marshes in six trials. Uptake rates ranged from 0.25–3.61 ng g−1 ww d −1. As with the wild fish, hydroperiod-within-site was a significant main effect that also interacted with sampling period. Survival exceeded 80%. Growth varied with season and hydroperiod, with greatest growth in short-hydroperiod marshes. The results suggest that dietary bioaccumulation determined mercury levels in Everglades aquatic animals, and that, although hydroperiod affected mercury uptake, its effect varied with season. ^

The Application of Fluorescent Nanocrystals to Study the Distribution of Neuropeptides in the Aedes Aegypti Mosquito

Semiconductor nanocrystals, also known as quantum dots (QDs), have been used in studies involving mice and human tissues, but never before in research on insects. We used QDs to study the distribution of two neuropeptides in the Aedes aegypti mosquito, the vector of both dengue and yellow fever. These neuropeptides play a significant role in the production of juvenile hormone, a hormone that controls biting behavior, metamorphosis, and reproduction throughout the life of the mosquito. The two neuropeptides allatostatin-C (AS-C) and allatotropin (AT) function as inhibitory (AS-C) and stimulatory (AT) regulators of juvenile hormone synthesis in the corpus allatum gland. In other insects, they also affect heart rate, gut movement, and nutrient uptake. Conjugating these neuropeptides to quantum dots via a streptavidinlbiotin link, we were able to expose the mosquito corpus allatum and abdomen to allatostatin-C and allatotropin and then to visualize their distribution under UV light using confocal and compound light microscopy. Histological sections of the whole mosquito, incubations of tissues with conjugates (in vitro), and microinjections of conjugates into the mosquito (in vivo) were performed. The results showed that quantum dots can be used to detect neuropeptide distribution in the mosquito. The more we understand about these neuropeptides and juvenile hormone, the more we can contribute to stopping the spread of infectious diseases, such as dengue and yellow fever.

Quantifying nitrogen cycling rates in freshwater marshes of the southern Everglades using nitrogen-15 tracer techniques

This dissertation research project addressed the question of how hydrologic restoration of the Everglades is impacting the nutrient dynamics of marsh ecosystems in the southern Everglades. These effects were analyzed by quantifying nitrogen (N) cycle dynamics in the region. I utilized stable isotope tracer techniques to investigate nitrogen uptake and cycling between the major ecosystem components of the freshwater marsh system. I recorded the natural isotopic signatures (δ15N and δ 13C) for major ecosystem components from the three major watersheds of the Everglades: Shark River Slough, Taylor Slough, and C-111 basin. Analysis of δ15 N and δ13C natural abundance data were used to demonstrate the spatial extent to which nitrogen from anthropogenic or naturally enriched sources is entering the marshes of the Everglades. In addition, I measured the fluxes on N between various ecosystem components at both near-canal and estuarine ecotone locations. Lastly, I investigated the effect of three phosphorus load treatments (0.00 mg P m-2, 6.66 mg P m-2, and 66.6 mg P m-2) on the rate and magnitude of ecosystem N-uptake and N-cycling. The δ15N and δ13C natural abundance data supported the hypothesis that ecosystem components from near-canal sites have heavier, more enriched δ 15N isotopic signatures than downstream sites. The natural abundance data also showed that the marshes of the southern Everglades are acting as a sink for isotopically heavier, canal-borne dissolved inorganic nitrogen (DIN) and a source for "new" marsh derived dissolved organic nitrogen (DON). In addition, the 15N mesocosm data showed the rapid assimilation of the 15N tracer by the periphyton component and the delayed N uptake by soil and macrophyte components in the southern Everglades.