Carbon (d13c) and Nitrogen (d15n) Isotopic Discrimination in Mangroves in Florida Coastal Everglades as a Function of Environmental Stress

Isotope signatures of mangrove leaves can vary depending on discrimination associated with plant response to environmental stressors defined by gra­dients of resources (such as water and nutrient limitation) and regulators (such as salinity and sul­fide toxicity). We tested the variability of man­grove isotopic signatures (d13C and d15N) across a stress gradient in south Florida, using green leaves from four mangrove species collected at six sites. Mangroves across the landscape studied are stressed by resource and regulator gradients repre­sented by limited phosphorus concentrations com­bined with high sulfide concentrations, respec­tively. Foliar d13C ratios exhibited a range from ­ 24.6 to –32.7‰, and multiple regression analysis showed that 46% of the variability in mangrove d13C composition could be explained by the differ­ences in dissolved inorganic nitrogen, soluble reac­tive phosphorus, and sulfide porewater concentra­tions. 15N discrimination in mangrove species ranged from –0.1 to 7.7‰, and porewater N, salin­ity, and leaf N:Pa ratios accounted for 41% of this variability in mangrove leaves. The increase in soil P availability reduced 15N discrimination due to higher N demand. Scrub mangroves (<1.5 m tall) are more water-use efficient, as indicated by higher d13C; and have greater nutrient use efficiency ratios of P than do tall mangroves (5 to 10 m tall) existing in sites with greater soil P concentrations. The high variability of mangrove d13C and d15N across these resource and regulator gradients could be a con­founding factor obscuring the linkages between mangrove wetlands and estuarine food webs. These results support the hypothesis that landscape fac­tors may control mangrove structure and function, so that nutrient biogeochemistry and mangrove-based food webs in adjacent estuaries should ac­count for watershed-specific organic inputs.

Application of biomarkers and compound specific stable isotopes for the assessment of hydrology as a driver of organic matter dynamics in the Everglades ecosystem

The Everglades is a sub-tropical coastal wetland characterized among others by its hydrological features and deposits of peat. Formation and preservation of organic matter in soils and sediments in this wetland ecosystem is critical for its sustainability and hydrological processes are important divers in the origin, transport and fate of organic matter. With this in mind, organic matter dynamics in the greater Florida Everglades was studied though various organic geochemistry techniques, especially biomarkers, bulk and compound specific δ13C and δD isotope analysis. The main objectives were focused on how different hydrological regimes in this ecosystem control organic matter dynamics, such as the mobilization of particulate organic matter (POM) in freshwater marshes and estuaries, and how organic geochemistry techniques can be applied to reconstruct Everglades paleo-hydrology. For this purpose organic matter in typical vegetation, floc, surface soils, soil cores, and estuarine suspended particulates were characterized in samples selected along hydrological gradients in the Water Conservation Area 3, Shark River Slough and Taylor Slough. ^ This research focused on three general themes: (1) Assessment of the environmental dynamics and source-specific particulate organic carbon export in a mangrove-dominated estuary. (2) Assessment of the origin, transport and fate of organic matter in freshwater marsh. (3) Assessment of historical changes in hydrological conditions in the Everglades (paleo-hydrology) though biomarkes and compound specific isotope analyses. This study reports the first estimate of particulate organic carbon loss from mangrove ecosystems in the Everglades, provides evidence for particulate organic matter transport with regards to the formation of ridge and slough landscapes in the Everglades, and demonstrates the applicability of the combined biomarker and compound-specific stable isotope approach as a means to generate paleohydrological data in wetlands. The data suggests that: (1) Carbon loss from mangrove estuaries is roughly split 50/50 between dissolved and particulate carbon; (2) hydrological remobilization of particulate organic matter from slough to ridge environments may play an important role in the maintenance of the Everglades freshwater landscape; and (3) Historical changes in hydrology have resulted in significant vegetation shifts from historical slough type vegetation to present ridge type vegetation. ^

Using Stable Isotopes to Investigate Trophic Interactions of Bottlenose Dolphins (Tursiops truncatus) in the Coastal Everglades

Top predators are best known for their ability to affect their communities through inflicting mortality on prey and inducing behavioral modifications (e.g. risk effects). Recent scientific evidence suggests that predators may have additional roles in bottom-up processes such as transporting materials within and across habitat boundaries. The Florida Coastal Everglades (FCE) is an “upside-down” oligotrophic estuary where productivity decreases from the mouth of the estuary to freshwater marshes. Research in the FCE suggest that predators can act as mobile links between disparate habitats and can potentially affect nutrient and biogeochemical dynamics through localized behaviors (e.g. American alligators and juvenile bull sharks). To date, little is known about bottlenose dolphins (Tursiops truncatus) in the FCE beyond broad-scale patterns of abundance. Because they are highly mobile mammals commonly found in coastal waters, bottlenose dolphins are an interesting case study for investigating the influence of ecology on the evolution of local adaptations. Within this influence lies the potential for investigation of the related roles those adaptations play in coastal ecosystems due to their high metabolic rates, movement capabilities, and tendency to display specialized foraging behaviors. Stable isotope analysis of biopsy samples were used to investigate habitat use, trophic interactions, and patterns of individual specialization in bottlenose dolphins to gain functional insights into ecosystem dynamics. δ13 C isotopic values are used to differentiate the relative importance of a food web to the diet of an organism, while δ15 N values are used to evaluate the relative trophic position of an organism. Dolphin δ13 C isotopic values seem to suggest that dolphins are foraging within single ecosystems and may not be moving nutrients across ecosystem boundaries while their δ15 N isotopic values appear to be of a top predator, at a similar level to bull sharks and alligators in FCE. Further research is necessary to provide vital insight into the large predators’ role in affecting the evolution of local adaptations. Conducting this research should also provide information for predicting how future changes occurring due to restoration dynamics (see CERP: evergladesplan.org) and climate change will affect the ecological roles of these animals.