Browsing by Subject "stable isotopes"

Sea turtles have experienced dramatic population declines during the last century as a consequence of direct harvest, by-catch in fisheries, and habitat loss. Despite almost 50 years of partial international protection, several populations of sea turtles are still at imminent risk of extinction. Our knowledge of their complex life histories is still far from complete; these knowledge gaps hinder our ability to provide scientific advice regarding their conservation and management. It is the very complexity of their life histories, which allows them to exploit widely separated habitats during development, often over the course of decades, which makes them inherently difficult to study. I used satellite telemetry (n=60) to investigate the movements and habitat use patterns of juvenile loggerhead (Caretta caretta), green (Chelonia mydas), and Kemp's ridley (Lepidochelys kempii) sea turtles on their summer feeding grounds in North Carolina estuaries. These turtles migrate into and out of the estuarine waters each spring and autumn, encountering a gauntlet of fishing gear on each journey. The by-catch of sea turtles is an important conservation issue in North Carolina, and throughout the world's oceans. I evaluated conservation measures established to reduce the by-catch of sea turtles in Pamlico Sound's autumnal large-mesh gill net fishery for southern flounder (Paralichthys lethostigma), using a spatially explicit predator/prey model. My findings indicated that species-specific habitat preferences contributed to a turtles' risk of encountering fishing gear and that areas of high by-catch are predictable from patterns of overlap between sea turtle habitat use and flounder fishing effort. I then examined how the behavior of green turtles affected their vulnerability to incidental capture in estuarine commercial fisheries. Individual green turtles interact with multiple gears per season as a result of strong site fidelity to habitats also preferred by fishers. Telemetry also allowed me to examine individual variation in movements, habitat use, and site fidelity patterns of juvenile loggerhead turtles, both within the estuary and as the turtles migrated out into the North Atlantic. I used these observations to test the hypothesis of a discrete ontogenetic shift in habitat and diet in juvenile loggerheads. Approximately one-third of large juvenile loggerheads tagged in North Carolina estuaries return to oceanic habitat, sometimes for several years, where they are vulnerable to by-catch in pelagic fisheries. This led me to conclude that the long held notion of a discrete ontogenetic habitat shift between the oceanic and neritic habitat was incorrect for juvenile loggerheads (and possibly also for green turtles). Finally, I explored variation in migratory destinations in these animals through multivariate analyses of carbon and nitrogen stable isotope ratios in blood plasma and red blood cells, and through analysis of sex, genetic, haplotype, body size, and remigration records, and described the trophic niche of these turtles with Bayesian isotope mixing models. Variation in migratory destination (oceanic or neritic habitat) was best described by stable isotope ratios of nitrogen and remigration tendency. Turtles that returned to the open ocean had significantly lower nitrogen ratios than those animals that remained in the neritic zone and their diets retained a substantial contribution of epipelagic prey items. The diet composition of neritic turtles, on the other hand, consisted primarily of estuarine benthic invertebrates during the summertime and autumn foraging season but shifted toward pelagic jellyfish, fish, and Sargassum during the overwintering period. Oceanic turtles likely came from open ocean regions prior to entering the summer foraging grounds while neritic turtles likely overwintered at the edge of the Gulf Stream. The agreement between the dietary compositions and migration patterns between the two groups of turtles suggest that these feeding and habitat use strategies were persistent characteristics in the turtles I sampled. My work has improved our understanding of sea turtle habitats in North Carolina estuaries and identified their migratory destinations and overwintering habitats. I hope that this work lays the groundwork for future studies that will explore how variation in habitat use and feeding strategies are manifested in life history traits that affect fitness directly, such as survivorship, growth rates, stage durations, and fecundity.

Whether through sea level rise, farmland abandonment, or wetland restoration, agricultural soils in coastal areas will be inundated at increasing rates, renewing connections to sensitive surface waters and raising critical questions related to environmental tradeoffs. Wetland restoration in particular is often implemented not only to promote wildlife habitat, but also to improve water quality through nutrient removal, especially in agricultural catchments. The microbial process of denitrification is the central mechanism of nitrogen removal in wetlands and flooded soils, and can be seen as a potential environmental benefit of flooding agricultural lands. While denitrification undoubtedly can remove nitrogen from soil and surface water, higher soil moisture or flooding in wetland soils can also increase the production of greenhouse gases, specifically nitrous oxide and methane, representing a potential environmental tradeoff. Understanding the likely benefits of denitrification and the likely greenhouse gas costs of wetland restoration could help inform environmental policies concerning wetland restoration.

Determining whether restored wetlands are larger sources of greenhouse gases compared to contrasting land use types (agriculture and forested wetlands) was the first goal of this dissertation (Chapter 2). We measured gas fluxes from soil and water to the atmosphere, and related environmental variables, in four sites over two years to estimate fluxes of the three major greenhouse gases. We found that carbon dioxide was the major contributor to the radiative balance across all sites, but that in the agricultural site and one of the forested wetland reference sites, nitrous oxide was the second most important contributor. Many studies have shown that methane is more important that nitrous oxide in most freshwater wetlands, as we found in the other forested wetland reference site and in flooded parts of the restored wetland. Overall, we did not find higher greenhouse gas fluxes in the restored wetland compared to agricultural soils or forested wetlands.

The controls over nitrous oxide are especially complex, because it can be produced by two complementary processes, nitrification and denitrification, which generally occur under different conditions in the environment. In Chapter 3, we determined the soil and environmental factors that best predicted nitrous oxide fluxes for a subset of our data encompassing gas fluxes measured in November 2007. We found that soil temperature and soil carbon dioxide flux, along with ammonium availability and denitrification potential, were good predictors of nitrous oxide (adj R2=0.81). Although the nitrous oxide model did not perform as well when applied to data from another sampling period, we expect to further develop our modeling efforts to include possible non-linear temperature effects and a larger range of environmental conditions.

In Chapter 4, we present results of a stable isotope tracer experiment to determine the relative contribution of nitrification and denitrification to nitrous oxide fluxes in these different land use types, and to determine the response of these processes to changing soil moisture. We added two forms of nitrogen-15 to intact soil cores to distinguish nitrification from denitrification, and subjected the cores to drainage or to a simulated rain event. We found that across the range of soil moisture, the fraction of nitrous oxide produced by denitrification did not change, but within each soil type there was a response to the simulated rain. In mineral soils, the nitrous oxide fraction increased with increasing soil moisture, with the highest mole fraction [N₂O/(N₂+N₂O)] in the agricultural soils, while in the organic soils there was no change or even a decrease. The fraction of nitrous oxide derived from coupled nitrification-denitrification increased with increasing soil moisture, and was much higher than that from denitrification alone in the more organic soils. This suggests that, in these saturated acid-organic soils, nitrification plays an important and underappreciated role in contributing to nitrous oxide fluxes from freshwater wetlands. The results from the laboratory experiment were consistent with patterns we saw in the field and help explain the differential contribution of nitrification and denitrification to nitrous oxide fluxes in different land use types in coastal plain wetlands of North Carolina.

Overall, we found that both nitrification and denitrification contribute to nitrous oxide fluxes in coastal plain wetlands in North Carolina, and that nitrification is an especially important source in acid-organic soils under both field-moist and saturated conditions. Although freshwater wetlands, with an average nitrous oxide mole fraction of 0.08, are generally seen as being insignificant sources of nitrous oxide, our study sites ranged from 0.10 to 0.30, placing them closer to agricultural fields (0.38; Schlesinger 2009). Although the ecosystems in our study produced more nitrous oxide than expected for freshwater wetlands, we found no significant tradeoff between the local water quality benefits conferred by denitrification and the global greenhouse gas costs in the restored wetland. These results suggest that, from a nitrogen perspective, wetland restoration in coastal agricultural lands has a net environmental benefit.