On the Use of Stable Isotopes to Elucidate Energy Flow Pathways from Organisms to Ecosystems

The primary goal of this dissertation was to explore the use of stable isotope analysis as a tool to understand how energy flows at several layers of biological organization from the individual to the ecosystem level. With the exception of phosphorus, the primary elements that are utilized by living organisms on earth (H, C, N, O, & S) have at least one stable isotope. The isotopes of carbon, nitrogen, and sulfur are particularly useful for following the flow of energy within and between organisms, because these elements are the primary components in proteins, carbohydrates, and fats. When used in combination and when significant contrast in isotope values exists, stable isotopes can be used to elucidate biological processes from the intercellular to global scale. Chapter one will provide an introduction to the fundamentals of stable isotope analysis and a review of the state of its use in biological systems. In chapter two, the usefulness of stable isotope to identify diet changes in wild gag grouper is determined with a repeated-measures diet-shift experiment on four adult gag held in the laboratory. Fish were initially fed a diet of Atlantic mackerel, Scomber scombrus, (mean 13C = -21.3 / ± 0.2, n= 20) for a period of 56 days and then shifted to a diet of pinfish, Lagodon rhomboids, (mean 13C = -16.6 / ± 0.6, n= 20) for the 256 day experiment. I developed a non-lethal surgical procedure to obtain biopsies of the muscle, liver, and gonad tissue monthly from the same four fish. I then determined the 13C value of each tissue by isotope-ratio mass spectrometry. For the gonad tissue, I used the relationship between C/N and lipid content to correct for the influence of lipids on  13C value. I observed a significant shift in the 13C values of all of the tissues sampled in the study. Carbon turnover rates varied among the three tissues, but the shift in diet from mackerel to pinfish was clearly traceable through analysis of  13C values. The turnover rates for muscle tissue were 0.005 / day-1, and for gonad tissue was 0.009 / day-1. Although it is generally thought that tissue turnover rates in ectotherms are driven primarily by growth, I found that metabolic rate can be a major factor driving tissue turnover in adult gag. Chapter three explores the application of stable isotope analysis to trace the transfer of secondary production from temperate, coastal seagrass habitat to offshore reefs. Seagrass meadows are among the most productive ecosystems in the marine environment. It has been speculated that much of this production is exported to adjacent ecosystems via the movements of organisms. My study utilized stable isotopes to track seagrass derived production into offshore food webs in the northeastern Gulf of Mexico. I found that gag grouper (Myctereoperca microlepis) on reefs as far as 90 km from the seagrass beds incorporate a significant portion of seagrass derived biomass. The muscle tissue of gag grouper, a major fisheries species, was composed on average of 19% ±10% seagrass-habitat-derived biomass. The timing of this annual seagrass subsidy appears to be important in fueling gag grouper egg production. The 34S values of gag grouper gonad tissues varied seasonally and were34S depleted during the spawning season, indicating that gag utilize the seagrass derived biomass to support reproduction. If such large-scale trophic subsidies are typical of temperate seagrass systems, then loss of seagrass production or habitat would result in a direct loss of offshore fisheries productivity. After showing that a significant quantity of secondary production is transferred across ecosystem boundaries in chapter four, this flux is placed in the context of the flux of organic nitrogen, the primary limiting nutrient in the Gulf of Mexico. The migration of a single species, pinfish (Lagodon rhomboids), exports the equivalent of 2.85 x 107 ± 9.32 x 106 kg of primary nitrogen to the northeastern Gulf of Mexico. This flux is greater than the amount of primary nitrogen fluxed by the Apalachicola River and slightly less than the amount of organic nitrogen fluxed via atmospheric deposition and fixation by Trichodesmium. Although fish flux is less than 3 % of the total quantity of material fluxed to the offshore environment by mass, gag grouper biomass contains 19 ± 10 % seagrass derived-material. Stable carbon isotope values of otoliths can provide long-term records of the environmental conditions experienced by fish. However, as shown in chapter five because otoliths contain carbon from metabolic sources, it is necessary to determine the proportion of metabolically derived carbon to properly interpret otolith 13C values. To evaluate the relative influence of dietary carbon and ambient seawater dissolved inorganic carbon (DIC) on the 13C isotopic composition of fish otoliths, juvenile red drum (Sciaenops ocellatus) were raised for 6 and 9 months in tanks with flow-through ambient seawater and fed diets differing by 2.1 / (p < 0.001) in carbon isotope composition (13C). At the end of the experiment, muscle tissue from the two treatments reflected the isotopic composition of the diets and differed by 2.1 / (p <0.001). Muscle tissue from both groups was enriched by 1.5 / in 13C relative to the respective diets. The carbon isotope values of otoliths were enriched ~16-17 / relative to the respective diets, indicating that DIC was the dominant source (85%-92%) for otolith carbon relative to carbon derived from diet. However, otoliths from the two treatments differed by 1.6 / and expressed 60% of the difference in the isotope values of the diets. This result indicates that the nutritional characteristics of food may be a factor influencing the incorporation of metabolic carbon in the otolith. This is one of the first studies to provide evidence that the nutritional value of food influences otolith 13C value.