Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Ernst B. Peebles, Ph.D.

Committee Member

Brad Rosenheim, Ph.D.

Committee Member

Chris Stallings, Ph.D.

Committee Member

Hannah Vander Zanden, Ph.D.

Committee Member

Kelton McMahon, Ph.D.

Keywords

carbon isotopes, nitrogen isotopes, spatial variability, stable isotope ecology

Abstract

Isoscapes are depictions of the spatial patterns of isotopic values in a given area. Isoscapes can be created using measurements form samples (empirical isoscapes) or using statistical models of spatial isotopic variation (modeled isoscapes). Isoscapes have a wide variety of potential applications though, in the realm of marine ecology, they are most often used infer ecological processes, food web linkages, the origin of samples, and the movements of marine organisms.

However, to use isoscapes for these applications, it is necessary to have isoscapes at spatial scales relevant to the application in question. It is also necessary to have isoscapes that are based on a material that allows them to be applied to the organism or process in question. To address these necessities, the following dissertation created regional Gulf of Mexico isoscapes using the δ13C and δ15N values of reef-associated mesopredators, so the isoscapes could be directly applicable to many fisheries species as well as other mesopredators of interest.

The objectives of this dissertation were (1) to create empirical δ13C and δ15N isoscapes for the Gulf of Mexico using fish muscle, (2) to create a statistical models for those δ13C and δ15N values using readily available remote sensing data, (3) to use those models to evaluate predicted temporal variability in those isoscapes, (4) to use the isoscapes and models to elucidate influential ecological processes for δ13C and δ15N baselines, and (5) to demonstrate one application of the isoscapes using Red Snapper eye lens stable isotopes to infer movement histories of individual fish. Care was taken that the methods and products presented here could be used in future studies, and important considerations and caveats for these methods and products are listed at the end of each chapter.

First, muscle samples from Red Snapper (Lutjanus campechanus) and Yellowedge Grouper (Epinephelus flavolimbatus) were collected from longlining research cruises in the Gulf of Mexico and analyzed for δ13C and δ15N values. Regressions were performed between each isotope and fish standard length to assess if there was a significant effect of trophic growth (increase in trophic position with fish size). All regressions were significant and, to prevent spatial differences in fish trophic position from overwhelming spatial differences in baseline isotopic values, residuals from the regressions were used to create isoscapes instead of the original data. Isoscapes were created for δ13C and δ15N values on the continental shelf of the Gulf of Mexico in areas reflecting the capture locations of each species. The Red Snapper δ13C isoscape depicted a depth gradient, wherein δ13C decreased with depth. This pattern was attributed to higher productivity and/or higher benthic basal resource availability inshore. The Yellowedge Grouper isoscape did not depict a depth gradient because Yellowedge Grouper were only captured close to the shelf edge. The δ15N isoscapes of both species depicted a pattern of higher δ15N values in areas near freshwater input with more eutrophic conditions and lower δ15N values in areas with more oligotrophic conditions. This pattern was attributed to differences in the sources of bioavailable nitrogen. In areas near freshwater input, it appeared rivers were delivering organic waste (sewage or livestock effluent) that tends to have higher δ15N values, whereas, in oligotrophic areas, most bioavailable nitrogen came from diazotrophic nitrogen fixation that tends to have lower δ15N values.

Next, statistical models were created for the spatial patterns in δ13C and δ15N values to evaluate temporal variability and further elucidate influential ecological processes. Potential predictor variables included satellite remote sensing products [CDOM, Chl, Kd (PAR), surface PAR, PAR(z), PIC, POC, and SST] and static variables (latitude, longitude, and depth) collected with each longlining deployment. Optional transformations (x2, Ln(x), 1/x, and √x) were applied to predictor variables, and predictor variables were standardized. Potential predictor variables were used in linear multiple regression analysis, wherein variables were selected using forward selection and AIC. All models were significant and explained at least a moderate amount of spatial isotopic variation. Overall, the δ15N models had higher R2 values and performed better when the model created with one species was used to predict the δ15N values of the other species. The selected variables and coefficients of the models suggested that the ecological explanations applied to the empirical isoscapes were correct with a few additions. The Red Snapper δ13C model included SST with a negative coefficient, which was attributed to SST being an influence on or a proxy for phytoplankton species composition. The selected variables and coefficients of the Yellowedge Grouper δ13C model suggest productivity is highly influential, and, in areas where productivity is lower and more spatially uniform, light environment is influential. Temporal variability of these isoscapes was assessed by gathering satellite products from all four seasons within an El Niño and La Niña year and using the statistical models to predict isoscapes for those time periods. Overall, the predicted isoscapes depicted very little temporal variation.

Finally, the δ13C and δ15N isoscapes were used in conjunction with Red Snapper eye lens stable isotopes to infer movement histories of individual fish. Fish eye lenses are proteinaceous spherical structures used to focus light within the eyes of fish. As the fish grows, the eye lens grows with it by adding successive layers (laminae) to the outside of the lens. After each layer is laid down, it undergoes attenuated apoptosis and becomes metabolically inert. Therefore, the eye lens is a conserved isotopic record of the isotopic conditions encountered by the fish, wherein the center is the oldest material, and the outermost layer is the newest material. Individual laminae were peeled off the eye lens and analyzed for δ13C and δ15N values. Linear regressions between the isotopic values of laminae and eye lens diameter indicated that Red Snapper underwent trophic growth, and the equation from that regression was used to create a predicted isotopic life history (IHL) plot for a hypothetical stationary fish. Deviation scores were calculated for each fish as the summed absolute values of the differences between the measured isotopic values within the fish’s eye lens and the predicted isotopic values for the hypothetical stationary fish. Deviation scores were evaluated along with the rho values from Spearman rank correlations of the δ13C and δ15N values within the eye lens of each fish to infer if a fish had undergone movement or remained relatively stationary throughout its life. If a fish had a low deviation score and a high rho value, it was inferred to have remained relatively stationary. Overall, this analysis and evaluation depicted a high level of individual variability in Red Snapper movement histories.

The major conclusions of this dissertation are (1) that spatial δ13C variation in the Gulf of Mexico is consistent with productivity and/or basal resource dependence, and δ15N spatial variation is consistent with bioavailable nitrogen sources, (2) that, based on the statistical models, there is very little temporal variability in δ13C and δ15N isoscapes, (3) that fish eye lens stable isotopes can be used in conjunction with isoscapes to infer possible movement histories, and (4) that Red Snapper generally undergo trophic growth with a high level of individual variability in movement histories. This dissertation provides isoscapes and isoscape models that have potentially broad applications, but applications relating to fisheries management and fish ecology are probably the most apt. The results from the Red Snapper movement histories have implications for fisheries management including the need for large sample sizes for Red Snapper studies to capture the full range of behaviors.

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