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Blue Crab Residency and Migration in the Mobile Bay Estuary: a Stable Isotope Study Investigating Connectivity

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Blue Crab Residency and Migration in the Mobile Bay Estuary: a Stable Isotope Study Investigating Connectivity BLUE CRAB RESIDENCY AND MIGRATION IN THE MOBILE BAY ESTUARY: A STABLE ISOTOPE STUDY INVESTIGATING CONNECTIVITY by ANTHONY JAMES VEDRAL BEHZAD MORTAZAVI, COMMITTEE CHAIR ROBERT H. FINDLAY JOHN F. VALENTINE A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Biological Sciences in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2012 Copyright Anthony James Vedral 2012 ALL RIGHTS RESERVED ABSTRACT The blue crab (Callinectes sapidus) is an important commercial species throughout Gulf of Mexico. We used carbon and nitrogen stable isotopes from fast and slow turnover tissues to investigate residency and migration of blue crabs in Mobile Bay. A laboratory diet switch experiment was conducted to estimate tissue turnover. By day 83 of the experiment, hepatopancreas tissue turnover averaged 94%, while muscle turnover averaged 43%. Results confirmed that hepatopancreas and muscle tissues are indicators of recent and past diets, respectively. Therefore, these two tissue types were sampled from individual crabs from the Delta, mid-bay (Fowl River), and coastal sites to investigate residency. Average divergence in δ13C values between the two tissues from crabs in the delta (-0.41‰) and Fowl River (-0.31‰) was small, while for crabs in the coastal sites such as Fort Morgan featured a large average divergence (2.39‰). The convergence of hepatopancreas and muscle tissues to similar δ13C values are indicative of residency, while a large divergence between the tissues is characteristic of migratory crabs. Additionally, we found that the Fowl River site is a hot spot for female crabs that delay their spawning migration to coastal waters. Blue crabs and other migratory species link the food webs in the delta and Gulf of Mexico. A greater understanding the role of migratory species as agents of connectivity is critical for fisheries management in response to climate and human induced changes. ii LIST OF ABBREVIATIONS AND SYMBOLS 12C carbon isotope with an atomic mass of 12 13C carbon isotope with an atomic mass of 13 14N nitrogen isotope with an atomic mass of 14 15N nitrogen isotope with an atomic mass of 15 C3 vegetation using the C3 photosynthetic pathway C4 vegetation using the C4 photosynthetic pathway cm centimeters c:n carbon to nitrogen ratio °C degrees Celsius δ The “delta” or isotopic value of the ratios of each isotope concentration in a sample compared to the standard DISL Dauphin Island Sea Lab = equal to g grams h-m hepatopancreas minus muscle km kilometers < less than L liters m meters iii mg milligrams mL milliliters mm millimeters n sample size p probability of an outcome under the null hypothesis as or more extreme than the observed value % percent ‰ per mil: unit delta values are reported in rpm revolutions per minute s seconds SD standard deviation psu practical salinity units iv ACKNOWLEDGMENTS This work was made possible through the excellent advice and insights from my advisor, Dr. Behzad Mortazavi, as well as my committee members Dr. Robert Findlay and Dr. John Valentine. Assistance with statistical analysis was generously provided by Dr. Christina Staudhammer. I would like to thank Rebecca Bernard and Agota Horel for their assistance in maintaining crabs during the feeding experiment as well as processing of samples. Additionally, thank you to the Valentine lab, Eric Sparks, and Marshall Johnson for their help in the field, as well as Tech Support at the Dauphin Island Sea Lab for their relentless work to help manage water conditions in my mesocosms. Finally, I would like to thank the University of Alabama, the Dauphin Island Sea Lab, and the Northern Gulf Institute. v CONTENTS ABSTRACT...................................................................................................... ii LIST OF ABBREVIATIONS AND SYMBOLS....................................................... iii ACKNOWLEDGMENTS................................................................................... v LIST OF FIGURES............................................................................................ vii 1. INTRODUCTION......................................................................................... 1 2. MATERIALS AND METHODS...................................................................... 9 3. RESULTS..................................................................................................... 13 4. DISCUSSION............................................................................................... 18 REFERENCES.................................................................................................. 35 APPENDIX...................................................................................................... 51 vi LIST OF FIGURES Figure 1. Study sites………………………………………………….............................................. 41 Figure 2. Lipid extraction………………………………………………………………………................. 42 Figure 3. Diet switch δ15N and δ13C values………………………………………...................... 43 Figure 4. Diet switch turnover…………………………………………………………….................... 44 Figure 5. Diet switch hepatopancreas-muscle (h-m) divergence….………………………. 45 Figure 6. Crab carapace length…………………………………………………………..................... 46 Figure 7. Site average δ15N and δ13C values………………………………………..................... 47 Figure 8. δ13C hepatopancreas-muscle (h-m) divergence...…………………................... 48 Figure 9. δ15N hepatopancreas-muscle (h-m) divergence ……….……......................... 50 vii INTRODUCTION The complexity of a deltaic food web results from nutritional inputs from in-situ production, marine, and terrestrial sources (Peterson et al. 1985, Deegan & Garritt 1997, Goecker et al. 2009). Terrestrial detritus, phytoplankton, and benthic algae are all key contributors to production within estuarine food webs (Peterson & Howarth 1987, Chanton & Lewis 2002). The proximity of a region within an estuary to terrestrial sources as well as marsh estuary exchange rates can influence the reliance of the food web on terrestrially derived material. During times of high river flow, a greater proportion of terrestrial material will enter the estuary (Abrantes & Sheaves 2010). As river flow decreases, marine waters will infiltrate further into the estuary. As a result, seasonal variation in river flow can alter the influx of terrestrial or marine based organic matter within such environments (Cifuentes 1991, Chanton & Lewis 2002). Additionally, migratory organisms can transport organic matter from the deltaic to the marine food web, or vice versa, throughout their life histories. These inputs can aid in fueling the food web in the case of prey species, or act as a predatory force if the migrating species are of a relatively higher trophic level. Such migrating organisms provide connectivity between food webs that are spatially and physically separated (Beger 2010). The life histories of many fish and invertebrate species encompass both the estuarine and marine environments. For many crustaceans, marshes and seagrass beds of upper estuaries act as a crucial nursery area for larval and juvenile development (Heck et al. 2001). 1 Shelter from predation and increased food availability creates an optimal environment for growth. Upon maturation, many of these animals migrate to offshore spawning areas where they aggregate in large numbers. Specific temperature and salinity ranges such as those found in certain coastal and offshore waters have been demonstrated to increase egg and larval survival (Holt et al. 1981) for estuarine species. Recruitment of larval stages back into the upper estuary completes the typical life history of estuarine species. The concept of connectivity asserts that almost all habitats (even those that seem isolated) are linked with other food webs through a multitude of abiotic vectors such as wind and water, as well as biotic vectors of mobile organisms feeding in one environment and moving into another (Polis et al. 1997). Connectivity occurs throughout a multitude of habitats, vectors, and scales, creating an intricate system of continuity among food webs (Sheaves 2009). Connectivity between aquatic habitats occurs abiotically through upwelling, sinking of organic material, currents, tides, and eddy-diffusion, while water and land habitats are connected through terrestrial runoff, allochthonous detritus, and localized flooding (Polis et al. 1997). On land, connectivity occurs through vectors such as wind and detritus transport through melted water and precipitation (Polis et al. 1997). Additionally, mobile organisms provide vectors of connectivity throughout each of these interfaces. The degree of connectivity between food webs varies through exchange rates, the ratio of interface perimeter to the area of total habitat (Polis & Hurd 1996), and mobility of organisms (Polis et al. 1997). Migrations play a key role in the life histories of many animals, with the most well- known including aquatic organisms such as salmon, whales, and waterfowl. Migrations can 2 occur over ranges of thousands of kilometers in the case of some birds (Henningsson & Alerstam 2005), whereas the daily migration of smaller creatures such as zooplankton can occur on the scale of meters (Zaret & Suffern 1976). Motives for migration vary among organisms. Food availability and weather are considered vital reasons for whale and many bird migrations (Boyle & Conway 2007). Fishes such as salmon return to the freshwater streams of which they were born to spawn (Miller
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