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STABLE ISOTOPE DICHOTOMY IN LOGGERHEAD TURTLES REVEALS PACIFIC-ATLANTIC OCEANOGRAPHIC DIFFERENCES

By

MARIELA E. PAJUELO

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2010

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© 2010 Mariela E. Pajuelo

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To my parents, Fortunato and Ricardina

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ACKNOWLEDGMENTS

I would like to express my gratitude and appreciation to all of those who

contributed to making my project possible one way or another. So, here it goes.

First, I especially thank Dr. Jeffrey A. Seminoff who introduced me to a fascinating

technique: Stable Isotope Analysis and encouraged me to develop this project. At Pro

Delphinus in Peru, I would like to thank Joanna Alfaro-Shigueto for giving me the

opportunity to get involved in marine conservation and everything this entails in a

country such as Peru, Jeffrey C. Mangel for proof-reading and commenting everything I

wrote, and Celia Cáceres Bueno for the invaluable help during field work. Collection of

samples would not have been possible without the enthusiastic help of fishermen

Francisco “Chaval” Bernedo, Mateo Mamani, and Miguel Cuentas. Pro Delphinus team

is also responsible for making work in the least desirable conditions fun. In Peru, I also

thank IMARPE-Ilo for letting me use their facilities to process samples.

My lab members Kim Reich, Hannah Vander Zanden, and Melania López-Castro,

were all incredibly great “chicas” and helped in various aspects of my research. Other

friends at the department Carlos Manchego, Patty Zárate, Jordan Smith and Elvis

Nuñez are thanked for making my time here fun. I warmly remember my friends outside

the department, Jorge Lingan, Andrea Pino, and Oriol Peñalver for their friendship and

the many good laughs (and e-mails) shared during the time I spent working on my

research project.

My parents Ricardina and Fortunato and siblings Eli, Ethel and Edgar deserve a

lot of gratitude for their continued moral support, encouragement, and unconditional

love. I also appreciate the support Lucas Majure has given me, and I love him very

much because of it.

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And most of all, I would like to thank my advisor, Dr. Karen A. Bjorndal, and Dr.

Alan B. Bolten for their patience, guidance, invaluable knowledge provided, and

encouragement throughout these years. It was their vision, along with that of Dr.

Seminoff, that led my project to its completion.

Last, I would like to acknowledge the support of Fulbright Scholarship (Peru),

Dexter Fellowship in Tropical Conservation Biology, Lerner-Gray Grants for Marine

Research at the American Museum of Natural History, and Brian Riewald Memorial

Fund at the University of Florida for grants awarded to conduct my research.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4  

LIST OF TABLES ............................................................................................................ 7  

LIST OF FIGURES .......................................................................................................... 8  

ABSTRACT...................................................................................................................... 9

CHAPTER

1 INTRODUCTION..................................................................................................... 11  

2 MATERIALS AND METHODS ................................................................................ 17  

Data and Sample Collection.................................................................................... 17  Sample Preparation and Analysis ........................................................................... 18  Statistical Analysis................................................................................................... 20  

3 RESULTS................................................................................................................ 23  

4 DISCUSSION.......................................................................................................... 30  

Differences between Ocean Regions...................................................................... 30  Interannual Variation of Loggerhead δ13C and δ15N within Ocean Regions............ 36  Relationship between δ15N and Turtle Size............................................................. 39  

5 CONCLUSIONS...................................................................................................... 42  

LIST OF REFERENCES................................................................................................ 43  

BIOGRAPHICAL SKETCH ............................................................................................ 51  

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LIST OF TABLES

Table page 3-1 Stable isotope ratios (δ15N and δ13C) of marine organisms in the southeast

Pacific and the northeast Atlantic........................................................................ 26  

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LIST OF FIGURES

Figure page 1-1 Schematic δ15N signatures in the upper water column in the presence of

nitrogen fixation and denitrification.. ................................................................... 16  

2-1 Collection sites of samples off southern Peru in the southeast Pacific. .............. 21  

2-2 Collection sites samples in Azorean waters in the northeast Atlantic. ................ 22  

3-1 Stable isotope ratios (δ15N and δ13C) of skin samples from juvenile loggerheads Caretta caretta in the southeast Pacific and in the northeast Atlantic ................................................................................................................ 27  

3-2 Comparison of stable isotopes ratios of the food web components in the southeast Pacific and in the northeast Atlantic. .................................................. 28  

3-3 Skin δ15N vs. curved carapace length in loggerhead sea turtles Caretta caretta in the southeast Pacific and in the northeast Atlantic. ............................ 29  

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the

Requirements for the Master of Science

STABLE ISOTOPE DICHOTOMY IN LOGGERHEAD TURTLES REVEALS PACIFIC-ATLANTIC OCEANOGRAPHIC DIFFERENCES,

By

Mariela E. Pajuelo

May 2010

Chair: Karen A. Bjorndal Major: Zoology

Denitrification and nitrogen fixation processes in the marine environment have

been intensively studied, particularly how these processes affect the delta^15N

signature of inorganic nutrients and organisms at the base of the food web. The

assumption that these delta^15N differences at the base of food webs are reflected in

higher trophic level organisms, however, has been neglected. In this study, I evaluated

whether an ocean basin delta^15N dichotomy was evident in oceanic juvenile

loggerhead turtles (Caretta caretta) by analyzing their stable isotope signatures in both

the Pacific and Atlantic oceans. Skin samples from oceanic juvenile loggerheads were

collected from Peruvian waters in the southeast Pacific and from waters around the

Azores Archipelago in the northeast Atlantic and analyzed for delta^15N and delta^13C.

Results that turtles in the two ocean regions have delta^13C signatures from -16.3 to -

16.7‰ reflect the oceanic feeding behavior of these loggerhead populations. The

delta^15N signatures in Pacific loggerheads are consistently higher (17.1 ± 0.9‰) than

those of Atlantic loggerheads (7.6 ± 0.5‰). This inter-ocean difference in delta^15N

values was also observed in organisms at the base of the food web in the two study

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areas. The characteristic predominant process of the nitrogen cycle in each ocean

region, which has an effect on the nitrogen composition at the base of the food web, is

subsequently transferred to higher trophic levels. Stable isotope signatures in high

trophic level organisms, such as oceanic sea turtles, can reveal differences in

oceanographic processes.

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CHAPTER 1 INTRODUCTION

Nitrogen fixation and denitrification are the two main processes by which nitrogen

flows into and out of the ocean, respectively (Gruber & Sarmiento 1997, Codispoti et al.

2001, Brandes & Devol 2002, Capone & Knapp 2007) (Fig. 1-1). Geographical

variations in the natural abundance ratio 14N:15N (expressed as δ15N) in marine

environments have been explained by distinctive characteristics of the nitrogen cycling

in those areas. Low δ15N values in food web baseline signatures (Minagawa & Wada

1986, Carpenter et al 1997, Montoya et al. 2002) and nitrogen pools (Wada & Hattori

1976, Liu et al. 1996, Montoya et al. 2002) (Fig. 1-1A) provide evidence of nitrogen

fixation, which are observed in the East China, Sargasso, and Arabian Seas, and the

western North Atlantic (Minagawa & Wada 1986, Wada & Hattori 1991, Carpenter et al.

1997, Gruber & Sarmiento 1997, Montoya et al. 2002, Deutsch et al. 2007). On the

other hand, denitrification evidenced by high δ15N values in particulate organic matter

(Wada & Hattori 1976, Saino & Hattori 1987, Hobson et al. 1994, Huckstadt et al. 2007),

zooplankton (Wu et al. 1997), and inorganic nitrogen (Saino & Hattori 1987, Voss et al.

2001) (Fig. 1-1B) has been observed in oxygen-depleted areas in the eastern tropical

North and South Pacific, and the Arabian Sea (Wada & Hattori 1976, Saino & Hattori

1987, Gruber and Sarmiento 1997, Robinson et al. 2007, Deutsch et al. 2007). Even

though these differences in δ15N at the base of the food web in marine systems have

been addressed by various studies (references cited above), few have suggested that

these baseline differences in δ15N are conserved up the food web (Minami & Ogi 1997,

Takai et al. 2000, Hatase et al. 2002, Wallace et al. 2006).

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Stable isotope analysis is an increasingly widespread approach to elucidating the

diet, trophic status, foraging habitat and migration patterns in marine animals (Hobson

et al. 1994, Rubenstein & Hobson 2004, Revelles et al. 2007, Cherel & Hobson 2007,

Newsome et al. 2009). This technique is based on the fact that food webs present a

range of stable isotope signatures of nitrogen and carbon that depend mostly, although

not exclusively, on the isotopic baseline signature of primary producers supporting that

food web. Thus, the tissues of the consumers exhibit an enrichment over their diet of

about 3-5 ‰ for δ15N and 0-1 ‰ for δ13C per trophic level (DeNiro & Epstein 1978, 1981,

Minagawa & Wada 1984, Post 2002). However, these commonly used values can be

quite variable (Bearhop et al. 2002, Reich et al. 2008) and will depend on the organism

and tissues studied (Martínez del Rio et al. 2009). Within a biological community,

enrichment in δ15N with increasing trophic level makes it useful to infer the trophic status

of an animal (Hobson et al. 1994, Post 2002), while the relatively small enrichment of

δ13C makes it useful to infer primary sources in a trophic web. In marine ecosystems,

δ13C is also useful to determine habitat use along three gradients: lower- vs. higher-

latitude, inshore (neritic) vs. offshore (oceanic), or pelagic vs. benthic (Rau et al. 1982,

Hobson et al. 1994, Cherel & Hobson 2007).

Endangered loggerhead turtles (Caretta caretta) are considered generalist

predators among sea turtle species (Bjorndal 1997). Recent analysis of diet in oceanic

juvenile loggerheads in the central north Pacific and the northeast Atlantic revealed that

these turtles are pelagic feeders, foraging opportunistically on pelagic gastropods and

barnacles, and cnidarians (Parker et al. 2005, Frick et al. 2009). These studies utilized

stomach contents that provided valuable species information of what turtles were

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feeding on at a particular point of time. Stable isotope analysis provides a

complementary tool to address dietary questions with the advantages that it (1) requires

a small amount of tissue (e.g., epidermis, blood, epidermal scale) that can be obtained

without harming the animal, (2) represents assimilated and not just ingested prey, and

(3) quantifies diet over extended periods of time (Hobson et al. 1996), making it a great

tool to further understand the trophic ecology of loggerhead turtles. Evaluating the

trophic status of a generalist consumer inhabiting known areas of nitrogen fixation or

denitrification could provide a broader insight on how these oceanographic processes

affect higher trophic levels.

When using stable isotopes, it is important to take into account the rate of isotopic

incorporation from the diet into the consumer tissue (i.e. discrimination factor), which will

depend mainly on the growth and catabolic turnover rate of the consumer tissue

(Martínez del Rio et al. 2009). Discrimination factors may be lower and the rate of

isotopic incorporation more rapid in fast growing animals than in non-growing animals

(Martínez del Rio & Wolf 2005, Reich et al. 2008, Martínez del Rio et al. 2009).

Because tissues have different turnover rates, different tissues can reflect dietary

information at different temporal scales. High turnover rate tissues such as blood

plasma or liver reflect recent diet, while metabolically less active tissue with slower

turnover rates such as skin, bone or scute reflect diet over long-term periods

(Rubenstein & Hobson 2004). Turnover rates have been estimated for fast growing,

small juvenile loggerhead turtle tissues, with skin tissue showing relatively slow nitrogen

and carbon turnover rates of around 45 days, and a δ15N discrimination value of 1.60‰

(Reich et al. 2008).

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A recent study by Wallace et al. (2006) revealed that leatherback turtles

(Dermochelys coriacea) from the Pacific and Atlantic Oceans showed significant

differences in the nitrogen isotopic signatures between ocean regions. In the same

study, a literature review of δ15N signatures revealed a pattern in the nitrogen values

between species occupying the same trophic level between denitrification and nitrogen

fixation zones. The authors suggested that nitrogen isotopic differences reflected

denitrification and nitrogen fixation processes affecting higher trophic levels. Along with

the Wallace et al. (2006) study, others have reported similar isotopic results on

conspecific species inhabiting different oceanographic areas (Minami & Ogi 1997,

Ishibashi et al. 2000, Takai et al. 2000), but none of these conducted food web sampling

to corroborate this observation.

Building on the study of Wallace et al. (2006), here I evaluate whether the

previously described trophic dichotomy in Pacific and Atlantic Ocean regions is evident

among other higher trophic organisms and seek to validate the assumption that

differences in the baseline δ15N signatures are preserved throughout the food web.

Because I wanted to evaluate integrated diet in juvenile loggerheads (which would

reflect the diet of an opportunistic feeder and therefore their relative trophic status), I

used skin tissues that should reflect dietary signatures over an intermediate time frame.

I analyzed the stable isotope ratios (δ13C and δ15N) of juvenile loggerhead turtles, the

base of the food web and other food web components in one area of the Pacific and

Atlantic oceans with evidence of prevalence of denitrification and nitrogen fixation

processes, respectively. Also, because sampling in the Pacific occurred during a

moderate El Niño event, we evaluate if these oceanic juvenile loggerheads can reveal

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any effect of El Niño event in their isotopic signatures. Finally, I provide insights on the

trophic status of juvenile loggerhead turtles as revealed by stable isotope analysis.

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Figure 1-1 Schematic δ15N signatures in the upper water column in the presence of (a) N2 fixation and (b) denitrification. Other processes controlling the isotopic signature are also shown. Solid heavy arrows represent inputs of nitrogen in the water surface through N2 fixation or upwelled nitrates (

€

NO3- ). Medium

solid arrows represent trophic transfer of nitrogen up the web, and light solid arrows represent losses of ingested nitrogen through solid (sinking particles) and dissolved excretes. Uptake of inorganic nitrogen by primary producers is shown by dashed lines (Modified from Montoya et al. 2007).

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CHAPTER 2 MATERIALS AND METHODS

Data and Sample Collection

Loggerhead turtle data were collected off southern Peru in the southeast Pacific

(SEP) (Fig. 2-1) from December 2003 to May 2007 as part of the small-scale fishery on-

board observer program of the Peruvian non-governmental organization Pro Delphinus

(Alfaro Shigueto et al. 2008). Turtle data in the northeast Atlantic (NEA) collected from

September to October 2002 and August to November 2003 (Fig. 2-2) were obtained

during an experimental fishery project evaluating various longline hook designs and the

resultant bycatch of loggerhead turtles in the Azores (Bolten et al. 2004). Data reported

here are from skin samples of 26 and 53 turtles from the SEP and NEA, respectively.

All turtle skin samples were collected from the dorsal surface of the neck of the turtle

using a 6 mm biopsy punch or razor blade. Samples were preserved in vials with dry

NaCl (SEP) or 70% EtOH (NEA), and neither preservative was found to affect the

isotope signatures (Seminoff & Hess, unpub data, Barrow et al. 2008). Data collected

for each loggerhead sampled included geographic location (latitude/longitude), date of

capture, and curved carapace length (CCL) from the nuchal notch to the posterior-most

tip (SEP) or to the posterior marginal notch (NEA) (Bolten et al. 1999). All turtles were

released with inconel tags applied to the trailing edge of each fore flipper.

In the NEA, food web organisms (e.g., red algae, crabs, barnacles) were gently

removed from the carapace of the turtles sampled and preserved in 70% ethanol. Also,

zooplankton samples were collected during September 2008 by oblique hauling bongo-

nets (mesh size 200 um) at 50m depth, frozen and then dried at 60°C before analysis.

In the SEP, tissue samples from other food web organisms (e.g., fish, squid) were

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collected for analysis during December 2008 with 6mm biopsy punches. To

characterize isotopically the base of the food web in the SEP, particulate organic matter

(POM, considered the first trophic level in marine environments) samples were also

collected off southern Peru in December 2008 by filtration of 4-8 liters of surface

seawater through pre-combusted (500 °C, 5h) Millipore quartz fiber filters. Food web

organisms and POM were stored frozen until dried at 60°C prior to sample preparation

and analysis.

Sample Preparation and Analysis

Turtle skin samples were first washed with deionized water and cleaned with

alcohol swabs to remove epibionts or any extraneous particles. The outermost layer of

the turtle epidermis was separated from the underlying tissue and finely diced with a

scalpel blade. Tissue samples from fish and squid were thawed and a small section of

muscle was separated from the skin, which was washed in deionized water and then

homogenized. For crabs, muscle tissue was obtained from abdominal segments, and

for barnacles, whole organisms (not including the shells) were analyzed. Barnacle

tissues were soaked in 1N HCl to remove carbonates. Red algae samples were rinsed

with deionized water and small invertebrates were removed. All turtle skin, fish, squid,

crab, barnacle, algae and zooplankton samples were dried at 60°C for 24 hours. Lipids

were extracted from turtle, fish, squid, crab, and zooplankton samples using petroleum

ether in a Dionex accelerated solvent extractor. All samples, except turtle samples,

were then ground to a fine powder using a mortar and pestle. POM filter samples were

dried at 60°C for 24 hours and washed with 1N HCl to remove carbonates and dried

again for analysis.

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For stable isotope analysis, approximately 500-600 µg of each sample was

weighed and sealed in a tin capsule. Between 1200-1500 µg was loaded for red algae.

POM filters were cut into halves that were also loaded in tin capsules. Samples were

then analyzed for carbon and nitrogen stable isotope ratios by combustion in a

COSTECH ECS 4010 elemental analyzer interfaced via a Finnigan-MAT ConFlow III

device (Finnigan MAT, Breman, Germany) to a Finnigan-MAT DeltaPlus XL (Breman,

Germany) isotope ratio mass spectrometer in the Stable Isotope Geochemistry Lab at

the University of Florida, Gainesville, FL, USA.

Stable isotope ratios, that compare the ratios of heavy and light isotopes of an

element in the sample to an international standard, are reported in the conventional

δnotation:

δX = [(Rsample/Rstandard) -1] x 1000

where δX is the relative abundance of 13C or 15N in the sample expressed in parts per

thousand (‰); Rsample and Rstandard are the ratios of heavy to light isotope (13C/12C and

15N/14N) in the sample and international standard, respectively. The standard used for

13C was Vienna Pee Dee Belemnite and for 15N was atmospheric N. Working standards

L-glutamic acid USGS40 (δ13C = -26.23 and δ15N = -4.52) were calibrated monthly

against international standards and were inserted in all runs at regular intervals to

calibrate the system. The analytical accuracy of our measurements, measured as the

SD of replicates of standards, was 0.20‰ for δ13C and 0.14‰ for δ15N (n = 28 and 28,

respectively).

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Statistical Analysis

Sea turtle data were tested for normality using the Shapiro-Wilk test. Carbon and

nitrogen stable isotope signatures in turtle skin samples were compared between ocean

regions and among years within ocean regions with Student’s t-tests. For evaluation of

nitrogen and carbon stable isotope signatures among years within ocean regions, I

compared values for turtles sampled in the Pacific in austral summer (December to

March) 2003-2004 and 2006-2007. Atlantic samples were compared between samples

collected during September-October 2002 and 2003. The relationships between stable

isotope values and body size were evaluated with linear regressions. The software

program R (http://www.r-project.org/) was used to analyze stable isotope and sea turtle

data. An α level of 0.05 was used to evaluate the significance of all tests.

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Figure 2-1. Collection sites of turtle skin (open circles), particulate organic matter (filled squares), and other organisms (crosses) samples off southern Peru in the southeast Pacific.

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Figure 2-2. Collection sites of turtle skin (open triangles), red algae (filled diamonds), and other organisms (crosses) samples in Azorean waters in the northeast Atlantic.

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CHAPTER 3 RESULTS

Values of δ13C from SEP loggerhead samples ranged from -14.6 to -17.4‰, and

NEA loggerhead samples ranged from -15.7 and -17.8‰; while values for δ15N ranged

from 15.2 to 18.7‰ in SEP samples and from 6.4 to 8.7‰ in NEA samples. The δ13C

and δ15N values for loggerheads were normally distributed (Shapiro-Wilk test, p > 0.05).

Although the NEA δ13C values were almost completely encompassed by the SEP

values (86% overlap of the data), NEA δ13C values were significantly lower than those

from the SEP (mean ± SD, SEP = -16.3 ± 0.7‰, NEA = -16.7 ± 0.5‰, t = 3.7, p < 0.001,

Fig. 3-1). Also, SEP samples showed significantly higher δ15N values than those from

the NEA (mean ± SD, SEP = 17.1 ± 0.9‰, NEA = 7.6 ± 0.5‰, t = 59.3, p < 0.001, Fig.

3-1).

In the SEP, POM samples had the lowest δ15N (12.1 ± 1.2‰) and δ13C (-22.4 ±

1.7‰) values (Table 3-1, Fig. 3-2). Other oceanic organisms had δ15N values between

14.9 ± 1.6‰ (Mahi mahi Coryphaena hippurus) and 18.6 ± 0.4‰ (jumbo squid

Dosidicus gigas) and δ13C values from -16.4‰ (juvenile blue shark Prionace glauca) to -

18.0 ± 0.3‰ (flying fish Exocoetus sp.) (Table 3-1, Fig. 3-2). A similar scenario was

found in the NEA with red algae presenting the lowest δ15N (3.7 ± 0.7‰) and δ13C (-20.6

± 0.7‰) values. Other food web components ranged from 4.4 ± 0.6‰ to 9.0 ± 0.6‰ for

δ15N and -20.4 ± 0.3‰ and -18.41 ± 0.6‰ for δ13C, with lowest values found in

zooplankton and highest values found in Columbus crab Planes minutus (Table 3-1,

Fig. 3-2).

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The difference in mean δ15N values between primary producers (POM and red

algae) and loggerheads was 4.9‰ and 4.0‰, while δ13C varied by 6.2‰ and 3.9‰, in

the SEP and NEA, respectively. It is important to note that red algae do not reflect the

base of the food web in the open ocean (unlike POM) and is only used as a proxy for

baseline isotopic signature. However, I might expect POM isotopic nitrogen and carbon

values in the NEA to be lower than those of red algae (as I will explain below).

Additionally, POM values are used here as approximates of what the baseline

signatures in the SEP are, since they were sampled in a different year from those of

SEP turtle samples. The difference of ~0.9‰ (4.9‰ – 4.0‰) between variations in δ15N

for primary producers and loggerheads is lower than the 15N discrimination value per

trophic level for juvenile loggerheads (1.60‰, Reich et al. 2008); thus, I estimate that

the relative trophic position in both loggerhead populations should be similar.

Turtles sampled in the SEP ranged from 35.9 to 81.4 cm curved carapace length

(CCL). Size range for loggerheads in the area is consistent with the size range

previously reported by Alfaro Shigueto et al. (2008). This size distribution corresponds

to immature individuals according to criteria cited by Limpus & Limpus (2003). I found a

significant positive correlation between δ15N and body size (R2 = 0.26, df = 24, p =

0.008, Fig. 3-3). However, no statistically significant relationship was found for δ13C and

turtle size (p = 0.354). Turtles in the NEA ranged in size from 33.1 to 80.3 cm CCL.

According to the minimum adult size criteria by Bjorndal et al. (2000), all of our turtles

correspond to juvenile individuals. Similar to loggerheads in the SEP, a significant

positive (although weak) relationship was found between δ15N and body size (R2 = 0.10,

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df = 51, p = 0.023, Fig. 3-3), and the relationship found between δ13C and body size was

not significant (p = 0.110).

Significant annual variation was found in loggerheads for δ13C in the SEP and for

both δ13C and δ15N in the NEA. In the SEP, δ13C values for samples collected during

2006-07 were significantly depleted relative to samples gathered in 2003-04 (2003-04 =

-15.8 ± 0.7‰, 2006-07 = -16.5 ± 0.4‰, t = 2.9, p = 0.004), but data showed a 67%

overlap. In the NEA, 2002 had significantly higher δ15N values than 2003 (mean ± SD,

2002 = 8.0 ± 0.4‰, 2003=7.5 ± 0.5‰, t = 3.6, p < 0.001) but with 84% overlap of data.

Carbon (δ13C) values were more depleted in 2003 (mean ± SD, 2002 = -16.2 ± 0.3‰,

2003 = -17.0 ± 0.4‰, t = 7.2, p < 0.001).

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Table 3-1. Stable isotope ratios (δ15N and δ13C) of marine organisms in the southeast Pacific and the northeast Atlantic. Values are mean ± SD

n δ15N(‰) δ13C(‰) Source SOUTHEAST PACIFIC POM N/A 12.1 ± 1.2 -22.4 ± 1.7 this study Mollusca Jumbo squid 3 18.6 ± 0.4 -17.6 ± 0.4 this study Chondrychthyes Blue shark 1 18.2 -16.4 this study Osteichthyes Anchovy 2 14.2 ± 0.9 -16.7 ± 0.1 Catenazzi & Donelly 2008 Mahi mahi 2 14.9 ± 1.6 -16.5 ± 0.1 this study Flying fish 3 16.9 ± 0.9 -18.0 ± 0.2 this study Mackerel 2 15.8 ± 1.4 -17.0 ± 0.8 this study Reptilia Loggerhead turtle 26 17.1 ± 0.9 16.3 ± 0.7 this study Leatherback turtle 17 15.4 ± 1.8 -19.1 ± 0.7 Wallace et al. 2006 Mammalia Sea lion 6 17.4 ± 0.9 -15.1 ± 0.4 Catenazzi & Donelly 2008 Sperm whale 12 18.6 to 20.3 -16.0 to -14.7 Marcoux et al. 2007 NORTHEAST ATLANTIC Red algae 5 3.7 ± 0.7 -20.6 ± 0.7 this study Zooplankton 3 4.4 ± 0.6 -20.4 ± 0.2 this study Crustacea Acorn barnacle 3 7.6 ± 0.1 -19.0 ± 0.6 this study Gooseneck barnacle 4 5.7 ± 1.6 -20.0 ± 0.5 this study Pedunculate barnacle 3 5.7 ± 0.7 -19.1 ± 0.6 this study Columbus crab 5 9.0 ± 0.6 -18.4 ± 0.5 this study Reptilia Loggerhead turtle 53 7.6 ± 0.5 -16.7 ± 0.5 this study Leatherback turtle

9.8 ± 1.5*

10.3** 10.4+

-19.4 ± 1.0* -19.6 ** -19.5+

Wallace et al. 2006 Maros et al. 2006 Caut et al. 2008

Aves Cory’s shearwater 35 13.2 -16.2 Ramos et al. 2009 Mammalia Sperm whale 1 13.2 -11.8 Mendes et al. 2007

Leatherback turtle values are from nesting females in *St Croix, Virgin Islands and **+French Guiana in the Atlantic.

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Figure 3-1. Stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) of skin samples

from juvenile loggerheads Caretta caretta off southern Peru in the southeast Pacific (open circles) and in Azorean waters in the northeast Atlantic (open triangles). POM values off southern Peru, and red algae and zooplankton samples from Azorean waters are also shown.

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Figure 3-2. Comparison of stable isotopes ratios of the food web components in the southeast Pacific (SEP; open circles) and in the northeast Atlantic (NEA; solid triangles). All SEP values are from waters off Peru except for those of leatherback sea turtles. Leatherback isotope values are from Wallace et al. (2006), those of anchovy and sea lion are from Catenazzi & Donnelly (2008), and those of sperm whale are from Marcoux et al. (2007). In the NEA, leatherback turtle isotopic values are from post-nesting females in St Croix, Virgin Islands in the Atlantic (Wallace et al. 2006). Values of sperm whale are from Mendes et al. (2007) and those from Cory’s shearwater are from Ramos et al. (2009). All values are mean ± range. 1. POM (SEP) or Red algae (NEA), 2. Zooplankton, 3. Gooseneck barnacle Lepas anatifera, 4. Pedunculate barnacle Conchoderma virgatum, 5. Acorn barnacle, 6. Loggerhead turtle Caretta Caretta, 7. Columbus crab Planes minutus, 8. Leatherback turtle Dermochelys coriacea, 9. Cory’s shearwater Calonectris diomedea borealis, 10. Sperm whale Physeter macrocephalus, 11. Anchovy Engraulis ringens, 12. Mahi mahi Coryphaena hippurus., 13. Chub mackerel Scomber japonicus, 14. Flying fish Exocoetus sp., 15. Sea lion Otaria flavescens, 16. Blue shark Prionace glauca, 17. Jumbo squid Dosidicus gigas.

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Figure 3-3. Skin δ15N vs. curved carapace length in loggerhead sea turtles Caretta caretta off (a) southern Peru in the southeast Pacific and (b) the Azores in the northeast Atlantic. Both relationships are significant, see text.

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CHAPTER 4 DISCUSSION

Differences between Ocean Regions

Based on satellite telemetry studies, juvenile loggerheads in the NEA feed in

oceanic, epipelagic zones (Santos et al. 2008). SEP juvenile loggerheads analyzed in

this study were all captured in the oceanic zone (Alfaro Shigueto et al. 2008). I would

expect to find similar δ13C in both SEP and NEA loggerhead populations reflecting their

oceanic feeding behavior. Although δ13C values were significantly depleted for NEA

loggerheads, carbon value ranges for both populations greatly overlap (86%) confirming

their oceanic and pelagic feeding strategy (Fig. 3-1). Overall, lower δ13C in NEA

loggerheads may be explained by the latitudinal variation observed in marine

phytoplankton δ13C, which decreases from the equatorial zones toward the polar

regions (Rau et al. 1982, Goericke & Fry 1994). This baseline latitudinal variation in

δ13C has been reflected in higher consumers such as squids and penguins (Takai et al.

2000, Cherel & Hobson 2007). However, this decline is more pronounced in the

southern versus northern hemisphere (-0.14 and -0.015‰ per degree in the southern

and northern hemisphere, respectively) and other factors may be playing a role for the

significant variation in carbon signatures. In our study, red algae δ13C values in the

Azores (38°N) were more enriched than values from POM off southern Peru (17°S).

Red algae in the NEA do not necessarily reflect the main primary producer in the open

ocean (as phytoplankton does) but serve as a proxy for a baseline isotopic signature in

Azorean waters. Variations in the thickness of the diffusive boundary layer between

phytoplankton (POM) and macroalgae (red algae) affect the diffusion of carbon during

carbon uptake (France 1995). This may translate into more depleted δ13C signatures

31

for phytoplankton compared to macroalgae because heavy isotopes (13C) are easily

discriminated through the thin boundary layer of the former. Thus, expected δ13C

values of POM from the Azores should be depleted relative to red algae which may

explain the more depleted δ13C values in NEA loggerheads.

Our study shows that δ15N values in SEP loggerheads were 9.4‰ higher than

NEA loggerhead values (Fig. 3-1). While δ15N is useful to determine the relative trophic

position of organisms within a community (Post 2002, Newsome et al. 2007), species

with known relative trophic status allow for comparison of their nitrogen signatures

between ecosystems. Large differences in the nitrogen signature of organisms

suggests differences in trophic levels, but diet studies in oceanic juvenile loggerhead

populations have shown that they are opportunistic predators, and as a result, both

populations should occupy a similar trophic level. For example, in Azorean waters and

the central north Pacific oceanic juvenile loggerhead turtles feed on various pelagic

organisms such as jellyfish, sea slugs, sea snails and pelagic barnacles (Parker et al.

2005, Frick et al. 2009), which are prey that generally feed at lower trophic levels.

Additionally, similar δ15N variation between primary producers and loggerhead

populations reported in this study suggest that they occupy a similar relative trophic

level. Therefore, I attribute this significant δ15N difference between SEP and NEA

loggerhead populations to different δ15N values at the base of the food chain.

Nitrogen isotopic signatures at the base of the food web are affected by

characteristic nitrogen cycle dynamics in the marine environment (Saino & Hattori 1987,

Carpenter et al. 1997, Montoya et al. 2002). In the tropical North Atlantic, nitrogen

fixation is the main process by which nitrogen is assimilated (Carpenter et al. 1997,

32

Montoya et al. 2002). However, few studies have addressed the characteristics of the

nitrogen dynamics in the northeast Atlantic, specifically in the waters of the Azores

region. Bourbonnais et al. (2009) analyzed the δ15N distribution in the water column in

the Azores Front region. They found clear evidence for nitrogen fixation and reported

dissolved organic nitrogen δ15N values of approximately 2.6‰. Because the isotope

fractionation during nitrogen fixation is small and atmospheric N is near 0‰, slightly

depleted values of δ15N have been reported for organic matter (Fig. 1a). Thus,

expected δ15N values of POM in the Azores should be close to 0‰ if nitrogen were

mainly supplied by nitrogen fixation (Minagawa & Wada 1986, Carpenter et al 1997,

Montoya et al. 2002) or relatively low if other sources of nitrogen were available (e.g.,

upwelled nitrates, Montoya et al. 2002). The δ15N of red algae was 3.7‰, which may

reflect the uptake of other sources of nitrogen besides that from nitrogen fixation.

However, as mentioned above, red algae were used as a proxy for isotopic baseline

signature in the Azores; expected POM δ15N may be lower than that from the

macroalgae. Differences in nitrogen uptake rates due to physiological characteristics of

phytoplankton and macroalgae (Montoya & McCarthy 1995, Needoba et al. 2003, De

Brabandere et al. 2007) may render significantly divergent δ15N signatures (Hobson &

Welch 1992, Bode et al. 2006). Phytoplankton discriminates against the heavy nitrogen

isotope (15N) during nitrate uptake, with an average isotopic discrimination value of ~

5‰ (Montoya & McCarthy 1995, Needoba et al 2003) while discrimination values for

macrophytes are suggested to be lower (DeBrabandere et al. 2007). This translates to

more depleted δ15N values in phytoplankton than in macroalgae. Moreover, the δ15N of

zooplankton, one trophic step above POM, in our study was 4.4‰ (Fig. 3-1) which is

33

similar to values reported for zooplankton in the nitrogen fixation tropical North Atlantic

(Montoya et al. 2002).

The SEP is characterized by a shallow oxygen-depleted water layer (~50-500 m)

off Peru, where high rates of denitrification occur (Tarazona et al. 2003, Robinson et al.

2007, including autotrophic denitrification: Ward et al. 2009). As nitrate is consumed

during denitrification, lighter isotopes of nitrogen are preferentially utilized, leaving

residual nitrate pools heavily enriched in 15N. The resultant high δ15N in the subsurface

denitrification layer is transmitted to surface waters through upwelling (Saino & Hattori

1987, Wada & Hattori 1991, Robinson et al. 2007). The constant supply of 15N enriched

nitrates in the surface ocean are later assimilated by phytoplankton (Fig. 1b) which in

turn show high δ15N signatures, as reflected in our POM samples (Fig. 3-1).

Deutsch et al. (2007) recently evaluated the rates of nitrogen fixation in the marine

environment through a novel analysis of ocean surface nutrients. They found that the

highest rates of nitrogen fixation are geographically coupled with that of denitrification.

In other words, the highest rates of nitrogen fixation are found in the Eastern Tropical

Pacific (ETP - where highest rates of denitrification occur) and not in the tropical North

Atlantic as previously thought. However, net rates of denitrification in the ETP are much

higher than fixation so that they obscure the isotopic evidence of nitrogen fixation, as

reflected in the high values of δ15N in our POM samples. It is not certain if, at a much

finer scale, areas of nitrogen fixation in the ETP overlap with that of denitrification. It is

assumed that the former areas are close to these high denitrification, high nutrient

waters, but whether there could be an overlap or not has not yet been revealed

(Deutsch et al. 2007).

34

The δ15N dichotomy revealed in loggerhead populations can be therefore

explained by the variations in nitrogen signatures at the base of the food web. To

further assess the effect of highest rates of nitrogen fixation and denitrification on the

food web, I compiled δ15N and δ13C values in primary producers and other organisms of

the food web in the SEP and in the NEA (Fig. 3-2). Isotopic signatures in the food web

of the SEP show that high nitrogen baseline values – driven by denitrification – are

conserved through several trophic levels. A similar pattern is observed in the NEA,

where low δ15N in red algae and especially zooplankton – due to nitrogen fixation – give

the food web overall low δ15N values when compared to the isotopic values in the SEP

food web.

Trophic structure analysis was not performed due to the limited number of

individuals and/or species sampled, different sampling dates, and/or the lack of predator

– prey related signatures (e.g., those of loggerheads and potential prey). However,

organisms analyzed in this study, as well as the collection of literature isotopic

signatures of organisms off southern Peru and the Azores, allows for a first insight into

the isotopic composition of the food web structure off the SEP and the NEA. In the

SEP, average δ15N signature of anchovy Engraulis ringens is low which reflects its diet

preference in the area of large zooplankton (Espinoza & Bertrand 2008). The δ15N

increases in larger fishes such as mackerel Scomber japonicus and flying fish

Exocoetus spp. Also as expected, higher trophic level species such as sea lion Otaria

flavescens, sperm whale Physeter macrocephalus, blue shark Prionace glauca and

jumbo squid Dosidicus gigas show much higher δ15N signatures (Fig. 3-2). A similar

pattern is observed in the Azores, where nitrogen signatures in filter feeding organisms

35

such as pedunculate barnacles (Conchoderma virgatum and Lepas anatifera) show

higher values than those from zooplankton but lower than those from crabs or

loggerhead turtles. Highest δ15N values are presented by sperm whale Physeter

macrocephalus and Cory’s shearwater Calonectris diomedea borealis which reflects

their diet preference of mainly cephalopods and fish, respectively (Clarke et al. 1993,

Granadeiro et al. 1998).

The difference in δ15N signatures between Pacific and Atlantic loggerheads found

in this study was higher than that reported by Wallace et al. (2006) between

leatherbacks in the same ocean regions (9.5‰ and 5.6‰ for loggerheads and

leatherbacks, respectively, Fig. 3-2). In fact, when comparing δ15N values between

species, loggerhead δ15N values are higher than leatherback δ15N values in the Pacific

(mean ± SD, loggerheads = 17.1 ± 0.9‰; leatherbacks = 15.4 ± 1.8‰), but lower in the

Atlantic populations (mean ± SD, loggerheads = 7.6 ± 0.5‰; leatherbacks = 9.8 ±

1.5‰). The specialist feeding behavior of leatherbacks on jellyfish and the generalist

feeding strategy of loggerheads, with commonly known intermediate and higher trophic

status respectively, allows the comparison of nitrogen signatures between these two

species. The higher δ15N in Atlantic leatherbacks than in Atlantic loggerheads (also

reported by Maros et al. 2006 and Caut et al. 2008) may be reflecting different oceanic

regions utilized by these species. I suggest that nesting leatherbacks from St. Croix

may be foraging in areas with low or no nitrogen fixation activity, where baseline

signatures δ15N are not close to zero. On the other hand, the δ15N values for

loggerheads and leatherbacks in the Pacific would be reflecting the feeding strategies of

these turtles revealed by conventional studies (Bjorndal 1997, Parker et al. 2005, Frick

36

et al. 2009) considering they were foraging in the same feeding ground or at least in

waters with similar baseline 15N. Most post-nesting leatherback turtles from the Pacific

coast of Costa Rica migrate to oceanic foraging areas in the South Pacific gyre

characterized by low productivity (Shillinger et al. 2008). However, early reports on

eastern Pacific leatherback migration and current data on bycatch in fisheries off Peru

reveal that these leatherbacks also utilize high-productivity waters off Peru (Eckert &

Sarti 1997, Alfaro-Shigueto et al. 2007).

Data compiled by Suryan et al. (2009) across a wide range of trophic levels in

several oceanic regions suggest that environmental forces on resource availability drive

variations in the life history of conspecifics. To what extent the differences in energy

flow in our study areas (through prevalence of nitrogen fixation in one and denitrification

in the other), along with other climatic factors affecting nutrient availability, may shape

the biology of these juvenile loggerhead populations is still poorly understood.

Interannual Variation of Loggerhead δ13C and δ15N within Ocean Regions

The geographic variability observed in δ15N of primary producers and POM due to

distinct marine nitrogen cycles may be affected by oceanographic/atmospheric changes

(Wada & Hattori 1991). In particular, the SEP experiences El Niño-Southern Oscillation

(ENSO) cycles with its warm El Niño (EN) phase. The main biological consequences of

EN events in the SEP (particularly off Peru) are linked to the decline of primary

productivity which produces a chain reaction that ultimately affects survival,

reproduction, and distribution of species at higher trophic levels (reviewed in Wang &

Fiedler 2006). Some of our samples from the SEP were collected during a moderate

EN event (season 2006-2007; n = 13) which allowed for comparison with a normal

37

condition season (2003-2004; n = 9). There is significant variability between seasons

2003-04 and 2006-07, as evidenced by the slightly more depleted values for δ13C in

2006-07. However, no significant effect of EN event was found on the δ15N values in

loggerheads as expected. Because the thermocline and oxygen-depleted layer

deepens and the upwellings cease during EN events, which disrupt the transport of

nutrient-rich waters (including 15N enriched nitrate) to the surface, primary production

declines significantly (Tarazona et al. 2003). Hence, I had expected to find significantly

reduced δ15N values in the 2006-07 loggerhead turtles. The EN event of 2006-07 may

not have had a strong effect on the δ15N, as a result of its moderate magnitude. In fact,

after the strong 1997-1998 EN event, favorable conditions for upwelling of highly

productive waters have not been affected by recent moderate EN events (Marzloff et al.

2009). On the other hand, due to the appearance of warmer water masses, EN events

in the SEP are also characterized by a shift in phytoplankton composition from mainly

centric diatoms to dinoflagellates (Sanchez et al. 2000, Tarazona et al. 2003). This shift

could entail a δ13C variation in the primary producers with further effect up the food web,

and it could explain the depleted δ13C values observed in the 2006-07 loggerheads. A

δ13C depletion in zooplankton has been observed in waters in which fast-growing centric

diatom production was low and presence of slow-growing dinoflagellates was high (Fry

& Wainwright 1991). Popp et al. (1998) suggested that phytoplankton cell geometry, as

well as growth rate, had an effect on their δ13C signature. Phytoplankton with higher

surface-area-to-volume ratios (e.g., pennate diatoms, dinoflagellates) have higher

isotopic fractionation values associated with photosynthesis and thus more depleted

38

δ13C, because photosynthesis discriminates against 13C. Further investigation of

variation of isotopic signatures should be conducted with stronger ENSO effects.

I found interannual variation in the δ15N and δ13C of loggerheads in the NEA,

although the differences were relatively small in both cases. The waters off the Azores

are influenced by the large-scale North Atlantic Oscillation (NAO) with reported effects

on abundance, biomass, and distribution of marine fauna due to variation in availability

of food resources (Drinkwater et al. 2003). The interannual variation observed in the

isotopic signatures in loggerheads, however, cannot be explored in relation to NAO, as

this is a long-term event for which one would need a larger data set to analyze. When

accounting for other factors that may explain this significant interannual variation in the

isotopic signatures such as turtle geographic position or sea surface temperature, I did

not find any correlation (data not presented).

Variability of δ15N in organisms can be a result of changes in their trophic positions

or changes in the baseline δ15N signatures. I lack baseline signatures in the years when

turtles in the NEA were sampled. However, assuming baseline nitrogen signatures did

not differ between years, I may suggest that 2002 loggerheads presented a relative

higher trophic level than 2003 loggerheads. Marked seasonality in primary production

with subsequent effects on isotopic signatures is expected in temperate and arctic

systems than in tropical and subtropical systems (Tamelander et al. 2009).

Nevertheless, Azorean waters are characterized by a high activity of eddies (Le Traon &

De Mey 1994). These oceanographic phenomena have known effects on higher trophic

levels through enhancement of primary production and biomass (Falkowski et al. 1991,

Oschlies & Garçon 1998). Satellite transmitter studies conducted by M. Santos

39

(unpublished data) suggest that the foraging behavior of loggerheads in Azorean waters

is related to eddies. Indeed, because of the opportunistic feeding behavior by

loggerheads (Frick et al. 2009), enhancement of productivity due to eddies may allow

them to consume a great variety of organisms, including several of higher trophic levels

(which will translate in higher loggerhead δ15N). Additionally, eddies may also change

the phytoplankton composition (Lochte & Pfannkuche 1987, Vaillancourt et al. 2003)

which in return will have an effect on δ13C signature (Fry & Wainwright, 1991).

However, although the nature and frequency of eddies in Azorean waters have been

investigated (Mouriño et al. 2003), their effect on the marine fauna has rarely been

studied (Huskin et al. 2001) and thus, I can only speculate on their effect for loggerhead

foraging ecology and isotopic signatures reported here. Further compound-specific

nitrogen isotope analysis of loggerhead tissues could help elucidate whether the

differences in δ15N are due to metabolic and trophic-level relationships or changes in

isotopic composition at the base of the food web without the necessity of food web base

sampling (Popp et al. 2007).

Relationship between δ15N and Turtle Size

Variability of δ15N in organisms within an ecosystem generally suggests changes

in trophic levels. However, it may also reflect differences in the nitrogen isotopic

composition at the base of the food web due to temporal variation of the δ15N of

nutrients. These need to be long term variations to have an effect on the isotopic

composition of higher trophic level or larger animals with low turnover rates for nitrogen

(Montoya et al. 2007). Additionally, within ecosystem spatial variation of the isotopic

composition of nutrients has been observed in deeper waters (Saino & Hattori 1987,

40

Montoya et al. 2002). Microbial and zooplankton consumption generate 15N-enriched

residual matter (Saino & Hattori 1987) that becomes the primary food source for deep-

water organisms. Thus, mesopelagic organisms may have overall higher δ15N values

(Rau et al. 1989), if one assumes that baseline signatures throughout the water column

are conserved up higher trophic levels (Graham et al. 2007).

I lack baseline δ15N signatures in the years when turtle samples were collected

and/or information on prey items consumed by these turtles to better determine whether

the δ15N differences observed are due to trophic change or temporal variation of

baseline signatures. However, the increase of loggerhead δ15N with size may be based

on three facts: turtle (1) growth rate, (2) gape size, and/or (3) feeding on mesopelagic

prey. Rapidly growing smaller juveniles (with higher growing rates) may show a lower

nitrogen discrimination value than larger juveniles (Martínez del Rio et al. 2005, Reich et

al. 2008). This translates to lower δ15N values in smaller juveniles even if they are

feeding on the same diet as larger juveniles. Nevertheless, larger turtles could be

feeding on higher trophic level organisms. Factors such as gape size may be playing

an important role in determining access to greater diversity and/or size of prey items

accessible for larger turtles. Allometry in the head of loggerhead turtles reported by

Kamezaki & Matsui (1997) may allow turtles to feed on bigger pelagic items of higher

trophic level. A higher trophic level in larger turtles has been already observed in

juvenile loggerheads in the Atlantic that move from oceanic to developmental neritic

zones where they feed mainly on demersal prey items (Bolten 2003). However, juvenile

loggerheads in the SEP and NEA inhabit exclusively oceanic waters (>200 m depth,

Alfaro Shigueto et al. 2008, Santos et al. 2008) where benthic feeding is not likely to

41

occur. Additionally, stomach studies in oceanic juveniles do not seem to support a

higher trophic level for larger turtles. Even though Frick et al. (2009) found a greater

diversity of prey species in the stomachs of larger loggerheads in Azorean waters, they

argued that this could be due to the larger volume of stomach contents in larger turtles

rather than a more diverse diet. Moreover, Parker et al. (2005) found that there was no

relation between the volume of stomach content or size of the prey ingested and the

size of the turtles (35-70 cm CCL) in the central north Pacific.

The third explanation for the enriched loggerhead δ15N in larger turtles may be

related to the consumption of mesopelagic organisms with enriched δ15N values.

Because POM δ15N increases with depth (Saino & Hattori 1987, Montoya et al. 2002),

feeding on mesopelagic prey would increase the δ15N in turtles compared to those with

an epipelagic feeding behavior. Mesopelagic prey items have been found in stomach

contents in oceanic loggerheads in the central north Pacific (Parker et al. 2005). This

feeding behavior has been also observed through satellite transmitter studies by

showing that oceanic loggerheads can dive and actively forage up to 100 m depth

(Polovina et al. 2003). In any case, stable isotopes generally cannot pinpoint the

specific diet items consumed and therefore, knowledge and sampling of potential prey

items is required to accurately explain trophic changes in loggerheads.

42

CHAPTER 5 CONCLUSIONS

In this study, loggerhead turtles revealed the ocean region differences in δ15N that

were also found in leatherback turtles, while δ13C were similar between turtle

populations, confirming their oceanic foraging behavior. The dichotomy in the δ15N at

the base of the food web and other organisms within the SEP and NEA informs us that

this difference in the baseline nitrogen signature is conserved through higher trophic

levels, thus reflecting characteristics of the nitrogen cycling regime in each ocean

region. Stable isotopes of carbon and nitrogen in higher trophic level organisms, such

as sea turtles, present the potential to elucidate changing oceanographic conditions, but

systematic and long-term sampling is needed. Also, the fact that both denitrification and

nitrogen fixation occur at highest rates in the ETP lead us to formulate some questions:

is this pattern observed in loggerheads and the food web of the SEP due to

denitrification alone? To what extent does denitrification and not nitrogen fixation affect

δ15N signatures? Can changing oceanographic conditions such as EN events with

effects on the extent of denitrification rates reveal differences in the nitrogen cycle and

can this be reflected in higher organisms? Additional compound-specific isotopic

analyses may explain variations in nitrogen source and hence help reveal changing

oceanographic conditions.

43

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BIOGRAPHICAL SKETCH

Mariela Pajuelo was born in Lima, Peru. She is one of four children of Fortunato

Pajuelo Matos and Ricardina Rubina Salazar. She attended Universidad Nacional

Mayor de San Marcos in Lima, Peru, and obtained her bachelor’s degree in biology with

emphasis on hydrobiology and fisheries science. Mariela joined Peruvian, non-

governmental organization Pro Delphinus in 2004 and has been involved in marine

conservation ever since. She was assigned her first sea turtle project in 2005, closely

working with fishermen, while testing new mitigation measures to avoid incidental

capture of sea turtles. She joined the Department of Zoology at the University of

Florida—Gainesville in 2007 and started working toward her master’s degree under the

supervision of Dr. Karen A. Bjorndal, looking at the trophic ecology of oceanic juvenile

loggerhead turtles. She completed her Master of Science degree in 2010.