Seasonal δ13C and δ15N isoscapes of fish populations along a continental shelf trophic gradient

Research papers

Seasonal δ13C and δ15N isoscapes of fish populationsalong a continental shelf trophic gradient

Kara R. Radabaughn, David J. Hollander, Ernst B. Peebles1

College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA

a r t i c l e i n f o

Article history:Received 31 October 2012Received in revised form21 May 2013Accepted 24 August 2013Available online 31 August 2013

Keywords:Stable isotopeC and N pathwaysIsoscapeMarine food webAnimal migration

a b s t r a c t

The West Florida Shelf, located in the eastern Gulf of Mexico, transitions from a eutrophic ecosystemdominated by the Mississippi River plume to mesotrophic and oligotrophic ecosystems off the coast ofpeninsular Florida. Three extensive trawl surveys in this region were used to acquire samples of fishmuscle, benthic algae from sea urchin stomach contents, and filtered particulate organic matter (POM) tocreate δ13C and δ15N isoscapes. Muscle δ15N from three widely distributed fish species, Synodus foetens(inshore lizardfish), Calamus proridens (littlehead porgy), and Syacium papillosum (dusky flounder),exhibited strong longitudinal correlations (Pearson′s r¼�0.67 to �0.90, po0.001) that coincided withthe principal trophic gradient, whereas δ13C values of fish muscle and benthic algae were correlated withdepth (Pearson′s r¼�0.34 to �0.73, po0.05). Correlations between isotopic values and surfaceconcentrations of chlorophyll and particulate organic carbon (POC) imply linkages between the isotopicbaseline and transitions from eutrophic to oligotrophic waters. The δ13C depth gradient and the δ15Nlongitudinal gradient were consistent between seasons and years, providing a foundation for futurestable isotope studies of animal migration in the Gulf of Mexico.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Isotopic spatial gradients have been found on marine continentalshelves in the δ13C and δ15N values of primary producers, consumers,and organic matter in surface sediments (Weffer and Killingley, 1986;Fry, 1988; Jennings and Warr, 2003; Alt-Epping et al., 2007; Nerotet al., 2012). In consumers, depth-related δ13C variation may bepartially due to consumption of different food sources in inshore andoffshore waters (Cherel and Hobson, 2007; Nerot et al., 2012).In primary producers, δ13C variation is linked to environmentalfactors regulating species composition, rates of primary production,δ13C signatures of aqueous CO2, and the magnitude of photosyntheticfractionation (Fry, 1988; Cooper and DeNiro, 1989; Hofmann et al.,2000; Liu et al., 2007). On a regional scale, marine δ13C values aregenerally higher in shallow waters and in areas with high primaryproductivity (Fry, 1988; Muscatine et al., 1989; Graham et al., 2010).

Spatial gradients in δ15N are also common in continental shelfwaters (Jennings andWarr, 2003; Alt-Epping et al., 2007; Olson et al.,2010; Nerot et al., 2012). Although the specific mechanisms affecting

baseline δ15N gradients on continental shelves vary, the root ofthis phenomenon is often linked to transitions from eutrophic ormesotrophic conditions to oligotrophic waters (Hansson et al., 1997;Alt-Epping et al., 2007; Harmelin-Vivien et al., 2008; Nerot et al.,2012). Diazotrophs such as the cyanobacterium Trichodesmiumprovide fixed nitrogen with δ15N values close to 0‰, which canlower the δ15N baseline in oligotrophic ecosystems (Carpenter et al.,1997; Montoya et al., 2002; Montoya, 2007). In contrast, δ15N ofnutrients and particulate organic matter (POM) in fluvial sourcesmay have higher isotopic values in the range of 5–9‰, particularly ifthe terrestrial runoff contains sewage or manure (Hansson et al.,1997; Kendall et al., 2001). The POM and dissolved nutrients in riverplumes can increase the δ15N baseline in eutrophic or mesotrophicwaters on continental shelves (Alt-Epping et al., 2007; Harmelin-Vivien et al., 2008; Dorado et al., 2012).

Geographic variability in isotope signatures can be displayedgraphically as isoscapes (West et al., 2010). Isoscapes are useful fordetermining the foundation of food webs or the trophic level of aspecies (Graham et al., 2010; Olson et al., 2010). Isoscapes are alsoused to trace the origin and migration of animals, as the isotopiccomposition of an animal′s diet is retained in certain body tissuesfor extended lengths of time (Rubenstein et al., 2002; Hobsonet al., 2010). An animal′s isotopic signature can be compared to theisotopic baseline to determine its most likely origin (Wunder andNorris, 2008; Hobson et al., 2010). For instance, the δD and δ13Csignatures of bird feathers have been used to estimate breeding

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Continental Shelf Research

0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.csr.2013.08.010

n Corresponding author.E-mail addresses: [email protected] (K.R. Radabaugh),

[email protected] (D.J. Hollander), [email protected] (E.B. Peebles).1 EBP and DJH conceived and designed the study. KRR conducted fieldwork,

isotopic analyses, and data analyses. KRR and EBP wrote the manuscript; DJHprovided editorial advice.

Continental Shelf Research 68 (2013) 112–122

latitudes using simple linear regressions of isotopic gradients(Rubenstein et al., 2002; Wunder and Norris, 2008).

Variations in the marine isotopic baseline provide a similaropportunity to estimate the movements of marine animals(Graham et al., 2010; Hobson et al., 2010). Empirical isoscapeshave been created for marine plankton (Graham et al., 2010) andphytoplankton δ13C values have been modeled at global scales(Hofmann et al., 2000). Consumers provide a time-averagedisotopic signal that is more stable than that of primary producers(Post, 2002), yet only a few isoscapes have been created usingmarine fish populations (Jennings and Warr, 2003; Graham et al.,2010; Olson et al., 2010). Isotopic analysis of fish populationspresents the opportunity to examine the temporal and spatialstability of these marine isotopic gradients.

1.1. West Florida Shelf

The West Florida Shelf (WFS, Fig. 1), located in the eastern Gulfof Mexico, exhibits a trophic gradient from highly eutrophicwaters associated with the Mississippi River and Mobile Bay

watersheds to mesotrophic and oligotrophic shelf waters off thecoast of peninsular Florida (Rabalais et al., 1996; Del Castillo et al.,2001). High volumes of nutrients in Mississippi River dischargelead to localized high primary productivity and seasonal anoxiczones near the river delta (Baustian et al., 2011). While much ofthe discharge from the Mississippi River is carried westward, someparcels of nutrient-rich water are carried east and then southalong the WFS (Del Castillo et al., 2001). This Mississippi Riverplume is evident in surface particulate organic carbon (POC) andchlorophyll concentrations on the WFS (Fig. 1b–c).

Along peninsular Florida, the WFS is a broad, gently slopingcontinental shelf with chlorophyll concentrations that decreaserapidly with distance offshore (Fig. 1c, Del Castillo et al., 2001).Diazotrophs such as Trichodesmium are an important nitrogensource in the oligotrophic central and southern regions of the WFS(Mulholland et al., 2006). The continental shelf consists of pre-dominantly sandy substrates, with approximately 35% hard bot-tom (Parker et al., 1983; Okey and Mahmoudi, 2002). The mainprimary producers in this region include phytoplankton, benthicmacroalgae, benthic microalgae, corals, and seagrass (Okey and

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Fig. 1. (a) Spatial extent of sampling on the West Florida Shelf during three time periods. Open circles are trawling sites from summer 2009, crosses are from summer 2010,and filled circles are from fall 2010. Isobaths are shown at 200 m intervals. (b) Particulate organic carbon concentration and (c) chlorophyll concentration in surface waters inthe summer of 2009. Note the irregular scale used for chlorophyll concentration.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122 113

Mahmoudi, 2002). The present study seeks to determine if theshift from eutrophic to oligotrophic waters on the WFS is reflectedin the δ13C and δ15N values of selected primary producers and fishspecies. Additionally, this study examines the spatiotemporalstability of these isotopic values to assess their potential use inanimal migration studies.

2. Materials and methods

2.1. Target species

Calamus proridens (littlehead porgy, family Sparidae), Syaciumpapillosum (dusky flounder, family Paralichthyidae), and Synodusfoetens (inshore lizardfish, family Synodontidae) were selected astarget species due to their abundance and wide distribution on theWFS. C. proridens is a benthic invertebrate feeder that associateswith structures such as reefs and hard bottom in relatively shallowwaters (Darcy, 1986; Okey and Mahmoudi, 2002). S. papillosum is abenthic invertebrate feeder that is commonly found on sand-dominated substrates (Okey and Mahmoudi, 2002). S. foetens is asolitary benthic predator that is also found on sandy substrates(Okey and Mahmoudi, 2002). S. foetens primarily feeds on demersaland pelagic fishes (Cruz-Escalona et al., 2005).

Samples of benthic algae were obtained from the stomachs ofthe variegated sea urchin, Lytechinus variegatus. L. variegatus isprimarily found on sand but is also present on hard substrates inoffshore waters (Cobb and Lawrence, 2005). L. variegatus isomnivorous, yet it exhibits dietary selectivity for algae overanimals (Cobb and Lawrence, 2005). L. variegatus feeds randomlyon the available species of algae, including drift algae. In the Cobband Lawrence (2005) study, the most common taxa of algaeconsumed by L. variegatus at offshore sites on the West FloridaShelf included Chlorophyta (Cladophora spp.), Phaeophyta (Dic-tyota spp., Sargassum spp.), Rhodophyta (Rhodomelaceae family,Kylinia crassipes, Lithothamnion spp.), Anthophyta (Thalassia testu-dinum), and epiphytic microalgae.

2.2. Sample collection

Samples were obtained during Southeast Area Monitoring andAssessment Program (SEAMAP) shrimp and groundfish cruises,which are part of a state/federal program designed to collectfisheries-independent monitoring data. Over 800 samples of thetarget fish species and primary producers were collected andanalyzed from water depths 9.0–102.4 m (Fig. 1a, ElectronicSupplementary material (ESM) 1). Fish and sea urchins wereobtained by deploying 12.8 m otter trawls with 4.1 cm stretchedmesh according to the fishery-independent sampling SEAMAPtrawl protocol (SEDAR27-RD-05). Trawling stations were deter-mined according to a stratified-random sampling design, withrandomization conducted independently for each cruise. Not allfish species or primary producers were collected at each trawlingstation. Samples were collected during three time periods: 10 to30-July-2009, 27-June to 14-July-2010, and 16-October to 12-November-2010. These time periods are hereafter referred to assummer 2009, summer 2010, and fall 2010.

Standard lengths of the three target species were recorded anda lateral muscle sample was collected midway between the dorsaland caudal fins and frozen at �20 1C at sea. Stomach linings andcontents of the sea urchin L. variegatus were removed, avoidingroe, and frozen. Due to the difficulty of isolating phytoplankton,a small size fraction of POM containing both phytoplankton anddetritus was used as a proxy to determine the isotopic baseline ofsurface waters (Harmelin-Vivien et al., 2008; Graham et al., 2010;Dorado et al., 2012). For POM samples, a bucket of surface water

was pre-filtered through a 125 μm sieve to remove most zoo-plankton biomass. A volume of 5–20 L of this water was thenfiltered through pre-combusted Whatman GF/F filters using aperistaltic pump at 10 psi, and the filters were frozen at �20 1C.The filtered amount varied due to variable phytoplankton con-centrations in inshore and offshore waters. POM samples were notcollected in duplicate.

2.3. Stable isotope analysis

Skin, scales, and visible blood vessels were removed fromfish muscle samples. The samples were dried for 48 h at 55 1Cand homogenized into powder using a stainless steel ball mill.0.5–1mg of powdered sample was weighed and packed into tin cups.13C/12C and 15N/14N were measured in duplicate on an ElementalAnalyzer (EA Carlo-Erba NA2500 Series II) coupled to a continuousflow Isotopic Ratio Mass Spectrometer (IRMS ThermoFinniganDeltaþXL) at the University of South Florida College of MarineScience in St. Petersburg, Florida. The lower limit of quantificationwas 12 μg of C or N. Calibrating standards were NIST 8573 andNIST 8574 L-glutamic acid Standard Reference Materials. Analyticalprecision, obtained by replicate measurements of NIST 1570aspinach leaves, was 0.0870.19‰ for δ15N and �27.2870.21‰for δ13C (average standard deviation of n¼307 replicates). δ13C isreported relative to the Pee Dee Belemnite standard and δ15N isreported relative to atmospheric N2, such that

δAð‰Þ ¼ ½ðRspl=RstdÞ–1� � 1000 ð1Þwhere δA is δ15N or δ13C and Rspl and Rstd are the isotopic ratios ofthe sample and standard.

Lipid correction was not necessary as the C:N ratios of all fishmuscle samples were below 3.5, thus lipid content was below 5%(Post et al., 2007). Within fish species, a correction for ontogeneticshifts in trophic level and/or diet was applied if there was asignificant relationship between δ15N or δ13C and the length of thefish (i.e., if linear regression slope po0.05, with each time periodand species assessed separately). This trend removal used theequation

δX′¼ δX–ðmlþbÞþδXm ð2Þwhere δX′ is the corrected δ15N or δ13C value, δX is the originalisotopic value for a fish of standard length l, and δXm is the mean ofall δX from a species. The slope m and intercept b are from thelinear regressions δX¼mlþb presented in Table 1. All datapresented in this manuscript utilize length-detrended isotopicvalues.

Sea urchin stomach contents, which contained algae, calciumcarbonate and silicious sand grains (quartz), were separated fromsea urchin stomach linings and tissues. Algae samples were nottaxonomically identified. Stomach contents were soaked in 0.5 NHCl for 24 h with additional acid added after 12 h. The algae werepoured off the top of the sample, leaving behind settled quartzsand. Algae were rinsed on a 35 μm Nitex mesh sieve, then soakedin de-ionized (DI) water for 1 h. Samples were dried, homoge-nized, and analyzed following the same procedure used for fishmuscle samples. For 17 urchins, stomach contents were dividedinto two samples to test the effect of the acidification procedure onalgal δ15N; the control samples were soaked in DI water instead ofHCl. The acidification procedure used to remove calcium carbonatefrom sea urchin stomach contents did not significantly alter algalδ15N (paired t test, n¼17, t¼�1.04, p¼0.31).

POM filters were rinsed with DI water to remove salt andwere cut in half. For δ13C analysis, half of the filter was acidifiedwith 0.5 N HCl then rinsed with DI water on a vacuum filtrationapparatus at 10 psi. POM samples were dried and analyzed follow-ing the aforementioned procedure. Both acidified and nonacidified

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122114

POM samples were analyzed in duplicate; POM samples with poorduplicate replication were omitted from analyses.

2.4. Isoscapes

Isoscapes were created using Surfer version 8.06.39. Ordinarypoint kriging was used with the default linear variogram modeland no drift. Most trawl locations had 2–3 individuals of eachspecies; their isotopic values (length-corrected, if applicable) wereaveraged for the creation of isoscapes. No nugget effect wasspecified; the gridding method was an exact interpolator for theaveraged data at each sampling location. Statistical analyses wereperformed using Statgraphics Centurion XVI Version 16.1.17. For allcorrelations and regressions, isotopic values of fish of the samespecies caught in the same trawl were treated as duplicates andaveraged to avoid pseudoreplication. With the exception of ESM1 and Table 1, all figures and tables within the manuscript utilizethese station-averaged values; thus all means of isotopic data arereferred to as “grand means.”

2.5. Remote sensing data

In order to compare isotopic trends with water-quality para-meters, POC and chlorophyll data were obtained from the Gio-vanni online data system, which is developed and maintained bythe NASA GES DISC at http://giovanni.gsfc.nasa.gov (Berrick et al.,2009). NASA GES DISC defines POC as organic particles that arecaught by a filter with pore size 0.7 μm. MODIS-Aqua data at a4 km resolution were obtained from the Ocean Portal′s OceanColor Radiometry Online Visualization and Analysis system. Satel-lite data were averaged for the calendar month of the start of datacollection and two months prior for summer 2009, summer 2010,and fall 2010. This three month time frame was selected to reflecttissue turnover time. For muscle tissue in adult fish, isotopic halflives in diet-switch studies are generally on the order of 1.5 monthsto 46 months (McIntyre and Flecker, 2006; Buchheister andLatour, 2010; Nelson et al., 2011). The locations for remote sensingdata were matched with stable isotope data to the nearest tenth ofa decimal degree using latitude and longitude.

3. Results

The POC and chlorophyll concentrations from summer 2009 areshown in Fig. 1b and c, respectively. The specific region sampledon the WFS varied during each time period (Fig. 1a). To comparetemporal stability, the grand means of isotopic values werecalculated within the area north of 27.01N and east of �85.31W(outlined region in Fig. 2a), as this region was extensively sampledduring all three time periods. As shown in Fig. 2b, species-specificgrand mean δ15N and δ13C values differed by less than 0.9‰ acrosstime periods (C. proridens and S. foetens ANOVA p-values o0.05,S. papillosum ANOVA p-values 40.05).

Each of the three fish species required corrections for signifi-cant relationships between isotopic composition and length dur-ing at least one of the time periods (Table 1). Isotopic spatialgradients are shown in the isoscapes of the three fish species(Figs. 3–5). The piscivorous S. foetens occupies a higher trophiclevel than S. papillosum and C. proridens; thus higher δ15N valuesare visible in Fig. 3 isoscapes. All three fish species had significantcorrelations between muscle δ15N and latitude and longitude(Table 2). These spatial trends, with higher δ15N in the northwest

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Fig. 2. (a) The square outlines the spatial extent sampled during all three timeperiods; this region was used to calculate (b) grand mean δ13C and δ15N values (7SD) of fish and primary producers across the three time periods. POM was notcollected in 2009.

Table 1Significant relationships between fish standard length (l, in cm) and muscle tissueδ13C and δ15N composition. Values from linear regressions used in Eq. (2) for lengthcorrection.

Fish species,time period

n fish Stableisotope

R2 Slopep-value

Linearregression

Calamus proridensSummer 2009 64 δ15N 0.024 0.216 None

δ13C 0.046 0.091 NoneSummer 2010 56 δ15N 0.042 0.132 None

δ13C 0.002 0.768 NoneFall 2010 61 δ15N 0.385 o0.001 δ15N¼8.04þ0.09� l

δ13C 0.020 0.278 None

Synodus foetensSummer 2009 104 δ15N 0.001 0.733 None

δ13C 0.418 o0.001 δ13C¼�19.45þ0.09� lSummer 2010 71 δ15N 0.004 0.612 None

δ13C 0.197 o0.001 δ13C¼�20.14þ0.10� lFall 2010 51 δ15N 0.301 o0.001 δ15N¼8.65þ0.087� l

δ13C 0.028 0.243 None

Syacium papillosumSummer 2009 183 δ15N 0.042 0.006 δ15N¼8.07þ0.07� l

δ13C 0.034 0.013 δ13C¼�18.35þ0.06� lSummer 2010 72 δ15N 0.004 0.588 None

δ13C 0.079 0.017 δ13C¼�19.06þ0.07� lFall 2010 69 δ15N 0.057 0.049 δ15N¼7.93þ0.06� l

δ13C 0.016 0.301 None

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122 115

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Fig. 3. Interannual and seasonal stability of Synodus foetens (inshore lizardfish) δ15N and δ13C isoscapes. 50 and 100 m isobaths are labeled with subsequent isobaths from 200 monward drawn at 200 m intervals. Black circles identify station locations. (a) Summer 2009 δ15N, (b) summer 2010 δ15N, (c) fall 2010 δ15N, (d) summer 2009 δ13C, (e) summer2010 δ13C and (f) fall 2010 δ13C. Color scales are consistent across all isoscape panels (Color isoscapes are available in the electronic version of this article).

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Fig. 4. Interannual and seasonal stability of Syacium papillosum (dusky flounder) δ15N and δ13C isoscapes. Symbols and isobaths as in Fig. 3. (a) Summer 2009 δ15N,(b) summer 2010 δ15N, (c) fall 2010 δ15N, (d) summer 2009 δ13C, (e) summer 2010 δ13C and (f) fall 2010 δ13C.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122116

region of the WFS and lower values near southwest Florida, areclearly visible in the δ15N isoscapes (Figs. 3–5). Longitudinal δ15Ncorrelations were significant for all three fish species during eachtime period (all p-values o0.01). S. foetens and S. papillosum δ15Nvalues also had significant positive correlations with surfacechlorophyll and POC concentrations (Table 2). Benthic algae andPOM had less consistent δ15N spatial trends (Figs. 6 and 7), andlacked many of the spatial correlations present in fish populations(Table 2).

There were significant negative correlations between waterdepth and δ13C for all three fish species and benthic algae(Table 2). These correlations, with higher δ13C values present inshallower waters, were significant for all three fish species duringeach time period (p-values o0.05). Fish muscle δ13C values alsodemonstrated significant positive correlations with surface POCand chlorophyll concentrations (Table 2). Simple linear regressionswere fit to δ15N as a function of longitude and δ13C as a functionof depth to examine the predictive capabilities of these

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Fig. 5. Interannual and seasonal stability of Calamus proridens (littlehead porgy) δ15N and δ13C isoscapes. Symbols and isobaths as in Fig. 3. (a) Summer 2009 δ15N, (b)summer 2010 δ15N, (c) fall 2010 δ15N, (d) summer 2009 δ13C, (e) summer 2010 δ13C and (f) fall 2010 δ13C.

Table 2Grand mean δ13C and δ15N values (7SD) of fish muscle tissue, benthic algae, and particulate organic matter (POM). Pearson product-moment correlation coefficientsdescribe the correlation between isotopic values, geographic data (depth, latitude, and longitude), and remote sensing data (chlorophyll and particulate organic carbon (POC)concentrations). Data from all locations and time periods combined.

Sample type Grand mean δ15N (‰) n Stations δ15N vs. depth δ15N vs. latitude δ15N vs. longitude δ15N vs. chlorophyll δ15N vs. POC

C. proridens 9.770.5 70 0.43nn 0.44nn �0.67nn �0.17 �0.24n

S. foetens 11.270.8 102 0.11 0.76nn �0.90nn 0.40nn 0.32n

S. papillosum 9.170.9 136 0.04 0.69nn �0.88nn 0.47nn 0.36nn

Benthic algae 4.070.8 47 �0.04 0.15 �0.24 0.32n 0.21POM 5.471.5 37 0.28 �0.01 �0.19 �0.24 �0.26

Sample type Grand mean δ13C (‰) n Stations δ13C vs. depth δ13C vs. latitude δ13C vs. longitude δ13C vs. chlorophyll δ13C vs. POC

C. proridens �16.070.8 70 �0.73nn �0.13 0.49nn 0.23 0.52nn

S. foetens �17.370.6 102 �0.43nn 0.03 0.10 0.28n 0.41nn

S. papillosum �17.571.0 136 �0.62nn 0.01 0.27n 0.29nn 0.39nn

Benthic algae �19.372.7 47 �0.34n �0.47nn 0.59nn 0.02 0.31n

POM �23.471.1 48 �0.03 0.19 �0.01 0.00 �0.02

n po0.05.nn po0.001.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122 117

characteristics on fish isotopic values (Fig. 8). Data utilized for thedepth vs. δ13C regression lines in Fig. 8b were restricted to 0–53 mto reflect the depth range shared by all three target fish species.

4. Discussion

4.1. δ13C correlations

δ13C depth gradients have previously been found in primaryproducers such as macroalgae, seagrass, and coral zooxanthellae(Weffer and Killingley, 1986; Cooper and DeNiro, 1989; Muscatineet al., 1989) and in consumers such as molluscs, seabirds, andsquid (Cherel and Hobson, 2007; Barnes et al., 2009; Nerot et al.,2012; Ruiz-Cooley and Gerrodette, 2012). While some of thisvariability is due to differences between inshore and offshorespecies, δ13C values of primary producers are also impacted by themagnitude of photosynthetic fractionation (εp). εp occurs whenplants selectively fix 12C over 13C during photosynthesis, resultingin primary producers with lower δ13C values relative to ambientCO2. Among other factors, the magnitude of εp is lower during highgrowth rates and in low aqueous CO2 concentrations (Popp et al.,1998; Hofmann et al., 2000). Thus, δ13C depth gradients in benthicmacroalgae, seagrass, and coral zooxanthellae have been attribu-ted to increased light availability in shallow waters leading tohigher growth rates and lower εp (Weffer and Killingley, 1986;Cooper and DeNiro, 1989; Muscatine et al., 1989). Regions of highproductivity may also drive down localized aquatic CO2 concen-trations and alter the δ13C values of available CO2 (Liu et al., 2007;Graham et al., 2010).

In the present study, correlations between δ13C values and bothPOC and chlorophyll concentrations support the idea that rapidalgal growth rates in shallow, nutrient-rich waters may havereduced εp by one of the processes mentioned above, resultingin higher δ13C values. δ13C correlations with POC concentrationwere consistently stronger than correlations with chlorophyll(Table 2). Although POC and chlorophyll concentrations are bothhigh along peninsular Florida and within the Mississippi Riverplume, chlorophyll concentrations decrease more rapidly than POCconcentrations (note different scales in Fig. 1b and c).

Variable primary producer taxa may also contribute to highδ13C values in shallow depths. While extensive seagrass beds aregenerally not found past 7 m of water depth on the WFS due tolight limitation, the seagrasses Halophila engelmannii and Halo-phila decipiens may occur in depths of 20 m or more (Zieman andZieman, 1989; Dawes et al., 2004). Seagrass δ13C values of �10 to�11‰ are typical (Bouillon et al., 2008). Such values are con-siderably higher than those of other marine primary producers,as the median δ13C values of marine microalgae and macroalgaeare �20.2‰ (range �23.0 to �18.0‰) and �18.9‰ (range �23.1to �16.4‰), respectively (Bouillon et al., 2008). The highestinshore values seen in the fish and benthic algae isoscapes in thisstudy were in the range of �14 to �16‰ (Figs. 3–6), so it ispossible that seagrass may have contributed to these inshorevalues either locally or through migration of prey from seagrass-rich areas landward of our survey areas, as suggested by Nelsonet al. (2012).

It is highly unlikely that terrestrial organic matter or settledphytoplankton blooms directly influenced the observed trends inδ13C, as the low δ13C in these materials would decrease, rather

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-86 -85 -84 -83 -82

TB

CH

Longitude-86 -85 -84 -83 -82 -81

TB

CH

E

25

26

27

28

29

30

Fig. 6. Interannual and seasonal stability of benthic algae δ15N and δ13C isoscapes. Benthic algae were collected from sea urchin stomachs. Letters indicate location of TampaBay (TB), Charlotte Harbor (CH), and the Everglades (E). Symbols and isobaths as in Fig. 3. (a) Summer 2009 δ15N, (b) summer 2010 δ15N, (c) fall 2010 δ15N, (d) summer 2009δ13C, (e) summer 2010 δ13C and (f) fall 2010 δ13C.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122118

than increase, δ13C values in nearshore waters (Chanton andLewis, 1999; Dorado et al., 2012; Radabaugh and Peebles, 2012).The average δ13C signatures of terrestrial C3 and C4 plants are�27‰ (range �33 to �20‰) and �13‰ (range �17 to �9‰),respectively (Boutton, 1991; Sharp, 2007). C4 Spartina grasses arepresent in northern Florida, but southern Florida coastlines aredominated by C3 mangroves (Dawes et al., 2004). Thus C4 plantsare unlikely to significantly contribute organic matter with highδ13C values to fluvial POM along the southern Florida peninsula.Even within the Apalachicola estuary, which is located on Florida′snorthern Gulf coast in an area that includes substantial Spartinamarshes, average δ13C values from POM are �26.7 to �22.4‰(total range �30.2 to �22.0‰) (Chanton and Lewis, 1999). Giventhese low δ13C values for fluvial POM, it is unlikely that terrestrialmaterial influenced δ13C values in our study as the highest valueswere found near shore.

4.2. POM isotopic values

Significant correlations that were present in benthic algae andfish isotopic values were absent in surface POM values (Table 2),even when each time period was examined independently. Corre-lations were also weaker in benthic algae than in fish. Primaryproducers have an inherently variable isotopic signal, whereasconsumers present a more stable, time-averaged signature (Post,2002). This may explain why gradients found in multiple species offish are not as clear at the primary producer level. Additionally, thePOM samples presented in this study were only obtained from

surface water. POM δ13C values vary with depth due to variablephotosynthetic fractionation and remineralization of sinkingorganic matter (Liu et al., 2007; Harmelin-Vivien et al., 2008).Thus, surface POM does not present the full spectrum of plank-tonic isotopic values in the water column.

Near the Everglades, POM δ15N values were high and δ13Cvalues were as low as �27.170.2‰ (Fig. 7a and c). These isotopicvalues are consistent with the isotopic signatures of terrestrialPOM (Boutton, 1991; Chanton and Lewis, 1999; Kendall et al.,2001), thus this region is likely influenced by outflow from theEverglades during the summer rainy season. Sampling in the fall of2010 did not extend to this region (Fig. 7b and d), so thepersistence of the low δ13C and high δ15N in this area is not known.

4.3. δ15N longitudinal/latitudinal gradients

Longitudinal and latitudinal gradients in fish muscle δ15N wereconsistent across all time periods, with lower δ15N in the south-eastern region of the study area. Widespread nitrogen fixation byTrichodesmium has been documented in the oligotrophic waters inthe central and southern region of the WFS (Mulholland et al.,2006), and nitrogen fixation has previously been suggested as anexplanation for low δ15N in this area (Macko et al., 1984; Doradoet al., 2012). The positive correlations between δ15N signatures ofS. foetens and S. papillosum with both POC and chlorophyllconcentrations (Table 2) are consistent with the theory thatnitrogen fixation in oligotrophic waters provides a source of lowδ15N to higher trophic levels.

Longitude

Latit

ude

25

26

27

28

29

30

-86 -85 -84 -83 -82

25

26

27

28

29

30

3

4

5

6

7

8

-86 -85 -84 -83 -82

-27-26-25-24-23-22

δ15N (‰)

δ13C (‰)

Fig. 7. Interannual and seasonal stability of surface particulate organic matter (POM) δ15N and δ13C isoscapes. Symbols and isobaths as in Fig. 3. (a) Summer 2010 δ15N,(b) fall 2010 δ15N, (c) summer 2010 δ13C and (d) fall 2010 δ13C.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122 119

δ15N variability along eutrophic-to-oligotrophic gradients havealso been linked to fluvial outflows (Hansson et al., 1997;Harmelin-Vivien et al., 2008; Nerot et al., 2012) and to oceanicupwelling (Olson et al., 2010; Ruiz-Cooley and Gerrodette, 2012).The correlations between δ15N values and both POC and chlor-ophyll concentrations may be linked to intrusions of the Missis-sippi River plume, which is carried along the WFS by the Gulf ofMexico Loop Current (Del Castillo et al., 2001). In the lowerMississippi River and its tributaries, average δ15N ranges from�3 to 9‰ for nitrate and �5 to 7‰ for POM, providing arelatively high δ15N source to the Gulf of Mexico and possibly tothe northwestern portion of our study area (Battaglin et al., 2001;Kendall et al., 2001; Dorado et al., 2012). δ15N values fromsedimentary organic matter near the mouth of the MississippiRiver have increased by approximately 1.5–2‰ over the pastcentury (Eadie et al., 1994). If current trends in eutrophicationcontinue, this source of high δ15N may become even moreprominent in the future.

Finally, denitrification can be a source of high δ15N (Altabetet al., 1999; Montoya, 2007). Under low oxygen conditions,bacteria use nitrate as an electron acceptor during respiration.The strong fractionation (20–30‰) during this denitrificationprocess results in higher δ15N for the residual nitrate (Altabetet al., 1999; Montoya, 2007). Although the surveyed region waseast of areas known to experience the chronic anoxia that facil-itates denitrification (Rabalais et al., 1996), westward water massesthat do experience anoxia and extensive denitrification are morelikely to mix with waters in the northwest part of the survey areathan the southeast part. The only explanation for fish δ15N spatialgradients that we do not believe to be likely is geographicdifferences in trophic level. Within-species variation in trophiclevel is generally related to increased fish length and mouth gape,yet the strong δ15N gradients persisted after removal of thispotential bias.

4.4. Fish and sea urchin site fidelity

The significant spatial isotopic gradients in fish muscle implythe three target fish species had relatively high site fidelities, asextensive movement by fish throughout the study area would havedestroyed the observed gradients. Prior studies have found thatthese three species can be somewhat mobile on continentalshelves. S. foetens moves between trawled and non-trawled areasin the northern Gulf of Mexico (Jeffers et al., 2008). Near theYucatan peninsula, C. proridens moves seasonally on continentalshelves (Darcy, 1986). While limited information is availableconcerning movement patterns of S. papillosum, other membersof the Syacium genus near the Yucatan peninsula have been foundto exhibit seasonal movements driven by spawning and changes inprimary productivity on the continental shelf (Sánchez-Gil et al.,1994). At some point in their life cycles, all three species of fish arecapable of moving tens of kilometers, primarily between inshoreand offshore waters on continental shelves (Darcy, 1986; Sánchez-Gil et al., 1994; Cruz-Escalona et al., 2005; Jeffers et al., 2008).Some of the small-scale variability in the fish isoscapes may bedue to fish mobility rather than to true differences in the isotopicbaseline. Nevertheless, the consistent isotopic gradients imply thespecimens analyzed during the present study must have main-tained a high degree of site fidelity during the months prior tosampling.

L. variegatus has also been found to move about on continentalshelves (Moore et al., 1963). However, the distance the algaesamples could travel within the urchin would be limited by therates of movement and digestion by the sea urchins. Moresignificant influences on the clarity of algal isotopic patterns maybe the impact of digestion or the inclusion of drift algae, whichL. variegatus is known to consume indiscriminately (Cobb andLawrence, 2005). L. variegatus is omnivorous (Cobb and Lawrence,2005), so some animal material may have been present in stomachcontents. However, membranes or other visible animal tissue weremanually removed during the algal preparation procedure.

4.5. Utility of isoscapes for animal migration studies

Previous studies have found seasonally variable δ15N and δ13Csignatures on continental shelves as a result of variable fluvialinput and primary productivity (Mariotti et al., 1983; Kürten et al.,2012). However, in this study, average isotopic values were fairlyconsistent interannually and seasonally for the target fish species,with grand means for both δ15N and δ13C varying by less than0.9‰ (Fig. 2b). This stability and the presence of consistent andsignificant spatial gradients provide the basis of a reliable isoscapefor the WFS, fulfilling the first requirement for using stable

C. proridens

S. foetens

S. papillosumδ13C = -15.77 - 0.05z, R2 = 0.27

δ13C = -16.47 - 0.03z, R2 = 0.22

δ13C = -14.34 - 0.06z, R2 = 0.53

δ13C

(‰)

Depth (m)

C. proridens

S. foetens

S. papillosumδ15N = -31.43 - 0.49L, R2 = 0.45

δ15N = -36.47 - 0.57L, R2 = 0.80

δ15N = -45.06 - 0.64L, R2 = 0.78

δ15N

(‰)

Longitude

-88 -87 -86 -85 -84 -83 -82 -817

8

9

10

11

12

13

14

15

10 20 30 40 50

-19

-18

-17

-16

-15

-14

-13

Fig. 8. (a) Longitudinal (L) gradients in fish muscle δ15N and (b) depth (z) gradientsin fish muscle δ13C (0–53 m depth; data from all time periods). All linear regressionslope p-values o0.0001.

K.R. Radabaugh et al. / Continental Shelf Research 68 (2013) 112–122120

isotopes as endogenous tracers of animal migration (Wunder andNorris, 2008; Hobson et al., 2010).

δ15N values may be useful for tracking longitudinal or latitu-dinal animal movements along the WFS, whereas δ13C values aremore relevant to tracking changes in depth. As shown in Fig. 8a,longitude can be used to explain 78–80% of the variability in δ15Nvalues of S. foetens and S. papillosum, respectively. Longitude is lessuseful for C. proridens, likely due to the limited range of this moretropical species (C. proridens were not collected west of �851W).Of the three fish species, C. proridens had the strongest relation-ship between δ13C values and depth, with 53% of variabilityexplained by depth (Fig. 8b). Depth explained only 22% and 27%of the variability in δ13C values of S. foetens and S. papillosum.While the simple linear regressions with longitude and depth areconvenient statistical descriptions of isotopic data, these gradientsare the product of covarying sources and processes. The incorpora-tion of additional water-quality parameters into a temporallydynamic model should explain more variability within the isotopicbaseline on the WFS, further improving the utility of isotope-basedmigration studies on the continental shelf.

5. Conclusions

The shift from eutrophic to oligotrophic waters on the WestFlorida Shelf is clearly present in the longitudinal and latitudinalgradients in fish muscle δ15N values. This δ15N gradient persistedafter accounting for variation in fish size; thus it is likely due tobaseline environmental factors such as variable nitrogen fixationor terrestrial input rather than variation in trophic level. δ13Ccorrelations with depth appear to be driven by high rates ofprimary production and/or changes in species composition, not byterrestrial input. The presence of isotopic gradients, both in thisand other studies, reinforces the need for wide spatio-temporalsampling and/or modeling when developing food web modelsbased on stable isotopes.

δ13C and δ15N spatial gradients in three target fish species werehighly consistent across two years and two seasons on the WestFlorida Shelf. The inherent variability in the isotopic compositionof primary producers makes their spatial gradients less reliablecompared to the isotopic composition of consumers. The consis-tency of the δ13C and δ15N gradients and the stability in meanisotopic values of these fish species provide a foundation for futurestudies of animal migration on the West Florida Shelf.

Acknowledgements

We thank the Florida Fish and Wildlife Conservation Commis-sion (FWC) for their generous cooperation during SEAMAP sur-veys, in particular Bob McMichael, Jr., Amanda Tyler-Jedlund, JennaTortorelli, Ted Switzer, and Ed Matheson. We thank the crew of theR/V Tommy Munro, operated by the University of SouthernMississippi. We also thank Kristen Murphy, Ethan Goddard, SheriHuelster, David Jones, Greg Ellis, Elon Malkin, Ralph Kitzmiller,Amy Wallace, and Michael Drexler of the University of SouthFlorida (USF) for help in sample collection and/or analysis. Wethank three anonymous reviewers for their constructive com-ments. This project partially fulfills requirements for the firstauthor′s doctoral degree and was supported by the Gulf Oceano-graphic Charitable Trust Fellowship, the William and Elsie KnightFellowship and in-kind assistance from the FWC and the USFPaleoLab.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.csr.2013.08.010.

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