Mar Biol (2014) 161:1593–1602 DOI 10.1007/s00227-014-2443-5

Original Paper

Stable isotope profiles from subtropical marine gastropods of the family : growth histories and relationships to local environmental conditions

Josiah Strauss · Anton Oleinik · Peter Swart

Received: 22 July 2013 / Accepted: 5 April 2014 / Published online: 1 May 2014 © Springer-Verlag Berlin Heidelberg 2014

Abstract and carbon stable isotope profiles were a measured MART of 9.4 °C. Carbon isotopes of T. gigan- constructed for two of large subtropical gastropods teus were ambiguous and reveal no significant relationships of the family Fasciolariidae— giganteus and with trends in nutrient (N and P), dissolved tulipa—from the Keys and the Baha- oxygen, and dissolved organic carbon, although they did mas, to evaluate their life history and to assess their poten- exhibit more negative values concomitant with landfall tial as paleoenvironmental proxies. Oxygen isotope pro- of Hurricane Irene and trended to increasing values with files revealed T. giganteus and F. tulipa grew their shells ontogeny that could reflect migration. Carbon isotopes in for 6 and 3 years, respectively. Both mollusks show faster F. tulipa were lower during winters, possibly reflecting sea- growth rates during the first half of their lifespan. Mean sonal or seagrass-mediated carbon cycling. annual (MAT) derived from oxygen iso- topes for T. giganteus were 26.5 °C and for F. tulipa were 26.7 °C, both matching instrumental MATs of 26.7 and Introduction 26.5 °C for the Florida Keys. Both shells, however, failed to record entire mean annual ranges (MART). Oxygen and carbon stable isotopes of biogenic marine yielded a calculated MART of 5.6 °C carbonates (e.g., skeletons, mollusk and brachio- compared with a measured MART of 9.3 °C, and T. gigan- pod shells and foraminiferal tests) have been widely used teus showed a calculated MART of 6.9 °C compared with as proxies for seawater temperatures and carbon cycling dynamics, respectively, throughout the Phanerozoic (Veizer et al. 1986; Hendry and Kalen 1997; Zachos et al. 2001; Communicated by C. Harrod. Grossman 2012). In more recent geologic history, such as the Quaternary to present, skeletal carbonates yielding J. Strauss Nicholas School of the Environment, Duke University, Durham, high-resolution, continuous records can be the most desir- NC 27705, USA able paleoclimate proxies. For example, bivalve shells are used to reconstruct seasonal climate variability (Patterson J. Strauss · A. Oleinik et al. 2010; Wanamaker et al. 2011) and coral skeletons are Department of Geosciences, Florida Atlantic University, Boca Raton, FL 33431, USA used evaluate periodicity of climatic events like the El Niño Southern Oscillation (Cobb et al. 2013). J. Strauss (*) Mollusks and warm water are the most commonly Dolan Integration Group, 2520 55th St, Suite 101, Boulder, used high-resolution marine proxies (Richardson 2001; CO 80301, USA e-mail: [email protected] Grottoli and Eakin 2007; Morrongiello et al. 2012). While mollusk shells do not yield records as consistently long P. Swart as corals, they have a unique advantages in that they are Marine Geology and Geophysics, Rosenstiel School of Marine not restricted by a limited environmental range, are gen- and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA erally plentiful, and are easy to collect and sample. While e-mail: [email protected] the majority of molluscan stable isotope studies focused

1 3 1594 Mar Biol (2014) 161:1593–1602 on bivalves, gastropods have also shown to be useful, but shallow, isolated carbonate bank located between the Flor- considerably less studied (Geary et al. 1992; Gentry et al. ida Keys and Cuba. The modern T. giganteus (Tg) shell was 2008; Tao and Grossman 2010). Gastropods represent a sig- collected live in Hawk Channel off Tavernier Key, Florida nificant component of global Cenozoic assemblages, Keys (25°01′N, 80°29′W), in 4 m of water from seagrass especially in the coastal plain Late Cenozoic sediments of patches in March 2004 (Fig. 1). These sites were selected the southeastern United States and Florida Peninsula. How- because of their moderately stable oceanographic condi- ever, only a few genera have been evaluated with regard to tions without significant fluctuations of salinity for mini- their growth histories (i.e., growth rate and life-span) and mal interference with isotopic data. The shells of Ft and Tg isotopically calculated temperatures using stable isotopes. measured 148 mm and 340 mm in length, respectively, and In this study, we examine the stable isotope chronologies based on the average adult shell size of these species and of two extant gastropods species collected from the Flor- thickening of the portion of the outer lip in both specimens ida shelf and the Bahamas: Triplofusus giganteus (Kiener (Abbott 1974). Both mollusks were identified as males 1840) (also referred to as the “Florida Horse ” or when extracted from their shells. gigantea (Kiener 1840)) and Fasciolaria After removal of soft tissues, specimens were soaked in tulipa (Linnaeus 1758) (commonly known as the “True 6 % sodium hypochlorite for 12 h to remove the periostra- ”). These mollusks both belong to the family Fascio- cum and any associated organic material. The sampling sur- lariidae, subfamily Fasciolariinae, of which twelve species face was then lightly polished with sandpaper and scrubbed of genera Fasciolaria and and one species of the with 30 % hydrogen peroxide to remove any remaining Triplofusus are known from the western Atlantic organic matter. Shells were then thoroughly rinsed with (Malacolog Version 4.1.1; Snyder 2003; Petuch 2013). All distilled-deionized water and dried at 40 °C for 6 h. mollusks of the family Fasciolariidae are carnivorous, feed- High-spired gastropods, such as Fasciolariinae, can- ing primarily on other gastropods and occasionally bivalves not be mounted on a flat surface for conventional micro- (Paine 1963a, b). These gastropods, mostly due to their milling, as it is usually done with flat-spired gastropods thick shells and resistance to abrasion, represent one of and bivalves (Goodwin et al. 2003; Kobashi and Grossman the most commonly sampled (Fasciolaria) and the largest 2003; Wanamaker et al. 2011). Due to their shape and size, (Triplofusus) gastropods of Pleistocene shell deposits of the Fasciolariine shells have to be sampled manually. Shells Ft Florida Peninsula (Petuch and Roberts 2007). Furthermore, and Tg were sampled under binocular microscope, using a these genera and occupy a range of marginal environments multi-speed dental drill with a 0.5-mm carbide dental bur. where other, more commonly used proxies (e.g., corals) Powder samples weighing approximately 200 µg were may not be available. T. giganteus is the largest known gas- drilled from the upper portion of the whorl, near the suture, tropod in the Atlantic (Abbott 1974), and although where shell is the thickest. Sixty-nine and 114 samples both Triplofusus and Fasciolaria play important roles in were collected from Ft and Tg, respectively (Fig. 2). Sam- the benthic ecology of the Florida Keys, Bahamas, and the pling of the shells for isotopic analyses adhered to methods Gulf of Mexico (Menzel and Nichy 1958; Paine 1963a), described by Wefer and Berger (1991), with special care very little is known about their longevity and character- being taken to not penetrate deeper than 1 mm, to avoid the istics of shell growth. Moreover, it is unknown whether inner layers of shell, which may have a different isotopic shells of these species are viable for paleoenvironmental composition. Shell Ft was sampled at an approximate reso- reconstruction. By comparing carbon and oxygen isotopic lution of 3 mm, while Tg, the much larger shell, was sam- records of chronologically calibrated modern specimens pled every 6.2 mm. with instrument-derived environmental records, we provide a preliminary evaluation of the potential for each species as Stable isotopes analysis an environmental proxy and shed insight into lifespans and growth characteristics. Stable isotope concentrations are expressed in the conven- tional delta notation: R − R nX 0 sample standard Materials and methods δ ( /00) = × 1,000 (1) Rstandard Sampling where n is the mass number of element X, and R is the ratio of heavy to light isotopes in both the sample and standard. The modern F. tulipa (Ft) shell was collected live in 10 Aliquots of powdered aragonite were analyzed for stable meters of water on the seagrass oxygen and carbon isotopes using a Kiel II automated car- meadow near the Anguilla Cays of Sal Bank, Bahamas bonate device attached to a Finnegan Delta Plus mass spec- (23°30′N, 79°36′W), in May 2003. The Cay Sal Bank is a trometer in the Stable Isotope laboratory of the Rosenstiel

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Fig. 1 Map illustrating sam- pling localities, shown by gray stars, for Triplofusus giganteus (Tg) in the Florida Keys and Fasciolaria tulipa from the Cay Sal Bank. Black rectangle (MR) shows location of The Molasses NOAA Buoy, where ambi- ent water temperatures were recorded. Photographs of shells Tg and Ft are shown, along with individual shell lengths

Fig. 2 Sampling schematic for gastropod shells. Shown is an illustration of Fasciolaria tulipa (Ft) with samples taken along the growth axis of the shell starting at the apex and ending at the margin of the aperture

School of Marine and Atmospheric Sciences, Univer- precision for all samples was better than 0.07 ‰ for δ18O ± sity of Miami. Oxygen and carbon isotopic compositions and 0.03 ‰ for δ13C. ± were reported in per mil units (‰) relative to the VPDB Calculated temperatures were derived from δ18O meas- (Vienna PeeDee Belemnite) carbonate standard. Analytical urements using the paleotemperature equation developed

1 3 1596 Mar Biol (2014) 161:1593–1602 by Grossman and Ku (1986) where coefficients have been KNMS-CD/index.htm). We focused on stations 223, 224, adjusted for Vienna standard mean ocean water (VSMOW): 227, 232, and 233, which were proximal to the Tg collec- tion site. ◦ = − 18 − 18 T C 21.8 4.34 δ Oar δ Osw (2)

18 where δ Oar is the isotopic composition (in ‰) of shell Results and discussion 18 aragonite and the δ Osw is the assumed oxygen iso- topic composition of seawater relative to Vienna Standard Environmental temperature and salinity Mean Ocean Water (VSMOW). For a modern seawater δ18O value, we use a constant value of 1 ‰, derived from The Molasses reef buoy reported summer tempera- regional observations within the NASA GISS Global Sea- tures between 30 and 31 °C, with mean winter tempera- water Oxygen-18 database (Schmidt et al. 1999). Using tures dropping to 22 °C and occasionally to 20 °C. The this equation, the analytical precision of the δ18O measure- mean annual temperature range (MART) in this region is ments represents a potential error of 0.3 °C. 9–10 °C. Winter temperatures were considerably lower ± than normal during early 1998, a result of the strong El Environmental temperatures, salinity, and water quality Niño contributing to cooler and wetter conditions over southeastern (Changnon 1999). Wetter con- Instrumental temperature observations for the Hawk ditions also caused lower salinities during the 1997–1998 Channel locality were obtained from NOAA observa- winter, with seafloor values near 34 psu. Lower salinities tion buoy MLRF1 at Molasses Reef (25.01″N, 80.38″W; between 32 and 33 psu were measured during the winters http://www.ndbc.noaa.gov/station_page.php?station m of 1999–2000 and 2001–2002. Overall, salinities of the = lrf1). Due to the lack of local detailed temperature, Molas- Florida Keys locality averaged 36.1 0.7 psu and ranged ± ses Reef temperature records were used for the Cay Sal from 32.3 to 37.8 psu from 1995 to 2011. The lowest salin- Bank locality as well. The Cay Sal Bank is situated 180 km ities were measured during winter months and the highest southeast of the Molasses Reef Buoy, and average seasonal observed during summer. temperatures at the sea surface and 10 m depth show gen- eral uniformity between the Cay Sal Bank and the Florida Oxygen isotopes and shell growth Keys (Locarnini et al. 2010) (Fig. 3). Salinity, dissolved oxygen (DO), chlorophyll A, dis- The oxygen isotope profiles for both species show cycli- solved organic carbon (DOC), total nitrogen (TN), and cal trends, reflecting years of shell deposition throughout phosphorous (TP) concentrations for Hawk channel waters the lifespan of each mollusk, where warmer temperatures were obtained from data made available by the Southeast- are indicated by lower δ18O values and cool temperatures ern Environmental Research Center (SERC) at Florida by higher δ18O values (Fig. 4). Shell Tg δ18O show a larger International University (http://serc.fiu.edu/wqmnetwork/F range and higher average than Ft (Table 1). Tg δ18O values range from 1.4 to 0.7 ‰ and exhibit mean of 0.4 ‰. The − δ18O values of Ft ranged from 1.0 to 0.5 ‰, with a mean − of 0.3 ‰. − Because both specimens were collected live, their δ18O profiles could be chronologically calibrated to instrumental records. The final samples (closest to the apertures) on both shells were assigned a collection date, and the remainder of growth was temporally calibrated by assigning the δ18O maxima and minima to days of coldest and warmest water temperature observations. This is similar to the profile tun- ing performed on Conus gastropods by Gentry et al. (2008). Chronologically calibrated δ18O profiles (Fig. 5) closely match profiles of seasonality. Intraseasonal increases in temperature during the first quarters of 2001 and 2003 trends were also captured in the Tg record. The pro- files revealed Tg had been precipitated over 6 years at an approximate average rate of 0.4 mm d 1, and shell Ft had Fig. 3 Temperature relationship between Florida Keys and Cay Sal − Bank sampling data. Seasonal temperature averages are from the been precipitated over nearly 3 years at approximately 1 2009 World Ocean Atlas (Locarnini et al. 2010) 0.3 mm d− . Assuming that, with the exception of growth

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Fig. 4 Cross-plots (left) and stable isotope profiles (right) of shells Tg and Ft. The nega- tive relationship between δ13C and δ18O is shown for Tg. No relationship was found for Ft. Inset on cross-plots shows shell average values with error bars of 1σ. The isotopic profiles of Tg show seven summers and six winters (marked as S or W), while the Ft captures three sum- mer and winter cycles

Table 1 Summary of δ18O and δ13C of gastropod shell carbonate Shell Species Spiral length (mm) Lifespan (years) Max δ18O Min δ18O Mean δ18O Max δ13C Min δ13C Mean δ13C

Tg Triplofusus giganteous 700 6 0.7 1.4 0.4 3.0 1.0 0.6 − − − Ft Fasciolaria tulipa 220 3 0.5 1.0 0.3 2.1 0.4 1.2 − − All values reported relative to the VPDB standard cessations, these gastropods grew their shells in a nearly Calculated temperatures continuous manner, as the δ18O curves suggest, then the 0.5 mm sampling diameter reflects roughly 2–3 days of Changes in the isotopic composition of seawater due fresh- growth. Thus, T. giganteus and F. tulipa have the potential water mixing and evaporation can introduce error in isotop- to yield records at sub-weekly scales by hand sampling ically calculated temperatures (Ingram et al. 1996; Strauss with a standard dental bur, and higher resolution records et al. 2012a). Salinity variation on the Cay Sal Bank is can likely be captured with more precise sampling. minimal due to limited freshwater influence. Conversely, The calibrated δ18O profiles revealed ontogenetic the southeastern Florida shelf experiences salinities as low changes in the growth of each shell (Fig. 6). Tg growth as 32 psu during winter months. Freshwater runoff from 1 18 rates varied between approximately 0.2–0.6 mm d− , with Florida waters has a relatively high δ O, so the impact of the first 3 years (excepting the second half of the 2nd year) freshwater discharge is not the same magnitude as other deposited faster than the latter three. Ft showed a similar localities (e.g., the northern Gulf of Mexico; Strauss et al. growth trend with overall slower growth rates, but also 2012b). An average of intercepts drawn from regressions 1 18 grew rapidly, to above 5 mm d− , in its final year. The of δ O and salinity for Florida Bay waters revealed the overall growth patterns of Ft and Tg share similarities with δ18O of freshwater discharge to be roughly 1 0.9 ‰ − ± modern Strombus (Wefer and Killingley 1980), Conus (Swart and Price 2002). If normal salinity is assumed to be (Kobashi and Grossman 2003; Gentry et al. 2008), and fos- 36 psu and δ18O equal to 1 ‰, a decline to 32 psu would be sil (Purton and Brasier 1997), where accelerated reflected by a lowering of shelf δ18O to 0.8 0.1 ‰. In the ± growth for the first 2–3 years of life were observed. Cli- context of temperature reconstruction, this would translate mate, food availability, sex, and ecological stress may play to a “warming” effect of 1 0.4 °C. ± an important role in the growth habits of these mollusks The tuned δ18O profiles (Fig. 5) reveal that summer tem- (G. Dietl, pers. comm.); therefore, limited conclusions on peratures are more accurately recorded in the shells than growth can be drawn from this study since only single male winter temperatures. Isotopically calculated mean annual specimens were examined. temperature (MAT) values closely matched instrumental

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Fig. 6 Shell growth rates, calculated from chronologically tuned pro- files, for shells Tg and Ft. Horizontal axis represents years of growth beginning from each shells apex

Table 2 Oxygen isotope temperatures calculated using Grossman and Ku (1986)

18 Shell δ Osw Max °C Min °C MART °C MAT °C

Tg 1.14 30.9 (31.5) 21.8 (20.3) 6.9 (9.4) 26.5 (26.7) Ft 1.14 29.3 (30.4) 22.8 (20.5) 5.6 (9.3) 26.7 (26.5)

18 The value for δ Osw is relative to VSMOW. Calculated mean annual temperature (MAT) and mean annual temperature range (MART) are compared with instrumental values (shown in parentheses), calculated over the span of individual shell growth

observations, with the isotopic MAT of each shell within 0.2 °C of the instrumental MAT (Table 2). Both shells underreport the MART by between 2.5 and 3.7 °C. Shell Tg captured an average of 73 % of the seasonal tempera- ture range, whereas Ft only captured an average of 60 % of the seasonal range. These results are comparable to results from Conus ermineus shells collected in the Gulf of Mex- ico that capture 69 % of the MART (Gentry et al. 2008). The inability of Tg and Ft to capture the complete MART is likely due to of slowed or discontinued shell growth cou- pled with a too low sampling resolution. For shell Tg, it Fig. 5 Chronologically tuned isotopic profiles of Tg and Ft.A ver- may also be in part due to lower shelf water δ18O during age daily water temperatures from Molasses Reef are illustrated underneath shell δ18O values. Both Ft and Tg capture the warmest winters. More promising, summer temperatures were cap- summer temperatures, but fail to capture temperatures below 23 °C. tured to within 1 °C in Tg, and Ft yielded similarly accu- Carbon isotope profiles are shown separately. Tg δ13C values increase rate temperatures during the second and third summers. Ft with shell age, Ft exhibits higher values during summers. The bot- underreported its first summer temperatures by approxi- tom plot shows chronologically calibrated calculated temperatures plotted against instrumentally measured temperatures. A 1:1 slope is mately 2 °C, however, that is likely due to uncertainty on shown in gray. The high scatter is likely due to poor time calibration the timing of first shell growth. between seasonal extremes, resulting in poorly matched temperatures Regressions of chronologically calibrated calculated during seasonal transitions temperatures against measured temperatures showed

1 3 Mar Biol (2014) 161:1593–1602 1599 significant relationships, but lacked the high level of cor- The most notable behavior of the Tg δ13C curve was a relation that might be expected based on visual comparison trend of increasing δ13C, to 3.0 ‰, toward the shell margin, of oxygen isotope profiles and temperature records (Fig. 5). where peaks become antiphase with δ18O during years 2002 Linear regressions fall close to a slope of one, with Tg and 2003. This trend may be indicative of shifting meta- yielding a regression model: bolic influence (vital effect); however, ontogenetic trends are reflected as decreased δ13C values in predatory Conus T = 0.94T + 1.91 m i (3) shells (Gentry et al. 2008) and other mollusks (Krantz et al. with an R2 0.61, and Ft showing a similar relationship: 1987; Lorrain et al. 2002, 2004). Thus, it is more likely that = increasing δ13C in Tg resulted from increasing δ13C of DIC. T = 0.99T + 0.32 m i (4) Transects of δ13C within particulate organic carbon (POC) with an R2 0.48, where T is the measured temperature, have shown inshore POC to be enriched in δ13C by as much = m and Ti is the isotopically calculated temperature (°C). The as 4 ‰ relative to reef POC (Lamb and Swart 2008), and scatter is most likely due to several factors: insufficient through oxidation, this may be reflected in the DIC pool. chronologic calibration, winter growth cessation under- Thus, the increasing δ13C of Tg may possibly reflect the reporting cooler temperatures, and influence of varying gradual migration of the mollusk inshore to shallower 18 δ Osw. The latter occurred mainly in shell Tg from the waters of Hawk Channel, where it was collected. Our field more marginal environments of the Florida Keys. Of note, observations also show that this is the area where the egg in both shells, calculated temperatures above approximately capsules of the T. giganteus are most commonly encoun- 28 °C are lower than measured values, while the major- tered, so it is possible that specimens reaching sexual matu- ity of calculated temperatures below 28 °C are higher than rity move in these waters for spawning. Unfortunately, measured temperatures (Fig. 5). These results correspond besides predator–prey relationships, little detail is known to the lower MART reported by both shells (Table 2). about the behavior of this species to further support this interpretation. Carbon isotopes The δ13C and δ18O of Tg exhibited a significant negative correlation (p < 0.01, r2 0.13) where: = While mollusk shell δ13C has been shown to mainly reflect 13 =− 18 + the δ13C of the dissolved inorganic carbon (DIC) pool of δ C 0.58δ O 0.4 (5) 13 ambient water (δ CDIC) (Gillikin et al. 2007; McCon- Focusing only on the last 250 mm of shell growth, where naughey and Gillikin 2008), interpretation is often difficult. δ18O and δ13C peaks were antiphase, results in a slightly This is because the δ13C reflects carbon cycling (Hayes improved model (r2 0.16), where DIC = et al. 1999), air–sea exchange (Broecker and Maier-Reimer 13 =− 18 + 1992), and freshwater mixing (Mook and Tan 1991). Fur- δ C 0.65δ O 1.0 (6) 13 18 thermore, metabolic (respired) CO2 can be incorporated While positive correlations between δ C and δ O of into the extrapallial fluid from which the shell is precipi- the shell have been observed in Gastropods from the Carib- tated, resulting in δ13C values substantially lower than the bean waters, where lowered salinity from freshwater run- DIC. In filter-feeding bivalves, the influence of the meta- off relate to lowered shell δ13C and δ18O, negative correla- bolic CO2 has been estimated to represent, at the most, tions have mainly been associated with upwelling of cooler, 10 % of the shell δ13C (McConnaughey and Gillikin 2008). low-δ13C waters (Bemis and Geary 1996) or the incorpora- Though the trophic level may influence shell δ13C, both tion of deeper low-δ13C shelf waters caused by increased grazing and carnivorous gastropods have yielded interpret- mixing-depth during winter (Purton and Brasier 1997). able environmental information (e.g., seasonal upwelling) Although not seasonal, upwelling events do occur on the from their δ13C records. (Geary et al. 1992; Strauss et al. eastern Florida shelf, and in the Florida Keys, it has shown 2012a, b; Tao et al. 2013). Both shells yield positive δ13C to drop water temperatures by about 6 °C along the reef values with similar δ13C ranges (Table 1). The δ13C val- tract (Leichter and Miller 1999). Some of the intraseasonal ues of Tg ranged from 1.0 to 3.0 ‰ and averaged 0.6 ‰. δ13C and δ18O variability within the shell may be related to − Values showed little variation over the initial 275 mm of this high-frequency upwelling. shell growth. From 275 to 350 mm, a clear negative excur- The negative correlation may also reflect increased sion exhibited values stabilizing at 0.8 ‰, following that, exchange of low-δ13C Florida Bay waters with the eastern − values slowly increased for the remainder of shell growth. Florida shelf waters. Florida Bay mollusk shells typically The Ft δ13C values ranged from 0.4 to 2.1 ‰ with a mean exhibit δ13C as low as 4 ‰ within enclosed bay waters − of 1.2 ‰. Three high-δ13C excursions from 75 to 90 mm, reflecting low δ13C of DIC caused by oxidation of terres- 120–135 mm, and 195–213 mm yielded values ranging trial organic material (Lloyd 1964). Similarly, coral δ13C from 1.8 to 2.1 ‰. records have revealed influence of Florida Bay waters on

1 3 1600 Mar Biol (2014) 161:1593–1602 the eastern Florida Keys shelf (Swart et al. 1996). We pre- oxidation of Thallasia leaf litter may have contributed to fer this scenario because of the subdued salinity effects on the lower δ13C values of Ft shell during winter months. seawater δ18O by high-δ18O freshwater runoff (Swart and Price 2002). Comparison of Tg carbon isotope records with SERC Conclusions water quality data shows that salinities were lowest in Hawk Channel during the winter months, with values We present oxygen and carbon isotope profiles of two approaching 33 psu. These low-salinity periods corre- modern fasciolariid shells from the Florida Keys and sponded to lower δ13C values in Tg during years 2002, Cay Sal Bank. Oxygen isotopes revealed the T. gigan- 2003, and 2004. Winter months were relatively dry com- teus sampled grew its shell for at least 6 years, and the F. pared to summer months, limiting the influence of fresh- tulipa grew its shell over 3 years. Both mollusks exhib- water runoff. Average salinities of Florida Bay waters from ited faster growth rates during the first half of the shell 1998 to 2005 ranged from 24 to 42 ‰, with lowest values growth span. Oxygen isotope calculated temperatures associated with wet El Nino winters of 2002 and 2004 and of both shells closely matched instrumentally measured landfall of Hurricane Irene in late 1999. The arrival of Hur- means, but failed to capture the entire seasonal tempera- ricane Irene to the shelf in fall of 1999 coincided with a ture ranges. We found the F. tulipa and T. giganteus shells significant decrease in Tg δ13C. This was concurrent with examined to capture 61 and 73 % of measured seasonal a reduction in salinity from 37 to 34 psu. Hurricane Irene temperature ranges, respectively, which is comparable to contributed to significant flooding on the mainland, leading isotopic temperature records of other gastropods. Both to large nutrient and labile organic carbon fluxes into Flor- species examined in the present study yielded accurate ida Bay (Davis and Yan 2004). The oxidation of hurricane- calculated temperatures. While regressions of chrono- fluxed organic carbon would have lowered δ13C of DIC in logically tuned isotopically calculated temperatures with Florida Bay waters. Hurricane Irene also induced eutrophi- measured temperatures show significant scatter, this is cation in Pamlico Sound of North Carolina (Paerl et al. likely related to limitations in the chronologic calibration 2001). The lower δ13C of Tg during this period may have during seasonal transitions, winter growth cessations, reflected hurricane-induced effects on the eastern Florida and variability of the oxygen isotopic composition of the shelf or the exchange of low-δ13C waters from Florida Bay. water. Of the two specimens we examined, T. giganteus While this low-δ13C event did correlate with an increase in provided a longer and more precise paleotemperature 1 chlorophyll A from 0.4 to 1 mg L− , DOC, TN, and TP did record than F. tulipa. It must be noted that this finding is not show any similar shifts. Overall, the Tg δ13C record did limited by the sampling and study of single specimens, not share any trends with SERC DO, DOC, and nutrient as growth rates and habits may be influenced by environ- data. ment and physiology. In shell Ft, high-δ13C excursions were separated by low- Interpretation of carbon isotopes of both shells were less δ13C values near or below 1 %, with heavier δ13C values certain. The δ13C record of F. tulipa was almost in antiphase persisting during the summers and lighter values during with δ18O values, possibly suggesting seasonal variations to the winter. The Cay Sal Bank is isolated from the main- the carbon cycle mediated by the expansive seagrass mead- land, so that it is practically free of freshwater effects on ows from which it was collected. However, we found no 13 13 13 δ CDIC. As a result, the δ C of Ft may reflect upwelling, correlation between T. giganteus δ C and nutrient, and in situ carbon cycling, and/or potential metabolic effects. DOC and DO concentrations, a singular event of decreased Upwelling primarily occurs on the northwestern region of values correlates with the landfall of Hurricane Irene, most the Cay Sal Bank bordering the Straits of Florida and is the likely due to a high flux of freshwater from the Everglades subject to the fluctuations within the Florida (Lee and Florida Bay. The T. giganteus shell also shows a trend et al. 1995). The influence of cold, low-δ13C waters would to increasing values with ontogeny, possibly reflecting lower both δ18O and δ13C of shell carbonate. The δ18O and migration to more inshore environments. δ13C values showed no significant relationship, suggesting limited or no upwelling influence. Because this specimen Acknowledgments We thank Amel Saied and Corey Schroeder of was collected at 10 m depth in expansive Thallasia testudi- the University of Miami RSMAS stable isotope laboratory for their 13 assistance with stable isotope measurements and Ethan Grossman for num meadows, the δ C values may track T. testudinum— providing a review of an early draft of the manuscript. This manu- mediated carbon cycling. Seagrass communities are some script was also improved by helpful comments from Gregory Dietl, of the most productive marine ecosystems, comparable Chris Harrod, and two anonymous reviewers. We thank the Depart- with mangroves in net primary productivity (Mateo 2006). ment of Fisheries of the Commonwealth of the Bahamas for issuing a permit to collect in Cay Sal Bank. This study was supported by a During the autumn continuing into winter, seagrass litter is student grant provided by the Conchologists of America and in part generally highest (Mateo and Romero 1997). The microbial by National Science Foundation Grant OPP 0095095.

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