Variations in the biological characteristics of the patagonian scallop (Zygochlamys patagonica) across the argentine shelf break front.
KEY WORDS: scallop, Zygochlamys, Patagonias, biological characteristics, spatial variations, traits, marine fronts, SW Atlantic
The biological characteristics of bivalve populations are strongly influenced by environmental variations (Bricelj & Shumway 1991, Thompson & MacDonald 2006, Blanchette et al. 2007) at different scales (macro, thousands of kilometers; meso, from meters to thousands of kilometers; and micro, from centimeters to meters; Orensanz (1986)). Classic studies in bivalves focused predominantly in costal and estuarial areas demonstrated the importance of environmental factors such as salinity, temperature, depth, and primary productivity on the metabolic processes of bivalves. Such environmental characteristics act as regulators of these processes at the macro and meso scales (Orensanz 1986, Bricelj & Shumway 1991, Dame 1996). However, for open and deep marine areas, where physical parameters have weaker variations, phytoplankton production in the overlaying water column generally becomes the most important factor to regulate the metabolic processes of bivalves (Dame 1996).
Temperature and food supply play a substantial role in shaping the life history characteristics of bivalves, such as growth, reproduction, and accumulation of reserves (see condition indices (Dame 1996, Pilditch & Grant 1999, Thompson & MacDonald 2006)). The range of environmental variations is one of the main factors driving the settlement of bivalve populations. Food availability may become the most important factor when temperature shows low variability (Thompson & MacDonald 2006) or over rides the effect of other factors (e.g., sublethal levels of hypoxia (Norkko et al. 2005). Indeed, several field studies have demonstrated the primacy of variations in food availability in determining observed growth rates (MacDonald & Thompson 1985, Bayne 2004, Heilmayer 2004) and/or influencing the reproductive cycle (Thompson & MacDonald 2006).
Variations in scallop biological characteristics have received significant attention because of their importance in aquaculture and worldwide fisheries (Orensanz et al. 2006). In this sense, special considerations regarding diet, condition indices, and reproductive cycles are influenced by both temperature and food availability (Barber & Blake 2006, Thompson & MacDonald 2006). Furthermore, an increased life span and a larger attainable size are also adaptations to variations in those parameters (Heilmayer 2004). Although most studies in scallop development show the important role of seasonal food availability on animal condition (Lorrain et al. 2002, Thompson & MacDonald 2006), it is not completely understood how mesoscale differences in food availability on the ocean floor could promote variation in their biological characteristics. Spatial differences in food availability are often imposed by oceanographic structures (e.g., upwelling, eddies, fronts) at distinct temporal and spatial scales (Mann & Lazier 1996), thus influencing the animal's development (Bonardelft et al. 1996, Menge et al. 1997).
The shelf break grounds of the Patagonian scallop, Zygochlamys patagonica (King & Broderip, 1832), in the Argentine Sea (36-48[degrees]S) are strongly associated with environmental variations (Bogazzi et al. 2005). On a macro scale, this species is associated with oceanographic frontal systems, likely resulting from the increased productivity observed in these areas and certain hydrographic mechanisms that can facilitate the retention and/or concentration of pelagic larvae (Bogazzi et al. 2005). Therefore, the mechanisms that influence primary productivity also appear to be responsible for different morphological traits, such as growth patterns, maximum age, and growth across beds (Lomovasky et al. 2008). On a meso scale, high scallop abundance and recruitment are located at the bed side closer to the Shelf Break Front (SBF) mean position, which matches with the highest mean chlorophyll a concentration (Mauna et al. 2008). In this sense, a generalized additive model shows that scallop abundance decreases at lower latitudes, lower chlorophyll a values, and greater depths (Gutierrez et al. 2008). Together with abundance and recruitment patterns, the scallop spawn could match larval development with the spring bloom to enhance larval survival chances (Campodonico et al. 2008). Regional evidence suggests that oceanographic features may affect the Patagonian scallop's biology even in a range of only a few kilometers. Therefore, we evaluated scallop characteristics (through size-mass relationships, conditions indices, and muscle and gonad isotopic signature variables) in relation to frontal position and phytoplankton concentrations across the SBF in the southwest Atlantic Ocean.
MATERIALS AND METHODS
The study was performed in an area influenced by the SBF (38-39[degrees]30' S, 55-56[degrees] W; Argentine Sea; Fig. 1). This is a long (>1,200 km) and narrow (<40-80 km in surface) thermohaline front that runs in a south-northeast direction along the Patagonian Shelf Break (Parker et al. 1997, Bogazzi et al. 2005, Franco et al. 2008). At the surface, the front closely matches the 200-m isobath position, and its displacements may be associated with areas of weaker slope at the bottom topography (Franco et al. 2008). This front is the consequence of encounters between diluted (33.2-33.8 psu) nutrient-poor shelf waters and the cold, saltier (33.8-34.2 psu) nutrient-rich waters of the Malvinas Current (Martos & Piccolo 1988, Lutz & Carreto 1991). The SBF presents strong gradients in thermal fields (>0.08[degrees]C/kin; see, for example, Martos and Piccolo (1988)) primarily during the summer, but it is weak in gradients of salinity (0.002 ups/km (Romero et al. 2006)). Across-front gradients of sea surface temperature (SST) and chlorophyll a were reported in this area (Mauna et al. 2008). The front showed seasonal displacements with onshore shifts during austral autumn and winter, and more offshore positions during summer and spring. The SBF annual mean position is located at the eastern boundary of the scallop northern ground, whereas the zonal displacement may reach -37 km, sweeping nearly the entire scallop ground during the year (Mauna et al. 2008). Studies have suggested that the upwelling processes at the shelf break could lead to the development of the high chlorophyll a satellite band associated with the SBF (Romero et al. 2006, Matano & Palma 2008). This band of high chlorophyll a presents a strong annual cycle, with maximum values during austral spring and summer (Romero et al. 2006).
[FIGURE 1 OMITTED]
Position and Variability of the Shelf Break Front
In areas containing the main Patagonian scallop beds (Bogazzi et al. 2005), we used satellite data to determine the mean position of the SBF and the surface chlorophyll a concentrations during August through October 2005. This period is based on previous studies about the time that scallop tissue need to integrate dietary isotope ratios (15 days-3 mo (Fila et al. 2001, Lorrain et al. 2002, Paulet et al. 2006)). Monthly averages of satellite-derived SST were used to study the frontal mean position and its spatial variations. Because the SBF develops near the transition from warm shelf waters to colder Malvinas Current waters, it is associated with a band of a negative maximum SST gradient (relatively strong offshore temperature decreases) near the shelf break (Franco et al. 2008). Zonal SST gradients were used along with a centered difference scheme based on monthly averages (MODIS/Aqua-derived SST) obtained from the National Aeronautics and Space Administration Physical Oceanography Distributed Active Center at the Jet Propulsion Laboratory (Pasadena, CA) (http://poet.jpl.nasa.gov).
Analyses of satellite-derived chlorophyll a concentrations (as a food supply proxy) were used based on monthly sea surface color images from standard mapped images, which were supplied by the Sea-viewing Wide Field-of-view Sensor (Sea WiFS). The standard mapped images are derived from level three monthly binned data and mapped with a resolution of 0.09[degrees] x 0.09[degrees] in a 2-dimensional array of an equidistant cylindrical projection (http://daac.gsfc.nasa.gov). Following Yoder (2000) and Romero et al. (2006), the satellite-derived sea surface chlorophyll a concentration will be referred thereafter to as CSAT (measured in milligrams per cubic meter).
Cross-Frontal Variation in Mass-Size, Condition Indices, and Isotopic Signatures of Zygochlamys patagonica
Samples were obtained from two transects nearly perpendicular to the SBF in a scallop ground. The northern transect (NT; located at 38044' LS) and the southern transect (ST; located at 38[degrees] 24' LS) were 34 km and 30 km in length respectively, and were separated by 40 km (Fig. 1). Samples were collected from 4 sampling stations--west (W), center west (CW), center east (CE), and east (E) at each transect by the Argentinean RV Capitdn Canepa October 19-20, 2005 (mean distance between them at each transect ~11 km and 9.6 km, SD = 3.3 and 4.7, respectively), with a nonselective dredge (1-cm mesh size). These sampling stations covered the spatial variation of the frontal position at the surface of approximately 40 km (Mauna et al. 2008).
Subsamples of 6-7 kg unsorted catch were randomly obtained and frozen at 20[degrees]C for laboratory measurements. Animals were sorted in the laboratory. To avoid differences in energy allocations in nonreproductive individuals, only those scallops with a shell height higher than 40 mm (n = 328) were used (Campodonico et al. 2008). Epibionts were removed by brushing the shell. Shell height (umbo to the ventral margin), gonad mass, and adductor muscle mass were also determined with precision [+ or -] 0.01 mm and [+ or -] 0.01 grams, in height and mass, respectively.
The null hypothesis of no differences in the slope of the relationship between tissue mass (gonad mass and adductor muscle mass) and height on scallops located across the front was evaluated with slope parallelism tests (Zar 1999). To enhance overall data set homogeneity, we restricted the scallop individuals to shell heights ranging from 45-65 mm. Multiple comparisons among slope tests (Zar 1999) were used to detect site-specific differences in slope for both tissue types.
To evaluate the relationship between the frontal position and scallop condition, we considered gonad and muscle tissue across the front. Thus, to evaluate the null hypothesis of no differences in the condition indices (CIs) across the front and between transects, we used a 2-way ANOVA (Zar 1999). The gonad relative condition index (RCI) was log-transformed to fulfill the analysis assumption.
Muscle and gonad RCIs were calculated using the following equation:
CI = mass/[height.sup.b]
where b is the slope of the regression of the corresponding overall mass size relation
log (mass) = a + b x log (height)
normalized to: RCI = (CI - mean CI)/SD of CI (Lasta et al. 2001).
To evaluate the relationship between frontal position and scallop diet, we obtained isotope-stable data from muscle and gonads sampled from stations across the front (Fig. 1). Four or 5 pools of at least 5 scallops each were processed, dried at 60[degrees]C, and ground to a fine powder. Stable isotope signatures for 813C and [[delta].sup.15]N were determined by mass spectrometry (Lajtha & Michener 1994) at the University of California-Davis Stable Isotope Facility. All isotopic data are given in the conventional delta notation, units of parts per thousand (%) relative to the Vienna Pee Dee Belemnite standard as follows:
[[delta].sup.13][C.sub.sample] = ([R.sub.sample]/[R.sub.standard] - 1) x 1,000 where R = [sup.13]C/[sup.12]C.
The carbon (C) isotope indicates relative contributions from differing food sources, whereas there is a stepwise enrichment of nitrogen (N) isotope at each trophic level (Peterson & Fry 1987). We considered muscle tissue a better indicator of food shifts than gonads because of the role of tissue as energetic storage, which was consistent with previous studies on this species (Botto et al. 2006). We also used the isotopic signatures of gonads, which provide information on different turnover rates with respect to muscle isotopic signature values (Lorrain et al. 2002). Gonads at W and E stations were not included in the analysis because of after-sample preparation difficulties. The C/N ratio of scallop muscle and gonad were also calculated from the C and N isotope stable concentration. Given C and N data heteroskedasticity (Levene's test, P < 0.01), ANOVA was still performed, because this test is robust to the heterogeneity of variance. However, the level of significance was set at [alpha] = 0.01 to reduce the risk of a type I error (Underwood 1997).
The null hypothesis of no differences in isotopic signatures and C/N ratio of scallop muscle and gonad across the SBF and between transects was evaluated with 2-way ANOVA (Zar 1999). Post hoc differences in averages between different stations were evaluated with a Bonferroni test (Zar 1999).
Position and Variability of the Shelf Break Front
The SBF is defined by the encounter of warm shelf waters with colder Malvinas Current waters, and it is associated with negative maximum SST zonal gradients near the shelf break region (Fig. 1). In the ST, the negative maximum SST zonal gradients were located primarily in the eastern positions of the scallop ground (CE and E stations), reaching gradients higher than 0.04[degrees]C/km during August and September (Fig. 2). The highest CSAT concentrations were located at the W station during August and October, whereas the highest values (>7 mg/ [m.sup.3]) during September were located at the E station, matching the mean front position (Fig. 2). In the NT, the maximum negative SST zonal gradients were located primarily in the CE sector of the scallop ground, reaching negative gradients higher than 0.08[degrees]C/kin during August (Fig. 2). High SST negative gradients (>0.04[degrees]C/kin) were located over the center (between CW and CE sectors) during September. The highest CSAT concentrations were reached in September (>7 mg/[m.sup.3]), located east of the CW sector, and CSAT concentrations were lower than 2 mg/[m.sup.3] and uniformly distributed across the ground during the other months. CSAT concentrations showed higher spatial variability than the SBF mean position (Fig. 2).
Cross-Frontal Variation in Mass-Size, Condition Indices, and Isotopic Signatures of Zygochlamys patagonica
All mass--size regressions (log transformed) were linear except for gonad mass-height at the CW station (Table 1). All gonad-height slopes, at both transects, were different among them (P < 0.001), showing a decreasing order in the ST from W to CE to E, and decreasing order in the NT from W to E to CE. In the ST, the slope for muscle-height relationships at CE and CW stations were equal, but differed from the rest to show a decreasing order slope from W to E to CW to CE (Fig. 3). In the NT, the muscle-height slopes showed a decreasing order from W to CE to E.
Both muscle and gonad RCI did not show differences across the front or between transects (Table 2).
Gonad and muscle C stable isotopic signatures and C/N ratios showed differences across the front and between transects. In some cases, there was an interaction between those last variables (Table 2). The 813C values showed no differences in gonads across the front or between transects, whereas muscle values showed an interaction between the front and transects. The lowest [[delta].sup.13]C values of muscle were located at the W station and there was no clear pattern at other stations (Fig. 4). The 815N values showed the same pattern in both gonad and muscle, with the higher values located at the NT and no detected interaction. No interaction was detected for the C/N ratio, which had different patterns in gonads and muscles. The gonad C/N ratio was influenced by the transect; the muscle C/N ratio increased its value from W to E, with the highest values found at the NT (Fig. 5, Table 2).
[FIGURE 2 OMITTED]
Our results show variations in the biological characteristics of the Patagonian scallop in relation to the SBF position. Scallops located away from the front have both the lowest C isotopic signature and a lower C/N ratio than those near the front. These variables, together with the N isotopic signature, differ between transects but show the same pattern across the front. However, the lack of shift in scallop organ conditions suggest that, during the time of year we sampled, spatial differences in food supply are not strong enough to have an impact on scallop development, as may happen at a seasonal scale.
The muscle/gonad mass-height relationship shows different slopes for the different stations across the front, indicating that these relationships may intersect at certain heights. Scallops at the W station on both transects have the highest slope, meaning that scallops located farther from the front have smaller initial muscle/gonad mass than scallops located closer to the front, whereas scallop shell heights increase in a reverse pattern. For the northern distribution of this species, there are differences in mass-size relationship among latitudes. Moreover, from these relationships the predicted mean weight for a shell height of 60 mm increased asymptotically with latitude (Gutierrez & Defeo 2005). In particular, gonad-height relationships that have lower slopes or no relationship at all (the CW station on the ST) suggest a balanced presence of maturation and spawning stages in the sampled scallops, which may be caused by asynchrony in spawning activity. Indeed, there is evidence of previous spawning asynchrony in the same region (Campodonico et al. 2008). Because of this asynchrony and the high variability of muscle and gonad mass throughout the year (Lasta et al. 2001, Campodonico et al. 2008), it is difficult to interpret which site has more favorable environmental conditions. Thus, differences in slope should be used solely as an indicator of better muscle and gonad condition, which must be corroborated with condition indices.
Reproductive strategy components, such as fecundity, timing of gametogenesis, and spawning, evaluated by condition indices, are often highly variable for scallop between sites or depths within a relatively small geographical region (Bricelj & Shumway, 1991). However, there were no differences between scallop muscle RCI or gonad RCI across the front, considering distances more than 10s of kilometers. This may be the result of high intrasite variability in reproduction synchrony, where part of the population supports gonad production through muscle energy transfer whereas another part starts the storage process. These mechanisms respond to a diphase between soma and gonad production, where soma is built in summer and early autumn (November to March) and gonads are built in late autumn and winter (July to September) (Lomovasky et al. 2007, Campod6nico et al. 2008). No variation in organ conditions is found at different estuaries, but differences in scallop isotopic signatures are found in Argopecten irradians (Lamark 1819) (Shriver et al. 2002). In this case, the lack of significant relationships between condition index and chlorophyll a, total suspended particulate matter, or inorganic matter are likely the result of excess food supply or the lack of significant temperature differences among sites (Shriver et al. 2002).
[FIGURE 3 OMITTED]
Although there is no available data on sediment organic matter content or total suspended particulate organic matter in the study area, chlorophyll a analysis all along the water column shows seasonal variation in chlorophyll a concentration at the bottom, with lower values during winter and summer (around 0.3 mg/[m.sup.3]) and higher values during spring (1-2 mg/[m.sup.3]) (Carreto et al. 1995). These studies suggest that an important amount of phytoplankton production reaches the bottom, supporting the benthic communities (Schejter et al. 2002, Bogazzi et al. 2005, Botto et al. 2006). Moreover, along the annual feeding cycle, Z. patagonica shows a predominance of phytoplanktonic diatoms in its stomach content, with maximum food ingestion in spring, which suggests that year-round, and especially after the spring phytoplankton bloom, oceanographic conditions allow sedimentation of food particles to the bottom (Schejter et al. 2002). Furthermore, stable isotope studies also show a phytoplanktonic diet composition in many suspension feeder species such as sponges and epibionts of Z. patagonica (Botto et al. 2006). Thus, high densities of diatoms in the study area (Lutz & Carreto 1991, Gayoso & Podesta 1996), the likelihood of high diatom sedimentation rate even during stratification conditions (Hansen & Josefson 2001), and the most dense bed of Z. patagonica associated with the SBF (Bogazzi et al. 2005) confirm that phytoplankton is an important food supply for the suspension feeders associated with the SBF. Thus, the lack of variation in Z. patagonica condition indices may be more related to the time of year rather than to the influence of the front position on food supply to the bottom.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
C stable isotope signatures of muscle suggest a differential diet composition across the front (Peterson & Fry 1987), whereas the C/N ratio suggests a change in diet quality (Shriver et al. 2002). Based on scallop muscle C signatures, our results suggest a phytoplanktonic food source at all stations, with differences in stations located farther from the front (W stations) caused by lower C enrichment. The idea of phytoplankton as a main food source for this species is supported by stomach content analyses showing phytoplanktonic diatoms as the main dietary component (Schejter et al. 2002) and stable isotope analysis in the area (Botto et al. 2006). Lower C-enriched muscle away from the front may be the result of a mixed diet that includes phytoplankton and detritus. Such muscle may also be caused by resting stages of benthic diatoms or C-depleted phytoplankton near the front, which grow at a higher rate and accumulate more of the heavier isotope (Fry & Wainright 1991). We discard the notion that diet shifts are caused by feeding selectivity, because there was no reduction in stable isotope signature variability (i.e., SD). Thus, evidence supports the hypothesis of a shift in food quantity and quality, which could be promoted by the frontal position.
In summary, despite the lack of shifts in scallop condition indices in relation to frontal position or CSAT at the surface, the C stable isotopic signature and C/N ratios of scallop muscle suggest that there are differences in food supply across the front. These results are a good example of the complicated interactions between oceanographic structures, food supply, and scallop characteristics, with front position playing an important role.
This work was supported by INIDEP, the Universidad Nacional de Mar del Plata EXA 437/08, the Fundacion Antorchas (Argentina; 13900-13), CONICET (Argentina PIP2851), and Glaciar Pesquera (all to O. I.); and a grant from the Inter-American Institute for Global Change Research CRN 2076, which is supported by the U.S. National Science Foundation (grant GEO-0452325). C. M. was supported by a PhD scholarship from CONICET-Glaciar Pesquera (Argentina). This is INIDEP contribution no. 1622.
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A. CECILIA MAUNA, (1) * BETINA J. LOMOVASKY, (2,4) BARBARA C. FRANCO, (3) MATIAS J. SCHWARTZ, (1) FLORENCIA BOTTO, (2,4) E. MARCELO ACHA, (1,2) MARIO L. LASTA (1) AND OSCAR O. IRIBARNE (2,4)
(1) Instituto Nacional de Investigacitn y Desarrollo Pesquero (INIDEP), Paseo V. Ocampo No 1, Mar del Plata, B7602HSA, Argentina; (2) Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Rivadavia 1906, Buenos Aires, 1033, Argentina," (3) Departamento Oceanografia, Servieio de Hidrografia Naval (SHN), Av. Montes de Oca 2124, Buenos Aires, D1270ABV, Argentina; (4) Lab. Eeologia, Departamento Biologia, FCEyN, Universidad Naeional de Mar del Plata, Funes 3250, Mar del Plata, B7602HSA, Argentina
* Corresponding author. E-mail address: cmauna-fuentes@inidep. edu.ar
TABLE 1. Regression parameters of the relationship between both muscle (M) and gonad mass (measured in grams) with shell height (H, measured in millimeters) across the Shelf Break Front at the north and south transects, following the relation log M = a + b log H. Station Transect Y a b r P n E North Muscle -3.49 2.33 0.88 <0.005 92 Gonad -2.64 1.63 0.41 <0.005 92 South Muscle -6.26 0.16 0.88 <0.005 61 Gonad -2.12 0.05 0.41 <0.005 61 CE North Muscle -3.54 2.35 0.89 <0.005 78 Gonad -1.15 0.78 0.30 <0.005 78 South Muscle -6.00 0.16 0.88 <0.005 131 Gonad -0.59 0.02 0.29 <0.005 131 CW South Muscle -5.83 3.57 0.89 <0.005 104 Gonad -3.13 1.62 0.04 0.13 104 W North Muscle -3.92 2.58 0.93 <0.005 23 Gonad -3.11 1.91 0.64 <0.005 23 South Muscle -6.09 3.74 0.93 <0.005 25 Gonad -9.28 5.16 0.52 0.01 25 Only scallops between 45-05 mm in height. No data in CW station at north transect. CE, central east; CW, central west; E, east; W, west. TABLE 2. Analysis of relative condition index (RCI), isotopic signatures, and C/N ratio on both scallop muscle and gonad across the Shelf Break Front between transects and among stations (2-way ANOVA, a = 0.01). Variable Location df MS Muscle RCI Transects 1 0.778 Stations 2 2.139 Transects X stations 2 0.628 [delta].sup.13]C Transects 1 0.138 Stations 3 1.500 Transects X stations 3 0.270 [delta].sup.15]N Transects 1 17.029 Stations 3 0.105 Transects X stations 3 0.052 C/N Transects 1 1.346 Stations 3 0.067 Transects X stations 3 0.004 Gonad RCI Transects 1 0.021 Stations 2 0.226 Transects X stations 2 0.139 [delta].sup.13]C Transects 1 0.046 Stations 1 0.264 Transects X stations 1 0.337 [delta].sup.15]N Transects 1 1.600 Stations 1 0.278 Transects X stations 1 0.004 C/N Transects 1 1.259 Stations 1 0.238 Transects X stations 1 0.017 Variable Location F P Muscle RCI Transects 0.751 0.386 Stations 2.066 0.127 Transects X stations 0.606 0.545 [delta].sup.13]C Transects 5.784 0.022 Stations 63.000 <0.005 Transects X stations 11.500 <0.005 [delta].sup.15]N Transects 767.730 <0.005 Stations 4.700 0.007 Transects X stations 2.400 0.089 C/N Transects 843.786 <0.005 Stations 42.200 <0.005 Transects X stations 2.500 0.073 Gonad RCI Transects 0.235 0.628 Stations 2.495 0.083 Transects X stations 1.541 0.215 [delta].sup.13]C Transects 1.100 0.321 Stations 6.000 0.027 Transects X stations 7.700 0.015 [delta].sup.15]N Transects 34.11 <0.005 Stations 5.94 0.028 Transects X stations 0.08 0.778 C/N Transects 42.484 <0.005 Stations 8.041 0.013 Transects X stations 0.572 0.462
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|Author:||Mauna, A. Cecilia; Lomovasky, Betina J.; Franco, Barbara C.; Schwartz, Matias J.; Botto, Florencia;|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2010|
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