Population structure, growth, and production of the wedge clam Donax hanleyanus (Bivalvia: Donacidae) from northern Argentinean beaches.ABSTRACT Donax hanleyanus Philippi, 1847 (Bivalvia: Donacidae) dominates fine to coarse sandy beach communities of the northern Argentinean Atlantic coast. The population biology of this intertidal wedge clam was studied by determining population structure, growth and production at the three locations Santa Teresita, Mar de las Pampas (both from December 2005 to December 2006) and Faro Querandi (from March 2005 to December 2006). Von Bertalanffy growth functions were established from length-frequency distributions using an asymptotic length ([L.sub.[infinity]) of 44 mm and the growth constants (K) of 0.46 and 0.47 [y.sup.-1] respectively of Mar de las Pampas and Faro Querandi. Compared with growth studies four decades ago, D. hanleyanus today is growing more slowly, but is reaching a higher maximum length. Longevity is estimated to be approximately five years. The present study confirms that the overall growth performance index is habitat-specific, grouping Donacidae into tropical/subtropical, temperate and upwelling species. The intertidal biomass of D. hanleyanus ranged between 0.04 and 1.32 g ash-free dry mass (AFDM) [m.sup.-2][yr.sup.-1]. Individual production revealed the highest value at 30 mm length (0.16 g AFDM [m.sup.- 2][yr.sup.-1]) and annual production ranged between 0.08 and 0.99 g AFDM [m.sup.-2][yr.sup.-1], resulting in renewal rate values (P/[bar.B]) between 0.82 and 2.16. The P/[bar.B] ratios of D. hanleyanus populations increased with decreasing latitude from temperate to tropical regions. Only at Santa Teresita D. hanleyanus was found living with the sympatric yellow clam Mesodesma mactroides. A significant negative correlation between abundances of both surf clams suggests that abundance peaks of D. hanleyanus are related with population crashes of M. mactroides. Spatial differences in abundance are significantly related to sand texture as confirmed by nonmetrical multidimensional scaling, but not to sea surface temperature. However, the decrease of D. hanleyanus seems to be principally related to human activities.
KEY WORDS: wedge clam, Donax hanleyanus, population dynamics, growth performance indices, grain size analysis, human impact
Exposed sandy beaches are some of the most dynamic environments in the world. Their invertebrate populations and communities are usually considered to be regulated mainly by beach morphodynamics depending on physical parameters such as wave energy, wind velocity, tide forces, and sand grain size. On a worldwide basis, Donacidae (super family Tellinacea) form by far the most diverse group inhabiting such highly dynamic environments including the genera Egeria, Iphigenia and Donax, with the latter being composed of 64 species (Pearse et al. 1942, Ansell 1983, Brown & McLachlan 1990, Wilson 1999). In some cases, they dominate the macrozoobenthic communities in number and biomass, which is also the case for the Donax populations studied (>95%: McLachlan et al. 1981, Arntz & Fahrbach 1991, Ieno & Bastida 1998).
Donax hanleyanus, also known as the wedge clam and in Argentina as "berberecho", can be found along the South American Atlantic coast from tropical (17[degrees]S Caravelas, Brazil) to temperate regions (37[degrees]S Punta Mogotes in Mar del Plata, province of Buenos Aires) (Penchaszadeh & Olivier 1975, Narchi 1978, Cardoso & Veloso 2003). This intertidal suspension-feeder represents the only Donax species occurring throughout the Argentinean sublittoral (Penchaszadeh & Olivier 1975). As is characteristic of donacids, this bivalve has adapted to live in exposed swash zones characterized by a fair amount of wave action aerating the sediment. This keeps organic detritus in suspension and allows for tidal migration (Mori 1938, Ansell & Trevallion 1969, Penchaszadeh & Olivier 1975, Narchi 1978, Ansell 1983). Although fossils of D. hanleyanus have been recorded in South America since the Querandinense period of marine transgression in the early Holocene (von Ihering 1907), live specimens of D. hanleyanus were reported from the Argentinean coast no earlier than 1960 (Penchaszadeh & Olivier 1975), from Brazil in 1949 (Lange de Morretes) and from Uruguay in 1951 (Barattini). Planktonic larvae of D. hanleyanus have been shown to cut across the Rio de la Plata with favorable ocean currents and settled on sandy beaches south of this estuary (Penchaszadeh & Olivier 1975).
Although several Donax clams are targeted by artisanal fisheries (McLachlan et al. 1996) and D. hanleyanus is edible (Veloso et al. 1953, Penchaszadeh & Olivier 1975), this species is not commercially exploited in Argentina. However, like most donacids D. hanleyanus plays an important role in the food web by linking benthic with planktonic ecosystems (Wade 1967, McLachlan et al. 1981, McLachlan & Lewin 1981, McDermott 1983, DeLancey 1989, Heymans & McLachlan 1996, Soares et al. 1997). This wedge clam is the main primary consumer in soft bottom communities and is in turn subject to predation by a wide variety of invertebrates, fish, birds, and mammals (Penchaszadeh & Olivier 1975, Peterson et al. 2000, Salas et al. 2001). Only a few studies on D. hanleyanus from Argentinean shores are published dealing with ecological aspects (Penchaszadeh & Olivier 1975), beach morphodynamics and their influence on wedge clam's abundance (Marcomini et al. 2002), anthropogenic influences (Dadon et al. 2001) and siphon regeneration (Luzzatto & Penchaszadeh 2001), however, none of the studies investigated the population dynamics.
Therefore the aim of this study is to (1) analyze the population structure of D. hanleyanus, (2) estimate its growth and production parameters, (3) compare these results with values of other Donax species, (4) examine the possible influences of sand grain size and sea surface temperature on abundance, (5) discuss the effect of mass tourism, and (6) reveal the relationship between Mesodesma mactroides and D. hanleyanus abundances.
MATERIAL AND METHODS
The population structure of the intertidal wedge clam D. hanleyanus was studied during alternate spring tides at three beaches (1) Santa Teresita (36[degrees]32' S, 56[degrees]4 'W), which is heavily influenced by mass tourism during the summer season (>1000 tourists 100 [m.sup.-1] coastline, pers. observ, and local tourist office "Secretaria de Turismo, Mar del Tuyu," Argentina), (2) Mar de las Pampas (37[degrees]19'S, 57[degrees]00' W), which is marginally influenced by humans (<10 tourists 100 [m.sup.-1] coastline, pers. observ.), and (3) Faro Querandi, which is unaffected by humans (<0.01 angler 100 [m.sup.-1] coastline, pers. observ.) and naturally protected (37[degrees]29'S, 57[degrees]07'W). All three open ocean beaches are located in the Province of Buenos Aires, Argentina (Fig. lA) and flow continuously into one another with a north-south shoreline orientation, which is stable on a long term basis (Marcomini & Lopez 1993).
[FIGURE 1 OMITTED]
Abiotic Factors and Anthropogenic Impacts
According to McLachlan's (1980) rating scale for exposure and Short and Wright's (1983) classifications of beach types, Santa Teresita is sheltered/dissipative, Mar de las Pampas exposed/intermediate, and Faro Querandi exposed/reflective (Table 1). The three sampling sites receive continuous wave action and are subject to semidiurnal tides, with a maximum tidal range of 1.6 m; a spring tide average of 1.7 m and a neap tide mean of 0.2 m. The sea surface temperature (SST) (mean [+ or -] SE) varies between 11[degrees]C [+ or -] 0.14[degrees]c in winter and 23[degrees]C [+ or -] 0.21[degrees] in summer (Fig. 2). The study sites ate composed of fine (Santa Teresita), medium (Mar de las Pampas) and coarse sands (Faro Querandi), with a mean particle diameter of 2.26 [phi], 1.43 [phi] and 1.05 [phi], respectively (Table 1). All three beaches ate affected by freshwater seepage as confirmed by satellite images and a southward current bringing water masses from the 290 km long and up to 220 km wide estuary of the Rio de la Plata. Mean salinity ranges between 31 and 34. All three beaches are well drained and oxygenated.
To characterize the physical attributes of the Donax belt, particle size analyses from the studied areas were carried out by sampling sediments with a plastic corer of 3.5 cm diameter to a depth of 10 cm. Sand samples were washed with freshwater overnight to remove salt and then dried at 70[degrees]C. Subsequently, any shell fragments were extracted and the remaining part of the samples was analyzed using a MacroGranometer settling tube (e.g., Flemming & Thum 1978, Flemming & Ziegler 1995) and the SedVar V6.2p software package (Brezina 1997), which is part of the system. The data processing software makes use of the more versatile equation of Brezina (1979) rather than that of Gibbs et al. (1971), which is applicable to smooth glass spheres only to calculate settling velocities. All textural parameters presented in this study were calculated using the percentile statistics of Inman (1952), whereas sediments were classified according to the Wentworth scale (1922).
Wave height was estimated by measuring the height of breaking waves (n = 10) with graduated poles against the horizon, and adding the result to the height difference between the location of the observer and the lowest point at which the backwash met the next incoming swash bore. The wave period was measured as the time interval between breakers (n = 50). The morphodynamic state of each site studied was described by the Dean parameter ([OMEGA]) (Dean 1973):
[OMEGA] = [H.sub.b] 100/WT, (1)
which is based on mean wave height [H.sub.b] (m) divided by wave period T (s) and sand fall velocity W (m [s.sup.-1]). The slope of the beach face was measured by the height difference (Emery 1961) between the drift and the water line. The swash period was estimated according to McArdle and McLachlan (1991). Salinity (Optech portable refractometer, model RSM) as well as the SST were measured monthly in situ at the three sample locations. For additional detailed information the SST was measured daily at Santa Teresita between October 2005 and December 2006 and was supplemented with data of the Argentinean Marine Institute (CEADO: Centro Argentino de Datos Oceanograficos, Servicio de Hidrografia Naval, Buenos Aires, Argentina), which operates a permanent weather station at this beach.
Data of tourists visiting the coast of Buenos Aires were provided by the local tourist office (Secretaria de Turismo, Mar del Tuyu, Argentina).
Sampling and Laboratory Procedures
Quantitative sampling of D. hanleyanus was carried out at monthly intervals (Santa Teresita and Mar de las Pampas: December 2004 to December 2006; Faro Querandi: March 2005 to December 2006) from a series of stations (4 m intervals). The latter were located along three transects separated by 20 m intervals and located perpendicular to the shoreline from the spring tide high water mark to the spring tide low water mark. At each station, three replicated sand samples (40 x 40 cm) were excavated to 35 cm depth using a 0.16 [m.sup.2] steel corer, and thereafter sieved individually on a 1-mm mesh. Maximum anterior-posterior shell length (apSL) of the retained wedge clams was measured to the nearest 0.1 mm with a digital vernier calliper (Mitutoyo, model 500-161U) to obtain monthly length-frequency distributions. For biomass analysis monthly subsamples of 35 randomly taken specimens from each of the three locations were stored in 70% ethanol. Thereafter soft tissues were removed to estimate shell-free wet mass (SFWM) by weighing after blotting on filter paper. Subsequently, samples were dried to a constant mass at 70[degrees]C, weighed again to determine shell-free dry mass (SFDM) and finally ignited in a muffle furnace at 550[degrees]C for 5 h to estimate ash-free dry mass (AFDM).
Growth, Growth Performance and Life Span
A series of 25 length-frequency distributions provided the basic information for growth estimates. A von Bertalanffy growth function (VBGF) (von Bertalanffy 1938) was fitted to the data using the electronic length-frequency analysis 'ELEFAN I' routine of the FISAT II program package (Gayanilo Jr. et al. 2005). Because of the pronounced seasonal changes in temperature (Fig. 2A), the modified version of the VBGF was used, which incorporates seasonal oscillation in growth:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (2)
where [L.sub.t] is the apSL (mm) at time t, [L.sub.[infinity]] the asymptotic maximum shell length, K is the curvature parameter, C is a constant, which defines the degree of seasonal oscillation (ranging from 0, which defines continuous non oscillating growth, to 1, when growth comes to a complete halt at the "winter point" (WP)), [t.sub.0] is the theoretical age at zero length and [t.sub.s] is the initial point of oscillation in relation to t = 0 and the WP. The different subroutines of ELEFAN (K-scan, response surface analysis) were used to identify the VBGF that best fits the monthly size-frequency data, using the Rn value as a criterion of fit.
[FIGURE 2 OMITTED]
Because of the absence of adult individuals at Santa Teresita, it was not possible to apply a VBGF (Gayanilo Jr. et al. 2005). Thus the monthly length-frequency distributions of Santa Teresita were converting to size-at-age data (SAD) and thereafter fitted to a general von Bertalanffy growth function (gVBGF) (Gulland & Holt 1959) applying Microsoft Excel's SOLVER routine (Brey 2001). Differences in growth among sites were checked by variance analysis of the residuals of the g VBGF.
To compare growth performance the growth index phi prime ([phi]') was used:
[phi]' = 2 [log.sub.10]([L.sub.[infinity]]) + [log.sub.10] K. (3)
This criterion was chosen because the negative correlation between K and [L.sub.[infinity]] invalidates direct comparisons between individual parameters (Pauly & Munro 1984). Finally the overall growth performance index:
OGP = log(K[[L.sub.[inifnity]].sup.3]), (4)
which represents growth rate at the point of inflexion of the size-growth curve (Pauly 1979), was calculated to compare growth of D. hanleyanus with other Donax species.
The theoretical life span [t.sub.max] (equal to relative age, because the larval period of this species is unknown, t = 0) of D. hanleyanus was estimated by the inverse von Bertalanffy growth equation (Taylor 1958):
t max = [ln [L.sub.95%] - ln([L.sub.[infinity]] - [L.sub.95%])]/K, (5)
where [L.sub.95%] represents 95% of the maximum shell length recorded during field sampling.
Biomass and Production
To compare biomass of D. hanleyanus with values of other Donax species, all published values were converted to AFDM (according to Brey et al. 1988), based on the empirically determined conversion factor of 0.186 (SFWM to AFDM for preserved D. hanleyanus). Parameters of the relationship between length and ash-free dry mass (AFDM) of D. hanleyanus were estimated by regression analysis
M = [aL.sup.b], (6)
where M is the AFDM (g), L is the shell length of the size class (mm), and a and b are constants. AFDM was determined for 400 specimens covering all size classes between December 2004 and December 2006.
Total annual production was calculated for D. hanleyanus belts at Santa Teresita, Mar de las Pampas (January-December 2005 and January-December 2006) and Faro Querandi (March-December 2005 and January-December 2006) by the mass specific growth rate method (Crisp 1984, Brey 2001). The latter requires information from size-mass relationships and the size-frequency distribution obtained from all pooled samples and the VBGF:
P = [summation] [N.sub.i][M.sub.i][G.sub.i](g AFDM [m.sup.-2][yr.sup.-1]), (7)
where [N.sub.i] is the average number of animals (number per [m.sup.2]) and Mi is the mean individual AFDM in length class i, and [G.sub.i] is the mass-specific growth rate:
[G.sub.i] = bK [([L.sub.[infinity]]/[L.sub.i]) - 1] ([yr.sup.-1]), (8)
where b is the exponent of the size-mass relation, K, [L.sub.[infinity]] are VBGF parameters and [L.sub.i] is the mean size in class i.
Mean annual biomass was estimated by:
[bar.B] = [summation] [N.sub.i][M.sub.i] (g AFDM [m.sup.-2]), (9)
and annual renewal rates (P/B ratios) of the D. hanleyanus populations were calculated from annual total production P and annual mean biomass [bar.B].
Spatial and temporal differences of the population abundance (one-way ANOVA) as well the influence of SST and anthropogenic factors on the population abundance (Spearman's rank order correlation) were analyzed using the statistical package SPSS version 17.0 (SPSS 2008). Differences were considered significant at a level of [alpha] = 5% (Zar 1999). The relationship between monthly mean abundance and mean grain size was analyzed through a nonmetric multidimensional scaling (nMDS), using square root transformed data and calculated Bray-Curtis similarities (PRIMER 2007) as used by several authors (e.g., Rumohr & Karakassis 1999, Jaramillo et al. 2001, Sousa et al. 2008). Correlation analysis was used to assess the relationships between renewal rate (P/[bar.B] ratios) and latitude of D. hanleyanus compared with those of several Donax species from other areas. The relation between the population abundance of D. hanleyanus and M. mactroides was estimated by linear regression analysis.
Abiotic Factors and Anthropogenic Impacts
Results of grain size and sand rail velocity analyses, as well as the associated parameters are summarized in Table 1. The nMDS analysis, based on monthly mean abundance data, identified two main areas of distinct granulometry: mean grain size 1.43 and 2.26 [phi] (at Mar de las Pampas and Santa Teresita, respectively, Figure 3A), and mean grain size 1.05 phi (at Faro Querandi, Figure 3B). The stress factor of 0.08 indicates an excellent representation with no prospect of misinterpretation (Clarke & Warwick 2001).
[FIGURE 3 OMITTED]
Mean annual SST (mean [+ or -] SE) was 17.6[degrees]C [+ or -] 0.17[degrees]C, with lowest temperatures of 9.8[degrees]C and 9.1[degrees]C in austral winter (June 2005 and July 2006, respectively), and highest values of 24.8[degrees]C and 24.5[degrees]C in austral summer (December 2005 and February 2006, respectively) (Fig. 2A). SST was not correlated with the abundance of D. hanleyanus at all three sampling sites (Spearman's rank order correlation: Santa Teresita [r.sub.s] = -0.013, P > 0.05, n = 25; Mar de las Pampas [r.sub.s] = 0.122, P > 0.05, n = 25; Faro Querandi [r.sub.s] = 0.077, P > 0.05, n = 22). However, at Santa Teresita the number ofvisiting tourists (> 1,000 tourists 100 [m.sup.-1] coastline, pers. observ, and local tourist office "Secretaria de Turismo, Mar del Tuyu," Argentina) was significantly correlated with the decrease of the wedge clam population during the summer months of both years (Spearman's rank order correlation, [r.sub.s] = -0.829, P < 0.05, n = 6). At Mar de las Pampas (<10 tourists 100 [m.sup.-l] coastline, pers. observ.) this correlation was not found (Spearman's rank order correlation, [r.sub.s] = 0.371, P> 0.05, n = 6), and at Faro Querandi no tourist effect was apparent (<0.01 angler 100 [m.sup.-1] coastline, pers. observ.).
During the study period of 25 mo, 21,405 D. hanleyanus were collected and measured: in the first year at Santa Teresita 4,230 ind., at Mar de las Pampas 1,545 ind. (December 2004-November 2005) and at Faro Querandi 3,911 ind. (March 2005-November 2005). During the second year 2,183 ind. were measured at Santa Teresita, 1,452 ind. at Mar de las Pampas and 8,524 ind. at Faro Querandi (December 2005-December 2006). The smallest recorded individual had an apSL of 3 mm at Santa Teresita and 4 mm at Mar de las Pampas and Faro Querandi. The largest individual measured 35 mm, 36 mm, and 40 mm respectively.
Mean annual D. hanleyanus abundance (mean [+ or -] SE) was 78 [+ or -] 42 ind. [m.sup.-2] (2005) and 40 [+ or -] 15 ind. [m.sup.-2] (2006) at Santa Teresita, 55 [+ or -] 23 ind. [m.sup.-2] (2005) and 50 [+ or -] 12 ind. [m.sup.-2] (2006) at Mar de las Pampas, and 84 [+ or -] 22 ind. [m.sup.-2] (2005) and 267 [+ or -] 77 ind. [m.sup.-2] (2006) at Faro Querandi. Maximal abundance was recorded in spring (2,475 ind. [m.sup.-2] at Santa Teresita in December 2004, 531 ind. [m.sup.-2] at Mar de las Pampas in December 2004, and 950 ind. [m.sup.-2] at Faro Querandi in November 2006).
The population abundance at Santa Teresita and Mar de las Pampas did not varied significantly between the studied years (ANOVA, [F.sub.1,22] = 2.427, P > 0.05; [F.sub.1,22] = 2.209, P > 0.05, respectively), but was significantly lower in 2005 compared with 2006 at Faro Querandi (ANOVA, [F.sub.1,22] = 11.891, P < 0.05). In all three beaches, population abundance differed significantly between months (Santa Teresita: [F.sub.11,13] = 1.171, P > 0.05; Mar de las Pampas: [F.sub.11,13] = 0.681, P > 0.05; and Faro Querandi: [F.sub.11,10] = 0.542, P > 0.05; ANOVA respectively). Monthly mean abundance was significantly higher in the reflective beach, Faro Querandi (ANOVA with Scheffe-procedure post hoc test, [F.sub.2,69] = 14.675, P < 0.05). The highest abundance of D. hanleyanus was recorded mainly in spring (at Santa Teresita [mean [+ or -] SE] 361 [+ or -] 149 ind. [m.sup.-2] in December 2004 and 95 [+ or -] 27 ind. [m.sup.-2] in December 2006, at Mar de las Pampas 125 [+ or -] 32 ind. [m.sup.-2] in December 2004 and 111 [+ or -] 71 ind. [m.sup.-2] in September 2005 and at Faro Querandi 190 [+ or -] 45 ind. [m.sup.-2] in November 2005, and 667 [+ or -] 205 ind. [m.sup.-2] in November 2006) (Fig. 2S, M, F). During the present study we did not find any evidence of migration of D. hanleyanus although the abundance may vary drastically at a given location (Fig. 2). Additional random sampling south and east of the transects did not reveal higher abundance compared with the monthly stations. Further random sampling in the highly dynamic wave breaking zone, taken with a diver-operated corer (Rumohr & Arntz 1982), confirmed that clams did not migrate into the surf zone.
The length-frequency distribution of D. hanleyanus of Santa Teresita, Mar de las Pampas and Faro Querandi showed distinct cohorts. One single new cohort was observed during both years (April 2005 and March 2006) at Santa Teresita. Cohorts were tracked up to 10 months reaching approximately 20 mm apSL before the cohort disappeared and new recruits were recorded (Fig. 4S). In March 2006 a new cohort was detected along with two additional cohorts at Mar de las Pampas and could be followed until the end of the study in December 2006 (Fig. 4M). Adult individuals co-occurring in two (February 2005 until March 2006) and three cohorts (April 2006 to October 2006) were found at Faro Querandi (Fig. 4F).
Growth and Life Span
The maximum sized valve of 44 mm was found at Faro Querandi. This length was used as a fixed [L.sub.[infinity]] value to calculate a seasonal VBGF with a growth constant K = 0.47 at Mar de las Pampas and K = 0.48 at Faro Querandi (Fig. 4M, F). Estimates of growth indicated moderate seasonal oscillations (C = 0.8), with slowest growth rates occurring in austral autumn (WP = 0.45 ~ mid-May). At Santa Teresita adult individuals were not abundant which prevented the reliable calculation of a VBGF with FISAT (Gayanilo Jr. et al. 2005). Thus monthly length-frequency distributions of Santa Teresita were converted to SAD and fitted to a g VBGF. Variance Analysis of the residuals (P > 0.05) revealed that growth parameters of Santa Teresita deviated strongly from those of populations inhabiting the beaches of Mar de las Pampas and Faro Querandi. Because of the lack of larger size classes and the resulting methodological bias, we decided to only compare growth parameters from the two latter populations.
[FIGURE 4 OMITTED]
The calculated life span ([t.sub.max]) was 4.96 y (Mar de las Pampas) and 5.18 y (Faro Querandi) (Table 2).
Biomass and Production
The observed relationship between length and calculated AFDM of D. hanleyanus [M.sub.(AFDM)] = (4.27 x [10.sup.-6]) x [L.sub.(mm).sup.3.19] (n = 2233, [r.sup.2] = 0.98) was used for production estimates.
Within the Donax belt these abundance values represent an average annual biomass [bar.B] of 0.21 g AFDM [m.sup.-2] [yr.sup.-1] (2005) and 0.04 g AFDM [m.sup.-2] [yr.sup.-1] (2006) at Santa Teresita, of 0.17 g AFDM [m.sup.-2] [yr.sup.-1] (both, 2005 and 2006) at Mar de las Pampas and of 1.32 g AFDM [m.sup.-2] [yr.sup.-1] (2005) and 0.96 g AFDM [m.sup.-2] [yr.sup.-1] (2006) at Faro Querandi (Fig. 5B-G). Individual production increased to its highest value at 30 mm length (0.16 g AFDM [m.sup.-2] [yr.sup.-1]) and decreased thereafter (Fig. 5A). The distribution of total annual production P and the abundance among the size classes are illustrated in Figure 5B-G. Annual production ranged between 0.08 and 0.99 g AFDM [m.sup.-2] [yr.sup.-l], depending on beach and year, and P/[bar.B] ratios were between 0.82 and 2.16.
A clear spatial pattern of D. hanleyanus inhabiting dissipative to reflective beaches was revealed from monthly mean abundance data. Mean grain size of sediments is dependent on currents, the mean slope, the exposure and thus the morphodynamic type of the beach (McLachlan & Brown 2006). Therefore mean grain size integrates over the latter parameters and is therefore a suitable factor to be analyzed for physical effects on beach organisms. The nMDS analysis distinguishing two main sites (Fig. 3), A: the first area (Santa Teresita and Mar de las Pampas) is characterized by a fine to medium textural group and smaller grain size (210-370 [micro]m, dissipative to intermediate beach morphology), and B: a second one (Faro Querandi, reflective beach morphology) by larger grain size (480 [micro]m) and a coarse textural group (Table 1). Thus, differences in the abundance of D. hanleyanus (Fig. 2) may be related to the compactness of the sand: the more compacted the sand, the more difficult for adult clams to burrow and consequently they can be washed away. This hypothesis is supported by Trueman (1971) stating that Donax is only secure against the drag of the waves when the shell is oriented with the current and at least two thirds of the shell is buried. In conformity with our work, Penchaszadeh and Olivier (1975) already stated in the 1970s that sediment characteristics are principle factors for the distribution of D. hanleyanus. In accordance, Alexander et al. (1993), McLachlan et al. (1995) and Nel et al. (2001) also described substrate-sensitivity of Donax species, although the clams are able to penetrate into sediments with a wide spectrum of grain sizes. In addition, Guillou and Bayed (1991) observed higher abundance of D. trunculus in sediments with larger mean grain sizes (from 190-230 [micro]m) and minimum abundance in sediments with smaller mean grain size ([less than or equal to] 160 [micro]m).
[FIGURE 5 OMITTED]
Sandy beaches such as Santa Teresita are prime sites for human recreation: especially during the holiday season, short intense anthropogenic disturbances can affect organisms at several biological scales, ranging from biogeochemistry and physiology up to the community level. Chandrasekara and Frid (1996) reported that human trampling changes the saltmarsh benthic fauna of the English coast. Schlacher and Thompson (2008) described a modification of the physical environment of two Australian sandy beaches and van der Merwe and van der Merwe (1991) found a negative effect on two intertidal Donax species in South Africa, both caused by off-road driving. In a Dutch intertidal mudflat, trampling affected the abundance and population dynamics of the clam Macoma balthica and the cockle Cerastoderma edule (Rossi et al. 2007). The authors assume that footsteps directly kill or bury specimens, provoking asphyxia. On Argentinean shores it is evident that human activities and urbanization in coastal areas affect the presence and abundance of D. hanleyanus (Marcomini et al. 2002). Some authors (Bastida et al. 1996, Dadon 2005) assume that human activities are probably one of the main factors interfering with the recovery of bivalve populations. The abrupt decrease of the D. hanleyanus population during summer especially at Santa Teresita (arrows in Fig. 2S) is significantly related to mass tourism. Up to one million tourists (Secretaria de Turismo, Mar del Tuyu, Argentina) visit the coast of Buenos Aires during each holiday season (Fig. 2B) and disturb the intertidal zone by intensive trampling. Therefore we argue that anthropogenic impact is one of the main causes of strong reductions in D. hanleyanus abundance at Santa Teresita. Further research should therefore focus on human activities as an important abundance oppressing factor.
The von Bertalanffy growth constant K varied insignificantly between 0.47 [yr.sup.-1] (Mar de las Pampas) and 0.48 [yr.sup.-1] (Faro Querandi). During the first two years of life, differences of growth parameters become obvious by comparing the present values (Table 3, values ml, m2) with those of Penchaszadeh and Olivier (1975) (Table 3, value m3). Forty years ago populations of D. hanleyanus grew faster within the first two years (K = 1.18) compared with those presently studied (K = 0.48), but reaehed a smaller maximal length (33.5 mm) (presently 44.0 mm). This difference in growth may be caused by a relaxed competition with the co-occurring M. mactroides population (Table 3, Fig. 8, see later). Thus, growth of D. hanleyanus apparently depends on this competition. Other studies have also demonstrated abundance-dependent growth for various bivalves (Broom 1982, Peterson 1982, Peterson & Black 1987, Peterson & Black 1988, Peterson & Beal 1989, Jensen 1993, Defeo & de Alava 1995, Weinberg 1998, Talman & Keough 2001).
Non-linear growth functions, such as the VBGF, are difficult to compare, whereas several authors (e.g., Pauly 1979, Munro & Pauly 1983, Moreau et al. 1986) demonstrated the suitability of the OGP index for inter and intraspecific comparisons. The OGP of the D. hanleyanus studied (4.60 at Mar de las Pampas and 4.61 at Faro Querandi) conforms well, with values calculated from data sets of the same species from the Brazilian (4.17 and 4.32: Cardoso & Veloso 2003), Uruguayan (4.46: Defeo 1996) and Argentinean coast (4.65: Penchaszadeh & Olivier 1975). OGP values (Fig. 6) from Uruguay (m4) and Brazil (m5m7) are conspicuously lower than those from Argentina (mlm3), which may be caused by the lower mean SST. This trend provides support to the latitudinal gradient hypothesis (LGH) postulated by Defeo and Cardoso (2002), which predicts a positive correlation between body size of species and latitude. Thus, subtropical beaches exhibit higher growth and mortality rates and shorter life spans than temperate beaches. However, the LGH cannot be generalized for invertebrates, because there are different, sometimes opposing, results for different taxa (cf. Schilder 1956, Jones & Simons 1983, Henmi 1993, Barnes & Arnold 2001, Hausdorf 2003, Clarke et al. 2004). The present data compilation from several studies confirmed the hypothesis of Laudien et al. (2003) that the OGP is habitat-specific (Fig. 6): species populating tropical/subtropical regions show the lowest OGP (2.84-3.68, group A), temperate species have intermittent OGP (4.17-4.91, group B), whereas species of upwelling areas show the highest OGP (5.06-5.65, group C). Even the effect of climate anomalies may be detected with the help of the auximetric grid, as indicated by the upwelling surf clam D. marincovichi sampled in Peru during and shortly after an El Nino event 1982 and 1983 (Fig. 6, values f2, f3) in comparison with the population sampled during normal upwelling years (Fig. 6, value f1) (Arntz et al. 1987). With an OGP of 3.36, calculated from Gil and Thome (2000), the Brazilian D. hanleyanus seems to be an exception (Fig. 6, value m5). Whereas all other computed OGP values for D. hanleyanus show that this is a temperate species, population parameters of the latter study classify this species as tropical/subtropical.
[FIGURE 6 OMITTED]
A few OGP values calculated from tropical/subtropical regions deflect from the model of the auximetric grid (Fig. 6), namely the growth performance of D. dentifer (e1: Palacios et al. 1983, e2: Riascos & Urban 2002), D. denticulatus (j2: Garcia et al. 2003, j3: Marcano et al. 2003), and D. striatus (k2: Rocha-Barreira de Almeida et al. 2002), classifying these species as temperate. Sparre and Venema (1997) state that growth estimates, based only on length-frequency data, may not always be useful for tropical species with several recruitment events through the year.
Most Donax species have a relatively short life span of 1-2 y (McLachlan 1979, Ansell 1983). Whereas Penchaszadeh and Olivier (1975) estimated a longevity of D. hanleyanus from the Argentinean coast of three years with a maximal length of 33 mm, the recent study indicates an even longer life span of approximately five years (maximal length of 44 mm) for the same species and region. In contrast to our estimation, Cardoso and Veloso (2003) noted for Brazilian D. hanleyanus only 1.5 y as the maximal life period. Similarly shorter maximal life spans were reported for various Donax species from temperate areas, such as D. vittatus (3.5 y) from the French coast (Ansell & Lagardere 1980) or D. deltoides (3.5 y) from South Australia (for data source see at Laudien et al. 2003). The recent value is however in accordance with D. trunculus from France (6 y, Guillou & Le Moal 1980, Guillou 1982) and D. serra (>5 y, de Villiers 1975) from South Africa. Lower values of longevity were reported for tropical/subtropical Donax species, D. faba and D. cuneatus from India probably does not reach 3 y (McLachlan et al. 1996) and D. striatus (McLachlan et al. 1996) and D. denticulatus (Velez et al. 1985) from Venezuela have a life span of only approximately 1.5 y. The analysis suggests a longer life span of temperate Donax species compared with tropical/subtropical Donax species (climate areas of Donax species are compared in Table 3 and Figure 6).
Biomass and Production
The intertidal biomass of the D. hanleyanus population ranged between 0.04 and 1.32 g AFDM [m.sup.-2] [yr.sup.-1] (Fig. 5). These values are in accordance with biomass data of Donax species from other temperate and tropical/subtropical areas (Ansell et al. 1978, Warwick et al. 1978, McLachlan 1979, McLachlan & van der Horst 1979, Wilson 1999). However, these values are in contrast to biomasses from upwelling systems (Laudien et al. 2003), which can be three orders of magnitude higher than the present values (Table 4). Four decades ago D. hanleyanus reached a biomass of 7.5 g AFDM [m.sup.-2][yr.sup.-1] (calculated from Penchaszadeh & Olivier 1975), which is approximately seven times higher than recent values.
Annual intertidal production of D. hanleyanus ranged between 0.08 and 0.99 g AFDM [m.sup.-2][yr.sup.-1] (Table 4, Fig. 5) and is therefore within the span (0.79-3.67 g AFDM [m.sup.-2][yr.sup.-1]) observed by Cardoso and Veloso (2003), but lower than that recorded by Gianuca (1985) (13.56 g AFDM [m.sup.-2][yr.sup.-1]). By comparing production, biomass and renewal rate among different Donax species from several geographical regions (Table 4, Fig. 7), it is evident that the renewal rates (P/B) increase from temperate to tropical regions, which was also observed for several other donacids (Ansell et al. 1978, Kalejta & Hockey 1991, Cardoso & Veloso 2003).
Presently D. hanleyanus numerically dominates the community at Santa Teresita. The sympatric yellow clam M. mactroides has been known to co-occur with D. hanleyanus populations since the seventies (Olivier et al. 1971, Penchaszadeh & Olivier 1975). However, M. mactroides was only abundant at Santa Teresita (mean annual abundance [mean [perpendicular to] SE] of 47 + 13 ind. [m.sup.-2] in 2005 and 29 [+ or -] 3 ind. [m.sup.-2] in 2006); in Mar de las Pampas and in Faro Querandi no more than single individuals were found during the whole sampling period (Herrmann 2009). In contrast, Olivier et al. (1971) found a mean abundance of 538 ind. [m.sup.-2] in a sampling section including Santa Teresita and 629 ind. [m.sup.-2] in a sampling section including Mar de las Pampas and Faro Querandi in 1968-1970. A significant negative correlation between abundances of these two suspension-feeders (Fig. 8) suggests that abundance peaks of D. hanleyanus are related with population crashes of M. mactroides. Penchaszadeh and Olivier (1975), Dadon (2005) as well as Defeo and de Alava (1995) observed similar inverse correlations between abundances of D. hanleyanus and M. mactroides from Argentinean and Uruguayan beaches. In agreement, surf clam populations from the Peruvian Pacific coast revealed the same relations; D. marincovichi (formerly called D. peruvianus) increased coincidentally with the collapse of M. donacium (Penchaszadeh 1971, Tarazona et al. 1985, Arntz et al. 1987). Defeo and de Alava (1995) hypothesized that interspecific competition may be the cause and Olivier et al. (1971) provided evidence by describing overlapping trophic niches of the two clams.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
In conclusion, lowest abundance of D. hanleyanus was observed at intermediate to dissipative beaches (Mar de las Pampas and Santa Teresita) composed of finer sediment. The present results connrm that the abundance of D. hanleyanus is influenced by sand grain size. Highest abundance was recorded at the reflective beach Faro Querandi characterized by coarse sediment. Decrease in D. hanleyanus abundance during the holiday season at Santa Teresita was significantly related to human trampling (mass tourism). D. hanleyanus abundance was negatively correlated with M. mactroides abundance, but was not significantly influenced by SST. In comparison with a 40-year-old data set, growth of D. hanleyanus is now slower, but specimens reach a larger maximal length and have a longer life span. The OGP index of D. hanleyanus conforms well to values calculated from several Donax species of other areas. Thus, as expected, the Argentinean wedge clam D. hanleyanus is classified into the temperate group of Donacidae on an auximetric grid. Presently the intertidal biomass of D. hanleyanus is seven times lower compared with data from the 1970s. Thus, without a sustainable management the conservation status of D. hanleyanus is endangered.
The authors thank Jose Alfaya, Sonia Cabrera, Maria Eugenia Segade Enrique, Juan Pablo Livore, my partner Sandra Noir and Soledad Zabala for field assistance, as well as the youngest helper Belen Alvela who measured SST off Santa Teresita day by day. The authors also thank Burghard W. Flemming for sediment analysis and the "Secretaria de Turismo at Mar del Tuyu" for providing tourist data. Thanks are also expressed to all Argentinean colleagues from the laboratory 19 at the 'Facultad de Ciencias Exactas y Naturales' (UBA) and from the laboratory 80 at the "Museo Argentino de Ciencias Naturales--Bernardino Rivadavia" (MACN). Finally the authors also thank Maggie Hasspacher for English editing. This study is part of the doctoral thesis of Marko Herrmann, partly supported by the University of Bremen and the "Deutscher Akademischer Austauschdienst (DAAD)."
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MARKO HERRMANN, (1) * DANIEL CARSTENSEN, (1) SONKE FISCHER, (1) JURGEN LAUDIEN, (1) PABLO E. PENCHASZADEH (2) AND WOLF E. ARNTZ (1)
(1) Alfred Wegener Institute for Polar and Marine Research, P.O. Box 120161, 27515 Bremerhaven, Germany; (2) Museo Argentino de Ciencias Naturales, Av. Angel Gallardo 470 3 piso lab. 80, C1405DJR Buenos Aires, Argentina
* Corresponding author. E-mail: email@example.com
TABLE 1. Characterization of physical and biological attributes of the three studied localities at north Argentinean Atlantic coast. Beach Features Santa Teresita Latitude 36[degrees]32'S Longitude 56[degrees]41'W Beach width (m) <80 Intertidal zone width (m) <70 Tidal range (m) 1.8 Mean grain size (phi/mm) (1) 2.26/0.21 Median grain size (phi) (1) 2.28 Sorting (1,2,3) Good (0.36) Skewness (1,2) -0.11 Textural group (5) Fine Mean slope of intertidal (%) 1/43 Exposure (4) Sheltered Morphodyn. Type (5) Dissipative Dean parameter ([OMEGA]) (6)* 1.92-2.53 D. hanleyanus belt (m) 30 Macrofauna richness (species) 5 Beach Features Mar de las Pampas Latitude 37[degrees]19'S Longitude 57[degrees]00'W Beach width (m) <70 Intertidal zone width (m) <60 Tidal range (m) 1.7 Mean grain size (phi/mm) (1) 1.43/0.37 Median grain size (phi) (1) 1.39 Sorting (1,2,3) Moderate (0.68) Skewness (1,2) +0.07 Textural group (5) Medium Mean slope of intertidal (%) 1/16 Exposure (4) Exposed Morphodyn. Type (5) Intermediate Dean parameter ([OMEGA]) (6)* 1.50-1.86 D. hanleyanus belt (m) 12 Macrofauna richness (species) 3 Beach Features Faro Querandi Latitude 37[degrees]29'S Longitude 57[degrees]07'W Beach width (m) <100 Intertidal zone width (m) <60 Tidal range (m) 1.7 Mean grain size (phi/mm) (1) 1.05/0.48 Median grain size (phi) (1) 0.99 Sorting (1,2,3) Moderate (0.51) Skewness (1,2) -0.11 Textural group (5) Coarse Mean slope of intertidal (%) 1/14 Exposure (4) Exposed Morphodyn. Type (5) Reflective Dean parameter ([OMEGA]) (6)* 3.28-3.93 D. hanleyanus belt (m) 10 Macrofauna richness (species) 3 (1) After Flemming and Thum (1978) and after Flemming and Ziegler (1995) (2) After Inman (1952) (3) After McLachlan and Brown (2006) (4) After McLachlan (1980) (5) After Short and Wright (1983) (6) After Dean (1973) * Calculated for specific seasonal SST from 9[degrees]C to 25[degrees]C TABLE 2. D. hanleyanus von Bertalanffy growth parameters: [L.sub.[infinity]] = asymptotic length (mm), K = curvature parameter ([y.sup.1]), C = constant for the amplitude of seasonal oscillation of growth, WP = point of lowest growth rate (y), Rn = goodness-of-fit index, [L.sub.95%] = length (mm) representing 95% of maximum shell length that was used to estimate [t.sub.max] = life span (y), [phi]a = standard growth index (according to Pauly & Munro 1984). [L.sub. Localities [infinity]] K C WP Mar de las Pampas 44 0.47 0.80 0.45 Faro Querandi 44 0.48 0.80 0.45 Localities [R.sub.n] [L.sub.95%] [t.sub.max] [phi]' Mar de las Pampas 0.202 40.11 4.96 2.96 Faro Querandi 0.350 40.62 5.18 2.97 TABLE 3. D. hanleyanus (mL, m2) in comparison with several other Donax species and their variation in growth performance using the von Bertalanffy growth parameter K ([y.sup.1]) and [L.sub.[infinity]] (mm) as well as the standard growth index [phi]' and the OGP. Climate areas (Ca) are: tropical/subtropical (A), temperate (B) and upwelling (C) species. Code letters (L.) are equivalent to Figure 1B. Datasets are published in Herrmann et al. (2008). L. Species Country Area m1 D. hanleyanus (2) Argentina Mar de las Pampas m2 D. hanleyanus (2) Argentina Faro Querandi m3 D. hanleyanus (2) Argentina Villa Gesell m4 D. hanleyanus (2) Uruguay Barra del Chuz m5 D. hanleyanus (2) Brazil Arroio Teixeira m6 D. hanleyanus (2) Brazil Marambaia m7 D. hanleyanus (2) Brazil Marambaia j1 D. denticulatus (2) Venezuela Araya Peninsula j2 D. denticulatus (2) Venezuela Playa Brava j3 D. denticulatus (2) Venezuela La Guardia e1 D. dentifer (2) Costa Rica Playa Garza, Puntarenas e2 D. dentifer (2) Colombia Bahia Malaga y1 D. cuneatus (2) India Palk Bay y2 D. cuneatus (2-5) India Miriya Bay, Ratnagiri z D. faba (2-5) India Gulf of Mannar k1 D. striatus (2) Venezuela Tucucas, Txori k2 D. striatus (2) Brazil Futuro Beach, Fortaleza w1 D. incarnatus (2-5) India Shertallai w2 D. incarnatus (2-5) India Cochin w3 D. incarnatus (2-5) India Goa w4 D. incarnatus (2) India Panambur f1 D, marincovichi (2) Peru Santa Maria del Mar f2 D. marincovichi (2) Peru Santa Maria del Mar f3 D. marincovichi (2) Peru Santa Maria del Mar n1 D. vittatus (2) France St. Trojan n2 D. vittatus (2) France St. Trojan n3 D. vittatus (2) France St. Trojan n4 D. vittatus (2) France Vertbois n5 D. vittatus (2-4) France Bay of Douarnenez p1 D. trunculus (2-4) France Vertbois p2 D. trunculus (2-4) France Vertbois p3 D. trunculus (2-4) France Vertbois p4 D. trunculus (2-4) France Vertbois p5 D. trunculus (2-4) France Vertbois p6 D. trunculus (2-4) France St. Trojan p7 D. trunculus (2-4) France St. Trojan p8 D. trunculus (2-4) France St. Trojan p9 D. trunculus (2) France Bay of Douarnenez p10 D. trunculus (2) France Camarque p11 D. trunculus (2) Spain Atlantic p12 D. trunculus (2) Spain Atlantic, El Barquero p13 D. trunculus (2) Spain Mediterano, Canera p14 D. trunculus (3) Spain Mediterano, Canera p15 D. trunculus (2) Portugal Faro p16 D. trunculus (2) Italy Apulia u1 D. deltoides (2) Australia Goolwa u2 D. deltoides (2) Australia New South Wales t1 D. serra (2) South Africa Elands Bay t2 D. serra (2) South Africa Elands Bay t3 D. serra (2) South Africa Melkbosstrand t4 D. serra (2) South Africa Melkbosstrand t5 D. serra (2) South Africa Elands Bay t6 D. serra (2) South Africa Koeberg t7 D. serra (2) South Africa St Francis Bay t8 D. serra (2) South Africa St Francis Bay t9 D. serra (2) South Africa Langestrand t10 D. serra (2) South Africa Paaltjies L. Lat. Long. Ca m1 37[degrees]19'S 57[degrees]00'W B m2 37[degrees]29'S 57[degrees]07'W B m3 37[degrees]16'S 56[degrees]59'W B m4 33[degrees]40'S 53[degrees]29'W B m5 29[degrees]38'S 49[degrees]56'W B m6 23[degrees]03'S 43[degrees]34'W B m7 23[degrees]03'S 43[degrees]34'W B j1 10[degrees]38'N 64[degrees]16'W A j2 -- -- A j3 11[degrees]00'N 64[degrees]00'W A e1 -- -- A e2 4[degrees]OS'N 77[degrees]16'W A y1 9[degrees]17'N 79[degrees]O5'E A y2 17[degrees]N 53[degrees]E A z -- -- A k1 -- -- A k2 03[degrees]42'S 38[degrees]27'W A w1 9[degrees]42'N 76[degrees]20'E A w2 10[degrees]00'N 76[degrees]12'E A w3 15[degrees]22'N 73[degrees]52'E A w4 12[degrees]27'N 74[degrees]48'E A f1 12[degrees]20'S 76[degrees]50'W C f2 12[degrees]20'S 76[degrees]50'W C f3 12[degrees]20'S 76[degrees]50'W C n1 45[degrees]50'N 1[degrees]20'W B n2 45[degrees]50'N 1[degrees]20'W B n3 45[degrees]50'N 1[degrees]20'W B n4 45[degrees]50'N 1[degrees]20'W B n5 48[degrees]OS'N 4[degrees]18'W B p1 45[degrees]50'N 1[degrees]20'W B p2 45[degrees]50'N 1[degrees]20'W B p3 45[degrees]50'N 1[degrees]20'W B p4 45[degrees]50'N 1[degrees]20'W B p5 45[degrees]50'N 1[degrees]20'W B p6 45[degrees]50'N 1[degrees]20'W B p7 45[degrees]50'N 1[degrees]20'W B p8 45[degrees]50'N 1[degrees]20'W B p9 48[degrees]OS'N 4[degrees]18'W B p10 43[degrees]27'N 4[degrees]26'W B p11 -- -- B p12 -- -- B p13 -- -- B p14 -- -- B p15 36[degrees]57'N 7[degrees]53'W B p16 41[degrees]55'N 15[degrees]26'E B u1 -- -- B u2 36[degrees]S 151[degrees]E B t1 32[degrees]19'S 18[degrees]21'E C t2 32[degrees]19'S 18[degrees]21'E C t3 33[degrees]42'S 18[degrees]26'E C t4 33[degrees]42'S 18[degrees]26'E C t5 32[degrees]19'S 18[degrees]21'E C t6 33[degrees]40'S 18[degrees]25'E C t7 33[degrees]59'S 25[degrees]20'E C t8 33[degrees]57'S 25[degrees]04'E C t9 22[degrees]47'S 14[degrees]33'E C t10 22[degrees]59'S 14[degrees]24'E C [L.sub. L. K [infinity]] [phi]' m1 0.47 44.00 2.96 m2 0.48 44.00 2.97 m3 1.18 33.50 3.12 m4 0.80 33.00 2.94 m5 0.09 28.82 1.90 m6 0.80 26.40 2.75 m7 0.90 28.50 2.86 j1 0.43 18.50 2.17 j2 1.48 31.47 3.17 j3 1.79 30.02 3.21 e1 0.30 46.00 2.80 e2 0.62 29.30 2.73 y1 0.06 22.87 1.48 y2 0.04 33.02 1.67 z 0.10 26.15 1.84 k1 0.29 20.20 2.07 k2 1.16 25.10 2.86 w1 0.09 29.04 1.86 w2 0.09 25.13 1.76 w3 0.09 20.06 1.55 w4 0.16 30.94 2.19 f1 1.00 46.00 3.33 f2 1.17 35.00 3.16 f3 0.70 42.00 3.09 n1 0.61 33.15 2.83 n2 1.32 29.76 3.07 n3 1.01 35.9U 3.11 n4 1.06 31.28 3.02 n5 0.68 38.00 2.99 p1 0.45 43.48 2.93 p2 0.70 36.33 2.97 p3 0.79 35.55 3.00 p4 0.68 32.25 2.85 p5 0.70 38.22 3.01 p6 0.72 38.41 3.02 p7 0.74 39.78 3.07 p8 0.77 39.70 3.09 p9 0.38 48.90 2.96 p10 0.96 35.99 3.09 p11 0.97 43.80 3.27 p12 0.55 52.84 3.19 p13 0.58 46.00 3.09 p14 0.71 41.80 3.09 p15 0.58 47.30 3.11 p16 0.30 47.56 2.83 u1 0.86 59.00 3.48 u2 1.59 56.00 3.70 t1 0.28 82.00 3.28 t2 0.43 75.00 3.39 t3 0.28 82.00 3.27 t4 0.32 78.00 3.29 t5 0.46 79.00 3.46 t6 0.47 85.00 3.53 t7 0.60 76.00 3.54 t8 0.65 67.00 3.47 t9 0.27 82.00 3.27 t10 0.47 78.00 3.46 L. OGP Source m1 4.60 Present study m2 4.61 Present study m3 4.65 Penchaszadeh and Olivier (1975) m4 4.46 Defeo (1996) m5 3.36 Gil and Thome (2000) m6 4.17 Cardoso and Veloso (2003) m7 4.32 Cardoso and Veloso (2003) j1 3.43 Velez et al. (1985) j2 4.66 Garcia et al. (2003) j3 4.69 Marcano et al. (2003) e1 4.19 Palacios et al. (1983) e2 4.19 Riascos and Urban (2002) y1 2.84 Nayar (1955) y2 3.19 Talikhedkar et al. (1976) z 3.25 Alagarswami (1966) k1 3.37 McLachlan et al. (1996) k2 4.26 Rocha-Barreira de Almeida et al. (2002) w1 3.33 Ansell (1972) w2 3.16 Ansell (1972) w3 2.85 Nair et al. (1978) w4 3.68 Thippeswamy and f1 4.99 Joseph (1991)Arntz et al. (1987) before El Nino f2 4.70 Arntz et al. (1987) [right arrow] during El Nino f3 4.71 Arntz et al. (1987) [right arrow] alter El Nino n1 4.35 Ansell and Lagardere (1980) n2 4.54 Ansell and Lagardere (1980) n3 4.6t Ansell and Lagardere (1980) n4 4.51 Ansell and Lagardere (1980) n5 4.57 Guillou and Le Moal (1980) p1 4.57 Ansell and Lagardere (1980) p2 4.53 Ansell and Lagardere (1980) p3 4.55 Ansell and Lagardere (1980) p4 4.36 Ansell and Lagardere (1980) p5 4.59 Ansell and Lagardere (1980) p6 4.61 Ansell and Lagardere (1980) p7 4.67 Ansell and Lagardere (1980) p8 4.68 Ansell and Lagardere (1980) p9 4.65 Guillou and Le Moal (1980) p10 4.65 Bodoy (1982) p11 4.91 Fernandez et al. (1984) p12 4.91 Maze and Laborda (1988) p13 4.75 Ramon et al. (1995) p14 4.71 Ramon et al. (1995) p15 4.79 Gaspar et al. (1999) p16 4.51 Zeichen et al. (2002) u1 5.25 King (1985) u2 5.45 Laudien et al. (2003) t1 5.19 de Villiers (1975) t2 5.26 de Villiers (1975) t3 5.18 de Villiers (1975) t4 5.19 de Villiers (1975) t5 5.36 Laudien et al. (2003) t6 5.46 Laudien et al. (2003) t7 5.42 Laudien et al. (2003) t8 5.29 Laudien et al. (2003) t9 5.18 Laudien et al. (2003) t10 5.32 Laudien et al. (2003) (1) Maximum length. (2) Age estimated from length-frequency distributions. (3) Age estimated from hyaline growth rings. (4) Age estimated from external growth rings. (5) Values calculated on monthly basis from raw data reported by the authors. TABLE 4. Secondary production (P, g AFDM [m.sup.-2][y.sup.-1]), biomass ([bar.B], g AFDM [m.sup.-2]) and renewal rate (P/[bar.B]) of D. hanleyanus at Santa Teresita (m1, m2), Mar del las Pampas (m3, m4) and Faro Querandi (m5, m6) between 2005 and 2006 in comparison with several Donax species from different geographical regions. Table modified from (Cardoso & Veloso 2003). Code letters (L.) are equivalent to Figure 1B and were used for visualizing the relationship between the production/to/biomass (P/[bar.B]) ratio and latitude in Figure 7. Datasets are published in Herrmann et al. (2008). L Species P m1 D. hanleyanus 0.45 m2 D. hanleyanus 0.08 m3 D. hanleyanus 0.20 m4 D. hanleyanus 0.15 m5 D. hanleyanus 0.99 m6 D. hanleyanus 0.79 m7 D. hanleyanus 0.76 m8 D. hanleyanus 3.67 t1 D. serra 356.60 t2 D. serra 273.20 t3 D. serra 637.30 t4 D. serra 166.90 u1 D. sordidus (1) 0.38 u2 D. sordidus (1) 0.15 u3 D. sordidus (1) 0.13 i D. variabilis (1) 1.96 j D. denticulatus (1,2) 31.39 p1 D. trunculus (1) 18.98 p2 D. trunculus (1) 39.99 n1 D. vittatus (1) 12.33 n2 D. vittatus (1) 17.33 n3 D. vittatus (1) 0.60 w D. incarnatus (1) 1.08 x D. spiculum (1) 0.87 L [bar.B] P/[bar.B] m1 0.21 2.18 m2 0.04 1.83 m3 0.17 1.18 m4 0.17 0.87 m5 1.32 0.75 m6 0.96 0.82 m7 0.48 1.59 m8 2.53 1.45 t1 288.80 1.20 t2 171.90 1.60 t3 545.90 1.20 t4 141.20 1.20 u1 0.21 1.78 u2 0.10 1.56 u3 0.12 1.03 i 0.26 7.64 j -- 5.84 p1 15.42 1.23 p2 39.35 1.02 n1 7.66 1.61 n2 10.43 1.66 n3 0.29 2.10 w 0.18 5.88 x 0.08 10.29 L Lat. Long. m1 36[degrees]32'S 56[degrees]41'W m2 36[degrees]32'S 56[degrees]41'W m3 37[degrees]19'S 57[degrees]00'W m4 37[degrees]19'S 57[degrees]00'W m5 37[degrees]29'S 57[degrees]07'W m6 37[degrees]29'S 57[degrees]07'W m7 23[degrees]03'S 43[degrees]34'W m8 23[degrees]03'S 43[degrees]34'W t1 22[degrees]47'S 14[degrees]33'E t2 22[degrees]47'S 14[degrees]33'E t3 22[degrees]59'S 14[degrees]24'E t4 22[degrees]59'S 14[degrees]24'E u1 33[degrees]57'S 25[degrees]40'E u2 33[degrees]57'S 25[degrees]40'E u3 33[degrees]57'S 25[degrees]40'E i 33[degrees]41'S 78[degrees]53'W j 10[degrees]8'N 64[degrees]16'W p1 45[degrees]50'N 1[degrees]20'W p2 45[degrees]50'N 1[degrees]20'W n1 45[degrees]50'N 1[degrees]20'W n2 45[degrees]50'N l[degrees]20'W n3 51[degrees]41'N 4[degrees]28'W w 9[degrees]42'N 76[degrees]20'E x 9[degrees]42'N 76[degrees]20'E L Source m1 Present study (Santa Teresita 2005) m2 Present study (Santa Teresita 2006) m3 Present study (Mar de las Pampas 2005) m4 Present study (Mar de las Pampas 2006) m5 Present study (Faro Querandi 2005) m6 Present study (Faro Querandi 2006) m7 Cardoso and Veloso (2003) m8 Cardoso and Veloso (2003) t1 Laudien et al. (2003) t2 Laudien et al. (2003) t3 Laudien et al. (2003) t4 Laudien et al. (2003) u1 McLachlan and van der Horst (1979) u2 McLachlan (1979) u3 McLachlan (1979) i Wilson (1999) j Velez et al. (1985) p1 Ansell and Lagardere (1980) p2 Ansell and Lagardere (1980) n1 Ansell and Lagardere (1980) n2 Ansell and Lagardere (1980) n3 Warwick et al. (1978) w Ansell et al. (1978) x Ansell et al. (1978) (1) Transformed values (g AFDM [m.sup.-2][yr.sup.-1]) by Ricciardi and Bourget (1998). (2) Maximum daily value (0.016 g AFDM [d.sup.-1] x 365 days).