© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Population structure of the copepods Centropages typicus and Temora stylifera in different environmental conditions
Laboratory of Biological Oceanography, Stazione Zoologica "A. Dohrn" Villa Comunale, IT-80121 Naples, Italy
*Correspondence to M. G. Mazzocchi: tel: +39 081 583 3212; fax: +39 081 764 1355. e-mail: grazia{at}szn.it.
The population structure (NI to adults) of the copepods Centropages typicus and Temora stylifera was investigated in parallel at two sites in the eutrophic and oligotrophic waters of the Gulf of Naples (Tyrrhenian Sea) with the aim of identifying the environmental factors that modulate the occurrence of these calanoids that are abundant in the neritic waters of the Mediterranean Sea. Monthly zooplankton sampling was conducted from May 1999 to June 2000 in 5–7 discrete depth layers in the upper 200 m with a 70-µm mesh net. Temperature, salinity, and fluorescence profiles were recorded by CTD casts. The remarkable differences observed in the temporal and spatial patterns of the two populations throughout their developmental cycles seem to result more from different responses to environmental factors than from different life-history traits. C. typicus was abundant in early to mid-summer and was distributed in the whole water column; T. stylifera followed in late summer–autumn and occurred preferentially in the upper 30 m. Temperature was the main regulating factor for T. stylifera. The availability of autotrophic food appeared relatively more important for C. typicus, although unequivocal signals were not observed, suggesting that more complex trophic interactions likely affect the dynamics of the two populations.
Keywords: Centropages typicus, distribution, Mediterranean, population structure, Temora stylifera
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Introduction |
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 Top Introduction Materials and methodsResultsDiscussionReferences |
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Copepods are important components of zooplankton communities in the neritic regions of the Mediterranean Sea, where a limited number of species account for the bulk of abundance and biomass during most of the year (Scotto di Carlo and Ianora, 1983). The calanoids Centropages typicus and Temora stylifera are among the dominant species in the coastal areas of the Western Mediterranean, where their populations occur with different seasonal cycles (Gaudy, 1962; Razoul, 1974; Seguin, 1981; Mazzocchi and Ribera d'Alcalà, 1995). In most regions, C. typicus shows a wide peak of abundance in late spring–summer, whereas T. stylifera shows a narrower peak in autumn. In a long-term study in the Gulf of Naples (Tyrrhenian Sea), alternate patterns of abundance between the two species have also been observed interannually (Mazzocchi and Ribera d'Alcalà, 1995), supporting the hypothesis of competition between Centropages and Temora as suggested by Raymont (1983) based on the observations of Gaudy (1962) and Bernard (1970).
Owing to their abundance, ease of collection, and adaptability to different laboratory conditions, C. typicus and T. stylifera have been thoroughly investigated for many aspects of their biology (Abou-Debs and Nival, 1983; Smith and Lane, 1985; Nival et al., 1990; Miralto et al., 1995; Halsband-Lenk et al., 2001). In the Gulf of Naples, the reproductive activity of wild females was monitored at annual and interannual scales (Ianora et al., 1989; Ianora and Buttino, 1990; Ianora et al., 1992; Ianora and Poulet, 1993), in parallel with the long-term study conducted on the temporal cycles of the whole populations at a coastal site (Mazzocchi and Ribera d'Alcalà, 1995; Ribera d'Alcalà et al., 2004).
The present work investigated the population structure of C. typicus and T. stylifera during a year at two sites in the Gulf of Naples, in coastal and offshore waters. The distribution of all developmental stages was analysed in discrete layers in the upper water column in relation to environmental parameters in order to identify the factors that affect the vertical and seasonal occurrence of the two species. The ultimate goal of our study was to learn if the apparent competition between C. typicus and T. stylifera was mainly determined by the environmental conditions, or by the intrinsic biological traits of the two species.
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Materials and methods |
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Zooplankton sampling was carried out monthly from May 1999 to June 2000 (except in December 1999 and April 2000) at two stations in the Gulf of Naples (Tyrrhenian Sea) (Figure 1). Stn MC, site of a long-term study, is located in the boundary region between the coastal and offshore systems (2 miles off the coast, 80 m depth) and is characterized by high concentrations of chlorophyll a (Chl a) (Ribera d'Alcalà et al., 2004); Stn L20 is located in the centre of the Gulf (300 m depth) and is permanently influenced by oligotrophic Tyrrhenian waters (Carrada et al., 1980). The upper water columns at the coastal and offshore sites are mainly occupied by the Coastal Surface Water and by the Tyrrhenian Surface Water, respectively, which differ primarily in their salinity properties (Carrada et al., 1980).
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The samples were collected between 08:00 and 12:00 h using a closing cylindrical–conical net (40 cm diameter, 120 cm length, 500 ml codend) with 70 µm mesh aperture to retain the first naupliar stages (NI) of C. typicus and T. stylifera (mean body sizes are 120 x 70 µm and 100 x 80 µm, respectively). Discrete depth layers of 10 m each were sampled vertically in succession in the upper 50 m at the coastal station and in the upper 70 m at the offshore station; the maximum depths were chosen for comparative purposes with the long-term mesozooplankton series that has been conducted at Stn MC since 1984 (Ribera d'Alcalà et al., 2004). The volume filtered (V=Ax
L, m3) was estimated taking into account the area of the net mouth (A) and the difference in winch readings (L). The thickness of the sampled layer (D) and the depth limits (Li, Lf) were computed considering the wire angle 
(D = L cos ). Attention was always paid to maintaining the wire angle below 10°. The clogging of the fine mesh net was prevented by towing for a short distance (10 m) at slow speed (0.2–0.3 m s–1) and the filtration efficiency was estimated to be 90–95% for all samples. The net was carefully rinsed after each tow and the content of the codend was immediately fixed and preserved in a 4% buffered formaldehyde–seawater solution. Temperature, salinity, and fluorescence values were recorded with a CTD system (Sea Bird Electronics 9/11-plus) deployed before each zooplankton tow series. To estimate the Chl a concentrations corresponding to the fluorescence readings, the binned (3 m) fluorescence data recorded at the same sampling dates at Stn MC in the long-term monitoring programme were linearly fitted with the Chl a concentrations measured in parallel from bottle samples. Each 0.05 fluorescence unit (f.u.) corresponded to 0.234 µg Chl a l–1 according to the regression equation: y=4.7737x–0.005 (r2=0.82). Since it is well known that the chlorophyll:fluorescence relationship can be variable, the calibration was intended to give an estimate of the true chlorophyll concentration. In the laboratory, specimens of the target species were counted under a dissecting microscope and all individuals were identified according to the developmental stage (NI to adults) and sex (CV and adults). The specimens were counted in the whole sample or in aliquots (10–20%), obtained with a large mouth graduated syringe after accurate mixing (modified Stempel pipette method). From each sample, we counted about 100 individuals for each of the 13 categories of the most abundant species.
To evaluate the possible influence of environmental factors on copepod distribution in space and time, the Spearman's rank correlation coefficient was computed on the species/stage composition and the environmental parameters at the two stations and in the depth layers (Stn MC, n=55; Stn L20, n=77). The multivariate principal component analysis (PCA) was applied to the whole data set of biological and environmental parameters, separately for Stn MC (matrix 55x9) and Stn L20 (matrix 77x9), according to the PASSTEC package (Ibanez and Etienne, 1998).
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Results |
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Environmental parameters
The water masses at the two sampling sites differed in their salinity properties; throughout the annual cycle, the coastal waters were always less salty (37.73–38.01 psu) than the offshore waters (37.85–38.12 psu). At both stations, the lowest values were recorded in May–June in the upper 20 m; the salinity profiles were fairly homogeneous from summer 1999 to the next spring.
The seasonal patterns of temperature were similar at the two stations (Figure 2a, b). The vertically averaged values ranged from 13°C in March to 23°C in August. The water column was stratified from May to October. Surface temperature values >20°C were recorded in the period May–October 1999, when the thermocline deepened from 20 m to 50 m depth. The water column was thoroughly mixed from November 1999 (18°C) to April 2000 (13°C).
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The distribution of autotrophic biomass, estimated from fluorescence profiles, differed markedly between the two stations (Figure 2c, d). At coastal Stn MC, high fluorescence values were recorded from the surface down to 20 m depth. Peaks >0.5 f.u. (>2.34 µg Chl a l–1) were recorded in the upper 10 m in spring and in September 1999, and in spring 2000. Low fluorescence values (<0.1 f.u., <0.47 µg Chl a l–1) were recorded in the water column from November to February. At Stn L20, very low fluorescence values (<0.05 f.u., <0.23 µg Chl a l–1) were recorded throughout the year; the highest value (0.15 f.u., 0.70 µg Chl a l–1) was registered at 30 m depth in August (Figure 2d).
Copepod distribution
Nauplii
Centropages typicus nauplii were present throughout the year at both stations, with four peaks of abundance (Figure 3a, b). By contrast, Temora stylifera nauplii were present with high abundance only from August to October and with extremely low numbers during the rest of the year (Figure 3a, b). C. typicus nauplii showed similar abundances at both sampling sites, with peaks in the range 130–201 ind. m–3. T. stylifera nauplii were much more abundant than C. typicus at both sites, and more abundant at Stn MC (up to 1724 ind. m–3) than at Stn L20 (600 ind. m–3).
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In C. typicus, early stages (NI–NIII) accounted for the largest portion (>50%) of the naupliar population on all sampling occasions, except in July (45.3%) and September (9.1%) 1999. In T. stylifera, all stages co-occurred in the water column at both stations only in the season of high abundance, from August to October, when the early stages (NI–NIII) accounted for 49.3–69.3% of the naupliar population. Only very few stages (mainly NIII and NIV) occurred during the rest of the year, when the total nauplii abundance was very low.
In C. typicus, all stages were more abundant in the upper 30 m during most of the year (Figure 4). However, NI–NV occupied a wider range of the water column, whereas NVI was confined within the upper 20 m (except in August–September 1999 at Stn L20). At Stn L20 in August, all nauplii NI were concentrated in the 20–30 m layer.
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In T. stylifera, from August to October, NI–NIII were present only in the upper 30 m and were mainly concentrated in the top 10 m (Figure 5). The older stages, particularly NVI, extended over a slightly wider depth range, although they were mainly concentrated in the upper 30 m.
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Copepodids
The seasonal distribution of copepodids paralled that of nauplii. C. typicus occurred throughout the year, with peaks of abundance in summer, whereas T. stylifera occurred with high abundance only from August to October (Figure 3c, d). In 1999, Temora was more abundant than Centropages at Stn MC, but the two species occurred with similar abundance at Stn L20.
In C. typicus, all stages were continuously present from May to October at Stn MC and from May to August at Stn L20. However, pronounced differences in stage composition were observed between the two stations in summer and late autumn. For example, in June and July, CIV–CV prevailed at Stn MC (66–75%), whereas CI–CIII prevailed at Stn L20 (78.9–82.3%); in August, CI–CIII prevailed at Stn MC (73.9%), whereas CIV–CV females and males prevailed at Stn L20 (60.6%); in November, the few copepodids were represented by CIV and CV female at Stn MC and by CI at Stn L20. In T. stylifera, all stages co-occurred only in August–October at both sites and in June 2000 at Stn L20. CI–CIV accounted for 82.9–98.1% of total copepodid abundance, whereas CV females and males always represented a small percentage (<10%).
In C. typicus, CI–CIII occupied the whole sampled water column during most of the year (Figure 6); however, they were more concentrated in the intermediate and deepest layers in spring–summer, and in the upper 20 m in winter. The older copepodids (CIV, CV) did not occur in the upper 20 m in summer at either station.
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In T. stylifera, CI–CIII were confined in the upper 40 m at both sampling sites (Figure 7). The older stages occurred deeper in the water column at Stn MC during the season of highest abundance; at Stn L20, they remained above the 40 m depth and only few individuals occurred deeper.
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Adults
The adults of C. typicus reached a peak of abundance in July 1999 (202 ind. m–3) at Stn MC, and in May 1999 (42 ind. m–3) at Stn L20 (Figure 3e, f). In T. stylifera, they occurred only in August–October 1999 and in June 2000 with low (<20 ind. m–3) and similar abundances at both stations (Figure 3e, f).
For both species, the sex ratio (female/male abundance) was fairly stable during the respective peak seasons but variable at low adult abundance. In C. typicus, females and males were almost equally represented at both stations (sex ratio = 1.3 ± 0.5 at Stn MC, 1.0±0.3 at Stn L20). In T. stylifera, females were slightly more abundant than males at Stn MC (1.2±0.3), while males were much more abundant at Stn L20 (0.4±0.2).
In C. typicus, adults were distributed throughout the water column at both sites during most of the study, but females and males were sometimes concentrated in different depth layers (e.g. in August and September at Stn MC, in June and October at Stn L20) (Figure 8). In T. stylifera, adults were mainly concentrated in the 30–50 m layer at Stn MC and in the upper 40 m at Stn L20 (Figure 8). When co-occurring in time, females and males were concentrated in the same depth layer only on a limited number of sampling dates (August and September at Stn MC, September and October at Stn L20).
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Statistical analysis
The Spearman's rank correlation between copepod abundance and environmental parameters (Table 1) showed that for C. typicus a significant correlation (p<0.01) occurred between nauplii and copepodid abundance and fluorescence at Stn MC and between the whole population and temperature at Stn L20. A significant negative correlation occurred between the whole population and salinity at Stn L20. For T. stylifera, all developmental stages were significantly correlated with temperature (p<0.01) at both sampling sites (Table 1).
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The results of the PCA analysis showed that C. typicus and T. stylifera were clearly separated in the plane of the first two axes, which accounted for 60.4% of the total variance for Stn MC and 65.8% for Stn L20 (Figure 9). For both stations, all developmental stages of T. stylifera were positioned close to the first axis, which clearly represented the temperature distribution. The second axis represented the fluorescence distribution only for Stn MC, and nauplii and copepodids of C. typicus were clustered close to it. Salinity did not represent any of the first three axes.
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Discussion |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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The present study investigated the seasonal and vertical distribution of Centropages typicus and Temora stylifera at two stations in the Gulf of Naples and showed that remarkable differences characterized the temporal and spatial patterns of these two species throughout the developmental cycle of their populations.
C. typicus occurred throughout the year, with overlapping generations, and peaks of abundance in early to mid-summer. T. stylifera followed in late summer and was strictly confined in a narrow temporal window until autumn, as regularly observed also at the interannual scale (Ribera d'Alcalà et al., 2004); all developmental stages showed the same timing of occurrence in both coastal and offshore waters, indicating that the spawning activity was synchronized in the whole population of the study area.
Both species were always present in the area; even Temora never disappeared although it occurred with very few individuals outside its typical season. They reproduced continuously throughout the year, similar to many other copepods in subtemperate regions, and with seasonal variability in fecundity (Abou-Debs and Nival, 1983; Ianora, 1998; Halsband-Lenk et al., 2001) and egg-hatching success (Ianora, 1998). Neither C. typicus nor T. stylifera undergo diapause phases in their life cycle in the Gulf of Naples; resting eggs have never been observed in multi-annual studies on the reproductive biology of the local populations (A. Ianora, pers. comm.). T. stylifera is actually not reported to be among the calanoids that produce resting eggs, differing from its congener T. longicornis and from C. typicus in the North Sea (Mauchline, 1998). The traits of its reproductive biology, along with the particular seasonal cycle of abundance, certainly raise the question of how T. stylifera gets through the winter–spring period in the Mediterranean, a puzzling question that has not yet been answered. In the Gulf of Naples, the optimal reproductive periods, with the highest egg production and viability, are December–April for C. typicus and August–October for T. stylifera (Ianora, 1998, her figures 1 and 4). Therefore, the two populations recruit at their highest rates in the coldest-mixed and warmest-stratified water columns respectively, indicating that reproduction was affected by different environmental conditions. This is in contrast to that reported for the populations of the same species in the northwestern Mediterranean, which showed strikingly similar reproductive patterns and responses to the same environmental factors, although different seasonal cycles of abundance (Halsband-Lenk et al., 2001). The seasonal occurrence of nauplii during our study reflected the seasonal pattern of egg recruitment for T. stylifera but not for C. typicus, which showed a more prolonged and variable occurrence, suggesting a more complex interaction with the environmental factors.
The nauplii were much more abundant in T. stylifera than in C. typicus, but it seems that this difference cannot be interpreted in terms of different reproductive traits. In fact, on a mean annual base, the two species show similar egg production rates and egg viability in the Gulf of Naples (Ianora, 1998; A. Ianora and Y. Carotenuto, pers. comm.), although Centropages is reported as the most fecund among the small broadcast spawner calanoids (Kiørboe and Sabatini, 1995). The quantitative differences between the two naupliar populations could be due to different vulnerability to predation. In comparison with C. typicus, T. stylifera nauplii are probably more protected against predation by their long posterior terminal spines and by the rows of ventral spines that arm stages from NIV onwards (Carotenuto, 1999). In the Gulf of Naples, the most intense hatching of C. typicus nauplii is in winter, the season of occurrence of carnivorous copepods (e.g. Candacia, Euchaeta) in the coastal zooplankton communities (our unpublished data), indicating a higher probability of encountering such potential predators. We cannot exclude, however, that different mortality rates in the naupliar populations could also be due to physiological differences depending on maternal and/or nauplii diet (Carotenuto et al., 2002).
A dramatic reduction in abundance from nauplii to copepodids and adults was observed during our study in the T. stylifera population but not in C. typicus. This indicated that the two juvenile populations differed in mortality rates and/or stage duration. Rearing experiments conducted at the same temperature (15°C) in Mediterranean populations showed that the stage duration in C. typicus and T. stylifera was similar for nauplii (11 and 12 days, respectively) and only slightly different for copepodids (14 and 11 days, respectively) (Halsband-Lenk et al., 2002). We may then hypothesize that T. stylifera copepodids and adults undergo higher mortality rates. A recent study on chaetognaths in the long-term series at Stn MC has shown that T. stylifera copepodids and adults were the calanoids most abundantly and frequently ingested by Sagitta spp. (P. Simonelli and M. G. Mazzocchi, unpublished data). We observed a 5–15 times difference in abundance from CIII–CV to adults also in the long-term series of zooplankton samples collected at Stn MC with a 200 µm Nansen net (113 cm mouth diameter). It should be noted, however, that the adult abundance of both species in the present study might not accurately reflect the real distribution patterns. It must be considered that the adult abundance has likely been underestimated in our samples owing to the possible avoidance of the small net by the more mobile stages in the populations.
C. typicus copepodids occurred throughout the year and were spread in the water column over a wide range of environmental conditions. T. stylifera copepodids were preferentially concentrated above the thermocline. It is worth noting that when co-occurring in the water column with similar abundance, as in August, the copepodid stages of the two species showed very different or opposite vertical distribution, being preferentially concentrated in different depth layers. This vertical partitioning was particularly remarkable at Stn L20.
Overall, both species were more abundant at the coastal, more eutrophic site, where nutrients are seldom limiting and support a rich and diversified autotrophic community (Ribera d'Alcalà et al., 2004). Naupliar stages of C. typicus and T. stylifera were mainly concentrated in the surface layers, where the possibility of encountering abundant food was higher, since the peaks of Chl a and phytoplankton cells in the area are mainly recorded in the upper 10 m (Ribera d'Alcalà et al., 2004). However, a significant correlation with fluorescence was found only for C. typicus nauplii and copepodids at Stn MC, although at the oligotrophic Stn L20 C. typicus CIII–CV were concentrated in the 20–50 m layer, where the fluorescence values indicated the presence of autotrophic biomass, and the occurrence of C. typicus NI at 30 m depth in August in correspondence with the maximum fluorescence value suggests that the eggs had probably been released by females that were recently concentrated in that layer. Although it is well known that all planktonic copepods have, to varying extents, omnivorous diets, it is reported that Centropages feeds at the DCM (Saiz and Alcaraz, 1990), but it can switch from herbivory to raptorial feeding on ciliates (Tiselius, 1989), whereas T. stylifera, which creates continuous feeding currents (Paffenhöfer, 1998), is likely primarily herbivorous (Turner, 1984). A more exhaustive comparison of feeding needs and performances between the two species in the area will be addressed with parallel grazing experiments on natural particle assemblages.
We cannot exclude that the differences observed in the distribution patterns between the two sampling sites could have been affected by advection processes taking place in the area, which seem to be, in any case, fairly weak throughout the year. A study on the wind and boundary-driven depth-averaged circulation (Gravili et al., 2001) has shown that northwestward currents (corresponding to an overall cyclonic Tyrrhenian circulation) characterize all seasons except summer, when much smaller currents with episodes of reversal are evident. The current displacement is generally weak (<1 cm s–1) throughout the Gulf area. Data are unfortunately lacking on the dynamics of different layers at small vertical scale.
In conclusion, the differences observed in the temporal and spatial distribution of C. typicus and T. stylifera during our study in the Gulf of Naples seem to result more from different responses to environmental factors than from different life-history traits. Temperature is the main regulating factor for the synchronized growth in summer and the vertical distribution of T. stylifera. C. typicus seems to avoid high temperature conditions. Our results suggest that availability of autotrophic food is relatively more important for C. typicus than for T. stylifera, but unequivocal signals were not recorded. It is reasonable to suppose that the spatial differences we observed in the population structure of the two target species might result from trophic factors that integrate more diverse and more complex interactions than the mere quantity of autotrophic food.
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Acknowledgements |
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We thank the captain and crew of the RV "Vettoria" for their collaborative support during the samplings and F. Corato for his help in the analysis of CTD data. We greatly appreciated the constructive comments and suggestions of D. L. Mackas and two anonymous referees who helped improve an early version of this manuscript.
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References |
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