© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Vertical and seasonal distribution of eight Clausocalanus species (Copepoda: Calanoida) in oligotrophic waters
Laboratory of Biological Oceanography, Stazione Zoologica "A. Dohrn" Villa Comunale, IT-80121 Naples, Italy
*Correspondence to À. Peralba: tel: +39 081 583 3212; fax: +39 081 764 1355. e-mail: peralba{at}szn.it; grazia{at}szn.it.
Copepods of the genus Clausocalanus Giesbrecht, 1888 are among the most abundant calanoids in the Mediterranean Sea, both in coastal and offshore regions. The vertical distribution of C. arcuicornis, C. furcatus, C. jobei, C. lividus, C. mastigophorus, C. parapergens, C. paululus, and C. pergens, which co-occur in the upper 200 m in the Gulf of Naples (Tyrrhenian Sea), was investigated during an annual sampling cycle conducted at an offshore station in 2002. The quantitative data on distribution of each species were analysed in relation to the environmental parameters. The patterns that we observed in the seasonal cycles and vertical distribution provided insights on the ecological niches of the eight Clausocalanus species.
Keywords: Clausocalanus, Mediterranean Sea, niche, seasonal cycle, vertical distribution
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Introduction |
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 Top Introduction Materials and methodsResultsDiscussionReferences |
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One of the most challenging topics in plankton ecology is the question of how large numbers of species can coexist in pelagic environments (e.g. Hutchinson, 1959; McGowan and Walker, 1985). Each species is assumed to occupy a unique niche (Grinnell, 1917) that has been considered an N-dimensional hyper-volume defined by interactions with the physical environment and with other organisms (Hutchinson, 1959; Giller, 1984).
In marine regions, the problem of species coexistence seems to be particularly interesting for copepods, which have successfully colonized the whole water column and dominate (numerically) the zooplankton communities under a very large variety of ecological conditions (Huys and Boxshall, 1991). The large number of copepod genera co-occurring in a certain depth layer, coupled with the occurrence of as many as 10–15 congeneric species (Mauchline, 1998), challenges the niche definition.
Among calanoid copepods, the genus Clausocalanus is one of the most common and abundant in the oceans, especially in subtropical–tropical regions (Frost and Fleminger, 1968; Webber and Roff, 1995). The 13 species described so far occur over a wide latitudinal range and for many of them the distribution ranges frequently overlap (Frost and Fleminger, 1968). Owing to its abundance among small calanoids in epipelagic waters, Clausocalanus has been the subject of several detailed studies. The horizontal distribution of Clausocalanus species was described for the surface layer (0–10 m) in the North Atlantic by Williams and Wallace (1975), while the vertical distribution was reported for Mediterranean sites by Hure and Scotto di Carlo (1970), Scotto di Carlo et al. (1984), and by Fragopoulu et al. (2001). The seasonal distribution was examined in the North Atlantic (Williams and Wallace, 1975) and in the Gulf of Naples, Tyrrhenian Sea (Hure and Scotto di Carlo, 1970; Mazzocchi and Ribera d'Alcalà, 1995). Despite its acknowledged numerical contribution to epipelagic communities, still very little is known about Clausocalanus ecology and especially biology; for example, feeding, reproduction, and development (Mazzocchi and Paffenhöfer, 1998). A study based on molecular analysis has recently been carried out on this genus, and is the first information on the phylogenetic relationships among Clausocalanus species (Bucklin et al., 2003).
In the Mediterranean Sea, eight Clausocalanus species have been reported (Razouls and Durand, 1991): C. mastigophorus, C. lividus, C. arcuicornis, C. parapergens, C. jobei, C. furcatus, C. pergens, and C. paululus. The coexistence of these congeners and recurrence of their seasonal patterns on a multi-annual scale (Mazzocchi and Ribera d'Alcalà, 1995) have prompted us to investigate their ecology and biology in greater detail.
We are currently carrying out a project focused on Clausocalanus aimed at: (1) understanding the mechanisms responsible for the success of Clausocalanus in oligotrophic epipelagic waters and (2) determining the extent of the niche separation of co-occurring species in the Mediterranean. The project covers the ecological, biological, and behavioural aspects of the target genus by combining field studies on distribution and laboratory experiments on feeding, reproduction, and metabolism.
In this article, the seasonal and vertical distributions of the eight Clausocalanus species occurring in the oligotrophic epipelagic waters in the open Gulf of Naples are presented in relation to the environmental parameters, thus allowing detailed description of their ecological features, i.e. basic information about their niche characterization.
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Materials and methods |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Quantitative zooplankton samples were collected monthly (biweekly when possible) from February 2002 to February 2003 at a site (Stn L20) located in the open system of the Gulf of Naples (>300 m depth). In the morning (10:00–12:00), single vertical hauls were taken in discrete depth layers using coupled WP-2 closing nets (57 cm mouth diameter) with 200 and 70 µm mesh aperture to quantitatively collect both adult specimens and developmental stages. Four to five depth layers were chosen in the 0–200 m water column according to environmental parameters recorded with a SCUFA® fluorometer (measurements of in vivo chlorophyll) interfaced with a Seabird CTD. The layers were identified according to different water-mass structures: above the thermocline, below the thermocline, at the deep chlorophyll maximum (DCM), and in deep homogeneous waters. When a mixed water column was encountered, the sampling strata were at standard depths (0–50 m; 50–100 m; 100–150 m; and 150–200 m). The fluorometer did not work in May, so the profile of autotrophic fluorescence was missed. Chlorophyll a concentration was measured by HPLC in water samples collected at discrete depths in Niskin bottles. Zooplankton samples collected with the 200 µm mesh net were fixed immediately after collection in a 4% buffered formaldehyde–seawater solution; samples collected with the 70 µm mesh net were preserved in 95% ethyl alcohol for future molecular analysis (as described by Bucklin, 2000). Counts of copepods, identification and counts of Clausocalanus were performed under a dissecting microscope using a Bogorov chamber. At least 1/2 of the entire sample was analysed taking repeated aliquots with a large mouth graduated syringe after accurate mixing (modified Stempel pipette method). Clausocalanus were identified to the species level for adult males and females, and to the genus level for copepodid stages. The total length of adult females and males was measured in 30 individuals for each Clausocalanus species under a dissecting microscope.
Here we present data of total adult population collected with the 200 µm mesh net in relation to the CTD profiles of environmental parameters and Chl a concentrations as a first step in analysing the ecological data collected until now for the ongoing project on Clausocalanus.
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Results |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Seasonal distribution
In the 0–200 m water column at Stn L20 from February 2002 to February 2003, the depth-average temperature ranged from 14.31°C (February 2003) to 16.32°C (October); salinity ranged from 38.06 psu to 38.26 psu, with its maximum value in summer; Chl a concentrations at the surface (0–10 m) ranged from 0.03 µg l–1 to 0.59 µg l–1, with its maximum in autumn (Figure 1).
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The abundance of Clausocalanus adults in the 0–200 m water column ranged from 22.0 to 116.6 ind. m–3 during the annual cycle (average 48.6 ind. m–3; s.d. ± 28.5). They accounted for 8.1–19.1% of the total copepod numbers (average 13.9%; s.d. ± 3.2) being by far the dominant genus in winter, when more than 20 copepod genera always occurred in very diversified communities (Figure 2).
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On annual average in the integrated water column, four species represented 89.4% of the genus abundance: C. paululus (34.8%), C. furcatus (27.7%), C. arcuicornis (19.4%), and C. pergens (7.5%). They were followed by C. parapergens (4.1%), C. lividus (2.9%), C. jobei (2.6%), and C. mastigophorus (1.0%). The relative importance of each species changed throughout the year (Figure 3), reflecting the seasonal cycle of species abundance (Figure 4). C. paululus, which was the smallest representative in our samples (females 0.66–0.74 mm, males 0.48–0.55 mm), was more abundant in winter; the population increased gradually in autumn and suddenly diminished in spring, with a numerical recovery in early summer (Figure 4a). Of the other small species, C. pergens (females 0.8–1.16 mm, males 0.58–0.70 mm) presented peaks of abundance in spring, one in late April and one in June, when its abundance was similar to C. paululus; during the rest of the year its abundance was much lower (Figure 4a). The highest abundance of C. arcuicornis (females 1.23–1.33 mm, males 0.97–1.16 mm) was recorded in spring; a second, much lower, peak occurred in autumn; C. furcatus (females 0.95–1.14 mm, males 0.71–0.94 mm) had a sudden and rapid increase in abundance in summer and a decline in mid-autumn (Figure 4a). Both species were nearly absent for the rest of the year. C. parapergens (females 1.07–1.38 mm, males 0.95–1.10 mm) and C. jobei (females 1.14–1.29 mm, males 0.94–1.05 mm) showed seasonal patterns similar to C. arcuicornis but with much lower abundance (Figure 4b). C. lividus (females 1.41–1.64 mm, males 1.19–1.36 mm) was mainly present in spring and occurred in negligible number during the rest of the year. C. mastigophorus (females 1.39–1.85 mm, males 1.11–1.35 mm), the least abundant species of the genus, appeared in winter (Figure 4b).
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Vertical distribution
The thermocline started to develop in late March in the upper 20 m and deepened throughout the season, reaching 70 m in December; the water column was completely mixed during winter (Figure 5a). The fluorescence profiles indicated the occurrence of a DCM between 50 and 110 m from April to November (Figure 5b).
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Environmental profiles of the water column (Figure 5) coupled with vertical profiles of species population distribution (Figure 6) show that C. paululus, C. pergens, C. arcuicornis, C. parapergens, and C. jobei avoided the surface (upper 25–50 m) when surface temperatures were close to 20°C or exceeded 20°C (Figure 5a). C. paululus (Figure 6a) and C. pergens (Figure 6b) were uniformly distributed in the water column in winter, concentrated in the upper 100 m in spring, and occurred below 50 m depth in summer. C. arcuicornis (Figure 6c) attained peak abundance in the upper layer during spring; in summer its distribution was concentrated at the DCM (Figure 5b) until mid-autumn, when the population rose again to the upper layer. C. parapergens (Figure 6d) and C. jobei (Figure 6e) generally occurred in association with the DCM (Figure 5b) from spring to autumn and thereafter they shifted to the upper layer, descending again during late winter. Unlike the previously mentioned species, C. furcatus was almost exclusively restricted to the upper 40 m (Figure 6f) above the thermocline; in late summer and autumn it was also present with low abundance down to 50 m. C. mastigophorus was localized in the upper layer during most of the year (Figure 6g). It moved to the 50–150 m depth layer only in summer; in late summer it started to rise again to the upper layer. Finally, C. lividus showed a varying vertical distribution (Figure 6h). Its peaks of abundance occurred in the upper layer from winter to spring, and in deeper layers in summer when it was concentrated under the thermocline and at the DCM (Figure 5b). This species almost disappeared from the water column from late summer to winter.
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Discussion |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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The present study provides a comprehensive picture of the distribution in time (annual) and space (vertical 0–200 m) of the adult populations of eight Clausocalanus species in relation to the environmental parameters in oligotrophic waters.
The study site had been previously characterized for its hydrography and described as a boundary location between the neritic and open water systems (Carrada et al., 1980). It was occupied by a homogeneous water column of Tyrrhenian Intermediate Water (TIW) in winter (T, S: 14°C and 38 psu) as a result of convective mixing; during spring and summer this water warms and remains over 50 m depth becoming the Tyrrhenian Surface Water (TSW) (T, S: 14–26°C and 38 psu). The core of TIW remains between 50 and 100 m until late autumn, when it is incorporated into the new production of TIW. The low Chl a values recorded during the present study confirmed that the site is characterized by permanent oligotrophic conditions as observed by Carrada et al. (1980), who reported autotrophic biomass seven to nine times lower than in coastal waters.
In this oligotrophic environment, Clausocalanus represented an important numerical component of the copepod communities throughout the year, and it dominated in winter, when the autotrophic biomass was particularly scarce. This was, in any case, sufficient to sustain numerous populations. It is likely that the feeding habits of Clausocalanus are not strictly based on autotrophic resources. Most calanoids have actually been shown to be omnivorous, but Clausocalanus seems to be particularly adapted to low phytoplankton concentrations. The feeding behaviour has been studied so far only for C. furcatus (Mazzocchi and Paffenhöfer, 1999). Differently from all other small calanoids, this species moves continuously and rapidly, exploring small areas in close succession; it does not create feeding current and captures the food particles after direct interception. All species of the genus show similar swimming behaviour (personal observations) and the same mode of catching food. We hypothesize that the direct interception of food particles does not allow an a priori selection of food categories, not at least on the basis of prey movement or size. The particle selection that has been observed (Mazzocchi and Paffenhöfer, 1999) should therefore result from short distance perception through chemical signals. As a consequence, Clausocalanus could be adapted to a wider range of food resources. Moreover, it has been observed that, in C. furcatus, feeding on phytoplankton cultures, egg production is higher at low, rather than at high, food concentration (Mazzocchi and Paffenhöfer, 1998).
In April, when the thermocline was not yet completely stabilized and the DCM occurred at 60 m, six Clausocalanus species were concentrated in the upper 0–20 m layer and only C. paululus and C. parapergens were localized deeper (20–50 m). From July to October, C. furcatus dominated the upper mixed layer, while the rest of Clausocalanus species had the core of their populations at the DCM. In autumn, when the DCM occurred deeper (60–130 m), the Clausocalanus populations were found in the upper layer. In winter, in a thoroughly mixed water column, the distribution of the species did not show preferential depths. It seems that seasonal changes in the structure of the water column create environmental conditions sufficiently diversified (in physical properties) to permit species coexistence.
The seasonal cycles of Clausocalanus species observed at Stn L20 were similar to those recorded during a long-term zooplankton study at a fixed coastal station (Stn MC) in the inner Gulf of Naples (Mazzocchi and Ribera d'Alcalà, 1995). However, the relative importance of the single Clausocalanus species for the total abundance of the genus differed between the two stations. C. pergens and C. furcatus were more abundant at the coastal than at the offshore site, while the opposite was observed for the remaining species. A similar comparison was done by Hure and Scotto di Carlo (1970) in their study on distribution and frequency of this genus at two stations (inshore and offshore) in the Gulf of Naples and in the South Adriatic Sea. These authors concluded that in the two regions: (1) Clausocalanus presented the same absolute abundance, but the genus was relatively more abundant in the Gulf of Naples, (2) the seasonal cycles were similar, and (3) the relative importance of the species changed from one region to the other but they had similar vertical distribution.
The seasonal cycles of C. paululus, C. pergens, C. arcuicornis, and C. furcatus, the four most abundant species in the Gulf of Naples, are similar in offshore (Stn L20, this study) and coastal (Stn MC, Mazzocchi and Ribera d'Alcalà, 1995) waters. Mazzocchi and Ribera d'Alcalà (1995) observed recurrent patterns in the seasonal cycles of these four species in the long-term series at Stn MC. Our study supports the observation that when the seasonal peaks of congeners overlap, the populations differ markedly in numbers. This suggests ecological differentiation among Clausocalanus species. In the North Atlantic, Williams and Wallace (1975) reported differences in the seasonal cycles and in the relative abundance of some Clausocalanus species. The total abundance of all species of Clausocalanus was mostly represented by C. pergens, C. lividus, and C. arcuicornis (80.4%). At our site, C. lividus never occurred in high numbers, whereas C. paululus was the most abundant species. Unfortunately, the mesh size used (300 µm) by Williams and Wallace (1975) did not allow them to collect C. paululus. Frost and Fleminger (1968) described the distribution of C. paululus as "apparently subtropical", but no data were available for the North Atlantic area, where its presence had recently been reported by Schnack-Schiel and Mizdalski (2002).
The medium-sized C. arcuicornis, C. parapergens, and C. jobei had the same period of highest abundance (spring), but C. arcuicornis was by far the dominant species. C. parapergens and C. jobei, which occurred with similar abundance, differed in vertical distribution, with the former occurring deeper in the water column. Studies conducted on the horizontal distribution of copepods in the Mediterranean Sea noted different abundances of these Clausocalanus species depending on the study areas (Hure and Scotto di Carlo, 1970; Regner, 1976; Pancucci-Papadopoulou et al., 1992; Fragopoulu et al., 2001). This suggests that the three species probably show different horizontal distribution at sub-basin scale.
C. furcatus, one of the most abundant species at Stn L20, presented a different seasonal cycle and occurred isolated in the upper layer in late summer. In the North Atlantic, C. furcatus represented only 2.3% of the genus with the peak abundance in September and November (Williams and Wallace, 1975). C. furcatus is a warm-water species and its distribution is typically superficial, above the thermocline (Fragopoulu and Lykakis, 1990; Paffenhöfer and Mazzocchi, 2003). This species shows high abundance also in the coastal Gulf of Naples in summer–autumn, when phytoplankton are at low concentrations (Mazzocchi and Ribera d'Alcalà, 1995; Ribera d'Alcalà et al., 2004), and in extremely oligotrophic waters such as the open Eastern Mediterranean (Siokou-Frangou et al., 1997), suggesting that the metabolic needs of this species are satisfied even at low levels of autotrophic biomass, as shown by preliminary feeding experiments in the laboratory (Mazzocchi and Paffenhöfer, 1998). Recent genetic analysis of Clausocalanus showed that C. furcatus is the most divergent species of the genus from the phylogenetic point of view (Bucklin et al., 2003). How this divergence reflects in the ecological and biological features is still unknown because of the lack of information on the other Clausocalanus species. In summer, all other Clausocalanus species moved to deeper layers, under the thermocline, suggesting that their populations were not favoured by high temperature.
C. mastigophorus and C. lividus, the largest Clausocalanus species present in the Mediterranean Sea, were very scarce at Stn L20, and attained peak abundance in spring in the upper layer. These two species are the only free-spawning Clausocalanus in the Mediterranean (Saiz and Calbet, 1999 for C. lividus; personal observations for both species). Their occurrence in the upper layers of the water column during their peak abundance might prevent egg loss from sinking, and ensure egg-hatching in the upper photic zone where nauplii can find more abundant food. In the North Atlantic, C. lividus was among the three most abundant species, but C. mastigophorus was the rarest species of the genus (Williams and Wallace, 1975). On the other hand, Fragopoulu et al. (2001) found that C. mastigophorus was the fifth most abundant species of the genus and C. lividus the rarest in the Eastern Mediterranean. These observations suggest different latitudinal gradients of abundance for the two species, although this does not appear from the qualitative maps of distribution drawn by Frost and Fleminger (1968). C. lividus might be more abundant in colder upper waters and C. mastigophorus in warmer upper waters.
In conclusion, the detailed analysis of the temporal and vertical occurrence of Clausocalanus species in relation to environmental parameters allowed us to draw a coherent picture of their distribution and make hypotheses regarding differentiation of their niches. The small species C. paululus and C. pergens differed in the numerical development of their populations. The medium-sized species C. arcuicornis, C. parapergens, and C. jobei presented similar seasonal cycles, but C. parapergens peak of abundance occurred deeper in the water column. C. arcuicornis and C. jobei were concentrated in the upper layer during their peaks, the former outnumbering the latter. C. furcatus differentiated from the other species and showed a narrow and quite isolated ecological niche. The large C. mastigophorus and C. lividus overlapped in seasonal and vertical distribution, but both species occurred with low abundance. This might suggest that our study area did not present the characteristics necessary for optimal development of their population.
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Acknowledgements |
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We thank the captain and crew of RV "Vettoria" for their collaboration and help during the sampling. Á.P. is grateful to C. Brunet for teaching her how to analyse the HPLC samples. The project on Clausocalanus is funded by the Italian programme ASTAPLAN-WP2.
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References |
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