ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 17, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(3):484-494; doi:10.1093/icesjms/fsn027
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This article appears in the following ICES Journal of Marine Science issue: 4th International Zooplankton Production Symposium: Human and Climate Forcing of Zooplankton Populations [View the issue table of contents]
Spatial distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast)
OceanBio Laboratory, Division of Biological Sciences, College of Arts and Sciences, University of the Philippines in the Visayas, 5023 Miagao, Iloilo, Philippines
Correspondence to M. M. P. Noblezada: tel: +63 33 3159271; fax: +63 33 3159271; e-mail: zoea21st{at}yahoo.com
Noblezada, M. M. P., and Campos, W. L. 2008. Spatial distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast). – ICES Journal of Marine Science, 65: 484–494.The composition, abundance, and distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast), from 31 stations along transects perpendicular to the coast were analysed. Samples were collected in April, 2001. In all, 26 species belonging to 14 genera were identified. Flaccisagitta enflata was the most abundant and frequently captured species at all stations, and constituted 41.9% of the total specimens. Most of the smallest diversity values were observed from areas affected by upwelling, although the greatest densities were observed at stations located within the upwelling zones. The occurrence of mesopelagic and bathypelagic species (Decipisagitta decipiens, Caecosagitta macrocephala, and Eukrohnia fowleri), in samples collected from upper water layers, could be explained by vertical transport caused by upwelling.
Keywords: bathypelagic, benthic, chaetognath, epipelagic, mesopelagic, northern Bicol Shelf, upwelling
Received 28 June 2007; accepted 13 January 2008; advance access publication 17 March 2008.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The northern Bicol Shelf (NBS), with a surface area of
11 600 km2, is the second widest shelf area in the Philippines after the northern Palawan Shelf (Amedo et al., 2002). Hydrographically, the NBS is one of the most complex regions of the Philippines. It is bounded on the east by the Philippine Sea and is part of the western boundary of the world’s largest ocean, the Pacific (Amedo et al., 2002; Udarbe-Walker et al., 2002). Several ocean currents affect the Philippine coast (Udarbe-Walker et al., 2002), including the North Equatorial Current (NEC), Kuroshio Current (KC), Mindanao Current (MC), Luzon Undercurrent (LU), and the Mindanao Undercurrent (MU; Qu et al., 1998; Figure 1). These currents are involved in the transport and exchange of water masses between the northern and southern hemispheres of the Pacific (Fine et al., 1994; Udarbe-Walker et al., 2002). They also exhibit interannual fluctuations linked to climatic variability (Lukas, 1988, 1998; Lukas et al., 1991; Qui and Lukas, 1996; Udarbe-Walker et al., 2002).
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Although oceanographic characterization of the region has been extensive, the results of biological and ecological research, especially of zooplankton communities, are lacking in the scientific literature. Zooplankton presence and abundance depend essentially on oceanographic characteristics such as currents and water masses. The present study site is located within an upwelling region. Although upwelling is known to enhance the nutrient distribution and primary production (Amedo et al., 2002; Primavera et al., 2002), the importance of upwelling in relation to zooplankton production and distribution is not well understood.
Chaetognaths occupy a prominent place among the total zooplankton (Reeve, 1971). Often, they belong with the top five most abundant zooplankton groups in the oceans, and their distribution and abundance can serve to corroborate variations in hydrographic conditions (Alvariño, 1964). Their close relationships with certain environmental variables (e.g. salinity, temperature, and dissolved oxygen) and their species-specific horizontal and vertical distribution make them excellent indicators of water masses (Bieri, 1959). The distribution patterns of some species have also been associated with hydrographic phenomena such as upwelling events. Certain species descend at convergence sites to depths greater than where they usually live, and species living at moderate depths rise towards the surface in regions of upwelling (Vinogradov, 1970). Mesopelagic and bathypelagic species like Decipisagitta decipiens, Caecosagitta macrocephala, and Eukrohnia fowleri are occasionally found in surface waters. These species normally do not reach these layers during their vertical migrations, so their presence has been considered an indicator of coastal upwelling events (Alvariño, 1964, 1965; Nair and Rao, 1973; Nair, 1977).
As active carnivores, chaetognaths form an important link in the marine food chain (Reeve, 1971). They feed mostly on copepods, crustacean larvae, and other chaetognaths, but occasionally on foraminiferans and fish larvae (Gray, 1961). Under conditions of low productivity, chaetognaths may strongly influence their prey populations (Øresland, 1990). Their roles as predators and competitors of larval fish have been evaluated in recent years (Baier and Purcell, 1997; Brodeur and Terazaki, 1999). Feigenbaum (1991) reports that the impact of chaetognath predation on fish larvae could be extensive because of the scarcity of fish larvae in the plankton, and could contribute to reductions of larval abundance during periods of fish production (Casanova, 1999).
Although the distribution and systematics of chaetognaths have been studied comprehensively in various parts of the world, relatively few studies have been conducted in the Philippines. Among these, Michael (1919), Bieri (1959), Alvariño (1967), and Rottman (1978) tackled taxonomic descriptions and distributional records. Others include Jumao-as and von Westernhagen (1975), Johnson (2005), and Cordero (2006), all of which are limited to specific basins, focusing on the western side and inland basins of the Philippines. The present study examined the species composition, abundance, and distribution of chaetognaths in relation to oceanographic conditions and processes along the Pacific coast of the Philippines, over the NBS.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Study area hydrography
The hydrographic features of the area during the survey are described by Amedo et al. (2002). The NEC bifurcates offshore of the NBS and gives rise to the north-flowing KC and the south-flowing MC (Figure 1). The bifurcation of the NEC occurs between 11°N and 14.5°N, and tends to shift to the north with increasing depth (Qui and Lukas, 1996). Numerical models indicate that the bifurcation may occur as far as 20°N for water of intermediate depths (Toole et al., 1990; Qu et al., 1998). The effects of these processes on the shelf areas are still unknown, but these energetic current systems can potentially impinge on the continental shelf and may give rise to secondary features such as upwelling (Amedo et al., 2002).
Distinct upwelling signals from temperature (colder areas) and large chlorophyll concentrations were observed off the NBS during the survey (Amedo et al., 2002). The upwelling area was detectable at a depth of 50 m at the shelf break between 14.75–15.0°N and 122.5–123.0°E (Figure 2), which is driven by a combination of prevailing alongshore winds that set up the Ekman layer transport offshore and a horizontal pressure gradient that sustains a borclinic jet. The strength of the upwelling varies with shifts in the bifurcation latitude. The bifurcation of the NEC occurs at a higher latitude during El Niño–Southern Oscillation years and at a lower latitude during La Niña years. Seasonally, the NEC bifurcates at the northernmost position in October and in the southernmost position in February (Qui and Lukas, 1996). Based on this, strong upwelling is likely during La Niña and the northeast monsoon season.
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Oceanographic sampling
An oceanographic cruise was conducted in the NBS between 1 and 11 April 2001 on the MV "DA-BFAR". The area investigated extended from 124.80–122.41°E and 11.25–15.79°N, covering 31 stations (Figure 1).
Zooplankton and ichthyoplankton were sampled using double oblique tows of a 60-cm bongo net with a 335-µm mesh to a maximum depth of 100 m (close to the thermocline depth) at deep stations or to 5 m above the bottom at shallow stations. A flowmeter was mounted at the mouth of the net to measure the volume of water filtered. The dates, station locations, bottom depth, and volume of water filtered are presented in Table 1. For each haul, the length of wire paid out was adjusted using the angle of declination to maintain the standard depth of sampling.
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Sample processing
Samples were preserved in 10% buffered formalin solution. Zooplankton biomass at each station was determined by measuring the displaced volume of each sample, and is expressed in ml m–3. Chaetognaths were sorted and identified to species level. Densities were calculated as the number of individuals 100 m–3. Identification was done under compound and dissecting microscopes, using the keys of Michael (1911), Alvariño (1967), Michel (1984), Pierrot-Bults and Chidgey (1988), Bieri (1991), and Casanova (1999).
Data analysis
The Shannon diversity index (H'; Shannon, 1948) was computed to compare species diversity among stations, as was species richness (S, total number of species recorded in each of the stations), where
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where Pi is the proportional abundance of the ith species (ni/N). Cluster analyses (Q and R) were performed to examine the distributional similarities of the different species, based on their relative abundance. The Q-mode cluster analysis was performed to form station clusters and the R-mode analysis to form assemblages of species showing similar relative abundances at the same stations. Cluster analysis was done using the program COMM (Piepenburg and Piatkovski, 1992).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Species composition, spatial distribution, and abundances
In all, 9029 specimens were examined, and 26 species from 14 genera were identified. Table 2 lists the species recorded, their mean densities, and relative abundance. Flaccisagitta enflata was the dominant species and made up 41.9% of all chaetognaths recorded. Among the top ten species were Aidanosagitta neglecta (12.5%), Serratosagitta serratodentata (10.2%), Sagitta bipunctata (7.3%), Ferosagitta ferox (5.7%), Zonosagitta bedoti (4.7%), Aidanosagitta oceanica (3.2%), Ferosagitta robusta (3.2%), Mesosagitta minima (2.3%), and Serratosagitta pacifica (1.7%). Together, these ten species constitute 92.7% of all chaetognaths recorded.
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The highest chaetognath abundances (3089 ind. 100 m–3) were recorded at stations located close to the shelf break, particularly in the central portion of the shelf (Figure 3). High overall abundance in this area is primarily attributed to large overall concentrations at two stations, 17 (1137 ind. 100 m–3) and 19 (3089 ind. 100 m–3), which are both located near the upwelling zone. Abundances then gradually decrease towards the east. However, chaetognath abundances were moderately large in the westernmost portion of the shelf. Overall density ranged from 2.8 to 3089 ind. 100 m–3, with a mean of 435 ind. 100 m–3.
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The average Shannon diversity index (H’) value was 4.07 (Table 3). In general, the NBS appeared to have fairly similar levels of diversity among stations, though species richness demonstrates a gradual change just east of the upwelling zone (Table 3). However, a substantial decrease in diversity and number of species occurred at station 11.
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The Q-mode cluster analysis revealed two major station clusters: the eastern and the western station clusters (Figure 4), formed primarily because of differences in densities. Many species were at all stations, but in varying abundances from one station cluster to another, resulting in station clusters that are not delineated by rigid boundaries. The assemblages characterizing the station clusters are therefore differentiated more by abundance and relative densities than by the presence or absence of any particular species. The R-mode cluster analysis defined two major species assemblages: one ubiquitous and the other with much smaller overall abundances, but also occurring much less frequently on the eastern half of the shelf, particularly near Catanduanes.
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The first cluster was made up of species of moderate to great overall abundance that occurred frequently in both western and eastern portions of the NBS. These included F. enflata, A. neglecta, S. serratodentata, F. ferox, and S. bipunctata, which displayed the largest concentrations in the central NBS region (Figure 5a–e). Ferosagitta robusta and Z. bedoti did not display marked spatial differences in abundance distribution, although Z. bedoti was frequently absent at the inner shelf stations (Figure 5f and g). Ferosagitta robusta displayed small abundances at the extreme western part of the study area (Figure 5g), whereas various unidentifiable juvenile chaetognaths were distributed rather evenly along the shelf (Figure 5h).
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The second species assemblage consisted of species that demonstrated moderate to small overall abundances, but occurred more frequently in the western portion of the shelf, and species having low overall frequency of occurrence and did not reveal recognizable or similar distribution patterns. Both A. oceanica and S. pacifica had high abundance in the central NBS region (Figure 6a and b). Mesosagitta minima and D. decipiens had rather evenly distributed densities (Figure 6c and d). Similarly, Aidanosagitta johorensis occurred only in moderate abundances, but was conspicuously absent around Catanduanes (Figure 6e). Aidanosagitta regularis, Serratosagitta tasmanica, Pterosagitta draco, and Zonosagitta nagae were not recorded at the inner shelf stations, and the latter two species were absent from the waters around Catanduanes, east on the NBS (Figure 6f–i). Caecosagitta macrocephala and Flaccisagitta hexaptera, on the other hand, were recorded at a few inner shelf stations, particularly in the central portion of the NBS (Figure 6j and k).
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The remaining rare species (Aidanosagitta bedfordii, Zonosagitta pulchra, Aidanosagitta septata, Parasagitta setosa, Spadella spp., Krohnitta pacifica, Krohnitta subtilis, and E. fowleri) occurred in patches of low densities solely in the central portion of the shelf and are not included in any of the major assemblages formed by the cluster analysis.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The overall species composition from the samples is consistent with the species reported in previous investigations in various areas in the region (Alvariño, 1967; Jumao-as and von Westernhagen, 1975; Rottman, 1978; Johnson, 2005). In the Hilutungan Channel, Jumao-as and von Westernhagen (1975) reported 13 species of chaetognaths, all of which have also been recorded in the present study. They also found F. enflata to be the most common and abundant species. Of the 13 species, 12 were described as epiplanktonic, and the only mesopelagic species was D. decipiens. Similarly, 22 species were recently reported from the Sulu Sea and Celebes Sea (Johnson, 2005), where F. enflata also dominated the chaetognath assemblages and contributed
39% of all chaetognaths recorded. All 22 species reported by Johnson (2005) have been found in previous investigations (Alvariño, 1967; Jumao-as and von Westernhagen, 1975; Rottman, 1978; Cordero, 2006). In this study, all previously reported species from the Philippines were found. The overall range of abundances observed in the study area is within the upper range of values reported in previous studies in other areas of the country (Alvariño, 1967; Jumao-as and von Westernhagen, 1975; Rottman, 1978; Johnson, 2005; Cordero, 2006). The NBS is divided into two regions: the eastern and the western regions do not appear to correspond strictly to the regions defined by the cluster analyses. The assemblages were generally similar because the diversity and number of species did not vary significantly. All H' values were relatively large compared with previous studies in the Philippines (Table 4), although there are some instances when diversity is inversely related to species number. Species diversity reveals peaks with greater species richness, regardless of dominance by any particular species. However, a slight variation in species richness was observed in the eastern portion of the NBS. Such a scenario seems to agree with the observed upwelling events in the region. In this case, the gradual change in species richness just east of the upwelling region can be related to this phenomenon.
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The vertical distribution of chaetognaths varies with latitude and in different regions because of the differences in temperature, salinity, dissolved oxygen, and other local hydrological conditions, as well as time of sampling. Consequently, it is not uncommon to see inconsistent and even conflicting results from previous studies. For instance, in the present study, S. serratodentata, S. tasmanica, F. hexaptera, K. subtilis, and K. pacifica are well represented in the epipelagic layer, but these species have been reported previously as mesopelagic species in the South Atlantic (Casanova, 1999), in the Caribbean Sea (Michel, 1984), and in the Philippines (Michael, 1919). Similarly, Johnson (2005) noted the occurrence of K. pacifica in mesopelagic waters of the Sulu and Celebes seas, and linked such occurrences with the resilience of a few individuals. Nevertheless, others have suggested that the presence of epipelagic species at great depths and of meso- and bathypelagic species at the surface are a result of downwelling and upwelling events. For instance, Stepien (1980) reported finding shallow-living zooplankton (including chaetognaths) at great depth in the Straits of Florida as a result of downwelling events. Alvariño (1964), on the other hand, attributed the presence of mesopelagic species of chaetognaths in the upper 100 m off San Diego, California, to upwelling.
Species reported as prevalent in certain regions may be categorized as exclusive to tropical, subtropical, temperate, and subtemperate regions or to the Pacific or Atlantic oceans. For example, S. serratodentata has been reported as common and typical of tropical and temperate Atlantic waters and, reportedly, does not occur in Philippine waters (Bieri, 1959). However, this species has been reported from the Pacific Ocean, Mediterranean Sea, and Indian Ocean (Grant, 1967), as well as in other seas (Pierrot-Bults and Chidgey, 1988). Michael (1919), Jumao-as and von Westernhagen (1975), and Cordero (2006) report this species in the western part and inland waters of the Philippines. Michael (1919) considered this species to be unusually variable, but might have been referring to the closely related S. pseudoserratodentata, which was not yet described at the time but has since been reported in the Pacific (Bieri, 1959).
Serratosagitta serratodentata is commonly misidentified as S. pacifica or, as in the case of Jumao-as and von Westernhagen (1975), S. tasmanica or S. pseudoserratodentata. In this study, the difficulty of distinguishing this species from S. pacifica was encountered, making thorough examination necessary. The following diagnostic features were verified in specimens examined: 9–10 (mostly 10) anterior teeth; 19–20 posterior teeth; 10–10.5 mm maximum body length; 6–7 hooks; 23–24% relative tail length; and seminal vesicles with a conspicuous trunk and a large knob with two horn-like prominences (Krohn, 1853). Although the condition of some specimens collected in this study had disintegrated with time, sufficient structures and characteristics remained intact to verify that they were indeed S. serratodentata. Some taxonomists, however, have considered S. pacifica to be a variety of S. serratodentata (Tokioka, 1940) because of similarities, and the specimens found on the NBS may be incorrectly identified.
Hyman (1959) argued that the majority of chaetognaths are epipelagic, whereas others are migratory species with a wide range of vertical movement in the water column (Bieri, 1959; Cheney, 1985). Many species display ontogenetic differences in vertical distribution, wherein young individuals live closer to the surface, and larger and mature individuals in deeper water. In general, diel vertical migrations cover distances of <50 m and are primarily within the upper 100 m (Cheney, 1985). Casanova (1999) describes diel migrations in F. enflata, S. serratodentata, and K. pacifica. Flaccisagitta enflata and S. serratodentata descend to deeper waters at noon, whereas K. pacifica does so during daylight in general. The diel migration might be attributed to feeding behaviour. In the Indian Ocean, the number of F. enflata with food in their guts increases towards noon (Øresland, 2000). The vast majority of the previous studies provide evidence of ontogenetic vertical migrations of mesopelagic and bathypelagic chaetognaths (Russell, 1931; David, 1955; Alvariño, 1964; Pearre, 1973; Pierrot-Bults, 1982). For example, in the Antarctic species S. gazellae, mature individuals reproduce below 750 m, whereas their young rise to the surface layer (David, 1955).
In practice, chaetognaths have been categorized by the area of their greatest concentration on a regular basis and over a large scale. In this study, of the 26 species recorded in the epipelagic zone of NBS, 22 have been previously described as epipelagic. The ocean, however, is a dynamic environment, so species can be moved around and displaced to areas where they are otherwise uncommon. The occurrence, though rare, of mesopelagic, bathypelagic, and benthic species (D. decipiens, C. macrocephala, E. fowleri, and Spadella spp.) in samples collected from upper layers, particularly near the upwelling zone, is one of the most interesting findings of this study. Decipisagitta decipiens and C. macrocephala have been previously reported to undergo ontogenetic migrations (Alvariño, 1964; Vinogradov, 1970; Kehayias et al., 1994; Kehayias, 2003). Most of the specimens collected in this study were of immature stages and found shallower, consistent with known ontogenetic vertical migration patterns. On the other hand, the bathypelagic species E. fowleri rarely or never reaches the epipelagic layer (Alvariño, 1964; Vinogradov, 1970). Its occurrence in the near-surface (100 m) water sample at station 16, with a depth of 500 m, can only be attributed to vertical transport. The benthic species Spadella spp. was collected at an adjacent station (17) located farther inshore on the shelf, with a depth of 150 m. Although spadellid chaetognaths are occasionally caught with plankton nets, this usually happens when the net strikes the seabed (e.g. Bieri, 1974; Bieri et al., 1987). This was not possible at this station (150 m bottom depth), because the depth of double oblique tows at any time during the survey did not reach >100 m. It is not known whether this spadellid species moves vertically in the water column. However, assuming that it is truly benthic, its occurrence in the plankton sample must be attributed to some means of vertical transport off the bottom, such as upwelling.
Stations 16 and 17 are located just inshore of the weak upwelling area reported by Amedo et al. (2002). Although indicators of upwelled water at this location are only evident 50–100 m below the surface, this is enough to transport deep-water organisms to depths sampled by the double oblique tows used in the survey. It is not known whether the processes bringing about the upwelling at this location extend to the shallower portions of the shelf, but the occurrences of E. fowleri and Spadella spp. recorded in this study are consistent with such an event. Similarly, chaetognath species assemblages, as measured by species richness and relative abundance, also reveal a gradual change just east of the upwelling area.
Fish eggs (Figure 7a) on the NBS in April 2001 were located away from the central portion of the shelf, where both chaetognath and larval fish concentrations (Figure 7b) were largest (Campos et al., unpublished data). Egg predation in F. enflata (Figure 8a) was greatest where fish egg concentrations were also largest, whereas predation on fish larvae (Figure 8b) closely followed the distribution of larval fish abundance as well. The reason for the distributions of fish eggs and larvae on the shelf is not clear. However, if we assume that fish stocks would benefit if their spawning sites were located upstream of food-rich areas where larvae will eventually be concentrated, the importance of the upwelling area in the central portion of the shelf becomes more apparent. Chaetognaths prey heavily on zooplankton, but rarely feed on fish eggs and larvae (Øresland, 1990; Feigenbaum, 1991; Baier and Purcell, 1997; Brodeur and Terazaki, 1999). Both eggs and larvae were found in the guts of F. enflata, the most abundant species recorded during the survey. Reeve (1971) stated that chaetognaths are mechano-receptive predators and feed only on living, moving prey. However, immobile or less mobile plankton like fish eggs, gastrula larvae, and radiolarians in chaetognath guts have been reported in several studies (Gray, 1961; Jumao-as and von Westernhagen, 1975; Cordero, 2006). The heads of chaetognaths possess structures that have been putatively described as chemoreceptors (Thuesen and Bieri, 1987; Thuesen et al., 1988), and these could play a role in choosing to ingest fish eggs, especially at high egg density. It is known that chaetognaths ingest prey once captured in the net (Sullivan, 1980), and more work is needed to determine the prevalence of chaetognath predation on fish eggs.
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The overall results of the study are consistent with the area of upwelling being a major hydrographic feature of the NBS, at least from the shelf break and farther offshore. This specific area is also where relatively large chlorophyll concentrations (Primavera et al., 2002), together with the largest values of total zooplankton and pelagic fish biomass (Tanay, 2002), and total larval fish concentrations (unpublished data) were also observed within the same period (April 2001). Upwelled water is generally associated with high production capacity and is the likely reason that this specific area is able to support large consumer concentrations. How this key feature affects the dynamics of productivity on and off the NBS merits further study. The present knowledge of the extent of geographical and vertical distribution of chaetognaths is not definitive, and many unexplored waters, particularly in high-diversity areas such as the Indo-West Pacific, remain to be fully characterized. It is hoped that this study will provide some useful data and will serve as a basis of comparison with future works in the Philippines and elsewhere.
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Acknowledgements |
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We thank the officers and crew of the MV "DA-BFAR" for assistance while sampling at sea, and members of the OceanBio and Marine Bio laboratories at the University of the Philippines in the Visayas for their valuable support. We gratefully acknowledge GLOBEC, PICES, ICES, and the local organizers of the 4th International Zooplankton Production Symposium in Japan, with special thanks to the Shibuya Foundation for providing a travel grant to MMPN. We acknowledge M. Terazaki for providing training on the identification of chaetognaths and comments by E. V. Thuesen that improved our manuscript. This study forms part of the Pacific Seaboard Research and Development programme funded by the Department of Science and Technology (DOST).
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References |
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