ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on January 23, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(3):469-476; doi:10.1093/icesjms/fsm194
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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]
Seasonal appearance of expatriated boreal copepods in the Oyashio–Kuroshio mixed region
1 Aquatic Resource Science Division, Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata, Kagoshima 890-0056, Japan
2 Laboratory of Marine Resources and Environment Planning, Department of Fishery Science and Technology, National Fisheries University, 2-7-1 Nagata-Honmachi, Shimonoseki 759-6595, Japan
3 Biological Oceanography Section, Mixed Water Region Fisheries Oceanography Division, Tohoku National Fisheries Institute, 3-27-5 Shinhama, Shiogama, Miyagi 985-0001, Japan
Correspondence to T. Kobari: tel: +81 99 286 4140; fax: +81 99 286 4133; e-mail: kobari{at}fish.kagoshima-u.ac.jp
Kobari, T., Moku, M., and Takahashi, K. 2008. Seasonal appearance of expatriated boreal copepods in the Oyashio–Kuroshio mixed region. – ICES Journal of Marine Science, 65: 469–476.Seasonal changes in abundance, biomass, and community structure of ontogenetically migrating copepods were investigated using monthly zooplankton samples from the Oyashio–Kuroshio mixed region (OKMR) to evaluate impacts of the copepod community expatriated from more northerly waters on carbon flow in southern areas. The copepod community comprised more than half the total zooplankton biomass and exhibited seasonal fluctuations, although they were minor components in terms of abundance. The ontogenetic migrants increased their abundance and biomass during May–July when Oyashio waters dominated at the surface. They were almost absent from 0 to 150 m during August to December. Predominant species were Eucalanus bungii and Metridia pacifica for abundance and Neocalanus cristatus for biomass. Seasonal changes in their abundance and stage composition indicated that surface development, dormancy, and reproduction were comparable in schedule with those reported in the Oyashio region. We suggest that the ontogenetically migrating copepods appearing in this area are populations transported with a southward Oyashio intrusion during surface development seasons, then by both downward migration and advection of dormant populations in the submerging Oyashio flow. We discuss impacts of the expatriated copepods on food availability for mesopelagic fish in the OKMR.
Keywords: community structure, copepods, ontogenetic migration, Oyashio–Kuroshio mixed region
Received 11 July 2007; accepted 22 November 2007; advance access publication 23 January 2008.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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In the western North Pacific Ocean, current and frontal structures are characterized by large seasonal and spatial variability. The Oyashio current flows southwest along the Kuril Islands and the Kuroshio turns northeast along the southeast coast of Japan (Figure 1). The oceanographic conditions are complicated and varied off the northeast coast of Japan, termed the Oyashio–Kuroshio mixed region (OKMR), where Subarctic and subtropical water masses interact (e.g. Yasuda, 2003). Because of meandering flow patterns and eddies, warm and cold core rings are sometimes entrapped in this area (Shimizu et al., 2001). It is also known that the Oyashio submerges and forms North Pacific intermediate water (NPIW) in the OKMR (Yasuda, 2003).
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Ontogenetically migrating copepods, including Eucalanus, Metridia, and Neocalanus species, are dominant components of mesozooplankton community throughout the Subarctic North Pacific Ocean (Mackas and Tsuda, 1999). They feed on various forms of particulate organic matter (Dagg, 1993; Gifford, 1993; Kobari et al., 2003) and are also major food items for epipelagic, mesopelagic, and demersal fish (Brodeur et al., 1999; Moku et al., 2000; Yamamura et al., 2002), whales (Kawamura, 1982), and seabirds (Russell et al., 1999). Thus, they are an important link in the energy flow between primary production and higher trophic levels in the Subarctic marine ecosystem.
In the 1960s, ontogenetically migrating copepods were found at mesopelagic depths in the southern areas of the Oyashio region, where surface layers are covered with warm Kuroshio waters (Omori, 1967; Omori and Tanaka, 1967). Oh et al. (1991) reported that Neocalanus cristatus appears in mesopelagic layers of Sagami Bay throughout the year but does not recruit there; apparently, they are transported to Sagami Bay with submerged Oyashio waters. However, we have little knowledge of the seasonal appearance of the boreal copepod community in the OKMR.
In the present study, we investigated seasonal changes in abundance, biomass, and community structure of the ontogenetically migrating copepods in the OKMR. From these results, we discuss the importance of the expatriated copepod community for carbon flow in the marine ecosystem in this region.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Sampling
Our analyses are based on zooplankton samples collected at station A (38°N 142°30'E) in the OKMR (Figure 1) from March 2000 to March 2001. Zooplankton samples were collected from 1000 m to the sea surface using a fast-sinking-mouth ringnet (modified 70 cm mouth diameter, 100 µm mesh opening; Kawamura, 1989). Additional samples were collected from 150 m to the sea surface using a North Pacific standard net (NORPAC: 45 cm mouth diameter, 350 µm mesh opening; Motoda, 1957). Flowmeters were mounted in the mouths of the nets to register the volume of water filtered. Both nets were towed vertically at 1 m s–1. Sampling time during the day was not standardized during this study. After collection, zooplankton samples were preserved immediately in 5% formalin-seawater buffered with sodium tetraborate.
Temperature, salinity, and density (
) profiles were determined with a CTD system on each sampling occasion. Water samples for chlorophyll a concentrations were taken from ten discrete depths (0, 10, 20, 30, 50, 75, 100, 125, 150, and 200 m) using a CTD-rosette multisampler (CTD-RMS). Each water sample was filtered through a Whatman GF/F filter. Chlorophyll pigments on the filter were extracted in 90% aqueous acetone, and chlorophyll a concentration was determined with a fluorometer (Turner Designs, 10AU) following Holm-Hansen et al. (1965).
Zooplankton from the deep-towed samples was sorted into five taxonomic groups (ontogenetically migrating copepods, other copepods, crustaceans other than copepods, gelatinous zooplankton, and others), and the groups were counted under a dissecting microscope. Ontogenetically migrating copepods were identified to species and included Eucalanus bungii, Metridia okhotensis, Metridia pacifica, Neocalanus cristatus, Neocalanus flemingeri, and Neocalanus plumchrus. All copepodite stages of these species were also counted. We could not distinguish copepodite stage I between N. flemingeri and N. plumchrus. Copepodite stages I and II for M. pacifica were likely underestimated in the samples collected using the NORPAC net because their bodies are smaller than the mesh openings. Although M. okhotensis and M. pacifica demonstrate a day–night migration (Padmavati et al., 2004), this behaviour was not significant for the present analysis because M. okhotensis was a minor component of the zooplankton community and M. pacifica demonstrated a similar pattern of seasonal abundance in both the 0–150 m and 0–1000 m strata (see Results).
Biomass estimation
Preserved E. bungii, M. okhotensis, and M. pacifica were briefly rinsed with distilled water, and 1–100 animals of each copepodite stage were pooled into preweighed aluminium pans. Dry weight was determined using a microbalance after drying at 52°C for 24 h. For the three Neocalanus species, we used stage-specific dry weights previously published by Kobari et al. (2003). Total carbon biomass of ontogenetically migrating copepod species in the 0–1000 m layer (mg C m–2) was estimated as the sum of stage-specific dry weight of each taxon (DWi: mg DW) multiplied by its abundance (N: ind. m–2) and carbon content:
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For zooplankton other than ontogenetically migrating copepods, mean animal biovolume was determined with size measurements of 30 arbitrarily selected organisms, assuming simple geometric shapes. The mean animal biovolume was converted to mean animal carbon weight using conversion factors of 0.003 pg C µm3 for gelatinous zooplankton and 0.06 pg C µm3 for the others (Parsons et al., 1984). Zooplankton biomass other than the ontogenetic migrants in the 0–1000 m stratum was estimated as the sum of the mean animal carbon weight of taxonomic group j (CWj: mg C) multiplied by its abundance (Nj: ind. m–2).
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Results |
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Oceanographic conditions
In April, the temperature fell below 5°C, and salinity was less than 33.6 throughout the upper water column, 0–150 m (Figure 2a and b). This cold, relatively fresh-water mass descended to 400 m during August to December when the surface layer was covered with warm, more saline waters. The sea surface temperature reached 24°C in August. Cold and less saline waters moved into the region again during January to February. The 26.7
isopycnal corresponded to the bottom of the cold, less saline water mass and was found at 200 m in April to July. It descended to 400 m during November and December (Figure 2c). Based on monthly composites of satellite images of sea surface temperature (Tohoku National Fisheries Research Institute, http://ss.myg.affrc.go.jp/kaiyo/temp/temp.html), we determined that warm core rings were not present around our sampling site throughout the study period.
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Chlorophyll a concentrations were < 0.3 mg m–3 above 50 m during May to June (Figure 2d). From July to October when a warm, saline water mass dominated at the surface, chlorophyll a was < 0.1 mg m–3 in the 0–30 m layer, and a subsurface maximum was evident at 50 m. Chlorophyll concentrations > 0.1 mg m–3 extended to 200 m from December to March, and they were as high as 0.7 mg m–3 at 75 m in March.
Zooplankton community
In the 0–1000 m water column, zooplankton abundance was high in May, July, August, and January (Figure 3). Although copepods, including ontogenetic migrants, numerically constituted 58–86% of total zooplankton abundance over the study period, ontogenetic migrants constituted < 33% of the total and were a minor component of zooplankton community. Gelatinous zooplankton increased in abundance during August to November and constituted up to 26% of zooplankton abundance. Seasonal patterns of zooplankton biomass were different from those of numerical abundance. Zooplankton biomass revealed a prominent peak of 24 g C m–2 in May, but it ranged from 2 to 9 g C m–2 in the other seasons. Ontogenetically migrating copepods were the predominant group contributing to zooplankton biomass, accounting for 53–92% of the total. Seasonal fluctuations of abundance and biomass for the zooplankton community were significantly correlated with those of the ontogenetically migrating copepods (Figure 4). The correlation coefficient for biomass was higher (r = 0.994, p < 0.01) than for abundance (r = 0.807, p < 0.01).
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Ontogenetically migrating copepods
Ontogenetically migrating copepods were abundant during May, July, and from January to March in both the 0–150 m and 0–1000 m layers (Figure 5), although they were nearly absent from the surface during September–November. Metridia pacifica was the most abundant species among the ontogenetic migrants, and E. bungii was second in importance above 1000 m. Neocalanus spp. increased in abundance during February–April, but they constituted less than 40% of the copepod community over the study period. Biomass of ontogenetic migrants demonstrated a prominent peak of 5.3 g C m–2 between 0 and 150 m and 20.9 g C m–2 between 0 and 1000 m during May. Although the copepod biomass was nearly zero between 0 and 150 m from August to December when the surface layer was covered with warm saline waters, biomass between 0 and 1000 m ranged from 1.6 to 7.3 g C m–2. Neocalanus cristatus and E. bungii dominated the biomass of the copepod community in the 0–1000 m layer over the study period, and M. pacifica contributed a somewhat smaller proportion (<40%).
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Population structure
Eucalanus bungii abundance peaked in both sample series during May, with increasing abundance of C1 and C2 (Figure 6). Copepodite stages CIII–CV dominated the population between 0 and 1000 m during August to February, although this copepod species was nearly absent or disappeared from the surface layer. Metridia pacifica increased in abundance in both sample series between May and July, when specimens younger than CIII were predominant. Between 0 and 1000 m, CV and CVI dominated the population from September to April.
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Abundance was great in both sampled strata from January to May for N. cristatus and N. flemingeri and from May to July for N. plumchrus (Figure 7). Although these copepod species occurred in the 0–1000 m stratum throughout the year, they were less abundant in the surface after August, and they disappeared from the upper water column during August–December. Stages CI and CII dominated the population of N. cristatus from January to March, although all copepodite stages were found throughout the study period. Seasonal succession of stage composition was more pronounced for N. flemingeri and N. plumchrus. Adults (CVI, mostly females) of N. flemingeri decreased from December to April, and CII to CIII increased in abundance during the same period. Neocalanus plumchrus demonstrated seasonal changes in stage composition, with CVI males and females dominating from November to April and an increasing abundance of young copepodites from May to July.
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Discussion |
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Source of the ontogenetically migrating copepods
An increasing abundance of young copepodites indicates recruitment from nauplii, followed by copepodite development from April to July for E. bungii and from May to July for M. pacifica (Figure 6). Because these copepods are absent or less abundant at the surface during August–January, stage composition in the 0–1000 m stratum prove that overwintering stages at depth are CIII to CV and CVI female for E. bungii and CV and CVI for M. pacifica. These findings indicate that seasonal patterns of abundance and population structure for the two species in the OKMR are comparable with those in the Oyashio region to the north (Padmavati et al., 2004; Tsuda et al., 2004; Shoden et al., 2005). On the other hand, young copepodites increased in abundance from January to May for N. cristatus and N. flemingeri and from May to July for N. plumchrus, revealing seasonality of development in their near-surface stocks (Figure 7). All Neocalanus spp. copepods were absent from 0 to 150 m samples from October to December, and stage composition in the 0–1000 m samples indicates that overwintering stages at depth were CV and CVI for N. cristatus and N. plumchrus and CIV and CVI (female) for N. flemingeri. Our findings for Neocalanus species are consistent with seasonal patterns of abundance and population structure in the Oyashio region, upstream of our sampling site (Kobari and Ikeda, 1999, 2001a, b; Tsuda et al., 1999, 2001, 2004). These results suggest that the ontogenetically migrating copepods occurring in the OKMR include local populations with life cycle patterns similar to those in the Oyashio region and populations advectively expatriated with the Oyashio waters in which they developed.
It has been recognized that southward Oyashio flow reaches the northeast coast of Japan during spring to early summer, crossing the Subarctic Front then submerging in the OKMR (Yasuda, 2003). Shimizu et al. (2004) report that 26.7 isopycnal floats deployed in the Oyashio region moved into the OKMR and reached the Kuroshio extension front, indicating a southward flow of submerged Oyashio water. Omori (1967) demonstrated progressively lesser abundance and deeper distribution of N. cristatus with increasing distance from the Oyashio region, and he concluded that the population appearing in the OKMR and Kuroshio was transported with the submerged Oyashio flow. Oh et al. (1991) described the submerged Oyashio flow in mesopelagic layers as the source of N. cristatus residing at depth in Sagami Bay. In our study, the ontogenetically migrating copepods collected from below 150 m accounted for 73–99% of the total biomass in the 0–1000 m water column over the study period (Figures 2 and 4). This indicates that the copepod community has a large biomass in mesopelagic layers, even when Oyashio waters are present at the surface. Moreover, the decline from the spring peak of abundance during summer is more pronounced for the populations in the OKMR than for those in the Oyashio region (Table 1). Based on these results, we suggest that there are two sources for the ontogenetically migrating copepods appearing at our study site (Figure 8). One is populations transported southwards with Oyashio intrusions during their surface development season. The other source is downward–migrating and dormant populations conveyed to the OKMR in submerged Oyashio flow. Considering the large biomass in the mesopelagic layer throughout the year, the latter source seems to be the dominant source of expatriated copepods in the OKMR.
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Importance of the expatriated copepods
The ontogenetically migrating copepods have long been an important focus for studies on the trophodynamics and biogeochemical cycles in Subarctic marine ecosystems. They feed not only on diatoms and flagellates but also on microzooplankton, sinking aggregates, and faeces (Dagg, 1993; Gifford, 1993; Kobari et al., 2003). Thus, they have important roles connecting the grazing and microbial foodwebs and concentrating carbon dispersed out of these foodwebs. Because they are major food resources for various animals (see Introduction), these copepods contribute to production at higher trophic levels. Also, it was recently emphasized that diel and ontogenetic migrations contribute to downward carbon flux through respiration and mortality at depth (Kobari et al., 2003; Harrison et al., 2004).
Although the ontogenetically migrating copepods in our study area are transported by south flows of Oyashio waters, they dominate zooplankton biomass throughout the year (Figure 3). Therefore, the expatriated copepod community must have significant impacts on energy flow in the marine ecosystem in the OKMR. Indeed, they are major food items there for epipelagic (Odate, 1994) and mesopelagic fish (Moku et al., 2000; Moku and Hidaka, 2002). However, little is known about the impacts of the expatriated copepod community on the energy flow of the marine ecosystem in more southern areas.
We can approximate the feeding impacts of myctophid fish on the copepod community in the OKMR (Table 2). Based on the results of Watanabe et al. (1999), myctophid biomass is estimated to be 18.5 g WW m–2 in the 0–700 m layer during July, using 10% for the catching efficiency of the otter trawls (Watanabe et al., 1999). Assuming a daily food requirement of 3.3% of body weight (Moku and Hidaka, 2002) and a contribution of copepods to stomach contents of 82.7% (MM, unpublished data), the myctophid feeding rate on the copepod community is estimated to be 0.5 g WW m–2 d–1. That could be an overestimate based on a higher daily feeding ratio for migrating myctophids and a higher contribution of copepods to stomach contents in non-migrants than migrants (Moku and Hidaka, 2002). In the present study, biomass of the expatriated and the other copepods between 0 and 1000 m is calculated as 111.9 and 10.4 g WW m–2, respectively. These estimates mean that the myctophid feeding requirement can be supported by the biomass of the expatriated copepod community during 222 d of the year, but copepods other than ontogenetic migrators could not meet the trophic demands of myctophids in July. Moku and Hidaka (2002) report that myctophid feeding is much greater than the zooplankton production rate in the water column from 0 to 150 m in the OKMR, and they attribute that to underestimation of zooplankton production. In the present study, however, we demonstrate that the expatriated copepod community residing below 150 m has a biomass (9.3 g C m–2) eight times greater than that of other copepods between 0 and 150 m during July (1.1 g C m–2). Therefore, our estimates suggest that the expatriated copepod community is the main source of food for myctophid fish in the OKMR.
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Acknowledgements |
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We are grateful to C. B. Miller for his kind review and H. Saito and A. Tsuda for valuable comments on an early draft of our manuscript. We extend thanks to the captain and crew of the RV "Takuyo-Maru" for their assistance with zooplankton sampling. Part of this study was supported by grants from the Japan Society for the Promotion of Science (18681003) and from the Fisheries Agency of Japan (Deep-sea Ecosystem and Exploitation Programme).
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