vertical distribution of ontogenetically migrating copepods in the western subarctic gyre t. kobari...

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Vertical distribution of ontogenetically migrating copepods in the Western Subarctic Gyre

T. Kobari1, D. K. Steinberg2, S. Wilson 2, K. Buesseler3, A. Tsuda4 & M. Kitamura5

1Kagoshima University, 2Virginia Institute of Marine Science, 3Woods Hole Ocean Institute, 4University of Tokyo, 5Japan Agency of Marine Science & Technology Center

Background

Copepod community concentrated their biomass near surface throughout the day. The distribution patterns of dominant copepods showed the vertical segregation which N. plumchrus occurred near surface, and E. bungii and N. cristatus preferred subsurface to mid-layers. A portion of the subsurface pair appeared in twilight zone. N. flemingeri has already started dormant below permanent halocline to 500-m depth. N. cristatus and N. plumchrus contributed to deep biomass.

Materials & Methods

Results

In the past decade it has become apparent that diel and seasonal migrating zooplankton contribute to carbon flux through their respiration, excretion, and mortality at depth. Ontogenetically migrating copepods are considered to have more important roles in total carbon flux in the subarctic Pacific compared with other oceans because they produce dormant stock to overwinter in the twilight zone. However, this process has been ignored from estimation of global carbon flux. Therefore, we analyze the vertical distribution of the ontogenetically migrating copepods in the Western Subarctic Pacific during late summer to clarify 1. Which species contributes to biomass of the ontogenetically migrants at each depth? 2. How much phytoplankton and other particles are consumed by them? 3. How much biomass will be decreased during overwintering in the twilight zone?

Fig.3. Species composition of abundance (left) and biomass (right) above 1000-m depth.

Fig.3. Vertical distribution (left) and its species composition of copepod biomass (right) above 1000-m depth.

Study site & period : K2 (46˚N, 160˚E) in the western subarctic Pacific : 31 July to 17 AugustSampling gear & layers : 9 layers above 1000-m depth by IONESSZooplankton samples & objective species : 4 day-night pairs (72 samples) : Ontogenetic migrating copepods (8 spp.)

Ontogenetically migrating copepods were observed abundantly and their biomass reached to 5.8 mgC m-2. Most contributed species was N. plumchrus. The three Neocalanus species, which have similar life cycle and die in the twilight zone, accounted for more than 88% of the copepod biomass during late summer.

Fig.4. Daily changes in the Weighted Mean Depth (WMD). Regression lines are significant (Spearman rank: p<0.05).

Dominant species showed no day-night pattern, but strong diel vertical migration was evident for M. pacifica. Gradual increase of the weighted mean depth for the subsurface pair indicated to start the ontogenetically vertical migration to the twilight zone.

Fig.5. . Estimated carbon flux of N. flemingeri by mortality and respiration during overwintering

Adult females contributed to the deep biomass of N. flemingeri. Based on the long dormant period (Kobari & Ikeda 2001: MEPS,209,243-255) and extremely low respiration rate (Evanson et al., 2000: MEPS,192,239-247), biomass loss in the twilight zone was estimated to be 307 mgC m-2 year-1. Mortality reached to 93% of the initial biomass. Such high mortality is caused by the life cycle pattern to die in the twilight zone.

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Fig.7. Vertical profiles of animal gut pigments.

Adult females of E. bungii and M. pacifica increased their gut pigments near surface, showing an actively grazing on phytoplankton. Such vertical patterns were not observed for N. cristatus and N. plumchrus.

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ConclusionNeocalanus copepods contribute to ontogenetically migrating biomass.The largest species seems to transport gut contents much deeper.They transforms non-phytoplankton to fecal pellets during late summer.Deep mortality could be one of significant carbon flux.

Ontogenetic vertical migrants contribute to carbon flux through grazing and overwintering mortality in the subarctic systems.

Community grazing rate on phytoplankton was estimated to be 1.4-2.6 mgC m-2 day-

1. Ingested phytoplankton carbon computed from gut pigments was much smaller than copepod respiration requirement. Considering with the low ambient chlorophyll (<0.8 mg m-3) and their food items in summer (Kobari et al. 2003: PO,57,3/4,279-298), the respiration requirement seems to rely on other particles such as protozoans and sinking particles. Faecal pellet production was estimated to be 76.9-156.9 mgC m-2 day-1.

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Table 1. Community feeding rates and faecal pellets productions in the water column above 150 m.

Parameter Resource 1.Aug 5.Aug 12.Aug 16.Aug Average

Copepod biomass 5659.8 2780.8 2803.6 2981.0 3556.3

Copepod respiration requirement a 367.7 180.5 182.0 193.6 231.0

Community grazing rate b Phytoplankton c 2.2 1.4 1.9 2.6 2.0

Other particles d 523.1 256.5 258.1 273.9 327.9

Ratio grazed Phytoplankton 0.4 0.6 0.7 0.9 0.6

Other particles 99.5 99.4 99.3 99.1 99.4

Faecal pellet production c Phytoplankton 0.7 0.4 0.6 0.8 0.6

Other particles 156.9 76.9 77.4 82.2 98.4

Units are mgC m-2 day-1, except copepod biomass (mgC m-2) and ratio grazed (%).

b Gut evacuation rate (hour-1)0.042: E. bungii and N. plumchrus, 0.048: M. pacifica, 0.045: N. cristatus (Tsuda et al. 2005: PO.,64,237-251)2.160: C. pacifica (Landry et al. 1994: MEPS,115,73-85)

c C/CHL ratio was assumed 50.d Assimilation coefficient was assumed 0.7 (Conover, 1966: LO,11,338-345)

a 6.5% of copepods biomass (Dagg et al., 1982: DSR.,29,45-63).

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