Life cycle strategies and seasonal variations in distribution and ...

Journal of Plankton Research Vol.16 no.ll pp.1543-1566, 1994

S.B.Schnack-Schiel and W.Hagen1 Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27515 Bremerhaven and 'Institut fur Polarokologie, Universitdt Kiel, Wischhofstrasse 1-3, Gebdude 12, D-24148 Kiel, FRG Abstract. The dominant Antarctic copepod species Calanoides acutus, Calanus propinquus, Rhincalanus gigas and Metridia gerlachei were investigated with respect to their abundance, vertical distribution, developmental stage composition, dry weight and lipid content. The specimens were sampled during three expeditions to the eastern Weddell Sea in summer (January/February 1985), late winter/early spring (October/November 1986) and autumn (April/May 1992) between 0 and 1000 m depth to follow the seasonal development of the populations. Three species were most abundant in April, only C.propinquus reached highest concentrations in February. A seasonal migration pattern was evident in all four species, but was most pronounced in C.acutus. In October/ November, they inhabited deeper water layers, their ascent started by mid-November and in midFebruary the species concentrated in the upper 50 m, except for M.gerlachei (50-100 m). Their descent was observed in April/May. The stage composition changed dramatically with season, the older developmental stages (CIII-CVI) dominated the populations in late winter/early spring, whereas younger stages (CI and CII) prevailed during summer (C.acutus, C.propinquus) or autumn (R.gigas, M.gerlachei). Only C.acutus ceased feeding in autumn and diapaused at depth. Strong differences between seasons were also detected in dry weight and lipid levels, with minima in late winter/early spring and maxima in summer {C.acutus, R.gigas) or autumn (C.propinquus, M.gerlachei). Lipid reserves seem to be most important for the older stages of C.acutus and C.propinquus. Based on these seasonal data, different life cycle strategies are suggested for the four species.

Introduction Zooplankton biomass in the Southern Ocean is usually dominated by the four large calanoid copepod species Calanoides acutus, Calanus propinquus, Rhincalanus gigas and Metridia gerlachei (e.g. Chojnacki and Weglenska, 1984; Boysen-Ennen etal., 1991; Hopkins etal., 1993). Data on their distribution and age composition were first given by Ottestad (1932, 1936), Ommanney (1936) and Andrews (1966). Many questions still remain, although our knowledge of the life histories of these species has improved considerably during the last few years (e.g. Marin, 1988; Atkinson, 1991; Huntley and Escritor, 1991, 1992; Schnack-Schiel etal., 1991; Bathmann etal., 1993; Drits etal., 1993; Hopkins et al., 1993). Calanoides acutus occurs in relatively high abundances in the entire Southern Ocean (Zmijewska, 1983; Hopkins, 1985a, 1987; Hubold and Hempel, 1987; Hopkins and Torres, 1988). Males occur only in deeper water layers in winter, where mating takes place. In spring, fertilized females ascend to the upper water layers to spawn and in summer the new generation thrives in the productive surface waters, while the older stages especially accumulate large lipid reserves in the form of wax esters (Hagen, 1988; Schnack-Schiel et al., 1991). In autumn, © Oxford University Press

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Life cycle strategies and seasonal variations in distribution and population structure of four dominant calanoid copepod species in the eastern Weddell Sea, Antarctica

S.B.Schnack-Schiel and W.Hagen

1544


feeding ceases and older copepodids descend to deeper water layers to overwinter in a resting stage (Andrews, 1966; Voronina, 1970; Hopkins, 1985b; Schnack-Schiel et al, 1991). Marin (1988) and Atkinson (1991) suggested a 1year life cycle. Calanus propinquus generally prefers colder water masses and does not occur in high densities in the Antarctic Circumpolar Current (Bathmann et al., 1993). During summer, the population is concentrated in the surface layer, where it feeds, develops and replenishes its extensive lipid reserves, mainly triacylglycerols (Schnack-Schiel et al., 1991; Hagen et al., 1993). According to Marin (1988) and Bathmann et al. (1993), part of the C.propinquus population remains in surface waters during winter and continues to feed. In contrast to C.acutus, this species mainly mates in surface layers and males are present throughout the summer. In the eastern Weddell Sea, Fransz (1988) found small amounts of eggs and nauplii of C.propinquus very early in the season (October-November) with virtually no phytoplankton in the water column (Gieskes et al., 1987). Major spawning, however, is believed to take place in December [Kosobokova (1992), in Drits et al. (1993)]. Marin (1988) described a 1-year life cycle, but based on physiological and biochemical measurements, Drits et al. (1993) suggested a 2year life cycle for C.propinquus. Rhincalanus gigas is a typical species of the Antarctic Circumpolar Current and is not very abundant in the Weddell Sea (Ottestad, 1932; Bathmann et al., 1993; Hopkins et al., 1993). Ommanney (1936) postulated that optimum spawning occurs between 1 and 4°C, but no spawning at temperatures 95% of the population, whereas CI specimens were not 1545


distribution pattern, age structure, dry weight and lipid data, we tried to elucidate the different life cycle strategies of these four copepod species and their adaptations to the pronounced seasonality of ice cover, light regime and primary production in high-Antarctic waters.


60"

50°

40'

30"

(b)

•68°

(a) 69"

70"

73°20' W

22*

21

19°

18°

17°

14° 13" 12" 11* 10* 9"

8" 7"

6"W

Fig. 1. Investigated area and locations of sampling stations (a, b) during three 'Polarstern' expeditions, (a) ANT III/3 (January/February 1985) and ANT V/3 (October/November 1986); (b) ANT X/3 (April/May 1992).

encountered and CII was rare in mid-April (0.2% of the population) (Figure 2b). The population was oldest in late winter/early spring and youngest in midFebruary (mean population stage [S] 5 and 2, respectively; Figure 2c). Adults made up between 39 and 50% of the population in late winter/early spring, 1546


70°

Life cycle strategies of calanoid copepods

Calanoides acutus late winter/early spring 1986 (a)

3.-5. 17.-24. November

autumn 1992

29. 15. January February

12.-13. 26. April April 2. May

12

sr io

E 3.

I

8

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^

i

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iMales DFemales BCV DCIV QCm HCII DDCI (C)

IS]

4.9 1.3

5.0 3.7

4.9

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no CS no CS

4.4

4.4

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Fig. 2. Total abundance (a) and relative abundance of developmental stages (b), mean population stage [S] and female:male ratio (c) of C.acutus.

whereas in summer and autumn their proportion comprised only 3-6%. Females always outnumbered males, which were only found in winter/spring. In mid-October, ~80% of the population was located in the Warm Deep Water between 1000 and 500 m (Figure 3). Towards mid-November, there was a gradual migration into upper water layers (Figure 3) and the population became older due to the ascent of fertilized females. In mid-October, < 1 % of the 1547


17.-21. October

summer 1985


late wtnterlearly spring 1986

.

17. 21. Oclok

-

3. 5. Nwcmber

-

17. 24. November

summer 1985 7.9. JpmrPy

IS. Febnray

autumn 1992

-

12. 1.9.

26. AprU2 May

Fig. 3. Vertical distribution of C.acutus as a percent of total numbers and temperature profile within the upper 1000 m.

population occurred in the top 50 m, whereas in mid-November this proportion had increased to 9%. In summer, the majority concentrated in the warm surface layer with 62% at the end of January and 90% in mid-February, mainly offspring of the new generation. The depth distribution in autumn was bimodal with a smaller fraction in the upper 90 m and a larger one in the Warm Deep Water layer. The developmental structure differed with depth, a younger fraction (CIII, CIV: 80-90%) was encountered in the surface layer and an older one in the deepest water layer (CIV, CV: 95%). During October/November, the dry weight of C.acurus increased from 127 ~g in stage CIV and 195 kg in CV to the adult stages (579 kg in females and 606 kg in males). The available summer data showed much higher dry weights of CV stages (729 pg) and femaIes (909 ~ g )which , were very similar to the autumn dry weights (CV: 755 pg; females: 899 kg). In autumn, stage CIII weighed 69 pg and CIV 187 pg (Figure 4a). In late winterlearly spring, lipid contents increased from stage CIV (12%DW) and CV (14%DW) to females (32%DW) and males (37%DW). Lipid levels were even higher in summer, with 45%DW in CV stages and 48%DW in females. In autumn, lipid contents were quite high in stages CIII (32%DW) and CIV (26O/oDW), while in CV and females (both 45%DW) lipid contents were very similar to the summer data. This lipid accumulation may at least partially explain the higher dry weights in summer and autumn (Figure 4b). Calanus propinquus Calanuspropinquus occurred with - 1 ind. m-3 in the upper 1000 m in October1 November and at the end of January. Two weeks later, in mid-February, abundance peaked with -6 ind. mA3and in autumn the population was recorded with -4 ind. m-3 (Figure 5a). In mid-October, copepodite stages CIII, CV and females formed 84% of the population (Figure 5b). Towards mid-November, the proportion of CIII increased, while that of females decreased. At the end of January, the population structure was bimodal, dominated by the youngest (CI) and the oldest (CV and females) stages. In mid-February, the C.propinquus population was represented mainly by CI copepodids (63%), whereas in autumn the


Rtlatlvc abundance (%)


1400

Calanoides acutus

Q

•o D.

1

Fig. 4. Dry weight (a) and total lipid content (b) of C.acutus; number of error bars = number of samples examined.

copepodite stages CIII-CV dominated with 90% (Figure 5b). The mean population stage changed only slightly during the late winter/early spring period. At the end of January, the population was about one copepodite stage younger and in mid-February more than two stages younger than in mid-November. In autumn, the population was again older with an intermediate mean population stage (Figure 5c). In late winter/early spring, adults represented between 20 and 30% of the population, whereas in summer and autumn they accounted for only 2-7%. Males occurred in higher numbers only in October/November; they were missing in autumn (Figure 5c). In October/November, the major part of the population inhabited the layer between 500 and 200 m (Figure 6). From mid-October to mid-November, there was an increase in abundance in upper water layers, mainly due to the ascent of CIII specimens. During all three sampling periods in late winter/early spring, 10-16% of the population was concentrated in the top 50 m. In summer and autumn, the majority of the population was located in this layer. The October/November data for C.propinquus showed an increase in dry weight from 52 u.g in stage CIII, 157 u.g in CIV, 773 jig in CV to 887 u.g in males and 1158 u.g in females. The summer dry weights ranged from 79 u.g in CII to 1692 u,g in females. Hence, dry weights of C.propinquus were clearly higher in summer as compared to the corresponding late-winter stages, only in the males 1549


a


Calanus propinquus late winter/early spring 1986 3.-5. 17.-24. November

summer 1985

autumn 1992


12.-13. 26. April April 2. May

(a)

i i

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0.6

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ioo

Males D Females SCV QCIV racill HCH (C)

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4.7

4.6

4.2

3.3

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7.5

3.8

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21.0

no Cf

Fig. 5. Total abundance (a) and relative abundance of developmental stages (b), mean population stage [S] and female:ratio (c) of C.propinquus.

was the difference not as pronounced. In autumn, dry weights ranged from 21 u,g in stage CII to 1640 u.g in females. They were somewhat lower than during summer, especially in stages CIII (69 u.g) and CV (814 u.g) (Figure 7a). The available lipid data from October/November showed a strong increase in total lipid content from CIII and CIV (19 and 16%DW, respectively) to CV (33%DW), but lipid contents decreased again in the adult stages (females: 1550


17.-21. October


Ccdanus propinquus late winter/early spring 1986 17. -21. October

3. - 5. November

summer 1985

17. - 2*. November

29. Januaiy

autumn 1992

IS. February

12. - 13. April

28. April2Mzy

0

-r

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40

-r

00

00

0

30

40

SO

SO

0 2 0 4 0

ao

o u

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o 20 40

GO

ao

ao ao

0 xo 40 ao ao

(re

Fig. 6. Vertical distribution of C.propinquus as a percent of total numbers and temperature profile within the upper 1000 m.

2000

Ccdanus propinquus

Fig. 7. Dry weight (a) and total lipid content (b) of C.propinquus; number on error bars = number of samples examined.

26%DW; males: 28%DW). In summer, copepodite stages CII (18%DW) and CIII (21% DW) had moderate lipid levels. Maximum values were reached in the CV stages (39%DW), which decreased again in the females (35%DW) and especially in the males (24%DW). Lipid levels were generally higher during summer and autumn, except for the males. The variability in lipid content was strongest in stages CV and females (Figure 7b). 1551


Relative abundance (%)

S.B.Schnack-Schie! and W.Hagen

Rhincalanus gigas

Rhincalanus gigas late winter/ early spring 1986 summer 1985 17.-21. October

3.-5. 17.-24. November


autumn 1992 12.-13. 26. April April 2. May

(a) V