population dynamics of four dominant copepods in prydz bay...

Chinese Journal of Oceanology and LimnologyVol. 29 No. 5, P. 1065-1074, 2011DOI: 10.1007/s00343-011-0230-4

Population dynamics of four dominant copepods in Prydz Bay, Antarctica, during austral summer from 1999 to 2006*

YANG Guang (杨光)1, 2, LI Chaolun (李超伦)1, **, SUN Song (孙松)1, 3

1 Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences,

Qingdao 266071, China2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China3 Jiaozhou Bay Marine Ecosystem Research Station, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071,

China

Received Aug. 25, 2010; revision accepted Nov. 28, 2010

© Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2011

Abstract Population dynamics of four dominant Antarctic copepods, Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas were studied based on zooplankton samples collected in the Prydz Bay during austral summer from 1999 to 2006. We found that C. acutus was the most abundant species among these four copepods, followed by C. propinquus, M. gerlachei and R. gigas. R. gigas occurred mainly in the warmer oceanic regions and showed distribution patterns discrete from the other three species, whose distribution in the whole survey area overlapped. By December 15th (about one month before our sampling) of the years 1999, 2003 and 2006, sea ice retreated earlier and polynyas existed in the neritic region one month before sampling. These periods were characterized by numerical dominance of C. acutus, C. propinquus and M. gerlachei, elevated proportions of Copepodite I and Copepodite II stages especially in the neritic region. While for the years 2000, 2002, and 2005, the ice edge located more northerly and polynyas did not exist in the neritic region, the copepods abundance was lower, indicating poor recruitment. Population structure of R. gigas was mainly composed of advanced stages Copepodite V and female during all cruises. Log10 (x+1) transformed densities of C. acutus, C. propinquus and M. gerlachei showed positive correlation with temperature and chlorophyll a concentration, while mean population stages of these copopods were negatively correlated with these environmental variables. Younger copepodite stages of C. acutus, C. propinquus and M. gerlachei appeared more often in neritic regions. We confirmed that the polynyas had a great contribution to phytoplankton blooms, which promote copepods reproduction and recruitment success. The study suggested that population dynamics of the four copepods have good correspondence with sea ice and polynya variations during all cruises of the Prydz Bay.

Keyword: copepods; population structure; inter-annual variations; Prydz Bay; Southern Ocean

1 INTRODUCTION

Copepods are key species in the Southern Ocean ecosystem, and their biomass can on occasion exceed that of krill (Errhif et al., 1997; Ward et al., 2007). Moreover, the grazing impact of copepods may represent more than half of the daily primary production in Southern Ocean (Conover and Huntley, 1991).

Calanoides acutus, Calanus propinquus, Metridia gerlachei, and Rhincalanus gigas were considered as the four dominant Antarctic copepod species for

their great contribution to both abundance and biomass of the whole zooplankton community in most Southern Ocean sectors (Schnack-Schiel and Hagen, 1994; Beaumont and Hosie, 1997). Many works have been conducted on the distribution, abundance, reproduction, feeding behavior, and

* Supported by National Key Technology Research and Development

Program of China (No. 2006BAB18B07), the National Natural Science

Foundation of China (No. 40821004), National Polar Project of China

(No. JDZX20110016), and the China International Polar Year (IPY)

Program

** Corresponding author: [email protected]

1066 Vol.29CHIN. J. OCEANOL. LIMNOL., 29(5), 2011

especially the life cycle patterns (Atkinson, 1998; Carli et al., 1999; Pasternak and Schnack-Schiel, 2001; Schnack-Schiel, 2001; Pane et al., 2004；Tanimura et al., 2008).

Sea ice, which covers up to 20×106 km2 during winter time and recedes to less than 4×106 km2 in austral summer, is a dominating and important feature of the Southern Ocean (Zwally et al., 1983; Lizotte, 2003). Adapting to the annual ice cycle and short primary production season, C. acutus and R. gigas perform seasonal ontogenetic vertical migration (Schnack-Schiel, 2001). They stayed in deep water with advanced copepodite stages during winter and migrated to surface water for grazing and reproduction in summer. While the species C. propinquus and M. gerlachei turn more omnivorous and remain largely active during winter (Atkinson, 1998). A one or two year life cycle strategy has been suggested for these species depending on different regions and environmental conditions (Chiba et al., 2002).

Inter-annual fluctuation in densities of these dominant species may result in zooplankton community variation (Hosie et al., 2000; Chiba et al., 2001; Hunt et al., 2007), which may have great impacts on the Southern Ocean ecosystem. However, many studies have focused on the seasonal or regional changes (Beaumont and Hosie, 1997; Schnack-Schiel et al., 2008). We need to provide more data on the inter-annual dynamics of copepod population in the Southern Ocean.

Based on zooplankton samples collected in the Prydz Bay during Chinese National Antarctic Research Expedition (CHINARE) from 1999 to 2006, we analyzed population dynamics of the four dominant copepod species during austral summer. Our main objectives were to record the regional and inter-annual variations of population abundance and structure of the four copepods, and estimate the effects of environment factors on the population dynamics.

2 MATERIAL AND METHOD

During six Chinese National Antarctic Research Expedition (CHINARE) cruises from 1999 to 2006, mesozooplankton along 70.5°E and 73°E transects in the Prydz Bay region were collected using NORPAC net (0.5 m2 net mouth, mesh 330 μm) towed from 200 m to surface (Fig.1). The sample time mainly focused on late January, while the oceanic regions (north of 1 000 m isobath) of 1999 and 2005, and neritic regions (south of 67.5°S) of 2002 were investigated in December and February, respectively (Table 1). Samples were preserved in 5% solution of buffered formalin. In the laboratory, four dominant copepods, Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas were extracted and counted by each copepodite stage at all stations. The mean population stage was calculated according to Ward et al. (1996):

S = (nCI+2nCII+…+6nCVI)/N

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Fig.1 Research area and sampling stations of all cruises

No.5 1067YANG et al.: Effect of environment variation on Antarctic copepods

where nCI, 2nCII, …, are the number of respective stages and N the total number counted.

Temperature and salinity profiles of all cruises were acquired by an SBE 911 plus CTD. For chlorophyll a measurements, 500 ml seawater samples were collected at a series of depths (0, 25, 50, 100, 150, and 200 m) and filtrated through GF/F membrane, extracted by 90% acetone in the dark for 24h, then measured by Turner Designs Fluorometer, Model 10. Unfortunately, chlorophyll a was not measured in the years 2003 and 2005. In this study, integrated temperature and chlorophyll a over the 200 m water column were used. The sea ice photos each half month were provided by National Oceanic and Atmospheric Administration, USA (NOAA).

Correlation analysis was employed to determine the influences of the environmental variables on copepod population structure. Before Pearson correlation analysis, densities data was log10 (x+1) transformed. To investigate the species associations, following the method of Field et al. (1982), inverse cluster analysis was performed on a station by species matrix where species were sub-divided into copepodite stages (copepodite stage I of R. gigas was omitted for the low densities). Data was first standardized before the Bray-Curtis measurement (Field et al., 1982). Following cluster analysis with Bray-Curtis index and UPGMA linkage, non-metric multidimensional scaling (NMDS) was used to map species associations in 2-dimensionl space (Hosie and Cochran, 1994).

Analyses were conducted using SPSS 16.0 and Primer 5.0, the sea ice photos were analyzed by ARCGIS 9.3.

3 RESULT

3.1 Environmental variation

Substantial inter-annual variation of the ice condition was found in the study area. By December

15th (about one month before our sampling) of 1999, 2003 and 2006, sea ice had retreated to a more southerly position than in 2000, 2002 and 2005 and a polynya of varying size had existed in the neritic region (Fig.2a, b, c, d). In 2000, 2002 and 2005, the ice edge located to the north and no polynya was found in the neritic region (Fig.2a, b, c, d).

Average temperature integrated over the upper 200 m was higher in the oceanic region compared with neritic region in all cruises, while the converse was observed for chlorophyll a concentration, which was always higher in the neritic region than the oceanic region (Fig.3). Higher temperature and chlorophyll a concentration of the neritic region in the years 1999, 2003 and 2006 generally indicated a great contribution of polynya (Fig.3).

3.2 Abundance and distribution

Calanoides acutus, with the densities varying from 0.16 to 775 ind./m3 during all cruises, was the most abundant species among these four copepods. In the years 1999, 2003 and 2006, the abundance was much higher than the other cruises, especially in the neritic region. The log10 (x + 1) transformed densities were positively correlated with surface temperature, surface chlorophyll a and integrated chlorophyll a, while the mean population stage was negatively correlated with temperature and chlorophyll a concentrations (Table 2). Population structure showed pronounced intra and inter-annual variation (Fig.4). Copepodite V (CV) and female dominated the population of oceanic regions during most cruises except the year 2003, when copepodite stages CI and CII had a greater contribution (Fig.4). In the neritic region, younger copepodite stages were always found with greater proportions (Fig.4).

Abundance of the species Calanus propinquus and Metridia gerlachei was similar, ranging from 0.09 to 84 ind./m3 for C. propinquus and 0.02 to 68 ind./m3 for M. gerlachei. Their density distribution patterns were similar to C. acutus, being higher in neritic regions compared with oceanic regions, and higher in the years 2003 and 2006 than other years (Fig.2b, c). The log10 (x+1) transformed densities of both species were positively correlated with surface temperature and the mean population stages were both negatively correlated with surface temperature (Table 2). While the mean population stage was negatively correlated with chlorophyll a for C. propinquus and the densities after log10 (x+1) transformed were positively correlated with chlorophyll a. Younger copepodite stages CI and CII were the dominating contribution to the whole

Table 1 Sampling time of different regions of the Prydz Bay during all cruises

Oceanic and shelf region Neritic region

1999 12/17–12/20, 1998 01/13–01/20, 1999

2000 01/22–01/26, 2000 01/22–01/26, 2000

2002 01/09–01/13, 2002 02/22–02/23, 2002

2003 02/05–02/08, 2003 02/05–02/08, 2003

2005 12/08–12/10, 2004; 01/28–01/31, 2005 01/31–02/01, 2005

2006 01/17–01/23, 2006 01/17–01/23, 2006


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Fig.2a Spatial density (ind./m3) distribution of Calanoides acutus sampled in all cruises Ice edge and Polynya region on Dec. 15 (about one month before our sampling) were shown.

population structure for C. propinquus in the whole survey region of all cruises (Fig.4). In all sampling stations of the years 2003, 2005 and neritic stations of 2002 and 2006, younger stages CI to CIII dominated the population of M. gerlachei (Fig.4). While in the years 1999, 2000 and oceanic regions of 2002 and 2006, advanced stages and female accounted for most of the population abundance (Fig.4).

Rhincalanus gigas, with densities varying from 0 to 7.5 ind./m3, was the least abundant species among these four copepods. R. gigas was mainly distributed in the oceanic regions and the population was seldom found in the neritic region (Fig.2d). The log10 (x+1) transformed densities were positively correlated with integrated temperature. The whole population structure was mainly composed of advanced stages CV and female during all surveys (Fig.4, Table 2).

3.3 Copepod associations

Three main clusters were identified by inverse analysis (Fig.5). The copepodite stages of R. gigas separated into a distinct cluster. Younger stages CI to CIV of C. acutus, C. propinquus and M. gerlachei formed one association, while advanced stages of these three copepods formed another (Fig.5).

4 DISCUSSION

Results showed that for the four copepods in Prydz Bay, C. acutus was most abundant, followed by M. gerlachei, C. propinquus and R. gigas, with the exception of 2003 when C. propinquus was a little slightly more abundant than M. gerlachei. These patterns agreed well with the previous results (Beaumont and Hosie, 1997).

C. acutus was considered one of the most typical herbivores in the Southern Ocean (Hopkins, 1987;


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Fig.2b Spatial density (ind./m3) distribution of Calanus propinquus sampled in all cruisesIce edge and Polynya region on Dec. 15 (about one month before our sampling) were shown.

Graeve et al., 1994; Michels and Schnack-Schiel, 2005), grazing in summer and diapausing at depth in winter (Voronina, 2003). In regions of ice retreating, the release of epontic cells during ice melt combined with stable water column is thought to be vital to phytoplankton bloom (Pakhomov and Froneman, 2004). Subsequently C. acutus would benefit from this for feeding and population recruitment. As shown in Table 2, with the rise of temperature and chlorophyll a, densities of the whole population would increase and the mean population stage would decrease. Chlorophyll a of the neritic region was usually higher than oceanic regions in Prydz Bay (Fig.3) which indicated different levels of phytoplankton bloom. The abundance of the whole population and proportions of younger copepodite stages were relatively higher in neritic regions compared with oceanic regions (Fig.2a, b, c, d). In 2003, the ice had retreated for nearly one month before our sampling, chlorophyll a was persistently high in January from OBPG SeaWiFS 8-Day Global

9-km products. Following this favorable environment, C. acutus may spawn earlier and younger stages were predominant in the whole survey area (Fig.4). In the years 1999, 2003, and 2006, a polynya existed in the neritic region from the sea ice photo. The chlorophyll a concentrations were higher than other years (Fig.3). This “oasis effect” (Hosie and Cochran, 1994) benefited C. acutus for reproduction and the population densities were higher than other cruises, especially the juvenile stages (Figs.2a, 4). Investigations in Ross Sea also suggested that the extended polynya favored the phytoplankton blooming (Arrigo et al., 2000; Catalano et al., 1997), which, in turn, promoted copepod population recruitment (Pane et al., 2004).

C. propinquus did not seem to diapause in winter, and the dominating lipid storage form of triacylglycerols (TAGs) that were considered as short-term energy sources indicated this species might feed ceaselessly through the whole life cycle (Pasternak et al., 2009). During austral summer, this


species mainly feeds on phytoplankton and the population was dominated by new generation CI and CII (Marin, 1988). Correlation patterns with environmental variables were similar to C. acutus (Table 2). Studies in eastern Weddell Sea showed that CI and CII specimens comprised a high proportion of the population in May, suggesting an extended reproduction period (Schnack-Schiel and Hagen, 1994). The proportion of CI and CII stages was also high in both oceanic and neritic regions during all surveys of this study (Fig.4). These results suggested that C. propinquus may spawn earlier in summer and continue to spawn throughout the summer period.

M. gerlachei seldom concentrates with maximum numbers at the surface water and has even been described as midwater species (Hopkins and Torres, 1988). Similar to C. propinquus, M. gerlachei was considered more omnivorous from the fatty acids composition (Pasternak et al., 2009). But in summer,

this species was classed as predominantly herbivorous (Huntley and Escritor, 1992). The inter-annual density dynamics patterns of the whole population were similar with C. acutus and C. propinquus (Fig.2a, b, c). The negative correlation of log10(x+1) transformed densities with integrated temperature may be due to the commonly neritic distribution of M. gerlachei for the integrated temperature of the neritic regions was lower than oceanic regions (Fig.3). The higher proportions of CII but lower proportions of CI to the whole population during all cruises were perhaps because the mesh size (330 μm) of the NORPAC net we used was too coarse to catch CI of M. gerlachei effectively (Chiba et al., 2002). The high proportions of females in the upper 200 m column of all regions indicated a prolonged spawning period (Schnack-Schiel and Hagen, 1994).

Compared with the other three copepods, R. gigas preferred warmer water and mainly distributed in

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Fig.2c Spatial density (ind./m3) distribution of Metridia gerlachei sampled in all cruisesIce edge and Polynya region on Dec. 15 (about one month before our sampling) were shown.


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the oceanic regions of Prydz Bay (Fig.2d). This was further demonstrated by the positive correlation between log10(x+1) transformed densities and integrated temperature (Table 2). Much dispute existed for the life cycle and reproductive period of R. gigas (Atkinson, 1998). One year life cycle in the warmer Antarctic Circumpolar Current (ACC) and two year in colder Weddell Sea were reported (Marin, 1988; Atkinson, 1991). Eggs and youngest copepodite stages were found occupying the surface water in early spring, summer and autumn (Schnack-Schiel and Hagen, 1994) which suggests that egg laying was protracted. The higher proportion of female and CV in the whole population suggests that R. gigas perhaps spawned later than other species in the Prydz Bay. A reproduction experiment conducted

Table 2 Pearson correlation between log10(x+1) transformed species abundances, mean population stage and environmental variables

Surface temperature Integrated temperature Surface chl-a Integrated chl-a

Calanoides acutus Log10(x+1) transformed densities 0.586** 0.378** 0.457**

Mean population stage -0.416** -0.277** -0.423** -0.479**

Calanus propinquus Log10(x+1) transformed densities 0.537**

Mean population stage -0.301** -0.366** -0.341**

Metridia gerlachei Log10(x+1) transformed densities 0.349** -0.251* 0.294* 0.312*

Mean population stage -0.228*

Rhincalanus gigas Log10(x+1) transformed densities 0.582**

Mean population stage

n 81 81 50 50

DF 1, 79 1, 79 1, 48 1, 48

Note: Only significant correlations are shown. *: P < 0.05; **: P < 0.01

aCIVaCIII

aCV

aFeStress: 0.1

bFe

dFe

dCII

dCIIIdCVdCIV

bCVcCIV

cCV

cFe

aCII

aCI cCIII

cCI

bCIbCIV

bCIIbCIII

cCII

1009080706050403020100

80

60

40

20

0

100

100

80

60

40

20

0

1999O 1999N 2000O 2000N 2002O 2002N 2003O 2003N 2005O 2005N 2006O 2006N

1999O 1999N 2000O 2000N 2002O 2002N 2003O 2003N 2005O 2005N 2006O 2006N

1999O 1999N 2000O 2000N 2002O 2002N 2003O 2003N 2005O 2005N 2006O 2006N

1009080706050403020100

1999O 2000O 2002O 2003O 2005O 2006O

Fe

a

c

b

d

CVCIVCIIICIICI

FeCVCIVCIIICIICI

FeCVCIVCIIICIICI

FeCVCIVCIIICIICI

Perc

enta

ge (%

)Pe

rcen

tage

(%)

Perc

enta

ge (%

)Pe

rcen

tage

(%)

Fig.4 Developmental stages of the four copepods in the oceanic and neritic region of all cruises a. Calanoides acutus; b. Calanus propinquus; c. Metridia gerlachei; d. Rhincalanus gigas; O: oceanic region; N: neritic region

Fig.5 NMDS inverse ordination plot comparing all developmental stages of the four copepods

a. Calanoides acutus; b. Calanus propinquus; c. Metridia gerlachei;

d. Rhincalanus gigas


during the summer of 1999 showed zero fecundity for R. gigas, while egg-production was found for C. acutus and M. gerlachei in the Prydz Bay (Zhang and Sun, 2002). To better know how R. gigas acclimatized to the bay environment, more field work should be conducted.

The distributions of all copepodite stages of R. gigas were discrete from the other three species, which were dispersed all over the Prydz Bay (Fig.5). It seems that temperature can account for the discrete distribution pattern of R. gigas which was scarce in colder continental shelf region (Bathmann et al., 1993). Temperature is also considered to be a controlling factor which affects the population composition of the four copepods in Prydz Bay (Beaumont and Hosie, 1997). Chlorophyll a is another widely accepted factor impacting population dynamics other than temperature. Though different life cycles were found for all these copepods, they were all considered mainly herbivorous in summer (Greave et al., 1994; Burghart et al., 1999). With the retreat of sea ice, primary production was enhanced at the ice edge region owning to the stable upper layer and actively growing sea ice microbes (Lancelot et al., 1993). These four copepods may take the advantages of seasonal phytoplankton stocks for active feeding and reproduction. So the variations in timing and extent of sea ice retreat would have great effect on phytoplankton bloom status and consequently, population dynamics of herbivorous zooplankton. In this study of the years 1999, 2003 and 2006, ice retreated earlier and a polynya had existed in the neritic region one month before our sampling which indicated earlier phytoplankton bloom. Herbivorous copepods benefited from this for population recruitment which could be shown from higher densities of the four copepods during these years (Fig.2a, b, c, d). As show from dissimilar developmental stages and population structure, these four species exhibited different patterns corresponding to their deviating life history trait and variable biotopes.

5 CONCLUSION

Population structure of the four dominant copepods exhibits a high degree of spatial and inter-annual variability in the Prydz Bay, Antarctica. Our results suggest that the population dynamics was mainly determined by water temperature, and the sea ice and polynya through their relationship with primary production features, which was consistent with the

results from other Antarctic coastal waters (Hopkins and Torres, 1988; Pane et al., 2004; Schnack-Schie et al., 2008; Voronina, 2003; Ward et al., 2007). As one of the most sensitive areas to climate change, sea ice is more dynamics in the Antarctic coastal areas, which is supposed to have more impacts on the pelagic ecosystems. Therefore, better understanding of the dynamics between key copepods and physical factors, such as sea ice and polynya’s, is important to forecast the marine ecosystem feedback to climate change.

6 ACKNOWLEDGEMENT

Thanks are due to all those who aided the collection of this data set: G T ZHANG, Y S ZHANG, P JI, W C ZHANG, X M PU, S Q WANG as well as the crew of the RV ‘Xuelong’. We would like to thank J ZHANG (Polar Research Institute of China), J X SHI (Ocean University of China) and C Y HU (The Second Institute of Oceanography, SOA) for providing the temperature, salinity and chlorophyll a data.

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