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Comparison of mesozooplankton communities from a pair of warm- and cold-core eddies off the coast of Western Australia
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Deep-Sea Research II 54 (2007) 1103–1112 www.elsevier.com/locate/dsr2
Comparison of mesozooplankton communities from a pair of warm- and cold-core eddies off the coast of Western Australia J. Strzeleckia,, J.A. Koslowa, A. Waiteb a
CSIRO Marine and Atmospheric Research, Underwood Avenue, Floreat, WA 6014, Australia School of Environmental Systems Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
b
Accepted 10 February 2007 Available online 27 June 2007
Abstract We examined the abundance and biomass, size, trophic and community structure of the mesozooplankton within a pair of warm- and cold-core eddies off the west coast of Australia in October 2003, about five months after their initial formation. Zooplankton assemblages were highly diverse in both eddies, with little dominance by any taxa. Assemblages were significantly different in the two eddies, and night assemblages differed from day. Contrary to initial expectation, mesozooplankton abundance and biomass were twice as high in the warm-core eddy as in the cold-core eddy. This was consistent with the higher phytoplankton and microzooplankton biomass and higher primary production in the warm-core eddy. The source water for the warm-core eddy was Leeuwin Current water with about a 10% admixture of shelf water. The plankton biomass and productivity of these water masses are substantially higher than of subtropical Indian Ocean water, the source water for the mixed layer of the cold-core eddy. Five months after eddy formation, the abundance and biomass of zooplankton in the eddies continued to reflect the values generally observed in their parent water masses. The trophic structure of the zooplankton in both eddies was dominated by carnivores (mostly chaetognaths). Although this apparent trophic structure may be an artefact of sampling oligotrophic waters with coarse-meshed (335 mm) nets, it points to a ‘lumpiness’ in the Sheldon particle spectrum. r 2007 Elsevier Ltd. All rights reserved. Keywords: Community structure; Mesoscale eddies; Zooplankton; Size distribution; Leeuwin Current; Trophic structure
1. Introduction Eddies are a major source of mesoscale variability in the plankton. They entrain shelf water, moving it offshore, and advect offshore water onto the shelf. As a consequence, species may be transported over large distances and into different habitats (The Ring Group, 1981; Flierl and McGillicuddy, 2002; Corresponding author. Tel.: +618 9333 6526; fax: +618 9333 6555. E-mail address: [email protected] (J. Strzelecki).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.02.004
Mackas and Galbraith, 2002; Batten and Crawford, 2005; Mackas and Coyle, 2005). Eddies may thus affect species abundance, biomass and community composition, as some become locally extinct and others become more abundant (Owen, 1981; The Ring Group, 1981). The biological composition of eddies is established by the source water mass but is then subject to the influx of surrounding communities into the eddy and the gradual trend toward a new equilibrium (Olson, 1991). In general, the effects of eddies on the zooplankton will depend on the phytoplankton and primary productivity of
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source waters, the eddy size, its age, the dynamics within the eddy and interactions with the surrounding waters, and the generation time of the organisms. Anticyclonic eddies often upwell nutrient-rich water into the euphotic zone and thus enhance primary production, which in turn supports increased zooplankton biomass and secondary production (Wormouth, 1985; Woodward and Rees, 2001). For example, the biomass of foraminifera was 18 times, primary production 50% and zooplankton biomass 1.3–1.8-fold higher in coldcore eddies than in the surrounding Sargasso Sea (The Ring Group, 1981). Cyclonic eddies, on the other hand, promote downwelling, with opposite effects on productivity. As a result, the general paradigm is that cold-core eddies are more productive and harbour a greater abundance of plankton relative to warm-core eddies (Biggs et al., 1997; Zimmerman and Biggs, 1999). Studies of hyperiid amphipods (Gasca, 2004), zooplankton and micronekton (Wormouth et al., 2000) and euphausiids (Castellano and Gasca, 1999) confirm this model. The oceanography off the west coast of Western Australia is dominated by the unique Leeuwin Current, an eastern boundary current that flows poleward and hence suppresses upwelling along the shelf break. It is 250–300 m deep and transports warm, low-salinity and low-nutrient tropical water southward along the Western Australian coast. The current flows all year but is stronger in the late austral autumn and winter when the opposing wind stress is the weakest (Cresswell, 1991). Warm-core eddies on the order of 100 km in diameter generally form in May—June, predominantly between 28 and 311S, before breaking off and drifting westward into the Indian Ocean (Fang and Morrow, 2003). The eddy kinetic energy of the Leeuwin Current is the highest of any eastern boundary current (Feng et al., 2003). These eddies induce an exchange of water between the Leeuwin Current, continental shelf, and open-ocean water (Cresswell, 1980). In October 2003, we investigated the mesozooplankton in a counter-rotating eddy pair seaward of the Leeuwin Current 150 km off the coast of Western Australia. The eddies formed from a meander structure of the Leeuwin Current in May 2003 and detached from the current in late August 2003. The two eddies were in close proximity during the cruise. The physical properties of the eddies and their phytoplankton dynamics are described in
detail by Feng et al. (2007) and Thompson et al. (2007), respectively, and Greenwood et al. (2007) provides a model of the phytoplankton dynamics within the two eddies. Our sampling was designed to examine the abundance, biomass, and community structure of the mesozooplankton within the pair of warm and cold-core eddies. 2. Methods 2.1. Sampling For details of how the eddies were tracked and their centres and perimeters located, see Waite et al. (2007). Mesozooplankton was sampled at the perimeter and core of the eddies (Fig. 1) with double-oblique tows to 150 m depth of bongo nets (60 cm diameter) with 355-mm and 100-mm mesh nets to sample both the macro- and mesozooplankton. Only samples from the coarse-meshed nets were included in this study. Sampling was not carried out within 1 h of sunrise and sunset. Plankton samples were fixed with 10% sodium tetraborate-buffered formalin immediately after collection. Preserved samples were returned to the laboratory for identification and measurement. The samples were quantitatively sub-sampled using a Hensen-Stempel pipette to obtain at least 100 adult copepods (0.5–2% of the total sample). Biovolume was estimated using image analysis (JImage) following Alcaraz et al. (2003). Organisms were transferred to a Bogorov tray and images were obtained using a Q-Imaging camera installed in a stereomicroscope and connected to a computer. The images at a magnification of 12 (270 pixels mm1) were captured and analysed using NIH-Image 1.62 (National Institute of Health, Bethesda, Md., USA) for the PC. The shape of the organism was assumed to be equivalent to an ellipsoid for crustaceans, a frustum cone or a cube for siphonophores and a cylinder for chaetognaths and appendicularians. We measured the nucleus of salps, the only part not affected by sample treatment or preservation, and the shape was assumed to be an ellipsoid (Alcaraz et al., 2003). It is known that fixatives cause shrinkage of organisms (Omori, 1978). However, since there is no correction factor suitable for all the organisms covered in this study, we did not attempt to make any corrections for this effect. To obtain size spectra the organisms were grouped into volume classes in an increasing octave scale from 2 mm3 to 41024 mm3. Size distributions were expressed as
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Fig. 1. Cold-core (CC) and warm-core (WC) eddy with bongo stations.
the density of organisms m3 in different volume classes regardless of their taxonomy. The raw data were normalized by the class width to get a function that does not depend on the partition size (Blanco et al., 1994), and the size spectra were displayed on log10–log10 scales. Zooplankton biovolume was converted to carbon (C) based on the relationship (Alcaraz et al., 2003): C ¼ 0.0699 volume. 2.2. Statistical analyses Community structure was analysed using nonparametric multivariate methods (Primer v.6). Prior to analysis the data were square-root transformed to enhance the contribution of the intermediately abundant or intermediately large species to the pattern. The Bray–Curtis similarity matrix, which reflects changes in relative abundance as well as species composition, was used to obtain multi-
dimensional scaling (MDS) ordinations, and community relationships were examined with twodimensional plots. The MDS was repeated until the same lowest stress was achieved. The simulation/permutation test ANOSIM (Warwick and Clarke, 1991) was used to compare the separation between groups. To determine the species contributing most to dissimilarities among groups, we used the program SIMPER (Warwick and Clarke, 1991). Organisms with a high average contribution and large ratio of average contribution to standard deviation of contribution were considered good discriminating organisms (Clarke, 1993). They not only contributed most to dissimilarity but did so consistently. A permutational MANOVA (PERMANOVA) (Anderson, 2005a) based on the Bray– Curtis distance measure was used to test if there was a statistical multivariate difference between assemblages. Groups may differ because of their location in multidimensional space, their relative dispersion
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or both. PERMDISP (Anderson, 2005b) was applied to each significant term to examine if groups differed because of the differences in dispersion among groups. Canonical analysis of principal coordinates (CAP) (Anderson, 2004) was used to test specific a priori hypotheses of differences between assemblages in the two eddies, the two habitats and between day and night. CAP provides information about differences in location among groups but does not provide information concerning the overall pattern of dispersion among groups (Anderson and Willis, 2003). Zooplankton biomass was correlated with environmental variables, phytoplankton biomass and primary production using non- parametric methods because of small sample sizes and non-linear but monotonic relationships. 3. Results 3.1. Abundance and biomass Mesozooplankton abundance from the 355-mm net samples was about twice as high in the warmcore eddy than in the cold-core eddy: 115 versus 54
individuals m3 (t[15] ¼ 4.48, po0.001). In both eddies, calanoid copepods dominated numerically, followed by cyclopoid copepods, chaetognaths and euphausiid larvae (Table 1). Appendicularians were relatively more important in the warm-core eddy, and ostracods and siphonophores in the cold-core. No species or genus of copepod dominated the assemblages, but the main groups were Clausocalanus spp., Oithona spp., Acartia spp., Ctenocalanus spp., Mesocalanus spp., Oncaea spp., and Corycaeidae in both eddies. In addition Pleuromamma gracilis, Mecynocera clausii and Calocalanus spp. contributed X2% in the warm-core eddy, and Lucicutia spp. in the coldcore eddy. Other species (or species groups) contributed less than 2% to the total abundance. There was a significant day–night difference in abundance only in the cold-core eddy (t[7] ¼ 5.98, p ¼ 0.002) (77 vs. 30 individuals m3 during the night and day, respectively) (Fig. 2(A)). Although the mean biomass was twice as high in the warm-core than the cold-core eddy (5.2 vs. 2.6 mg C m3, respectively), this was of marginal significance (t[15] ¼ 4.11, p ¼ 0.06). In the cold-core eddy biomass was significantly higher at night
Table 1 Mesozooplankton composition, abundance and biomass in the cold-core and warm-core eddies Eddy
Number of taxa
Mean abundance (ind. m37SD)
Most important contributors by abundance (%)
Warm core
72
115728
Cold core
69
54727
5.273.0 Clausocalanus spp. (10.4), calanoid copepodite (9.8), Oithona spp. (6.8), appendicularian (6.4), chaetognaths (6.2), Acartia spp. (3.9), euphausiid calyptopis (3.7), Ctenocalanus spp. (3.0), other calanoid copepods (26.4), other cyclopoid copepods (5.9) 2.671.8 Chaetognaths (9.5), Clausocalanus spp. (8.8), Acartia spp. (7.6), ostracods (6.5), siphonophores (6.1), Lucicutia spp. (5.4), Corycaeus spp. (5.3), calanoid copepodites (5.2), Oithona spp. (4.6), Ctenocalanus spp. (3.3), Oncaea spp. (3.3), other calanoid copepods (20.0), other cyclopoid copepods (11.42)
Only organisms contributing X3% are listed.
Mean biomass mg (C m37SD)
Most important contributors by biomass (%) Chaetognaths (56.5), calanoid copepods (16), siphonophores (7.9), euphausiid furcilia (4.9), appendicularians (3.6) Copila spp. (3.5)
Chaetognaths (48.4), siphonophores (15.2), calanoid coepods (14.0), Copila spp. (9.3)
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Table 2 Probabilities (P) based on permutational multivariate analysis of variance (PERMANOVA) testing for differences in the zooplankton assemblages between the eddies, between their centre and periphery (habitat), and between day and night (time), based on analysis of the abundance (individual m3) and biomass (mg C m3) data for groups comprising the assemblages
Fig. 2. (A) Abundance of mesozooplankton (mean individuals1 m37standard deviations) in centre (C), perimeter (P), day (D, open circles), and night (N, closed circles) in two eddies. (B) Biomass of mesozooplankton (mg C m37standard deviations) in centre (C), perimeter (P), day (D, open circles), and night (N, closed circles) in two eddies.
(4.0 mg C m3) than during the day (1.3 mg C m3) (t[7] ¼ 10.78, p ¼ 0.02), but there was no significant difference between the centre (2.2 mg C m3) and the perimeter (3.1 mg C m3). The biomass pattern was different in the warm-core eddy, with no significant difference between day (5.7 mg C m3) and night (4.7 mg C m3) but a significant difference between the centre (7.3 mg C m3) and the perimeter (3.2 mg C m3) (t[7] ¼ 6.43, p ¼ 0.04). The usual diurnal biomass pattern was reversed in the warmcore eddy centre, where biomass in the day was higher (Fig. 2(B)). Chaetognaths usually dominated the net zooplankton biomass, and cyclopoid and calanoid copepods, siphonophores, and euphausiid larvae were important at most stations.
Factor
P abundance
P biomass
Eddy Habitat Time Eddy habitat Eddy time Habitat time Eddy habitat time
0.00 0.16 0.00 0.09 0.14 0.28 0.58
0.00 0.35 0.00 0.02 0.05 0.01 0.14
Fig. 3. Results of nonparametric multidimensional scaling analysis based on zooplankton abundance showing associations of mesozooplankton assemblages between day (D) and night (N), centre (C) and perimeter (P) within the cold (C) and warm (W) core eddies with superimposed clusters from a dendogram at similarity levels of 55% (green line), 65% (blue line), and 75% (red line).
3.2. Community composition 3.2.1. Abundance Analysis of the abundance data indicated that the mesozooplankton assemblages were different in the two eddies (Table 2, Fig. 3). The difference was due to their location in multidimensional space only and not their dispersion (PERMANOVA, p ¼ 0.0001, PERMDISP, p ¼ 0.5). Appendicularians, ctenophores, calanoid copepodites, Clausocalanus spp. and Oithona plumifera were good discriminators of the eddies and occurred consistently in greater abundance in the warm core eddy. There was a shift in assemblage structure from day to night, and
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the day was more variable than the night assemblage (PERMDISP, p ¼ 0.03). Only 17 species groups out of 53 that contributed to the dissimilarity between day and night assemblages were more abundant during the day than at night. Calocalanus spp., appendicularia, euphausiid calyptopis were more abundant during the day, and Pleuromamma spp., Scolecithricella spp., Nannocalanus minor, Clausocalanus spp., Lucicutia spp., Ctenocalanus spp., Oithona spp., ostracod and euphausiid furcilia were consistently more abundant at night. All these taxa except Oithona spp. are vertical migrators. 3.2.2. Biomass The eddies were less clearly differentiated based upon analysis of the zooplankton biomass data (Fig. 4). PERMANOVA indicated that there were significant multivariate interactions of the factor eddy with both habitat and time (Table 2). Differences thus varied depending upon when and where sampling took place: whether at the centre or perimeter of the eddies (habitat) and whether during the day or night (time). There was also a significant interaction between the factors habitat and time (Table 2). Pairwise a posteriori comparisons revealed significant differences between the centres of the two eddies (PERMANOVA, p ¼ 0.05) but not between their perimeters, which may reflect the greater isolation of the eddy centres; the perimeters
were more affected by exchange with surrounding waters. The perimeter and centre were not significantly different within each eddy. Day assemblages were significantly different between the two eddies (PERMANOVA, p ¼ 0.03) but night assemblages were not (PERMANOVA, p ¼ 0.1). Day and night assemblages were significantly different within each eddy (PERMANOVA, p ¼ 0.05 and p ¼ 0.02). Several species of copepods contributed to the dissimilarity between the two eddies. Candacia spp. and Sapphirina spp. had a higher biomass in the cold-core eddy and Paraeucalanus sewelli, P. gracilis, Clausocalanus spp., Mesocalanus spp., Lucicutia spp., and Haloptilus longicornis had a higher biomass in the warm-core eddy. The copepods Pleuromamma spp., Sapphirina spp., N. minor, P. sewelli, Clausocalanus spp., Calanus spp., and Lucicutia spp. contributed to over two-thirds of the dissimilarity between day and night and all had greater biomass at night than during the day. 3.3. Size spectra The median biovolume of organisms in both eddies was between 64 and 128 mm3 (Fig. 5(A)). There were no significant effects of day and night within or between eddies in the size spectra, although the modal size shifted from 64 mm during the day to 128 mm at night in both eddies (Fig. 5(B)), due to a higher abundance of copepod nauplii and small copepods such as M. clausii, O. plumifera, Bestiolina spp. during the day and larger diel migrators at night, such as Pleuromamma spp. and Sapphirina spp. There were no significant differences in the size structure of the zooplankton assemblages between the centre and perimeter of the eddies (Fig. 5(C)). The slopes of the normalized spectra did not differ significantly between the assemblages within the warm and cold-core eddies (Fig. 6). 4. Discussion 4.1. Abundance, biomass and assemblage analyses
Fig. 4. Mesozooplankton associations based on non-parametric multidimensional scaling analysis of the zooplankton biomass data. Samples are shown to be from day (D) or night (N) and the centre (C) or perimeter (P) of the cold (C) and warm (W) core eddies, with superimposed clusters from a dendrogram at similarity levels of 40% (green line), 50% (blue line), and 65% (red line).
The mesozooplankton was more abundant and had higher biomass in the warm-core than in the cold-core eddy. This is contrary to the generally observed pattern of higher plankton abundance and biomass in cold-core eddies. However, this pattern reflects the origin of the water masses.
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Fig. 5. Zooplankton distribution across 11 volume classes (octave scale) of the zooplankton biovolume spectrum from (A) the warm-core eddy (WC) and the cold-core eddy (CC) (B) warm-core eddy day (WC D) and night (WC N), and the cold-core eddy day (CC D) and night (CC N), and (C) the warm-core eddy centre (WC C) and perimeter (WC P), and the cold-core eddy centre (CC C) and perimeter (CC P).
Fig. 6. Normalized biovolume size spectra and fitted straight line of the entire mesozooplankton community in the warm-core eddy (WC) and the cold-core eddy (CC).
The warm-core eddy was formed predominantly from Leeuwin Current water with about a 10% admixture of shelf waters (Feng et al., 2007), which are inherently more productive than the subtropical Indian Ocean water that comprised the mixed layer of the cold-core eddy (Tranter, 1962; Longhurst, 1998; Lourey et al., 2006). Although the deep waters of the cold-core eddy were influenced by the Leeuwin Undercurrent (Rennie et al., 2007; Meuleners et al., 2007), the upwelling within this eddy was insufficient to penetrate this lens of near-surface subtropical water and significantly influence the nutrient levels and primary productivity within the euphotic zone (Feng et al., 2007). Thus the initial concentrations and productivity of the phytoplankton and zooplankton were likely to have
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been substantially higher initially in the warm than the cold-core eddy, a condition quite different to that found off western boundary currents, such as the Gulf Stream, where cold-core eddies are often composed of relatively productive slope water (The Ring Group, 1981). Greenwood et al. (2007) estimate that 75% of the primary production within the warm-core eddy at the time of the cruise was based on its entrainment of initially more productive water. The biomass of zooplankton in the cold-core eddy (2.6 mg C m3) was remarkably close to that observed by Tranter and Kerr (1969) in offshore Indian Ocean waters at 30–321S, 1101E in October–November 1962: 0.6 mg C m3 at night and 4.3 mg C m3 in the day, for a mean of 2.5 mg C m3 (see their Table 1, with wet weight converted to carbon, assuming dry weight is 10% wet weight and carbon is 33% dry weight Wiebe et al., 1975). The abundance of zooplankton in the warm-core eddy (115 individuals m3) was also comparable to that observed in Leeuwin Current water at 300 and 1000 m depth in December 2002 off Two Rocks near Perth: 124 individuals m3 (Keesing et al., 2006). Such comparisons are crude but indicate that the abundance and biomass of the zooplankton in the cold- and warm-core eddies were comparable to those found in their parent water masses at a similar time of year. 4.2. Size spectra, community and trophic structure The differences between the assemblages were due to different proportions and abundances of the taxa rather than to differences in a few indicator species. Over 30 taxa were required to account numerically for 70% of the dissimilarity between two eddies. There were no dominant taxa; rather most taxa were present in both eddies but in different densities. However, the lack of clear differentiation between the composition of the eddies may be due to many taxa not having been identified to species. Some species of copepods were recorded from only one eddy but not enough is known about copepod biogeography in the region to determine whether they are indicative of Leeuwin Current or oceanic waters. Herbivorous/omnivorous taxa are commonly observed to dominate the trophic structure of zooplankton, with chaetognaths generally comprising 10–30% of the biomass of copepods (Steele, 1974; Bone et al., 1991). In contrast, both eddies
Fig. 7. Biomass of herbivores, omnivores and carnivores in the warm- and cold-core eddies. Trophic groups were classified according to Boltovskoy (1999), Boxshall and Halsey (2004), Bradford and Jillett (1980), Bradford et al. (1983), BradfordGrieve, 1994, 1999). Corycaeus, Oncaea and Oithona, as well as all ‘herbivorous’ copepods were classified as omnivores, since almost all copepods can feed on microzooplankton, metazoans, detritus and phytoplankton and switch depending on available food.
were strongly dominated (60–70%) by carnivorous zooplankton, predominantly chaetognaths (Fig. 7). Such a preponderance of carnivorous relative to herbivorous/omnivorous zooplankton is not sustainable but may have resulted from the reduced productivity of the eddies, leading to the grazing down of the herbivorous/omnivorous component of the plankton. Alternatively this apparently anomalous trophic structure may be, at least in part, a sampling artefact. Oligotrophic waters are generally dominated by small phytoplankton (o5 mm), as observed in these eddies (Waite et al., 2007), and by relatively small zooplankton (Gallienne and Robins, 1998). The herbivorous/omnivorous copepods may thus be relatively more underrepresented in samples obtained with coarse-meshed (4200 mm) nets in oligotrophic than in more productive waters, leading to the apparent dominance of the larger chaetognaths. Clark et al. (2001) observed a transition to similarly chaetognath-dominated zooplankton samples in passing from productive to oligotrophic portions of the North Atlantic, which they ascribed to sampling with a coarse-meshed net. However, the data seem to indicate an inherent ‘lumpiness’ in the Sheldon size spectrum for zooplankton in oligotrophic waters, with a gap in the biomass of intermediate-sized herbivore/omnivores and high relative abundance of carnivores, which was in fact observed in the original particle
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size spectrum described from the Sargasso Sea by Sheldon et al. (1972, see their Fig. 11). Acknowledgements We thank C. White for the identification of mesozooplankton. Fig. 1 was created by M. Feng. Comments by D. McKinnon, P. Thompson and an anonymous reviewer greatly improved the manuscript. Thanks to the master and crew of the R.V. Southern Surveyor for their assistance at sea. Funding was received from the University of Western Australia, the Faculty of Engineering, Computing and Mathematics (UWA) Strategic Fund, the Waite et al. Collaborative Project and Koslow Biophysical Project from the Strategic Research Fund for the Marine Environment, and the Wealth from Oceans CSIRO flagship. References Alcaraz, M., Saiz, E., Calbet, A., Trepat, I., Broglio, E., 2003. Estimating zooplankton biomass through image analysis. Marine Biology 143 (2), 307–315. Anderson, M.J., 2004. CAP: a FORTRAN computer program for canonical analysis of principal coordinates. Department of Statistics, University of Auckland, New Zealand. Anderson, M.J., 2005a. PERMANOVA: a FORTRAN computer program for permutational multivariate analysis of variance. Department of Statistics, University of Auckland, New Zealand. Anderson, M.J., 2005b. PERMDISP: a FORTRAN computer program for permutational analysis of multivariate dispersions (for any two-factor ANOVA design) using permutation tests. Department of Statistics, University of Auckland, New Zealand. Anderson, M.J., Willis, T.J., 2003. Canonical analysis of principal coordinates: a useful method of constrained ordination for ecology. Ecology 84 (2), 511–525. Batten, S.D., Crawford, W.R., 2005. The influence of coastal origin eddies on oceanic plankton distribution in the eastern Gulf of Alaska. Deep-Sea Research II 52, 991–1009. Biggs, D.C., Zimmerman, R.A., Gasca, R., Sua´rez-Morales, E., Castellanos, I., Leben., R.R., 1997. Note on plankton and cold-core rings in the Gulf of Mexico. Fishery Bulletin 95 (2), 369–375. Blanco, J.M., Echevarrı´ a, F., Garcı´ a, C.M., 1994. Dealing with size-spectra: some conceptual and mathematical problems. Scientia Marina 58 (1–2), 17–29. Bone, Q., Kapp, H., Pierrot-Bults, A.C., 1991. Introduction and relationships of the group. In: Bone, Q., Kapp, H., PerrotBults, A.C. (Eds.), The Biology of Chaetognaths. Oxford University Press, Oxford, pp. 1–4. Boltovskoy, D., 1999. South Atlantic Zooplankton. Backhuys Publishers, London. Boxshall, G.A., Halsey, S.H., 2004. An Introduction to Copepod Diversity. The Ray Society, London.
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Greenwood, J.E., Feng, M., Waite, A.M., 2007. A onedimensional simulation of biological production in two contrasting mesoscale eddies in the south eastern Indian Ocean. Deep-Sea Research II, this issue [doi:10.1016/ j.dsr2.2006.10.004]. Keesing, J.K., Heine, J.N., Babcock, R.C., Craig, P.D., Koslow, J.A., 2006. Strategic Research Fund for the Marine Environment Final Report. The SRFME core projects, vol. 2, Strategic Research Fund for the Marine Environment, CSIRO, Australia, 274pp. Longhurst, A.R., 1998. Ecological Geography of the Sea. Academic Press, San Diego, CA. Lourey, M.J., Dunn, J.R., Waring, J., 2006. A mixed-layer nutrient climatology of Leeuwin Current and Western Australian shelf waters: seasonal nutrient dynamics and biomass. Journal of Marine Systems 59, 25–51. Mackas, D.L., Coyle, K.O., 2005. Shelf-offshore exchange processes, and their effects on mesozooplankton biomass and community composition patterns in the northeast Pacific. Deep-Sea Research II 52, 707–725. Mackas, D.L., Galbraith, M.D., 2002. Zooplankton distribution and dynamics in a North Pacific eddy of coastal origin: I. Transport and loss of continental margin species. Journal of Oceanography 58, 725–738. Meuleners, M., Pattiaratchi, C.P., Ivey, G., 2007. Numerical modeling of the mean flow characteristics of the Leeuwin Current system. Deep-Sea Research II, this issue [doi:10.1016/ j.dsr2.2007.02.003]. Olson, D., 1991. Rings in the ocean. Annual Review of Earth and Planetary Sciences 19, 283–311. Omori, M., 1978. Some factors affecting dry weight, organic weight and concentrations of carbon and nitrogen in freshly prepared and in preserved zooplankton. Internationale Revue Der Gesamten Hydrobiologie 63, 261–269. Owen, R.W., 1981. Fronts and eddies in the sea: mechanisms, interactions and biological Effects. In: Owen, R.W. (Ed.), Fronts and Eddies in the Sea. Academic Press, London. Rennie, S.J., Pattiaratchi, C.B., McCauley, R.D., 2007. Dynamics of the surface and sub-surface currents off southwestern Australia: a numerical study. Deep-Sea Research II this volume. Sheldon, R.W., Prakash, A., Sutcliffe Jr., W.H., 1972. The size distribution of particles in the ocean. Limnology and Oceanography 17, 327–340.
Steele, J.H., 1974. The Structure of Marine Ecosystems. Harvard University Press, Massachusetts. The Ring Group, 1981. Gulf Stream cold core rings: their physics, chemistry, and biology. Science 212 (4499), 1091–1100. Thompson, P.A., Pesant, S., Waite, A.M., 2007. Contrasting the vertical differences in the phytoplankton biology of a dipole pair of eddies in the south-eastern Indian Ocean. Deep-Sea Research II, this issue [doi:10.1016/j.dsr2.2006.12.009]. Tranter, D.J., 1962. Zooplankton abundance in Australasian waters. Australian Journal of Marine and Freshwater Research 13, 106–129. Tranter, D.J., Kerr, J.D., 1969. Seasonal variation in the Indian Ocean along 1101E. Australian Journal of Marine and Freshwater Research 20, 77–84. Waite, A.M., Thompson, P.A., Pesant, S., Feng, M., Beckley, L.E., Domingues, C.M., Gaughan, D., Hanson, C.E., Holl, C.M., Koslow, T., Meuleners, M., Montoya, J.P., Moore, T., Muhling, B.A., Patersona, H., Rennie, S., Strzelecki, J., Twomey, L., 2007. The Leeuwin Current and its eddies: an introductory overview. Deep-Sea Research II, this issue [doi:10.1016/j.dsr2.2006.12.008]. Warwick, R.M., Clarke, K.R., 1991. A comparison of some methods of analysing changes in benthic community structure. Journal of the Marine Biological Association of the UK 71, 225–244. Wiebe, P.H., Boyd, S., Cox, J.L., 1975. Relationships between zooplankton displacement volume, wet weight, dry weight, and carbon. Fishery Bulletin 73, 777–786. Woodward, E.M.S., Rees, A.P., 2001. Nutrient distributions in an anticyclonic eddy in the northeast Atlantic Ocean, with reference to nanomolar ammonium concentrations. Deep-Sea Research II 48, 775–793. Wormouth, J.H., 1985. The role of cold-core Gulf Stream rings in the temporal and spatial patterns of euthecosomatous pteropods. Deep-Sea Research 32, 773–788. Wormouth, J.H., Ressler, P.H., Cady, R.B., Harris, E.J., 2000. Zooplankton and micronekton in cyclones and anticyclones in the Northeast Gulf of Mexico. Gulf of Mexico Science 18 (1), 23–34. Zimmerman, R.A., Biggs, D.C., 1999. Patterns of distribution of sound-scattering zooplankton in warm- and cold-core eddies in the Gulf of Mexico, from a narrowband acoustic Doppler current profiler survey. Journal of Geophysical Research 104, 5251–5262.
Deep-Sea Research II 54 (2007) 1103–1112 www.elsevier.com/locate/dsr2
Comparison of mesozooplankton communities from a pair of warm- and cold-core eddies off the coast of Western Australia J. Strzeleckia,, J.A. Koslowa, A. Waiteb a
CSIRO Marine and Atmospheric Research, Underwood Avenue, Floreat, WA 6014, Australia School of Environmental Systems Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
b
Accepted 10 February 2007 Available online 27 June 2007
Abstract We examined the abundance and biomass, size, trophic and community structure of the mesozooplankton within a pair of warm- and cold-core eddies off the west coast of Australia in October 2003, about five months after their initial formation. Zooplankton assemblages were highly diverse in both eddies, with little dominance by any taxa. Assemblages were significantly different in the two eddies, and night assemblages differed from day. Contrary to initial expectation, mesozooplankton abundance and biomass were twice as high in the warm-core eddy as in the cold-core eddy. This was consistent with the higher phytoplankton and microzooplankton biomass and higher primary production in the warm-core eddy. The source water for the warm-core eddy was Leeuwin Current water with about a 10% admixture of shelf water. The plankton biomass and productivity of these water masses are substantially higher than of subtropical Indian Ocean water, the source water for the mixed layer of the cold-core eddy. Five months after eddy formation, the abundance and biomass of zooplankton in the eddies continued to reflect the values generally observed in their parent water masses. The trophic structure of the zooplankton in both eddies was dominated by carnivores (mostly chaetognaths). Although this apparent trophic structure may be an artefact of sampling oligotrophic waters with coarse-meshed (335 mm) nets, it points to a ‘lumpiness’ in the Sheldon particle spectrum. r 2007 Elsevier Ltd. All rights reserved. Keywords: Community structure; Mesoscale eddies; Zooplankton; Size distribution; Leeuwin Current; Trophic structure
1. Introduction Eddies are a major source of mesoscale variability in the plankton. They entrain shelf water, moving it offshore, and advect offshore water onto the shelf. As a consequence, species may be transported over large distances and into different habitats (The Ring Group, 1981; Flierl and McGillicuddy, 2002; Corresponding author. Tel.: +618 9333 6526; fax: +618 9333 6555. E-mail address: [email protected] (J. Strzelecki).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.02.004
Mackas and Galbraith, 2002; Batten and Crawford, 2005; Mackas and Coyle, 2005). Eddies may thus affect species abundance, biomass and community composition, as some become locally extinct and others become more abundant (Owen, 1981; The Ring Group, 1981). The biological composition of eddies is established by the source water mass but is then subject to the influx of surrounding communities into the eddy and the gradual trend toward a new equilibrium (Olson, 1991). In general, the effects of eddies on the zooplankton will depend on the phytoplankton and primary productivity of
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source waters, the eddy size, its age, the dynamics within the eddy and interactions with the surrounding waters, and the generation time of the organisms. Anticyclonic eddies often upwell nutrient-rich water into the euphotic zone and thus enhance primary production, which in turn supports increased zooplankton biomass and secondary production (Wormouth, 1985; Woodward and Rees, 2001). For example, the biomass of foraminifera was 18 times, primary production 50% and zooplankton biomass 1.3–1.8-fold higher in coldcore eddies than in the surrounding Sargasso Sea (The Ring Group, 1981). Cyclonic eddies, on the other hand, promote downwelling, with opposite effects on productivity. As a result, the general paradigm is that cold-core eddies are more productive and harbour a greater abundance of plankton relative to warm-core eddies (Biggs et al., 1997; Zimmerman and Biggs, 1999). Studies of hyperiid amphipods (Gasca, 2004), zooplankton and micronekton (Wormouth et al., 2000) and euphausiids (Castellano and Gasca, 1999) confirm this model. The oceanography off the west coast of Western Australia is dominated by the unique Leeuwin Current, an eastern boundary current that flows poleward and hence suppresses upwelling along the shelf break. It is 250–300 m deep and transports warm, low-salinity and low-nutrient tropical water southward along the Western Australian coast. The current flows all year but is stronger in the late austral autumn and winter when the opposing wind stress is the weakest (Cresswell, 1991). Warm-core eddies on the order of 100 km in diameter generally form in May—June, predominantly between 28 and 311S, before breaking off and drifting westward into the Indian Ocean (Fang and Morrow, 2003). The eddy kinetic energy of the Leeuwin Current is the highest of any eastern boundary current (Feng et al., 2003). These eddies induce an exchange of water between the Leeuwin Current, continental shelf, and open-ocean water (Cresswell, 1980). In October 2003, we investigated the mesozooplankton in a counter-rotating eddy pair seaward of the Leeuwin Current 150 km off the coast of Western Australia. The eddies formed from a meander structure of the Leeuwin Current in May 2003 and detached from the current in late August 2003. The two eddies were in close proximity during the cruise. The physical properties of the eddies and their phytoplankton dynamics are described in
detail by Feng et al. (2007) and Thompson et al. (2007), respectively, and Greenwood et al. (2007) provides a model of the phytoplankton dynamics within the two eddies. Our sampling was designed to examine the abundance, biomass, and community structure of the mesozooplankton within the pair of warm and cold-core eddies. 2. Methods 2.1. Sampling For details of how the eddies were tracked and their centres and perimeters located, see Waite et al. (2007). Mesozooplankton was sampled at the perimeter and core of the eddies (Fig. 1) with double-oblique tows to 150 m depth of bongo nets (60 cm diameter) with 355-mm and 100-mm mesh nets to sample both the macro- and mesozooplankton. Only samples from the coarse-meshed nets were included in this study. Sampling was not carried out within 1 h of sunrise and sunset. Plankton samples were fixed with 10% sodium tetraborate-buffered formalin immediately after collection. Preserved samples were returned to the laboratory for identification and measurement. The samples were quantitatively sub-sampled using a Hensen-Stempel pipette to obtain at least 100 adult copepods (0.5–2% of the total sample). Biovolume was estimated using image analysis (JImage) following Alcaraz et al. (2003). Organisms were transferred to a Bogorov tray and images were obtained using a Q-Imaging camera installed in a stereomicroscope and connected to a computer. The images at a magnification of 12 (270 pixels mm1) were captured and analysed using NIH-Image 1.62 (National Institute of Health, Bethesda, Md., USA) for the PC. The shape of the organism was assumed to be equivalent to an ellipsoid for crustaceans, a frustum cone or a cube for siphonophores and a cylinder for chaetognaths and appendicularians. We measured the nucleus of salps, the only part not affected by sample treatment or preservation, and the shape was assumed to be an ellipsoid (Alcaraz et al., 2003). It is known that fixatives cause shrinkage of organisms (Omori, 1978). However, since there is no correction factor suitable for all the organisms covered in this study, we did not attempt to make any corrections for this effect. To obtain size spectra the organisms were grouped into volume classes in an increasing octave scale from 2 mm3 to 41024 mm3. Size distributions were expressed as
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Fig. 1. Cold-core (CC) and warm-core (WC) eddy with bongo stations.
the density of organisms m3 in different volume classes regardless of their taxonomy. The raw data were normalized by the class width to get a function that does not depend on the partition size (Blanco et al., 1994), and the size spectra were displayed on log10–log10 scales. Zooplankton biovolume was converted to carbon (C) based on the relationship (Alcaraz et al., 2003): C ¼ 0.0699 volume. 2.2. Statistical analyses Community structure was analysed using nonparametric multivariate methods (Primer v.6). Prior to analysis the data were square-root transformed to enhance the contribution of the intermediately abundant or intermediately large species to the pattern. The Bray–Curtis similarity matrix, which reflects changes in relative abundance as well as species composition, was used to obtain multi-
dimensional scaling (MDS) ordinations, and community relationships were examined with twodimensional plots. The MDS was repeated until the same lowest stress was achieved. The simulation/permutation test ANOSIM (Warwick and Clarke, 1991) was used to compare the separation between groups. To determine the species contributing most to dissimilarities among groups, we used the program SIMPER (Warwick and Clarke, 1991). Organisms with a high average contribution and large ratio of average contribution to standard deviation of contribution were considered good discriminating organisms (Clarke, 1993). They not only contributed most to dissimilarity but did so consistently. A permutational MANOVA (PERMANOVA) (Anderson, 2005a) based on the Bray– Curtis distance measure was used to test if there was a statistical multivariate difference between assemblages. Groups may differ because of their location in multidimensional space, their relative dispersion
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or both. PERMDISP (Anderson, 2005b) was applied to each significant term to examine if groups differed because of the differences in dispersion among groups. Canonical analysis of principal coordinates (CAP) (Anderson, 2004) was used to test specific a priori hypotheses of differences between assemblages in the two eddies, the two habitats and between day and night. CAP provides information about differences in location among groups but does not provide information concerning the overall pattern of dispersion among groups (Anderson and Willis, 2003). Zooplankton biomass was correlated with environmental variables, phytoplankton biomass and primary production using non- parametric methods because of small sample sizes and non-linear but monotonic relationships. 3. Results 3.1. Abundance and biomass Mesozooplankton abundance from the 355-mm net samples was about twice as high in the warmcore eddy than in the cold-core eddy: 115 versus 54
individuals m3 (t[15] ¼ 4.48, po0.001). In both eddies, calanoid copepods dominated numerically, followed by cyclopoid copepods, chaetognaths and euphausiid larvae (Table 1). Appendicularians were relatively more important in the warm-core eddy, and ostracods and siphonophores in the cold-core. No species or genus of copepod dominated the assemblages, but the main groups were Clausocalanus spp., Oithona spp., Acartia spp., Ctenocalanus spp., Mesocalanus spp., Oncaea spp., and Corycaeidae in both eddies. In addition Pleuromamma gracilis, Mecynocera clausii and Calocalanus spp. contributed X2% in the warm-core eddy, and Lucicutia spp. in the coldcore eddy. Other species (or species groups) contributed less than 2% to the total abundance. There was a significant day–night difference in abundance only in the cold-core eddy (t[7] ¼ 5.98, p ¼ 0.002) (77 vs. 30 individuals m3 during the night and day, respectively) (Fig. 2(A)). Although the mean biomass was twice as high in the warm-core than the cold-core eddy (5.2 vs. 2.6 mg C m3, respectively), this was of marginal significance (t[15] ¼ 4.11, p ¼ 0.06). In the cold-core eddy biomass was significantly higher at night
Table 1 Mesozooplankton composition, abundance and biomass in the cold-core and warm-core eddies Eddy
Number of taxa
Mean abundance (ind. m37SD)
Most important contributors by abundance (%)
Warm core
72
115728
Cold core
69
54727
5.273.0 Clausocalanus spp. (10.4), calanoid copepodite (9.8), Oithona spp. (6.8), appendicularian (6.4), chaetognaths (6.2), Acartia spp. (3.9), euphausiid calyptopis (3.7), Ctenocalanus spp. (3.0), other calanoid copepods (26.4), other cyclopoid copepods (5.9) 2.671.8 Chaetognaths (9.5), Clausocalanus spp. (8.8), Acartia spp. (7.6), ostracods (6.5), siphonophores (6.1), Lucicutia spp. (5.4), Corycaeus spp. (5.3), calanoid copepodites (5.2), Oithona spp. (4.6), Ctenocalanus spp. (3.3), Oncaea spp. (3.3), other calanoid copepods (20.0), other cyclopoid copepods (11.42)
Only organisms contributing X3% are listed.
Mean biomass mg (C m37SD)
Most important contributors by biomass (%) Chaetognaths (56.5), calanoid copepods (16), siphonophores (7.9), euphausiid furcilia (4.9), appendicularians (3.6) Copila spp. (3.5)
Chaetognaths (48.4), siphonophores (15.2), calanoid coepods (14.0), Copila spp. (9.3)
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Table 2 Probabilities (P) based on permutational multivariate analysis of variance (PERMANOVA) testing for differences in the zooplankton assemblages between the eddies, between their centre and periphery (habitat), and between day and night (time), based on analysis of the abundance (individual m3) and biomass (mg C m3) data for groups comprising the assemblages
Fig. 2. (A) Abundance of mesozooplankton (mean individuals1 m37standard deviations) in centre (C), perimeter (P), day (D, open circles), and night (N, closed circles) in two eddies. (B) Biomass of mesozooplankton (mg C m37standard deviations) in centre (C), perimeter (P), day (D, open circles), and night (N, closed circles) in two eddies.
(4.0 mg C m3) than during the day (1.3 mg C m3) (t[7] ¼ 10.78, p ¼ 0.02), but there was no significant difference between the centre (2.2 mg C m3) and the perimeter (3.1 mg C m3). The biomass pattern was different in the warm-core eddy, with no significant difference between day (5.7 mg C m3) and night (4.7 mg C m3) but a significant difference between the centre (7.3 mg C m3) and the perimeter (3.2 mg C m3) (t[7] ¼ 6.43, p ¼ 0.04). The usual diurnal biomass pattern was reversed in the warmcore eddy centre, where biomass in the day was higher (Fig. 2(B)). Chaetognaths usually dominated the net zooplankton biomass, and cyclopoid and calanoid copepods, siphonophores, and euphausiid larvae were important at most stations.
Factor
P abundance
P biomass
Eddy Habitat Time Eddy habitat Eddy time Habitat time Eddy habitat time
0.00 0.16 0.00 0.09 0.14 0.28 0.58
0.00 0.35 0.00 0.02 0.05 0.01 0.14
Fig. 3. Results of nonparametric multidimensional scaling analysis based on zooplankton abundance showing associations of mesozooplankton assemblages between day (D) and night (N), centre (C) and perimeter (P) within the cold (C) and warm (W) core eddies with superimposed clusters from a dendogram at similarity levels of 55% (green line), 65% (blue line), and 75% (red line).
3.2. Community composition 3.2.1. Abundance Analysis of the abundance data indicated that the mesozooplankton assemblages were different in the two eddies (Table 2, Fig. 3). The difference was due to their location in multidimensional space only and not their dispersion (PERMANOVA, p ¼ 0.0001, PERMDISP, p ¼ 0.5). Appendicularians, ctenophores, calanoid copepodites, Clausocalanus spp. and Oithona plumifera were good discriminators of the eddies and occurred consistently in greater abundance in the warm core eddy. There was a shift in assemblage structure from day to night, and
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the day was more variable than the night assemblage (PERMDISP, p ¼ 0.03). Only 17 species groups out of 53 that contributed to the dissimilarity between day and night assemblages were more abundant during the day than at night. Calocalanus spp., appendicularia, euphausiid calyptopis were more abundant during the day, and Pleuromamma spp., Scolecithricella spp., Nannocalanus minor, Clausocalanus spp., Lucicutia spp., Ctenocalanus spp., Oithona spp., ostracod and euphausiid furcilia were consistently more abundant at night. All these taxa except Oithona spp. are vertical migrators. 3.2.2. Biomass The eddies were less clearly differentiated based upon analysis of the zooplankton biomass data (Fig. 4). PERMANOVA indicated that there were significant multivariate interactions of the factor eddy with both habitat and time (Table 2). Differences thus varied depending upon when and where sampling took place: whether at the centre or perimeter of the eddies (habitat) and whether during the day or night (time). There was also a significant interaction between the factors habitat and time (Table 2). Pairwise a posteriori comparisons revealed significant differences between the centres of the two eddies (PERMANOVA, p ¼ 0.05) but not between their perimeters, which may reflect the greater isolation of the eddy centres; the perimeters
were more affected by exchange with surrounding waters. The perimeter and centre were not significantly different within each eddy. Day assemblages were significantly different between the two eddies (PERMANOVA, p ¼ 0.03) but night assemblages were not (PERMANOVA, p ¼ 0.1). Day and night assemblages were significantly different within each eddy (PERMANOVA, p ¼ 0.05 and p ¼ 0.02). Several species of copepods contributed to the dissimilarity between the two eddies. Candacia spp. and Sapphirina spp. had a higher biomass in the cold-core eddy and Paraeucalanus sewelli, P. gracilis, Clausocalanus spp., Mesocalanus spp., Lucicutia spp., and Haloptilus longicornis had a higher biomass in the warm-core eddy. The copepods Pleuromamma spp., Sapphirina spp., N. minor, P. sewelli, Clausocalanus spp., Calanus spp., and Lucicutia spp. contributed to over two-thirds of the dissimilarity between day and night and all had greater biomass at night than during the day. 3.3. Size spectra The median biovolume of organisms in both eddies was between 64 and 128 mm3 (Fig. 5(A)). There were no significant effects of day and night within or between eddies in the size spectra, although the modal size shifted from 64 mm during the day to 128 mm at night in both eddies (Fig. 5(B)), due to a higher abundance of copepod nauplii and small copepods such as M. clausii, O. plumifera, Bestiolina spp. during the day and larger diel migrators at night, such as Pleuromamma spp. and Sapphirina spp. There were no significant differences in the size structure of the zooplankton assemblages between the centre and perimeter of the eddies (Fig. 5(C)). The slopes of the normalized spectra did not differ significantly between the assemblages within the warm and cold-core eddies (Fig. 6). 4. Discussion 4.1. Abundance, biomass and assemblage analyses
Fig. 4. Mesozooplankton associations based on non-parametric multidimensional scaling analysis of the zooplankton biomass data. Samples are shown to be from day (D) or night (N) and the centre (C) or perimeter (P) of the cold (C) and warm (W) core eddies, with superimposed clusters from a dendrogram at similarity levels of 40% (green line), 50% (blue line), and 65% (red line).
The mesozooplankton was more abundant and had higher biomass in the warm-core than in the cold-core eddy. This is contrary to the generally observed pattern of higher plankton abundance and biomass in cold-core eddies. However, this pattern reflects the origin of the water masses.
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Fig. 5. Zooplankton distribution across 11 volume classes (octave scale) of the zooplankton biovolume spectrum from (A) the warm-core eddy (WC) and the cold-core eddy (CC) (B) warm-core eddy day (WC D) and night (WC N), and the cold-core eddy day (CC D) and night (CC N), and (C) the warm-core eddy centre (WC C) and perimeter (WC P), and the cold-core eddy centre (CC C) and perimeter (CC P).
Fig. 6. Normalized biovolume size spectra and fitted straight line of the entire mesozooplankton community in the warm-core eddy (WC) and the cold-core eddy (CC).
The warm-core eddy was formed predominantly from Leeuwin Current water with about a 10% admixture of shelf waters (Feng et al., 2007), which are inherently more productive than the subtropical Indian Ocean water that comprised the mixed layer of the cold-core eddy (Tranter, 1962; Longhurst, 1998; Lourey et al., 2006). Although the deep waters of the cold-core eddy were influenced by the Leeuwin Undercurrent (Rennie et al., 2007; Meuleners et al., 2007), the upwelling within this eddy was insufficient to penetrate this lens of near-surface subtropical water and significantly influence the nutrient levels and primary productivity within the euphotic zone (Feng et al., 2007). Thus the initial concentrations and productivity of the phytoplankton and zooplankton were likely to have
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been substantially higher initially in the warm than the cold-core eddy, a condition quite different to that found off western boundary currents, such as the Gulf Stream, where cold-core eddies are often composed of relatively productive slope water (The Ring Group, 1981). Greenwood et al. (2007) estimate that 75% of the primary production within the warm-core eddy at the time of the cruise was based on its entrainment of initially more productive water. The biomass of zooplankton in the cold-core eddy (2.6 mg C m3) was remarkably close to that observed by Tranter and Kerr (1969) in offshore Indian Ocean waters at 30–321S, 1101E in October–November 1962: 0.6 mg C m3 at night and 4.3 mg C m3 in the day, for a mean of 2.5 mg C m3 (see their Table 1, with wet weight converted to carbon, assuming dry weight is 10% wet weight and carbon is 33% dry weight Wiebe et al., 1975). The abundance of zooplankton in the warm-core eddy (115 individuals m3) was also comparable to that observed in Leeuwin Current water at 300 and 1000 m depth in December 2002 off Two Rocks near Perth: 124 individuals m3 (Keesing et al., 2006). Such comparisons are crude but indicate that the abundance and biomass of the zooplankton in the cold- and warm-core eddies were comparable to those found in their parent water masses at a similar time of year. 4.2. Size spectra, community and trophic structure The differences between the assemblages were due to different proportions and abundances of the taxa rather than to differences in a few indicator species. Over 30 taxa were required to account numerically for 70% of the dissimilarity between two eddies. There were no dominant taxa; rather most taxa were present in both eddies but in different densities. However, the lack of clear differentiation between the composition of the eddies may be due to many taxa not having been identified to species. Some species of copepods were recorded from only one eddy but not enough is known about copepod biogeography in the region to determine whether they are indicative of Leeuwin Current or oceanic waters. Herbivorous/omnivorous taxa are commonly observed to dominate the trophic structure of zooplankton, with chaetognaths generally comprising 10–30% of the biomass of copepods (Steele, 1974; Bone et al., 1991). In contrast, both eddies
Fig. 7. Biomass of herbivores, omnivores and carnivores in the warm- and cold-core eddies. Trophic groups were classified according to Boltovskoy (1999), Boxshall and Halsey (2004), Bradford and Jillett (1980), Bradford et al. (1983), BradfordGrieve, 1994, 1999). Corycaeus, Oncaea and Oithona, as well as all ‘herbivorous’ copepods were classified as omnivores, since almost all copepods can feed on microzooplankton, metazoans, detritus and phytoplankton and switch depending on available food.
were strongly dominated (60–70%) by carnivorous zooplankton, predominantly chaetognaths (Fig. 7). Such a preponderance of carnivorous relative to herbivorous/omnivorous zooplankton is not sustainable but may have resulted from the reduced productivity of the eddies, leading to the grazing down of the herbivorous/omnivorous component of the plankton. Alternatively this apparently anomalous trophic structure may be, at least in part, a sampling artefact. Oligotrophic waters are generally dominated by small phytoplankton (o5 mm), as observed in these eddies (Waite et al., 2007), and by relatively small zooplankton (Gallienne and Robins, 1998). The herbivorous/omnivorous copepods may thus be relatively more underrepresented in samples obtained with coarse-meshed (4200 mm) nets in oligotrophic than in more productive waters, leading to the apparent dominance of the larger chaetognaths. Clark et al. (2001) observed a transition to similarly chaetognath-dominated zooplankton samples in passing from productive to oligotrophic portions of the North Atlantic, which they ascribed to sampling with a coarse-meshed net. However, the data seem to indicate an inherent ‘lumpiness’ in the Sheldon size spectrum for zooplankton in oligotrophic waters, with a gap in the biomass of intermediate-sized herbivore/omnivores and high relative abundance of carnivores, which was in fact observed in the original particle
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