Epipelagic mesozooplankton communities in the northeastern Indian Ocean off Myanmar during the winter monsoon
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Abstract: The northern Andaman Sea off Myanmar is one of the relatively high productive regions in the Indian Ocean. The abundance, biomass and species composition of mesozooplankton and their relationships with environmental variables in the epipelagic zone (~200 m) were studied for the first time during the Sino-Myanmar joint cruise (February 2020). The mean abundance and biomass of mesozooplankton were (1916.7±1192.9) ind./m3 and (17.8±7.9) mg/m3, respectively. A total of 213 species (taxa) were identified from all samples. The omnivorous Cyclopoida Oncaea venusta and Oithona spp. were the top two dominant taxa. Three mesozooplankton communities were determined via cluster analysis: the open ocean in the Andaman Sea and the Bay of Bengal (Group A), the transition zone across the Preparis Channel (Group B), and nearshore water off the Ayeyarwady Delta and along the Tanintharyi Coast (Group C). Variation partitioning analysis revealed that the interaction of physical and biological factors explained 98.8% of mesozooplankton community spatial variation, and redundancy analysis revealed that column mean chlorophyll a concentration (CMCHLA) was the most important explanatory variable (43.1%). The abundance and biomass were significantly higher in Group C, the same as CMCHLA and column mean temperature (CMT) and in contrast to salinity, and CMT was the dominant factor. Significant taxon spatial variations were controlled by CMCHLA, salinity and temperature. This study suggested that mesozooplankton spatial variation was mainly regulated by physical processes through their effects on CMCHLA. The physical processes were simultaneously affected by heat loss differences, freshwater influx, eddies and depth.
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Key words:
- mesozooplankton /
- Myanmar /
- epipelagic zone /
- physical processes /
- water column mean chlorophyll a
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Figure 5. Column mean values of temperature (a), salinity (b), dissolved oxygen concentration (c), and chlorophyll a concentration (d) from 200 m (or the bottom) to the surface. CMT: column mean temperature; CMS: column mean salinity; CMDO: column mean dissolved oxygen; CMCHLA: column mean chlorophyll a.
Figure 6. Mesozooplankton community clusters according to taxonomic abundance datasets. Three groups (A: open ocean, B: transition zone, C: nearshore water) were divided at 50% similarity; Group B was divided into two subgroups (B-1 and B-2) at 60% similarity, while Group C was divided into three subgroups (C-1, C-2 and C-3) at 61% similarity. Green stars indicate Group A; light and dark blue triangles indicate Groups B-1 and B-2, respectively; yellow, orange and light orange circles indicate Groups C-1, C-2 and C-3, respectively.
Figure 8. Relationship between ten environmental variables and the mesozooplankton community by interactive-forward-selection RDA. Green stars indicate Group A; light and dark blue triangles indicate Groups B-1 and B-2, respectively; yellow, orange and light orange circles indicate Groups C-1, C-2 and C-3, respectively. MLD: mixed layer depth; SST: sea surface temperature; SSS: sea surface salinity; SDO: surface dissolved oxygen concentration; SCHLA: surface chlorophyll a concentration; CMT: column mean temperature; CMS: column mean salinity; CMDO: column mean dissolved oxygen concentration; CMCHLA: column mean chlorophyll a concentration.
Figure 9. Diagram of “Var-part-3groups-simple-effects-tested-FS” variation partition analysis. a, b, and c represent parts individually controlled by physical, chemical and biological factors, respectively; d represents the part controlled jointly by physical and chemical factors; e represents the part controlled jointly by chemical and biological factors; f represents the part controlled jointly by physical and biological factors; g represents the part controlled by physical, chemical and biological factors collectively. Values under the letters represent explained percentage. Physical factors include depth, mixed layer depth, sea surface temperature, sea surface salinity, column mean temperature, and column mean salinity; chemical factors include surface dissolved oxygen and column mean dissolved oxygen; and biological factors include surface chlorophyll a and column mean chlorophyll a.
Figure 10. Relationship between ten environmental variables and mesozooplankton taxa by interactive-forward-selection redundancy analysis. C. farrani: Clausocalanus farrani; E. concinna: Euchaeta concinna; C. dentata: Cypridina dentata; P. robusta: Pleuromamma robusta; C. plumulosus: Calocalanus plumulosus; M. phasma: Mormonilla phasma. MLD: mixed layer depth; SST: sea surface temperature; SSS: sea surface salinity; SDO: surface dissolved oxygen concentration; SCHLA: surface chlorophyll a concentration; CMT: column mean temperature; CMS: column mean salinity; CMDO: column mean dissolved oxygen concentration; CMCHLA: column mean chlorophyll a concentration.
Table 1. Spatial variations in mesozooplankton abundance (ind./m3) and dry biomass (mg/m3)
Group Subgroup Abundance in subgroup Dry biomass in subgroup Abundance in groups Dry biomass in groups A − − − 394.7±80.9b 4.0±0.4b B B-1 1519.8±268.7 16.3±4.7 1265.7±343.8b 15.2±4.7b B-2 1011.6±177.2 14.2±5.2 C C-1 4332.7±171.6 26.3±6.2 2958.5±1030.3a 24.8±3.7a C-2 2116.0±298.0 23.6±3.3 C-3 2848.1±481.1 25.0±3.9 Note: Different lowercase letters (a, b) in the same index indicate significant differences among groups (p<0.05). − represents no data. Table 2. Spatial variations in the relative abundances (%) of the mesozooplankton taxa
Taxonomic groups Order Differences in Order Differences in Taxonomic groups Group A Group B Group C Group A Group B Group C Subclass Copepoda Calanoida 38.2±0.6 37.3±8.1 44.7±4.7 81.4±7.6 79.9±5.0 85.9±5.7 Cyclopoida 40.2±5.9 39.0±6.5 40.8±8.0 Harpacticoida 0.4±0.1 0.5±0.3 0.3±0.3 >Mormonilloida 2.6±1.2ab 3.2±2.2a 0.1±0.3b Subclass Malacostraca Amphipoda 0.1±0.1 0.2±0.2 0.0±0.1 0.3±0.4 0.6±0.4 0.2±0.3 Mysida 0.0±0.0 0.0±0.0 0.0±0.0 Euphausiacea 0.2±0.3 0.4±0.3 0.1±0.1 Decapoda 0.0±0.0 0.0±0.0 0.0±0.1 Cumacea 0.0±0.0 0.0±0.0 0.0±0.0 Subclass Phyllopoda Cladocera − − − 0.2±0.3 0.1±0.2 0.0±0.0 Subclass Ostracoda − − − − 2.1±0.6ab 3.6±2.6a 0.9±0.6b Class Appendicularia − − − − 8.7±4.7 8.9±3.7 7.6±5.2 Class Thaliacea − − − − 0.5±0.0ab 1.0±0.7a 0.1±0.2b Class Polychaeta − − − − 0.3±0.0 0.2±0.3 0.2±0.3 Phylum Chaetognatha − − − − 1.7±0.8ab 1.9±0.7a 0.8±0.6b Phylum Cnidaria − − − − 0.4±0.1 1.2±0.6 0.4±0.6 Phylum Ctenophora − − − − 0.0±0.0 0.0±0.0 0.0±0.0 Phylum Mollusca − − − − 0.8±0.1 0.3±0.2 0.6±0.7 Phylum Protozoa − − − − 0.5±0.5 0.2±0.2 0.4±0.3 Note: Different lowercase letters (a, b) in the same index indicate significant differences among groups (p<0.05). − represents these taxonomic groups are not subdivided by order. Table 3. Spatial variations in the relative abundances (%) of the dominant species
Taxa Dominant species Group A Group B Group C Copepoda Oncaea venusta 23.6±4.2 19.9±6.5 22.9±6.2 Copepoda Oithona spp. 10.6±1.2 12.0±3.0 13.9±5.0 Copepoda Paracalanus aculeatus 10.8±2.6 7.8±3.0 13.8±6.9 Appendicularia Oikopleura spp. 7.7±4.5 8.4±3.5 7.5±5.2 Copepoda Clausocalanus farrani 6.0±0.3ab 3.0±1.6b 6.9±3.0a Copepoda Subeucalanus subtenuis 0.7±0.6 2.3±2.0 3.9±2.9 Copepoda Clausocalanus furcatus 2.9±1.7 5.2±2.5 2.8±2.3 Copepoda Euchaeta larva 1.7±0.0 1.6±0.7 2.7±2.4 Copepoda Clausocalanus/Paracalanus larva 1.4±0.4 2.5±2.0 2.6±2.6 Copepoda Neocalanus larva 2.8±1.0 2.3±1.4 2.5±1.4 Copepoda Acrocalanus gibber 0.9±0.3 1.8±1.0 1.9±1.2 Copepoda Farranula gibbula 2.8±0.1 1.7±1.2 1.4±0.9 Copepoda Triconia conifera 0.7±0.6 2.2±1.3 1.0±0.6 Copepoda Euchaeta concinna 0.7±0.6ab 0.2±0.2b 1.0±0.9a Copepoda Canthocalanus pauper 0.5±0.2 1.2±0.7 1.0±0.8 Copepoda Mormonilla phasma 2.6±1.2ab 3.2±2.2a 0.1±0.3b Ostracoda Cypridina dentata 0.1±0.2ab 1.7±1.9a 0.2±0.3b Copepoda Temora turbinate 0.3±0.4 1.5±1.8 0.6±1.6 Copepoda Scolecithricella nicobarica 0.7±0.6 1.3±0.8 0.7±0.5 Ostracoda Euconchoecia aculeata 0.9±0.1 1.0±0.7 0.5±0.5 Copepoda Corycaeidae larva 0.2±0.2 1.0±0.7 0.5±0.7 Copepoda Calocalanus plumulosus 1.5±0.9a 0.3±0.4ab 0.1±0.2b Planktonic larva Copepoda nauplius larva 1.3±0.6 0.4±0.4 0.9±0.6 Copepoda Pleuromamma robusta 1.0±1.0a 0.3±0.1b 0.1±0.1b Appendicularia Fritillaridae spp. 1.0±0.2 0.5±0.5 0.1±0.1 Chaetognatha Serratosagitta pacifica 1.0±0.6 0.4±0.3 0.5±0.5 Note: Dominant species were defined as having a relative abundance of greater than 1% in at least one group. Different lowercase letters (a, b) in the same index indicate significant differences among groups (p<0.05). Table 4. Spatial variations in the environmental variables by the Kruskal–Wallis test
Environmental variables Group A Group B Group C Depth/m 2468.5±53.0a 1473.6±920.3a 89.1±35.0b MLD/m 32.5±9.2 28.4±14.9 14.4±6.3 SST/℃ 27.0±1.5 26.3±0.9 27.6±0.8 SSS 32.1±0.1 32.2±0.5 31.7±0.6 SDO concentration/(mg·L−1) 6.4±0.1 6.5±0.1 6.3±0.1 SCHLA concentration/(µg·L−1) 0.3±0.0 0.4±0.1 0.6±0.3 CMT/℃ 21.0±0.9ab 20.3±0.6b 23.6±1.4a CMS 34.0±0.1a 34.1±0.2a 33.6±0.3b CMDO concentration/(mg·L−1) 2.6±0.2 2.2±0.4 3.0±0.8 CMCHLA concentration/(µg·L−1) 0.3±0.1ab 0.3±0.0b 0.7±0.2a Micro SCHLA/% 7.1 8.5 20.8±8.0 Nano SCHLA/% 15.9 13.7 19.6±5.5 Pico SCHLA/% 77.0 77.7 59.6±11.6 Note: Different lowercase letters (a, b) in the same index indicate significant differences between groups (p<0.05). Size-fractionated chlorophyll a concentrations were measured only at representative stations (M7 in Group A; M9 in Group B; M1, M2, M13, M14 in Group C). A Kruskal–Wallis test was not performed for size-fractionated Chl a. MLD: mixed layer depth; SST: sea surface temperature; SSS: sea surface salinity; SDO: surface dissolved oxygen; SCHLA: surface chlorophyll a; CMT: column mean temperature; CMS: column mean salinity; CMDO: column mean dissolved oxygen; CMCHLA: column mean chlorophyll a. Micro SCHLA, Nano SCHLA and Pico SCHLA represent proportions of micro- (>20 µm), nano- (2−20 µm), and pico- (0.7−2 µm) surface chlorophyll a concentration to total surface chlorophyll a concentration, respectively. Table 5. Results of interactive-forward-selection redundancy analyses
Environmental variables Figure 8 Figure 10 Explains/% pseudo-F p p(adj) Explain/% pseudo-F p p(adj) CMCHLA 43.1 11.4 0.006 0.06 30.0 6.4 0.002 0.020 CMDO 12.7 4.0 0.030 0.3 5.2 1.4 0.232 1 CMT 12.6 5.2 0.008 0.08 6.6 1.8 0.148 1 CMS 6.6 3.2 0.030 0.3 10.5 2.5 0.018 0.162 SCHLA 2.6 1.3 0.260 1 5.6 1.6 0.158 1 SDO 2.4 1.2 0.282 1 1.5 0.4 0.822 1 SST 3.6 2.0 0.144 1 8.6 2.2 0.038 0.304 Depth 1.0 0.5 0.662 1 2.2 0.6 0.778 1 SSS 1.2 0.6 0.610 1 3.3 0.9 0.490 1 MLD 0.2 <0.1 0.980 1 5.1 1.6 0.186 1 Note: MLD: mixed layer depth; SST: sea surface temperature; SSS: sea surface salinity; SDO: surface dissolved oxygen concentration; SCHLA: surface chlorophyll a concentration. CMT: column mean temperature; CMS: column mean salinity; CMDO: column mean dissolved oxygen concentration; CMCHLA: column mean chlorophyll a concentration; adj: adjusted. Table 6. Species number, abundance, and dry biomass of zooplankton in Indo-Pacific waters
Research area Layer Species number Mesh size Sampling time Abundance/(ind.·m−3) Dry biomass Reference Northern Andaman Sea off Myanmar 0−200 m (or bottom) 213 200 μm February 1916.7
(337.5−4454.0)17.8 mg/m3 (3.7−30.7 mg/m3)
6.1 mg/m3 (in terms of C);
845.7 mg/m2 (in terms of C)this study surface layer − 200 μm Spring 106.1−945.0 4.4−38.6 mg/m3 Jyothibabu et al. (2014) Other Indian waters off North coastal Andhra Pradesh surface layer 112 200 μm January, April, May, November 4473.0 27.8 mg/m3 Rakhesh et al. (2006) off Rushikulya Estuary 0−bottom (<200 m) 93 120 μm January−June 340.0−6550.0 − Mohanty et al. (2010) Kodiakkarai coastal waters surface layer 121 158 μm twelve months − − Damotharan et al. (2010) Open water in Pacific, BOB, and Arabian Sea western boundary currents in the subtropical North Pacific 0−200 m − 160 μm Winter 206.6 (35.1−456.8) − Dai et al. (2016) western tropical
Pacific Ocean0−200 m 259 200 μm Summer 146.7 4.9 mg/m3 Yang et al. (2017) southern BOB 0−200 m 187 505 μm Spring 33.4 − Li et al. (2017) western BOB 0−bottom of thermocline − 200 μm Winter − 777.0 mg/m2 (in terms of C) Jyothibabu et al. (2008) southwestern BOB influenced by a cyclonic eddy mixed layer − 200 μm Winter 760.9 (337.5−4454.0) 35.9 mg/m3 (in terms of C) Jayalakshmi et al. (2015) northern side of cyclonic eddy in central BOB mixed layer − 200 μm Winter 277.0 20.0 mg/m3 (in terms of C) Sabu et al. (2015) western Arabian Sea 0–150 m − 333 μm February 289.9 14.7 mg/m3 (in terms of C) Koppelmann et al. (2003) central Arabian Sea 0–150 m − 333 μm February 371.3 13.1 mg/m3 (in terms of C) Koppelmann et al. (2003) BOB western BOB 0−bottom of thermocline − 200 μm Winter − 777±433 mg/m2 (in terms of C) Jyothibabu et al. (2008) Spring − 223±236 mg/m2 (in terms of C) Jyothibabu et al. (2008) southwestern BOB Summer − 628±499 mg/m2 (in terms of C) Jyothibabu et al. (2008) Winter 70−4288 8.9−35.9 mg/m3 (in terms of C) Jayalakshmi et al. (2015) western BOB mixed layer − 200 μm Spring 2−5340 1.7−162.6 mg/m3 (in terms of C) Fernandes and Ramaiah (2019) Summer 25−4621 1.3–31.0 mg/m3 (in terms of C) Fernandes and Ramaiah (2009) Autumn 100−2482 2.4−53.6 mg/m3 (in terms of C) Fernandes and Ramaiah (2013) Note: Conversion factors for deriving zooplankton carbon biomass from the displacement volume of zooplankton used were as follows: (1) 1 mL zooplankton = 75 mg dry weight; (2) 1 mg dry weight zooplankton = 0.342 mg carbon of zooplankton (Fernandes and Ramaiah, 2009). − represents no data. -
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