Oithona similis, Claus, 1866

Fernandes, Veronica, Received, Nagappa Ramaiahb, Online, Published & Version, Final, 2019, Spatial structuring of zooplankton communities through partitioning of habitat and resources in the Bay of Bengal during spring intermonsoon, Turkish Journal of Zoology 43 (1), pp. 68-93 : 74-75

O. similis , and P. indica were present at all stations and depths sampled.

3.7. Copepod diversity measures

3.7.1. Alpha diversity (Shannon diversity H’, species richness d, and evenness J)

Alpha diversity for copepod species varied greatly with depth and between stations. Diversity in the central bay ranged from 1 to 5 and was generally higher in the mixed layer, especially at CB1 ( Figure 11 ). In the western bay H’ varied from 1.7 to 4.4 and a steady northward decline was noticed in the thermocline. The H’ values were the lowest in the 200–300 m strata in both transects with drastically low values in the north in the central bay.

Species richness ranged from 0.5 to 4.8 in the central and from 1.5 to 4.4 in the western bay. The maximum value of species richness was found in the mixed layer at CB1 and in the thermocline at WB1. In both transects, the ‘d’ was greatly diminished in the 200–300 m stratum, decreasing northwards, especially between CB3 and CB5.

3.7.2. Beta diversity

The relative variability of the copepod species assemblage within each of the strata sampled showed higher values of MVDISP in the 200–300 m stratum, indicating higher beta diversity ( Figure 12 ). Beta diversity was higher also in the central bay.

3.8. Copepod community structure

Multivariate analysis of data of all the 129 copepod species combined from all the stations and depths in the central and western bay distinguished through the 30% cut-off level of the Bray–Curtis similarity dendrogram yielded three clusters. Most of the thermocline (T) and OMZ (O) strata clustered into Group I, those from the mixed layer

(M) strata and a few thermocline (T) strata into Group II, and OMZ strata from three stations CB3, CB4, and CB5 as Group III ( Figure 13 ). The same was confirmed by two-dimensional ordination of the samples by NMDS (stress: 0.1; Figure 14), suggesting that the depths within each of the three groups shared similar copepod species communities. One-way ANOSIM testing further substantiated the observed significant differences between these groups of assemblages (global R: 0.826; P: 0.1%) in the study area.

Similarity percentage (SIMPER) analysis based on abundance estimates of all copepod species revealed the percent contribution of species to intragroup similarity (‘characterizing species’) as well as intergroup dissimilarity (‘discriminating species’). Table 4 summarizes information on (characterizing) species that contribute foremost to the average similarity together with their percent contribution to the total copepod abundance within each group. There was little variation in the intragroup similarities (36.47%– 82.27%).

Group I characterized 25 species with average similarity 49.6%, of which the top 8 most-contributing species are listed in Table 4. These are O. venusta , C. arcuicornis , O. similis , P. indica , L. flavicornis , C. speciosus , C. catus , and E. monachus , contributing over 50% of the average similarity of Group I. Similarly, 20 species from depths between 200 and 500 m characterized Group II, of which the top 7 species contributed to over two-thirds of the average similarity. The composition of assemblages in the subsurface Group II was not very different from the surface Group I. However, the abundances of species in Group II were low and the relative importance of O. venusta (17%), M. minor (15%), P. indica (11%), and O. similis (10%) was higher. Group III was characterized by just 3 species of copepods with an average similarity of 82.27%. Except for a few specimens of these species that were found in the three stations CB3, CB4, and CB5 at 200–300 m depth, no other copepod was found here. The reason for this is probably the intensely low oxygen concentration in this zone.

Table 5 shows the key copepod species that differentiate between the assemblages within MLD–thermocline (Group I) and thermocline– 500 m (Group II) strata, thus identifying them as ‘discriminating species’ in this study. As seen in the tables, the average dissimilarity between the surface and subsurface groups is smaller when compared with Group III. Though O. venusta is the predominant species at most depths and stations, M. minor , E. monachus , C. arcuicornis , C. catus , O. similis , Corycaeus speciosus , Acartia negligens , A. gracilis , O. venusta , Cosmocalanus darwinii , and Acrocalanus monachus are good discriminating species for being abundant in the mixed layer and having low to nil abundance in the strata below.

Copepods like Conaea gracilis , Heterostylites major , Heterorhabdus sp. , Gaussia princeps , and Haloptilus acutifrons are also good discriminators as they were found in deeper waters and never in the surface strata. The exclusive presence of the copepods Gaetanus kruppii , Gaussia princeps , and Pleuromamma xiphias in the 200– 300 m zone ( Figure 15 ), where DO concentration was 5–10 µM, is indicative of them as discriminators of deep water hypoxia.

Figures

Figure 11. Variation in Shannon diversity (H’), species richness (d), and evenness (J’) of copepods in different depth strata in the upper 500 m of the central (a) and western (b) Bay of Bengal.

Figure 12. Variation in multivariate dispersion (MVDISP) indices between different depth strata (9 stations data combined) and between the central and western transects in the Bay of Bengal.

Figure 13. Multivariate cluster analysis of the data of all 129 copepod species combined from all the stations and depths in the central and western bay using the 30% cut-off level of Bray–Curtis similarity. Cluster/Group I are assemblages mostly from the mixed layer (M) and thermocline (T) from central and western transects. Group II comprises assemblages found between the thermocline and 500 m and Group III includes only a few species found exclusively from 200–300 m depth at stations CB3–CB5.

Figure 15. Microphotographs of copepods Pleuromamma indica (A), P.xiphias (B), Gaetanus kruppii (C), and Gaussia princeps (D) from the 200–300 m stratum (oxygen minimum zone) between CB3 and CB5. The scale bar is in millimeters.