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Study of Dental Fluorosis in Subjects Related to a Phosphatic Fertilizer Indian Journal of Geo Marine Sciences Vol. 46 (07), July 2017, pp. 1371-1380 Diversity and abundance of epipelagic larvaceans and calanoid copepods in the eastern equatorial Indian Ocean during the spring inter-monsoon Kaizhi Li, Jianqiang Yin*, Yehui Tan, Liangmin Huang & Gang Li Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China *[E-mail: [email protected]] Received 20 August 2015; revised 29 September 2015 This study investigated the species composition, distribution and abundance of larvaceans and calanoid copepods in the eastern equatorial Indian Ocean. In total, 25 species of larvaceans and 69 species of calanoid copepods were identified in the study area. Although the average diversity and evenness indexes of larvaceans were lower than those of calanoid copepods, the abundance of larvaceans was higher than that of calanoids with means of 40.1±14.9 ind m-3 and 28.4±9.1 ind m-3, respectively. Larvacean community was numerically dominated by Oikopleura fusiformis, Oikopleura longicauda, Oikopleura cophocerca, Fritillaria formica and Fritillaria pellucida, accounting for 83% of total larvacean abundance. The calanoid community was dominated by the following five species, represented 61% of calanoid copepods: Clausocalanus furcatus, Clausocalanus farrani, Acartia negligens, Acrocalanus longicornis as well as the copepodite stage of Euchaeta spp. This study highlights that the importance of larvaceans in the eastern equatorial Indian Ocean. [Keywords: Appendicularians, Calanoids, Water mass, Monsoon, Indian Ocean] Introduction It has been clear that small organisms of marine Clausocalanus) and the cyclopoid genera (such as zooplankton have historically been under-sampled by Oithona, Oncaea and Corycaeus1). Most calanoid coarse-mesh nets1,2. Larvaceans and copepods are copepods are considered as ‘herbivorous’ zooplankton important constituents of the mesozooplankton, and with a preference for larger particles. Small calanoid play pivotal roles from the microbial ecosystem to copepods often form a dominant zooplankton group in higher trophic levels in the tropical oligotrophic tropical pelagic environments11. They have a great waters. Larvaceans (or Appendicularians), an influence on the efficiency of trophic coupling important group of zooplankton, appear to show little between the primary producers and other larger taxonomic diversity with only 65 species currently pelagic carnivores1,12. Although the important roles of described due to their general frailty2. In recent years, larvaceans and copepods in marine food webs are it has become obvious that the remarkable filtration well studied, little is known about their diversity, systems and rapid population growth of larvaceans abundance or distribution, especially in tropical may be contributing to their population sizes equaling oligotrophic waters. or exceeding those of copepods3-8. Larvaceans play a Monsoons play a vital role in determining the pivotal role, from the microbial ecosystem to upper various physical and chemical features of the northern trophic levels, and their elaborate ‘houses’ enable Indian Ocean. The circulation of the upper layers them to be efficient filter feeders of picoplankton and undergoes drastic seasonal variations, with the nanoplankton9,10. southwest monsoon current occurring during May- Copepods, with a length < 1 mm, are the most September, and the northeastern monsoon current abundant metazoans on Earth, including adults and during December-February13. Another important copepodites of the calanoid genera (such as current is the Wyrtki Jet, located between 2°S and Paracalanus, Pseudocalanus, Acartia, and 2°N in the equatorial Indian Ocean. The Wyrtki Jet is 1372 INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017 an intensive and narrow eastward surface current that (unit: m3). The filtered water volume ranged from occurs twice a year during the monsoon transition 38.70 to 79.98 m3. Trawl winch speed was about 0.5– periods in spring (April-May) and fall (October- 1m/s. The samples were preserved in a 5% formalin- November)14. The Wyrtki jet and the equatorial seawater solution for later analysis. A subsample, undercurrent may advect the Arabian Sea’s salty (10% of the sample), was counted under a water eastward along the equator and thereby change stereomicroscope?after fraction with a stempel- the upper ocean salt and nutrient levels in the eastern pipette. All specimens for larvaceans and copepods Indian Ocean13-15. For example, two distinct water were identified to species level when possible, based masses have been identified below the subsurface on the status of taxonomic information currently waters located north and south of the equator, with the available19-24. first consisting of low-salinity waters from the Bay of Bengal and the second consisting of high-salinity and nitrate deficient waters from the Arabian Sea16. The distribution and abundance patterns of mesozooplankton of the Bay of Bengal and the Arabian Sea have been described, with the former being closely influenced by fresh water inflow and cold eddies17, and the latter being influenced by seasons, upwelling, and oxygen concentrations18. However, little information is available on the mesozooplankton from the eastern equatorial Indian Ocean, especially for larvaceans and copepods, which are important intermediates between the classical and microbial food webs. Fig. 1—The location of sampling stations in the eastern equatorial Therefore, this study investigated the species Indian Ocean. The triangles represent the location of stations on the latitude (5°N-5°S) scale, and circles represent the location of composition and abundance of larvaceans and stations on the longitude (80°-95°E) scale. calanoid copepods in the eastern equatorial Indian Ocean, along with the environmental conditions Vertical profiles of temperature and salinity were influencing their distribution. measured using a CTD (SBE911 Plus, USA) at each station, as described in detail by Xuan et al.15. To Materials and Methods determine chlorophyll a (Chl a) concentration, Sampling and laboratory procedures seawater from 7 depths (0 m, 25 m, 50 m, 75 m, 100 The survey region was located between 5°N and m, 150 m, and 200 m) at each station was filtered 5°S along the longitude from 80° to 95°E, and through a 0.70 μm cellulose Whatman GF/F filter. For included 26 sampling stations (Fig. 1). The cruise was determination of surface picophytoplankton cell supported by the National Science Foundation of fractions, prefiltered samples (3 μm pore-size China, and conducted by the Shiyan 1 scientific polycarbonate filter) were filtered. Detailed research vessel at the South China Sea Institute of information on collection and measurement of Chl a Oceanology.Stations I1-I5 were located around samples has previously been documented by Li et Sumatra Island, 16 stations (I6-I21) lay along the al.25. equator, and the remaining 5 stations (I22-I26) were located between the region south of Sri Lanka and the Data analysis equator. Sampling stations were located at 1° Species richness (S) was calculated as the number latitudinal intervals between 5°S and 5°N as well as of taxa observed in a given sample. The Shannon- 1°longitudinal intervals between 80° and 95°E , Wiener diversity index (H′) and Pielou’s index of except for I1-I5. evenness (J′) were calculated in accordance with Zooplankton were collected at each station once s with a planktonic net (50 cm mouth diameter, 160 μm Ma26: H PlnP , J H , where p is the i i lnS i mesh opening) towed vertically from 200 m depth to i1 the surface during 12–24 April 2011. The net mouth proportion of individuals from a sample unit was equipped with a flowmeter (Hydro-Bios) to belonging to species i. The three biodiversity determine the volume of filtered water in each tow parameters (S, H′ and J′) were assessed for spatial LI et al.: LARVACEANS AND CALANOIDS IN THE EASTERN INDIAN OCEAN 1373 differences for both larvacean and calanoid copepod Species diversity species. The Shapiro-Wilk test was used to examine A total of 25 species of larvaceans were identified differences in physical and biological parameters with belonging to two families and five genera (Table 1). respect to the latitude (combined I1-I5 and I22-I26) and longitude stations (I6-I21), using a significance level of P<0.05. Pearson’s correlation analysis was used to determine which environmental factors (surface sea temperature, salinity, and Chl a) may have been influencing larvacean and calanoid copepod diversity indexes and abundance (after being log-transformed). The distribution of species and stations in relation to physical and biological factors was explored by canonical correspondence analysis (CCA). Results Environmental conditions Results on hydrography and Chl a concentration previously published in Xuan et al.15 and Li et al.25 have been briefly redescribed here. Warm water with Fig.2—Variation in temperature (°C) (a-b), salinity (PPT) (c-d), a temperature higher than 28°C was present in the and chlorophyll a concentration (mg m-3) (e-f) at surface (0 m) upper 50 m layer (Figs. 2a-b), and a strong vertical and 50 m, 100 m, 150 m, and 200 m depth. temperature gradient was located at depths of 50-150 m. The temperature at the 100 m layer decreased Three other taxa (Oikopleura spp., Fritillaria spp. sharply from 2°N to 3°S (Fig. 2a), and from 80° to and Euchaeta spp.) were also identified as genera 95°E (Fig. 2b). The salinity distribution was much because of their damaged or immature morphological more complex than that of temperature due to the characteristics. Fritillaria formica comprised two influences of different water masses. Low-salinity subspecies: Fritillaria formica digitata, and surface water was observed south of Sri Lanka (I22- Fritillaria formica tuberculata, with the former being I26) and off Sumatra Island (I1-I10), and extended more abundant than the latter. Similarly, Fritillaria southward from the Bay of Bengal (Figs. 2c-d). pellucida consisted of F.
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