Inter-annual Variations In Summer Zooplankton Community Structure And Feeding Ecology Of Dominant Species In Prydz Bay, Antarctica

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330μm mesh, 0.5 m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than in either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill, E. crystallorophias, and large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.Population dynamics of four dominant Antarctic copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas), three euphausiid larvae (Euphausia superba, Thysanoessa macrura and Euphausia crystallorophias) and Salpa thompsoni were studied based on zooplankton samples collected in 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 dsicrete from the other three species, whose distribution in the whole survey area overlapped. In December the 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 didn't exist in the neritic region, the copepods abundance was lower which indicated 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 positively correlation with temperature and chlorophyll a concentrations, 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. Euphausiid larvae in Prydz Bay were mainly composed of E. superba, T. macrura and E crystallorophias. Larvae of T. macrura were distributed wide and relatively abundant. Larvae of E. crystallorophias were mainly neritic distributed. Salps, mostly Salpa thompsoni, were only distributed in separate station and the abundance was very low. In years with earlier and larger scale ice retreat, the abundance of T. macrura and E. superba were higher and developmental stages were older. The frequency and abundance of S. thompsoni also increased. We confirmed that the polynyas had a great contribution to phytoplankton blooms which promote reproduction and recruitment success of copepods and E.crystallorophias. The study suggested that population dynamics of euphausiids larvae and salps have good correspondence with sea ice and polynya variations during all cruises of Prydz Bay.Feeding of dominant zooplankton species in Prydz Bay showed distinct spatio-temporal variations. In late spring, C. acutus was inactive and the ingestion rate was only 1.30μgC female^-1 day^-1. In early summer, phytoplankton bloom dominated by Thalassiosira spp. and Fragilaria spp. happened and the biomass of microplankton reached to 247.67μgC/L. Daily ingestion rates of E. crystallorophias, C. acutus, M. gerlachei and Ctenocalanus citer were 129.69μgC/ind, 8.06μgC/ind, 11.41μgC/ind and 3.54μgC/ind, respectively. Biomass of microplankton in stations of the shelf slope and oceanic regions was lower ( < 20μgC/L) and the zooplankton cultured predated on ciliates preferentially. In late summer, microplankton of shelf and neritic regions was lower than 10μgC/L and mainly composed of Pseudo-nitzschia spp. and ciliates. C. acutus cultured in the neritic region predated on ciliates preferentially and zooplankton in the shelf slope stations mainly grazed on Pseudo-nitzschia spp.Fatty acids of dominant zooplankton species in Prydz Bay were analysed using multivariate analysis based on samples collected in Prydz Bay during the 26th Chinese National Antarctic Research Expedition. Zooplankton fatty acids showed dominant inter-species differences. The major fatty acids of C. acutus were EPA, 20:1w9, DHA and 16:1w7. Fatty acids of R. gigas were mainly composed of EPA, 16:1w7, 18:1w9 and DHA. E. crystallorophias had high levels of 18:1w9, EPA, 16:0 and DHA, while 16:0, EPA, 14:0 and DHA were higher in E. superba. Fatty acids of C. propinquus and M. gerlachei overlapped and they both had higher levels of DHA, EPA and 16:0. Significant intra-species and spatio-temporal variations existed in fatty acids composition of E. superba and C. acutus. Meanwhile, special fatty acids composition had good accordance with feeding and trophic levels of dominant zooplankton species. Different fatty acids biochemical pathways indicated that zooplankton species of the Prydz Bay utilize diffetent strategy adapting to the poor environmental conditions of the Southern Ocean.Previous research mostly focused on zooplankton community structure and population distribution of the dominant species—E. superba. We discussed the inter-annual dynamics of community and population structure, and besides, the relationship with environmental factors. Meanwhile, we did some elementary research about the feeding activity and fatty acids composition of dominant species in Prydz Bay, Antarctica, which provided data about zooplankton lifestyles living in poor polar environments. Future work would focus on response of polar marine ecosystems to global change through analyzing long-term community dynamics. Besides, lifestyle strategies would also be researched from the physiological and biochemical points of view.