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During the austral summer 2004, an intensive multidisciplinary survey was carried out in the Indian Ocean sector of the Southern Ocean to study the main hydrographic features and the associated productivity processes. This sector includes circumpolar zones and fronts with distinct hydrographic and trophic regimes, such as the Subtropical Zone (STZ), Subtropical Frontal Zone (STFZ), Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), North Subtropical Front (NSTF), Agulhas Retroflection Front (ARF), South Subtropical Front (SSTF), Subantarctic Front, Surface Polar Front (SPF), and Subsurface Polar Front. Seasonal variations in the solar irradiance and day length, stratification, lack of micronutrients like iron and increased grazing pressure are the major factors that influenced or constrained biological production in this region. Even though broad differences in these controlling factors exist in time and space between the zonal regions, the upper 1000 m of the water column of the main zones, STZ, STFZ, SAZ, PFZ, supported almost identical standing stocks of mesozooplankton, 0.43, 0.47, 0.45 and 0.49 ml m -3 , respectively, during the austral summer. This unexpected similarity can be explained either through the functioning of the microbial loop within STZ, STFZ and SAZ and the multivorous food web ecology within the PFZ. Dominance of ciliates in the microzooplankton community may be one factor resulting in the maintenance of a high mesozooplankton standing stock in SAZ. In contrast to the zones, frontal regions showed wide differences in hydrography and biological characteristics. The SSTF and SPF were far more biologically productive than that of NSTF and ARF. KEY WORDS: Fronts · Zones · Microzooplankton · Mesozooplankton · Microbial loop · Southern OceanResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 389: 97-116, 2009 et al. (1991) reported that seabirds and mammals may transfer 20 to 25% of photosynthetically fixed carbon into the atmosphere via respiration after consuming macrozooplankton and micronekton. Therefore, zooplankton occupies an important position in the Southern Ocean carbon cycle.The major current system in the Indian Ocean sector of the Southern Ocean is the Antarctic Circumpolar Current (ACC). The principal mechanism determining the different properties within the zones and along the frontal systems is the wind-induced transport (from the westerlies) that drives the uninterrupted eastward flowing ACC. Deacon (1933Deacon ( , 1937 was the first to outline the frontal systems in the Southern Ocean and suggested that they are circumpolar and other studies followed (Lutjeharms 1981, Deacon 1982, Belkin & Gordon 1996, Holliday & Read 1998, Froneman et al. 2000, Pollard & Read 2001. Along the 45°E longitude the main features are (from north to south), the existence of the North Subtropical Front (NSTF), Agulhas Retroflection Front (ARF), South Subtropical Front (SSTF), Subantarctic Front (SAF) and Polar Front (PF); the PF has both Su...

Winter cooling and persistent mixing for more than a quarter of year (November to early March) along the North Eastern Arabian Sea (NEAS) results in nutrient enrichment of the euphotic column thereby triggering biological production. Hydrographic characteristics of NEAS during Late Winter Monsoon (LWM) and Early Spring Inter Monsoon (ESIM) and the influence on biological production are overviewed here. Winter convective mixing signatures were evident during LWM with low SST (24°C), high SSS (36.4), deep mixed layers (>100 m) and increased surface nitrate (~1 µM). Open ocean waters observed high chlorophyll a (1–2 mg m−3) and microphytoplankton abundance (1.2–1.5 × 104 cells l−1). Diatoms and green Noctiluca scintillans were the major microphytoplankton identified. ESIM observed gradual stabilization of water column with curtailment of winter signatures and strengthening of Noctiluca scintillans blooms. Mesozooplankton biomass was higher during LWM and decreased towards ESIM with intensification of Noctiluca blooms. However during ESIM, abundance of gelatinous zooplankton occurred in the bloom region. Inter-annual variations were observed in the biological responses along with the hydrographic changes. Thus the convective process during winter monsoon and stabilization of the water column during ESIM plays a significant role in the production pattern of NEAS.

The present communication reports on the occurrence of a multi-species diatom bloom in the upwelled waters along the southwest coast of India. During the late summer monsoon season (September 2009) a multi-species diatom bloom with a pale green discoloration of the sea surface was observed in the coastal waters of southwest coast of India. The bloom spread over an area of approximately 15 km2 along the coastal waters off Kannur (Lat. 11°59.471 N, Long. 75°03.446 E). Total diatom cell density of the bloom area was 16 × 104 cells l−1. Proboscia (=Rhizosolenia) alata (Brightwell) Sandstrom constituted 90% of the total phytoplankton population. Other phytoplankton groups that contributed to the bloom population included Chaetoceros spp., Pseudo-nitzschia spp., Rhizosolenia spp., Coscinodiscus sp., Leptocylindrus danicus, Thalassiosira sp., and Bacteriosira sp. Among these Pseudo-nitzschia multiseries, a toxic species with the ability to produce potent neurotoxin domoic acid, was observed with a cell density of 4 × 103 cells l−1. Surface chlorophyll a concentration of the bloom region was 14.1 μg l−1. Nutrient concentrations of the bloom area were 0.01 μmol l−1 for NO2-N, 0.1 μmol l−1 for NO3-N, 0.83 μmol l−1 for PO4-P and 11.44 μmol l−1 for SiO4.

Characteristics of a cold-core eddy and its influence on the mesozooplankton community were studied along the central (87° E) Bay of Bengal during winter monsoon (November 2008) based on in situ data. The thermo-haline structure and the satellite-derived sea level anomaly maps showed the presence of a cyclonic eddy between 16° N and 20° N. The nutrient enhancement due to the eddy pumping in the euphotic column (∼ 50 m) had resulted in high chlorophyll a concentration, a factor of 8 times higher than that outside the eddy, which led to higher mesozooplankton biovolume (0.35 ± 0.36 ml m(-3)) and abundance (276 ± 184 ind m(-3)). The northern cyclonic eddy (NCE) seems to exist for approximately 6 months between July and January. During summer, the NCE is forced by local wind stress curl and the resultant Ekman pumping, whereas during fall and early phase of the winter, it is sustained by westward propagating semi-annual Rossby waves. The longer existence of NCE in the study region, which originated 6 months prior to the present observation, provides a favourable environment for the mesozooplankton community to grow and reproduce, resulting in noticeable increase in the biovolume. Hence, the persistent and longer existence of NCE significantly influences the biological production of the generally oligotrophic BoB, making it locally biologically 'active'.