M. Sudhakar

Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf

Observations along 10 shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June–July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone, is comparable to that during the preceding spring intermonsoon (SIM). Again, as usual, the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/saturations of 2012 on the midshelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net autotrophic (gross primary production to community respiration ratio, GPP/R:1.11 ± 0.84) as organic production exceeded consumption.

Hydrobiological responses of the North Eastern Arabian Sea during late winter and early spring inter-monsoons and the repercussions on open ocean blooms

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 × 10 4 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.

An updated checklist of echinoderms of the southeastern Arabian Sea

The southeastern Arabian Sea (SEAS) is a distinct ecosystem within the northern Indian Ocean, with unique physical, chemical and biological attributes. This paper provides comprehensive documentation on echinoderm diversity in the SEAS at depths of 20 to 1500 m, based on systematic surveys of over 300 sites. A total of 76 species were recorded in the surveys, which included one new species, seven new records for Indian waters and 39 species previously known from nearby areas and recorded for the first time from SEAS. With the addition of the new records from the present study to the 133 species previously reported from the SEAS, the species richness of echinoderms in the SEAS stands at 180 species. Updated checklists of echinoderms in the SEAS are provided.

Occurrence of Marbled Shrimp Saron marmoratus (Olivier, 1811) (Decapoda: Caridea: Hippolytidae) in Lakshadweep Archipelago, India

Two juvenile specimens of Saron marmoratus (Olivier, 1811) were collected from the inter tidal lagoon area of Eastern side of Agatti Island, Lakshadweep on November 4, 2013. These shrimp are brown and slightly green in color with yellow and white speckled spot and having a typical tuft of cirri on the back. Marbled shrimps are highly demanded and good priced in the marine aquarium trade. This is a new record to the Lakshadweep waters.