P. Sabu

Bio: P. Sabu is an academic researcher from National Centre for Antarctic and Ocean Research. The author has contributed to research in topics: Phytoplankton & Monsoon. The author has an hindex of 12, co-authored 22 publications receiving 510 citations. Previous affiliations of P. Sabu include National Institute of Oceanography, India & Council of Scientific and Industrial Research.

Intrusion of the Bay of Bengal water into the Arabian Sea during winter monsoon and associated chemical and biological response

Abstract: This work was supported by the Department of Ocean development under the program Marine Research-Living Resources (MR-LR). Jayu Narvekar acknowledges the financial support from CSIR.

Variability in biological responses influenced by upwelling events in the Eastern Arabian Sea

Abstract: This study documents the result of field observations from the southeastern Arabian Sea during onset, peak and late phases of summer monsoon. The pronounced variations in phytoplankton standing stock (chlorophyll a), primary production, meso zooplankton standing stock and physico-chemical variables were examined according to the different stages of upwelling i.e., newly upwelled (Type-1), mature (Type-2) and aged upwelling (Type-3). Type-1 stage was observed along the inshore waters off 8–10°N during the onset of summer monsoon and along north of 13°N during peak and late summer monsoon. This stage was characterized by high wind stress (∼ 0.72 dynes/cm2) and upsloping of isotherms and isolines of nitrate towards the coast. Type-2 stage was identified along the inshore regions of 8–11.5°N during peak summer monsoon with relatively stable and nutrient rich (> 2 µM NO3) cold (> 27.7°N) surface waters. Type-3 stage was observed along 8°N transect during the late summer monsoon, which was characterized by relaxed wind stress. Upwelling signatures were not prominent along this transect. Relatively higher biological production was observed during Type-2 stage. Distribution of meso zooplankton biomass followed the same trend as that of chlorophyll a and primary production. Northward propagation of upwelling was observed as summer monsoon proceeded. Spatial variations in biological production were observed in the Arabian Sea as a consequence of the variations in the intensity of upwelling.

The response of microzooplankton (20-200 μm) to coastal upwelling and summer stratification in the southeastern Arabian Sea

Abstract: During the late summer monsoon (LSM), coastal upwelling and river runoff have increased the nutrient concentration in the inshore region of the southeastern Arabian Sea. This has resulted in elevated chlorophyll-a (av. 40±18 mg m−2), phytoplankton abundance (av. 474±116×106 ind. m−2) and primary production (av. 917±616 mg C m−2 d−1). Diatoms were the major component of the phytoplankton community (av. 60±8% of the total abundance) during this period, followed by dinoflagellates (18±12%). However, the inshore locations of 10°N and 13°N transects behaved differently, with an exceptional abundance of phytoflagellates (>1010 ind. m−2). The microzooplankton (MZP) community was comprised of heterotrophic dinoflagellates (av. 60%), ciliates (av. 30%) and copepod nauplii (av. 5%); these showed marked variation in abundance between stations. The highest abundance (av. 283×104 ind. m−2) was at the inshore location of 10°N transect, where phytoflagellates were abundant. Contrasting to the LSM, intense surface layer stratification and depletion of nitrate (with a nitracline at 60 m depth), observed in the southeastern Arabian Sea during the spring intermonsoon (SIM) period, caused low phytoplankton abundance (av. 141±86×106 ind. m−2), chlorophyll-a (av. 19±11 mg m−2) and primary production (av. 146±68 mg C m−2 d−1). High temperature (>29 °C) and nitrate-depleted surface waters favoured the proliferation of Trichodesmium erythraeum at most of the locations. Total abundance of MZP during the SIM was markedly low (av. 20×104±13×104 ind. m−2), as was the species richness and diversity (0.36±0.02 and 0.57±0.15, respectively) compared to the LSM (1.17±0.31 and 2.72±0.34, respectively). During the SIM, the MZP community that occurs in the southeastern Arabian Sea is important, since smaller individuals are widespread and form the majority of phytoplankton community. The present study points also to the fact that the MZP could play an important role, even in nutrient-enriched environment, if smaller phytoplankton are abundant.

Red tide of Noctiluca miliaris off south of Thiruvananthapuram subsequent to the ‘stench event’ at the southern Kerala coast

Abstract: ‘Red tides’ refer to the discoloration of the ocean surface caused by the blooming of some planktonic organisms. A few species of dinoflagellates (a group of protists) periodically form red tides along the west coast of India and Noctiluca miliaris is the most frequent one. Noctiluca red tide observed on 29 September 2004, seems to be significant since the phenomenon has occurred within a fortnight of the stench event that had created panic among people living along the southern Kerala coast. Mass fish kill was noticed on 17 September 2004 along the Trivandrum coast, with foul smell coming from the sea. Many people, especially children, who got exposed to the stench, were hospitalized due to vomiting and nausea. Initial reports indicated that the causative organisms for the stench and fish kill were Cochlodinium sp. and Gonyaulax diegensis. Later, detailed study on samples collected from all along the southern Malabar coast reported it was due to a holococolithophore bloom. In the present communication, we describe the favourable environmental conditions that prevailed in the region of the Noctiluca red tide. During the 229 cruise of FORV Sagar Sampada, we witnessed large patches of Noctliluca bloom centred about 25 nautical miles off south of Trivandrum (off Thumba – lat. 8°19′N, long. 76°30′E). On the calm sea surface, the bloom mostly existed as large patches, with a length of 9–11 km and a width of 2–4 km (Figure 1 a). The bloom was located along the continental slope and rarely found uniformly spread on the sea surface (Figure 1 b). We sampled the bloom region for many relevant hydrographical and biological parameters. The conductivity temperature depth (CTD) profiler recorded the temperature and salinity of the water column. Nutrient (nitrate, phosphate and silicate) concentration of the study area was analysed by a SKALAR SAN PLUS autoanalyser. Primary productivity, chlorophyll a, phytoplankton composition and microzooplankton were analysed following standard procedure. Mesozooplankton samples were collected from the mixed layer with a multiple plankton net. Surface samples collected using a plastic bucket were examined microscopically for counting the Noctiluca cells. Horizontal distribution of temperature, salinity and dissolved oxygen showed some interesting features in the bloom area (Figure 2). Relatively low temperature and high salinity, known to be favourable for the proliferation of Noctiluca, prevailed in the bloom area. Relatively lower concentrations of dissolved oxygen found in the bloom area were apparently due to the largescale respiration by Noctiluca. Low nutrient concentration observed in the southernmost part of the west coast (bloom region) indicates the retreating phase (weakening phase) of upwelling in the area (Figure 2). Upwelling weakens in the southernmost part of the west coast of India by October and as a result, the high concentrations of nutrients initially available at the surface waters get exhausted due to autotrophic production. As Noctiluca is a total heterotroph, it is unlikely that the reduced nutrient concentrations were due to its proliferation. The weather was calm during the cruise with clear sky and the wind was southwesterly and moderate, with a speed of 3.92 ms. Abundance of Noctiluca cells in the bloom patches exceeded 9 × 10 l. Phytoplankton in the bloom area was relatively lower in abundance (4250 cells l) compared with other coastal stations (av. 5000 cells l). Major components of the phytoplankton community of the bloom area were Thalassiosira sp. (1160 cells l), Coscinodiscus sp. (480 cells l), Rhizosolenia sp. (360 cells l), Nitzschia sp. (320 cells l), Peridinium sp. (360 cells l), Gymnodinium sp. (140 cells l) and Ceratium sp. (100 cells l). Chlorophyll a concentration was 0.6 mg m in the bloom region, which was relatively less compared to the other coastal stations (av. 0.9 mg m). Primary production was also lower in the bloom area (217 mgC m d) compared with other coastal stations (av. 350 mgC m d). Relatively low chlorophyll a and primary production may be due to the low availability of nutrients in the region. Abundance of microzooplankton (ciliates) in the bloom region was low (19 l) compared to the other coastal stations (30 l). Interestingly, mesozooplankton biomass was higher in the bloom area (1052 ml 1000 m) compared with the adjacent stations (756 ml 1000 m). Zooplankton community was

Seasonal variations and trophic ecology of microzooplankton in the southeastern Arabian Sea

Abstract: The seasonal ecological response of microzooplankton in the southeastern Arabian Sea is presented. During the spring intermonsoon period (March), stratification and depletion of nitrate in the surface waters (nitracline was at 60 m depth) caused low integrated chlorophyll a (av. 19 ± 11.3 mg m -2 ) and primary production (av. 164 ± 91 mgC m -2 d -1 ). On the other hand, nutrient enrichment associated with coastal upwelling and river influx during the onset and peak summer monsoon resulted in high integrated chlorophyll a (av. 21 ± 6 mg m -2 and av. 29 ± 21 mg m -3 respectively) and primary production (av. 255 ± 94 mg Cm -2 d -1 and av. 335 ± 278 mgC m -2 d -1 respectively). During all three periods, diazotropic cyanobacterium Trichodesmium erythraeum dominated in the nutrient depleted surface waters. A general increase in abundance of larger diatoms was evident in the surface waters of the inshore region during monsoon periods. The microzooplankton abundance was found to be significantly higher during the spring intermonsoon (av.241 ± 113 x10 3 ind.m -2 ) as compared to onset of summer monsoon (av. 105 ± 89 x10 3 ind.m -2 ) and peak summer monsoon (av.185 ± 175 x10 3 ind.m -2 ). Microzooplankton community during the spring intermonsoon was numerically dominated by ciliates while heterotrophic dinoflagellate was the dominant ones during the monsoon periods. The high abundance of ciliates during the spring intermonsoon could be attributed to the stratified environmental condition prevailed in the study area which favors high abundance of smaller phytoplankton and cyanobacteria, the most preferred food of ciliates. On the other hand, the dominance of heterotrophic dinoflagellates during the monsoon periods could be linked to their ability to graze larger diatoms which were abundant during the monsoon periods. The overall results show low abundance of microzooplankton in the eastern Arabian Sea during the monsoon periods mainly due to a decline in ciliates abundance. This decline during the monsoon periods could be the result of (a) low abundance of smaller phytoplankton and (b) high stock of mesozooplankton predators (av. 245 ml 100 m -3 ).

Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization

Abstract: If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Residual-mean solutions for the Antarctic Circumpolar Current and its associated overturning circulation

Abstract: Residual-mean theory is applied to the streamwise-averaged Antarctic Circumpolar Current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane. Simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated (residual) meridional overturning circulation.

Acquired phototrophy in aquatic protists

Abstract: Acquisition of phototrophy is widely distributed in the eukaryotic tree of life and can involve algal endosymbiosis or plastid retention from green or red origins. Species with acquired phototrophy are important components of diversity in aquatic ecosystems, but there are major differ- ences in host and algal taxa involved and in niches of protists with acquired phototrophy in marine and freshwater ecosystems. Organisms that carry out acquired phototrophy are usually mixotrophs, but the degree to which they depend on phototrophy is variable. Evidence suggests that 'excess car- bon' provided by acquired phototrophy has been important in supporting major evolutionary innova- tions that are crucial to the current ecological roles of these protists in aquatic ecosystems. Acquired phototrophy occurs primarily among radiolaria, foraminifera, ciliates and dinoflagellates, but is most ecologically important among the first three. Acquired phototrophy in foraminifera and radiolaria is crucial to their contributions to carbonate, silicate, strontium, and carbon flux in subtropical and trop- ical oceans. Planktonic ciliates with algal kleptoplastids are important in marine and fresh waters, whereas ciliates with green algal endosymbionts are mostly important in freshwaters. The photo- trophic ciliate Myrionecta rubra can be a major primary producer in coastal ecosystems. Our know- ledge of how acquired phototrophy influences trophic dynamics and biogeochemical cycles is rudi- mentary; we need to go beyond traditional concepts of 'plant' and 'animal' functions to progress in our understanding of aquatic microbial ecology. This is a rich area for exploration using a combina- tion of classical and molecular techniques, laboratory and field research, and physiological and ecosystem modeling.

Surface freshwater from Bay of Bengal runoff and Indonesian Throughflow in the tropical Indian Ocean

Abstract: According to recent estimates, the annual total continental runoff into the Bay of Bengal (BoB) is about 2950 km 3, which is more than half that into the entire tropical Indian Ocean (IO). Here we use climatological observations to trace the seasonal pathways of near surface freshwater from BoB runoff and Indonesian Throughflow (ITF) by removing the net contribution from precipitation minus evaporation. North of 20 degrees S, the amount of freshwater from BoB runoff and ITF changes with season in a manner consistent with surface currents from drifters. BoB runoff reaches remote regions of the Arabian Sea; it also crosses the equator in the east to join the ITF. This freshwater subsequently flows west across the southern tropical IO in the South Equatorial Current.

Biology of the Southern Ocean

Abstract: Biology of the Antarctic Seas II Edited by George A. Llano. (Antarctic Research Series, Vol. 5. Publication No. 1297.) Pp. xi + 280. (Washington, D.C.: American Geophysical Union of the National Academy of Sciences—National Research Council, 1965.) $12.