Hydrography and biogeochemistry of the north western Bay of Bengal and the north eastern Arabian Sea during winter monsoon

The north eastern Arabian Sea and the north western Bay of Bengal within the Indian exclusive economic zone were explored for their environmental characteristics during the winter monsoons of 2000 and 2001 respectively. The two regions were found to respond paradoxically to comparable intensities of the atmospheric forcing. There is an asymmetry in the net heat exchange of these two basins with atmosphere because of the varying thickness of barrier layer. During winter, the convective mixing in the Arabian Sea is driven by net heat loss from the ocean, whereas the Bay of Bengal does not contribute to such large heat loss to the atmosphere. It appears that the subduction of high saline Arabian Sea water mass is the mechanism behind the formation of a barrier layer in the northeast Arabian Sea; whereas that in the Bay of Bengal and the southeast Arabian Sea are already established as due to low saline water mass. The weak barrier layer in the Arabian Sea yields to the predominance of convective mixing to bring in nitrate-rich waters from the deeper layers to the surface, thereby supporting enhanced biological production. On the other hand, the river discharge into the Bay of Bengal during this period results in the formation of a thick and stable barrier layer, which insulates vertical mixing and provide oligotrophic condition in the Bay

Biogeochemical and ecological impacts of boundary currents in the Indian Ocean

Monsoon forcing and the unique geomorphology of the Indian Ocean basin result in complex boundary currents, which are unique in many respects. In the northern Indian Ocean, several boundary current systems reverse seasonally. For example, upwelling coincident with northward-flowing currents along the coast of Oman during the Southwest Monsoon gives rise to high productivity which also alters nutrient stoichiometry and therefore, the species composition of the resulting phytoplankton blooms. During the Northeast Monsoon most of the northern Indian Ocean boundary currents reverse and favor downwelling. Higher trophic level species have evolved behavioral responses to these seasonally changing conditions. Examples from the western Arabian Sea include vertical feeding migrations of a copepod (Calanoides carinatus) and the reproductive cycle of a large pelagic fish (Scomberomorus commerson). The impacts of these seasonal current reversals and changes in upwelling and downwelling circulations are also manifested in West Indian coastal waters, where they influence dissolved oxygen concentrations and have been implicated in massive fish kills. The winds and boundary currents reverse seasonally in the Bay of Bengal, though the associated changes in upwelling and productivity are less pronounced. Nonetheless, their effects are observed on the East Indian shelf as, for example, seasonal changes in copepod abundance and zooplankton community structure. In contrast, south of Sri Lanka seasonal reversals in the boundary currents are associated with dramatic changes in the intensity of coastal upwelling, chlorophyll concentration, and catch per unit effort of fishes. Off the coast of Java, monsoon-driven changes in the currents and upwelling strongly impact chlorophyll concentrations, seasonal vertical migrations of zooplankton, and sardine catch in Bali Strait. In the southern hemisphere the Leeuwin is a downwelling-favorable current that flows southward along western Australia, though local wind forcing can lead to transient near shore current reversals and localized coastal upwelling. The poleward direction of this eastern boundary current is unique. Due to its high kinetic energy the Leeuwin Current sheds anomalous, relatively high chlorophyll, warm-core, downwelling eddies that transport coastal diatom communities westward into open ocean waters. Variations in the Leeuwin transport and eddy generation impact many higher trophic level species including the recruitment and fate of rock lobster (Panulirus cygnus) larvae. In contrast, the transport of the Agulhas Current is very large, with sources derived from the Mozambique Channel, the East Madagascar Current and the southwest Indian Ocean sub-gyre. Dynamically, the Agulhas Current is upwelling favorable; however, the spatial distribution of prominent surface manifestations of upwelling is controlled by local wind and topographic forcing. Meanders and eddies in the Agulhas Current propagate alongshore and interact with seasonal changes in the winds and topographic features. These give rise to seasonally variable localized upwelling and downwelling circulations with commensurate changes in primary production and higher trophic level responses. Due to the strong influence of the Agulhas Current, many neritic fish species in southeast Africa coastal waters have evolved highly selective behaviors and reproductive patterns for successful retention of planktonic eggs and larvae. For example, part of the Southern African sardine (Sardinops sagax) stock undergoes a remarkable northward migration enhanced by transient cyclonic eddies in the shoreward boundary of the Agulhas Current. There is evidence from the paleoceanographic record that these currents and their biogeochemical and ecological impacts have changed significantly over glacial to interglacial timescales. These changes are explored as a means of providing insight into the potential impacts of climate change in the Indian Ocean

Ecobiogeography, Spatial and Temporal Variations of Microzooplankton Along the East Coast of India

In the present study an attempt has been made to understand the microzooplankton community along the easr coast of India. Most of the earlier studies projected Bay of Bengal as an oligotrophic system where phytoplankton growth is limited by a number of factors among which nutrients are the foremost. Hence it is logical to consider that the most of the primary production in the Bay of Bengal could be contributed by small sized phytoplankton harnessing the available resources, which in turn can be utilized effiency by the microzooplankton only. Hence microzooplankton could play in transferring primary organic carbon to higher tropic levels in this region

Response of phytoplankton to heavy cloud cover and turbidity in the northern Bay of Bengal

An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being similar to 600 mu mol m(-2) s(-1) on sunny days and declining to similar to 50 mu mol m(-2) s(-1) on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41-80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR

On the accuracy of assessing copepod size and biovolume using FlowCAM and traditional microscopy

1261-1264In this paper, based on the biovolume estimation of different genera of copepods, we present the fact that only the Area-based Diameter (ABD) algorithm of FlowCAM has the efficiency to measure the bio-volume of copepods better than traditional microscopy. Also, we have demonstrated that the efficiency of the Equivalent Spherical Diameter (ESD) algorithm of FlowCAM over traditional microscopy is lesser while assessing the copepod biomass, and it depends on the morphological characteristics of various copepod genera. The ESD algorithm overestimates (8-140 times) while traditional microscopy method underestimates (2-8 times) the copepod biovolume, chiefly because of the inclusion of the entire image field for volume estimation in the former case and avoidance of extended body parts such as appendages in the latter case. These observations have special implications in aquatic environmental monitoring as many of the modern researchers prefer FlowCAM as a better tool to accurately quantify plankton biomass

Response of phytoplankton to heavy cloud cover and turbidity in the northern Bay of Bengal

An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being similar to 600 mu mol m(-2) s(-1) on sunny days and declining to similar to 50 mu mol m(-2) s(-1) on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41-80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR

Waning of plankton food web in the upstream region of the Cochin backwaters during the Southwest Monsoon

1145-1154Heterotrophic bacteria (0.2-2 µm), pico-phytoplankton (0.2-2 µm), nano-phytoplankton (2-20 µm), micro-phytoplankton (>20 µm), heterotrophic nanoflagellates (2-20 µm), and micro-zooplankton (20 – 200 µm) present in the Cochin backwaters were quantified during the Southwest Monsoon. High fresh water influx during the period transformed the Cochin backwaters into an extensive fresh water zone with high levels of nutrients (nitrate av. 20 µM; phosphate av. 3 µM), which favoured high phytoplankton abundance (av. 31 ± 9 x 104 No. L-1) and chlorophyll a (av. 5.2 ± 3.2 mg m-3). Autotrophic pico-plankton and nano-plankton contributed the majority (av. 69 ± 16%) of the total chlorophyll a while heterotrophic bacteria was found to be high throughout the study area (av. 3 ± 0.6 x 108 No. L-1). Abundance of heterotrophic nano-flagellates and micro-zooplankton were markedly lower in the freshwater dominant region (av. 0.5 ± 0.3 x 106 No. L-1 and av. 3 ± 1 No. L-1) as compared to the downstream region (av. 6 ± 3 x 106 No. L-1 and av. 3222 ± 3619 No. L-1). This clearly indicated a weak microbial food web in a major part of the Cochin backwaters during the Southwest Monsoon. The ecological implication is that it leads to inefficient transfer of bacterial and pico/nano-phytoplankton biomass to higher trophic levels. This study provides further evidences to our earlier observation that most of the heterotrophic bacteria and phytoplankton carbon in monsoonal estuaries gets transported to the coastal regions during the Southwest Monsoon

Seasonal variability in biological carbon biomass standing stocks and production in the surface layers of the Bay of Bengal

369-379As a part of the Bay of Bengal Process Study, several biological characteristics were investigated during summer monsoon (SuM; July-August 2001), fall intermonsoon (FIM, September-August 2002), spring intermonsoon (SpIM; April-May 2003) and northeast monsoon (NEM, Dec 2005-January 2006). Chl a carbon during SuM, FIM, SpIM and NEM averaged 688, 767, 1212 and 1057 mg C m-2 in the western Bay (WB) and 518, 904, 789 and1023 mg C m-2 in the central Bay (CB). Primary productivity (PP) averaged 326, 281, 366 and 280 mg C m-2 d-1 in the WB and 144, 306, 241 and 375 mg C m-2d-1 in the CB respectively. Shallow euphotic depths (~<55m) apparently are not conducive for (PP) in the 0-120 m column. Heterotrophic bacterial carbon biomass was sizable and averaged 282, 662, 95 and 288 mg C m-2 in the WB and was 677, 227, 104 and 333 mg C m-2 in the CB during SuM, FIM, SpIM and NEM respectively. They appear very crucial in governing the abundances of microheterotrophs in the southern and, open-ocean regions of the Bay. Microzooplankton were much poorer in abundance than those in the Arabian Sea except during SpIM. Their maximum carbon biomass (WB: 233; CB: 327 mg C m-2 in the top 120 m was observed during SpIM. In the upper 200 m, the mesozooplankton biomass did not show much variation between the season (WB: 222, 906, 2428 and 988 mg C m-2 and CB: 587, 1036, 821 and 808 mg C m-2 d-1 respectively during SuM, FIM, SpIM and NEM). Incidentally it was mostly decoupled with chl a and PP. Cold-core eddies observed during most sampled seasons seem to bear an enhancing influence on the overall biological productivity processes

Oscillating environmental responses of the eastern Arabian Sea

67-75Characteristics of two distinct physical processes, the coastal upwelling and convective overturning, which enhance phytoplankton productivity in the west coast of India, are discussed in this study. Systematic in-situ data collected during the two contrasting monsoon (summer and winter) periods were utilized for the study. During summer monsoon, the colder (27°C), less oxygenated (3-N >2 μM) waters were evidenced in the upper layers of the southwest coast of India, along with higher level of primary production (surface - 76 mgC m-3d-1 & column -1629 mgC m-2d-1) and chlorophyll a (surface - 1.2 mgm-3 & column -45.7 mgm-2). In contrast, during winter, the upper thermohaline layer of the northeastern Arabian Sea exhibited colder sea surface temperature (SST, 25-26°C), and deeper mixed layer depth (MLD, >70 m). This area was distinguished by less oxygenated (2μM) and high productive (surface - >10 mgC m-3d-1 & column - >800 mgC m-2 d-1) waters. I can be stated that the physically forced chemical changes occurred in the upper layers of both southeastern and northeastern Arabian Sea, appeared in an oscillating manner with respect to monsoonal changes, seemed to trigger the enhancement of phytoplankton productivity of the area

Intense blooms of <i>Trichodesmium erythraeum</i> (Cyanophyta) in the open waters along east coast of India

165-167Two blooms of Trichodesmium erythraeum were observed during April 2001, in the open waters of Bay of Bengal and this is the first report from this region. The locations of the bloom were off Karaikkal (10°58N, 81°50E) and off south of Calcutta (19° 44N, 89° 04), both along east coast of India. Nutrients (nitrate, phosphate, silicate) concentration in the upper 30 m of the water column showed very low values. High-integrated primary production (Bloom 1- 2160 mgC m-2 d-1, Bloom 2-1740 mgC m-2 d-1) was obtained in these regions, which indicated the enhancement of primary production in the earlier stages of the bloom. Very low NO3-N concentrations, brownish yellow bloom colour, undisturbed patches and high primary production strongly suggested that the blooms were in the growth phase. Low mesozooplankton biomass was found in both locations and was dominated by copepods followed by chaetognaths