Search

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 chlorophylla(1–2 mg m−3) and microphytoplankton abundance (1.2–1.5 × 104cells l−1). Diatoms and greenNoctiluca scintillanswere the major microphytoplankton identified. ESIM observed gradual stabilization of water column with curtailment of winter signatures and strengthening ofNoctiluca scintillansblooms. Mesozooplankton biomass was higher during LWM and decreased towards ESIM with intensification ofNoctilucablooms. 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 dynamics of Northeastern Arabian Sea (NEAS) is governed by convective mixing in winter and stratification during the inter-monsoon spring seasons, wherein blooms of green Noctiluca scintillans is a recurrent phenomenon. The study investigated the role of microzooplankton (MZP) in mediating the microbial food web in bloom waters of Noctiluca scintillans. The winter monsoon and inter-monsoon spring periods for the years 2009, 2011 and 2012 were considered for the present study as the occurrence and intensity of bloom fluctuates between these seasons. The MZP mediation seems to be through the top-down controls (preferential grazing) on ciliates (CTS) in bloom waters, consequently supporting high mesozooplankton standing stock and inevitably higher stock of copepods. Among the three years of observations, this relation was noticeable for the year 2012, as the intense bloom of Noctiluca (3 ×107 cells L−1) coincided with the highest number of copepods. On the contrary, in the non-bloom waters, a simple classical food web seems to be operational as diatoms dominated the phytoplankton community. Furthermore, the functional role of two CTS species viz. Codonellospsis ecaudata and Steenosemella ventricosa were most influential in structuring the community and microbial food web in the bloom events. The study suggests that during the bloom of Noctiluca scintillans, MZP mediated food web seems to be relatively significant in sustaining the high secondary production in the NEAS.

Understanding the forcing mechanisms that determine the coastal ecosystem dynamics has always been a challenging topic of research. The Arabian Sea (AS) coastal ecosystem in the western half of the northern Indian Ocean is influenced by the seasonally reversing atmospheric forcing and structured by distinct water masses along its depth. Although AS differs greatly from its eastern counter-half, Bay of Bengal (BoB) in the thermohaline characteristics, the inter-basin exchanges of the water masses between these ocean basins add complexity to its hydrography. Hence, the present study was attempted to understand the influence of the intrusion of the BoB water in structuring the zooplankton community of the coastal AS. In the mixed layer depth, a conspicuous spatial heterogeneity was noticed in the distribution and abundance of the zooplankton community contributed by the cumulative influence of the higher phytoplankton biomass and the advection of population through BoB water intrusion regulated by the coastal circulation in the southern part. The study also assessed the interaction of the zooplankton community with the specific water masses defined by distinct thermohaline characteristics along the vertical profile of the AS coastal waters. Three distinct water masses were identified within the 200 m depth of AS each categorized by discrete salinity and density features. The distinct groups of the dominant taxon, Copepoda identified through redundancy analysis demonstrate the species-specific preferences of the community to different water mass characteristics in the AS. This study addressing how hydrographic characteristics shape the macroecology of zooplankton in the coastal AS, will add new insight in understanding the dynamic ecosystems of the northern Indian Ocean.

The processes of upwelling off the southern tip (the Kanyakumari coast) and the west coast of India are highly localised features with different forcing mechanisms, and they cannot be treated as a uniform wind-driven upwelling system. Off the Kanyakumari coast, upwelling is due to the south-west monsoon winds that are tangential to the coast. Along the west coast, the area between 8° N and 9° N represents the shadow zone to the influence of the remote forcing on the upwelling; the latter is forced by the longshore wind stress. Moderate to relatively intense upwelling occurs along the Kollam to Mangalore coast (9° N to 13° N) due to the combined action of the longshore wind stress, the coastally trapped Kelvin waves, and the offshore propagating Rossby waves. North of this area (13° N to 15° N), upwelling is weak due to weak wind stress and is closely confined to the coastal belt. This results from the suppressive effect of the southwards-flowing Arabian Sea high saline water on the process of upw...