Southern Bay Research Articles

North Spanish fisheries: discards, causes and reduction measures

North Spanish fisheries: discards, causes and reduction measures

Collaborative Observations of Boundary Currents, Water Mass Variability, and Monsoon Response in the Southern Bay of Bengal

Collaborative Observations of Boundary Currents, Water Mass Variability, and Monsoon Response in the Southern Bay of Bengal

Formation of the southern Bay of Bengal cold pool

A pool of relatively cooler water, called here as the southern Bay of Bengal cold pool, exists around Sri Lanka and southern tip of India during the summer monsoon. This cold pool is enveloped by the larger Indian Ocean warm pool and is believed to affect the intraseasonal variations of summer monsoon rainfall. In this study, we have investigated the mechanisms responsible for the formation of the cold pool using a combination of both satellite data sets and a general circulation model of the Indian Ocean. Sea surface temperature (SST) within the cold pool, after the steady increase during the February-April period, decreases first during a pre-monsoon spell in April and then with the monsoon onset during May. The onset cooling is stronger (similar to 1.8 ) than the pre-monsoon cooling (similar to 0.8 ) and culminates in the formation of the cold pool. Analysis of the model temperature equation shows that SST decrease during both events is primarily due to a decrease in incoming solar radiation and an increase in latent heat loss. These changes in the net heat flux are brought about by the arrival of cloud bands above the cold pool during both periods. During the pre-monsoon period, a cloud band originates in the western equatorial Indian Ocean and subsequently arrives above the cold pool. Similarly, during the monsoon onset, a band of clouds originating in the eastern equatorial Indian Ocean comes over the cold pool region. A lead-lag correlation calculation between daily SST and rainfall anomalies suggest that cooling in SST occurs in response to rainfall events with a lag of 5 days. These sequence of events occur every year with certain amount of interannual variability.

Ocean–atmosphere processes driving Indian summer monsoon biases in CFSv2 hindcasts

This paper analyzes the role of the Indian Ocean (IO) and the atmosphere biases in generating and sustaining large-scale precipitation biases over Central India (CI) during the Indian summer monsoon (ISM) in the climate forecast system version 2 (CFSv2) hindcasts that are produced by initializing the system each month from January 1982 to March 2011. The CFSv2 hindcasts are characterized by a systematic dry monsoon bias over CI that deteriorate with forecast lead-times and coexist with a wet bias in the tropical IO suggesting a large-scale interplay between coupled ocean–atmosphere and land biases. The biases evolving from spring-initialized forecasts are analyzed in detail to understand the evolution of summer biases. The northward migration of the Inter Tropical Convergence Zone (ITCZ) that typically crosses the equator in the IO sector during April in nature is delayed in the hindcasts when the forecast system is initialized in early spring. Our analyses show that the delay in the ITCZ coexists with wind and SST biases and the associated processes project onto the seasonal evolution of the coupled ocean–atmosphere features. This delay in conjunction with the SST and the wind biases during late spring and early summer contributes to excessive precipitation over the ocean and leading to a deficit in rainfall over CI throughout the summer. Attribution of bias to a specific component in a coupled forecast system is particularly challenging as seemingly independent biases from one component affect the other components or are affected by their feedbacks. In the spring-initialized forecasts, the buildup of deeper thermocline in association with warmer SSTs due to the enhanced Ekman pumping in the southwest IO inhibits the otherwise typical northward propagation of ITCZ in the month of April. Beyond this deficiency in the forecasts, two key ocean–atmosphere coupled mechanisms are identified; one in the Arabian Sea, where a positive windstress curl bias in conjunction with warmer SSTs lead to a weakening of Findlater jet and the other in the east equatorial IO where a remote forcing by the predominantly westerly bias in the western-central equatorial IO in the summer strengthen the seasonal downwelling Kelvin wave that in turn deepens the thermocline in the eastern IO. The equatorial Kelvin wave continues as a coastal Kelvin wave and disperses as Rossby waves off Sumatra and induces positive SST and precipitation biases in the eastern and southern Bay of Bengal. This study shows that the biases that first appear in winds lead to a cascade of coupled processes that exacerbate the subsequent biases by modulating the evolution of seasonal processes such as the annual Kelvin and Rossby waves and the cross-equatorial vertically integrated moisture transport. While this analysis does not offer any particular insights into improving the ISM forecasts, it is a foundational first step towards this goal.

Dynamics of dissolved and particulate phosphorus influenced by seasonal hypoxia in Green Bay, Lake Michigan

Despite major investments in point source reductions, portions of the Great Lakes, like Green Bay, remain hypereutrophic and are subject to persistent seasonal hypoxia. Phosphorus (P) is generally a limiting nutrient in the Great Lakes ecosystem, but not all P species are equally bioavailable, and the dynamics of nutrients and their correlation to algal bloom remain poorly characterized, in part, due to a lack of adequate quantification of P chemical speciation. During summer 2014, water samples were collected from seasonally hypoxic Green Bay for measurements of dissolved and particulate inorganic and organic P to examine P cycling dynamics along a steep nutrient gradient ranging from Fox River inflow dominated eutrophic waters in the southern bay to mesotrophic northern waters near the bay's connection with open Lake Michigan. River-derived dissolved and particulate P was quickly removed from the water column in southern Green Bay through biological uptake and sedimentation. Concentrations of phosphate or dissolved inorganic P (DIP) dramatically decreased from 828±216nM in the Fox River, comprising 57±1% of the total dissolved P, to 24±9nM in northern Green Bay where dissolved organic P (DOP) became predominant (>80%). Generally low phosphate concentrations and extremely high dissolved organic C/P ratios (2090±1160 in August 2014) suggested high DOP turnover rates and active transformation between DOP and DIP through organic degradation during P-limited conditions in Green Bay. Elevated DIP levels were accompanied by low dissolved oxygen in deeper waters (10–15m) of central Green Bay where hypoxia-development occurred, suggesting the release of DIP through particle regeneration under hypoxic conditions enhanced by lateral transport and sediment resuspension. High partition coefficients (Kd) of both inorganic and organic P and their significant negative correlation with suspended particulate matter concentrations indicated the particle-reactive nature of P in freshwater environments and may imply that DOP could also be bioavailable under P-limitation.

Validation of the first annual increment deposition in the otoliths of European anchovy in the Bay of Biscay based on otolith microstructure analysis

In order to validate the first annual increment deposition in European anchovy otoliths, early juveniles were captured in October 2012 in the southern Bay of Biscay. These individuals were maintained under a continuous feeding regimen in a sea cage over a period of 6 months. From October 2012 to January 2013, lengths increased slightly or remained stable at ~9.8cm. After this period, standard length increased significantly up to a mean value of 12.0cm in April 2013. Likewise, the age of anchovies was estimated based on otolith microstructure analysis. The estimated age varied from 96 days (for individuals sampled in October 2012) to 293 days (for anchovies sampled in April 2013). A daily increment deposition rate was confirmed in otoliths of individuals maintained in the sea cage during the winter. The general otolith daily growth pattern showed that increment widths increased rapidly and were broadest between 51 and 56 days, with a mean of 19.1µm. Thereafter, the widths decreased steadily to 1.5µm and remained almost constant until the end of the experiment. The present study also revealed that the first translucent band formation started in autumn and was completed by spring.

Southern Bay of Bengal currents and salinity intrusions during the northeast monsoon

AbstractShipboard velocity and hydrographic profiles collected in December 2013 along with drifter observations, satellite altimetry, global ocean nowcast/forecast products, and coupled model simulations were used to examine the circulation in the southern Bay of Bengal as part of ongoing international research efforts in the region. The observations captured the southward flowing East India Coastal Current (EICC) off southeast India and east of Sri Lanka. The EICC was approximately 100 km wide, with speeds exceeding 1 m s−1 in the upper 75 m. East of the EICC, a subsurface‐intensified 300 km‐wide, northward current was observed, with maximum speeds as high as 1 m s−1 between 50 m and 75 m. The EICC moved low‐salinity water out of the bay and the subsurface northward flow carried high‐salinity water into the bay during typical northeast monsoon conditions during a time period when the central equatorial Indian Ocean was experiencing a westerly wind burst related to the Madden‐Julian Oscillation (MJO) event. While the northward subsurface high‐salinity flow has previously been observed during the southwest monsoon, it was observed during the northeast monsoon. The observations are consistent with northward high‐salinity subsurface flow in numerical model solutions. The analysis suggests that direct forcing along the equator may play a significant role for high‐salinity intrusions east of Sri Lanka.

Combining Small-Vertebrate, Marine and Stable-Isotope Data to Reconstruct Past Environments.

Three very different records are combined here to reconstruct the evolution of environments in the Cantabrian Region during the Upper Pleistocene, covering ~35.000 years. Two of these records come from Antoliñako Koba (Bizkaia, Spain), an exceptional prehistoric deposit comprising 9 chrono-cultural units (Aurignacian to Epipaleolithic). The palaeoecological signal of small-vertebrate communities and red deer stable-isotope data (δ13C and δ15N) from this mainland site are contrasted to marine microfaunal evidence (planktonic and benthic foraminifers, ostracods and δ18O data) gathered at the southern Bay of Biscay. Many radiocarbon dates for the Antoliña’s sequence, made it possible to compare the different proxies among them and with other well-known North-Atlantic records. Cooling and warming events regionally recorded, mostly coincide with the climatic evolution of the Upper Pleistocene in the north hemisphere.

Seasonality in molecular and cytometric diversity of marine bacterioplankton: the re-shuffling of bacterial taxa by vertical mixing.

The 'cytometric diversity' of phytoplankton communities has been studied based on single-cell properties, but the applicability of this method to characterize bacterioplankton has been unexplored. Here, we analysed seasonal changes in cytometric diversity of marine bacterioplankton along a decadal time-series at three coastal stations in the Southern Bay of Biscay. Shannon-Weaver diversity estimates and Bray-Curtis similarities obtained by cytometric and molecular (16S rRNA tag sequencing) methods were significantly correlated in samples from a 3.5 year monthly time-series. Both methods showed a consistent cyclical pattern in the diversity of surface bacterial communities with maximal values in winter. The analysis of the highly resolved flow cytometry time-series across the vertical profile showed that water column mixing was a key factor explaining the seasonal changes in bacterial composition and the winter increase in bacterial diversity in coastal surface waters. Due to its low cost and short processing time as compared with genetic methods, the cytometric diversity approach represents a useful complementary tool in the macroecology of aquatic microbes.

Differential zooplankton aggregation due to relative vorticity in a semi-enclosed bay

The relationship between a mesoscale cyclonic eddy and the abundance and distribution of zooplankton in the Bay of La Paz, Gulf of California, in the summer of 2009 is presented. The water density distribution, the geostrophic current velocity and the vertical component of relative vorticity indicate a mesoscale cyclonic eddy located in the Alfonso Basin, with a diameter of ∼30 km. The zooplankton trophic groups along the bay presented a radial distribution. Using a Bray–Curtis similarity cluster analysis, three different zones were delimited: 1) the Alfonso Basin where the cyclonic eddy is centered, 2) the strip from southern bay to the Boca Grande, which is the periphery of the eddy, and 3) outside the bay. The results indicated that the mesoscale cyclonic eddy might be the dominant physical feature influencing the horizontal distribution of the different trophic groups of zooplankton in the Bay of La Paz. Our results confirm that the mesoscale cyclonic eddy retains a biological composition that is markedly different from that of its surroundings. Zooplankton biomass was substantially higher inside the eddy than at its periphery. However, outside the bay; the biomass was comparable to that found in the center of the eddy. The trophic group composition exhibits a predominance of herbivorous zooplankton in the center of the eddy, a predominance of omnivorous zooplankton in its periphery, and a high abundance of carnivorous zooplankton in the region adjacent to the Gulf of California.

Geophysical-petrological modeling of the lithosphere beneath the Cantabrian Mountains and the North-Iberian margin: geodynamic implications

Cenozoic contractional deformation in the North-Iberian continental margin (southern Bay of Biscay) led to the uplift of the Cantabrian Mountains and the northward subduction of part of the thick continental crust, down to at least ~55km depth beneath the coastline, and perhaps even ~30–40km deeper. In order to provide a more constrained model of this unique structure and gain insight into the factors controlling its evolution, we performed an integrated geophysical-petrological modeling of the lithosphere along a 470km-long, N-S transect down to 400km depth. The methodology used allows for fitting gravity anomalies, geoid undulations, surface heat flow, elevation and seismic velocities with a realistic distribution of densities and seismic velocities in the mantle and the subducting lower crust, which are dependent on chemical composition, pressure and temperature. Two models are presented, with variable maximum depth for the crustal root: 60km (Model A) and 90km (Model B). Results indicate that both models are feasible from the geophysical point of view, but the shallower root agrees slightly better with tomographic results. The thickness of the thermal lithosphere in Model A varies from 125–145km south of the Cantabrian Mountains to 170km beneath the crustal root and 135–140km beneath the central part of the Bay of Biscay. Model B requires a thicker thermal lithosphere beneath the crustal root (205km). Low seismic velocities beneath the Bay of Biscay Moho and in the mantle wedge above the crustal root are explained by the addition of 1–2wt% of water. Input from dehydration reactions in the subducting lower crust is ruled out in Model A and has a very minor influence in Model B. We therefore interpret the water to have percolated from the seafloor during the formation of the margin in the Mesozoic. A later basaltic underplating was also inferred. A tentative evolutionary model (to a great extent governed by these petrological processes) is proposed, implying a minimum shortening close to 100km from the Latest Cretaceous to the present.

Changes in local oceanographic and atmospheric conditions shortly after the 2004 Indian Ocean tsunami

This study examines changes in the local oceanographic and atmospheric conditions over the southern Bay of Bengal and adjacent Indian Ocean waters after the 2004 Indian Ocean tsunami based on satellite remote sensing data and atmospheric reanalysis fields. After the tsunami that occurred on 26 December 2004, the accumulated rainfall had a notably increase (600 mm per month) in January of 2005 over deep waters to the southeast of Sri Lanka. This rainfall increase after the tsunami was accompanied with cooling in the sea surface temperature (SST) (up to −2 °C). Four-day averaged SST anomalies had a noticeable increase (1–4 °C) after the tsunami over the deep waters to the southwest of the epicenter. Series of ocean atmospheric and biological variables changed successively after the change of SST. The chain of causality between the tsunami and the changes in the local atmospheric conditions is suggested.

Dust deposition in the eastern Indian Ocean: The ocean perspective from Antarctica to the Bay of Bengal

AbstractAtmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple‐source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66 ± 60 mg m−2 yr−1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m−2 yr−1 at 18°S) and in the southern Bay of Bengal (2500 ± 570 mg m−2 yr−1). Our total deposition fluxes, which have an uncertainty on the order of a factor of 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.

Winter mixing and coastal upwelling drive long-term changes in zooplankton in the Bay of Biscay (1993–2010)

Long-term studies are necessary to gain insight into the influence of external climate forcing on zooplankton dynamics. Using monthly time series (1993–2010) collected in the Cantabrian Sea (southern Bay of Biscay), we studied long-term trends and seasonal cycles of zooplankton abundance and biomass along a coastal-ocean gradient. We also analysed changes in environmental variables such as sea surface temperature (SST) and chlorophyll a concentration. Zooplankton abundance and biomass increased during the study period, with greater increases in biomass towards the coast and for larger size classes. These results contrast with the decrease in biomass and size expected under global warming, which is shown in the region by the rise in summer SST, suggesting the operation of other processes. Indeed, winter mixing and coastal upwelling were key drivers of zooplankton dynamics in spring and autumn, respectively. In these seasons, zooplankton inter-annual increases were stronger. Including these hydrographic processes in future models of zooplankton dynamics for the Bay of Biscay and other temperate areas will improve their accuracy. Our results highlight the importance of disentangling the main mechanisms behind long-term changes in zooplankton, especially within the context of climate change.

Ischnomesus harrietae sp. nov., a new benthic asellote (Crustacea: Isopoda: Ischnomesidae) from bathyal bottoms of the southern Bay of Biscay.

A new species of Ischnomesidae (Crustacea: Isopoda: Asellota), Ischnomesus harrietae sp. nov. is described from the southern Bay of Biscay. This new species is distinctive due to the presence of numerous pedestal setae arranged in longitudinal rows on pereonite 5. Because of this morphological peculiarity, it can be easily distinguished from the four other Ischnomesus species previously reported from bathyal/abyssal bottoms of the European continental margin. Within its known distributional area, the new species inhabits sandy and muddy bottoms between 619 and 1099 m, with a maximum abundance of 41.8 individuals per 100 m2 recorded at approximately 700 m on the Arcachon Plateau. Another new species is also reported, Ischnomesus sp.1, represented by one specimen only and briefly described. An identification key to European species of Ischnomesus is provided.

Photosynthetic parameters and primary production, with focus on large phytoplankton, in a temperate mid-shelf ecosystem

Annual variability of photosynthetic parameters and primary production (PP), with a special focus on large (i.e. >2 μm) phytoplankton was assessed by monthly photosynthesis-irradiance experiments at two depths of the southern Bay of Biscay continental shelf in 2003. Integrated chl a (22–198 mg m−2) was moderately dominated by large cells on an annual basis. The March through May dominance of diatoms was replaced by similar shares of dinoflagellates and other flagellates during the rest of the year. Variability of photosynthetic parameters was similar for total and large phytoplankton, but stratification affected the initial slope αB [0.004–0.049 mg Cmg chl a−1h−1 (μmol photons m−2s−1)−1] and maximum photosynthetic rates PmB (0.1–10.7 mg Cmg chl a−1h−1) differently. PmB, correlated positively with αB only for the large fraction. PmB tended to respond faster to ambient irradiance than αB, which was negatively correlated with diatom abundance in the >2 μm fraction. Integrated PP rates were relatively low, averaging 387 (132–892) for the total and 207 (86–629) mg C m−2 d−1 for the large fraction, probably the result of inorganic nutrient limitation. Although similar mean annual contributions of large phytoplankton to total values were found for biomass and PP (∼58%), water-column production to biomass ratios (2–26 mg Cmg chl−1 d−1) and light utilization efficiency of the >2 μm fraction (0.09–0.84 g Cg chl−1mol photons−1m2) were minimum during the spring bloom. Our results indicate that PP peaks in the area are not necessarily associated to maximum standing stocks.

Characterizing catches taken by different gears as a step towards evaluating fishing pressure on fish communities

To implement an ecosystem approach to fisheries management, there is a need to characterize the total pressure exerted by fisheries at the community level. French onboard observer data were used to derive catch metrics and compare fishing distribution across community components between two sites in the Southern Bay of Biscay. Sample-based rarefaction curves were used to standardize metrics across different active and passive gears, and correct for sample size differences. Six metrics for species, length and functional catch composition were tested. Length and functional metrics were found the most relevant metrics to highlight differences in catches between gears, sites, and gear-site interactions. Significant differences were found between gears, mainly in mean length and proportion of piscivores. None of the gears had the most diverse catch across all metrics. Small differences were found between sites, mainly in length range and species richness.

Dissolved silicon and its isotopes in the water column of the Bay of Bengal: Internal cycling versus lateral transport

The concentration of dissolved Si and its isotope composition are measured in the Bay of Bengal (BoB) region of the northern Indian Ocean; the isotope data are the first data set from the northern Indian Ocean. The measurements are made in eight depth profiles closely along the 87°E transect (GIO1 section of the international GEOTRACES program) and in a few samples from the northern shelf of the bay. Dissolved Si in the water column varies from ∼0.6 to ∼152.5μmol/kg, whereas the δ30Si data cover a range +1.2‰ to +3.6‰. The depth profiles of dissolved Si show generally lower values in the surface increasing with depth, whereas the pattern reverses in the case of δ30Si. These vertical distribution patterns of Si and δ30Si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing Si biogeochemistry in the upper layers (top ∼1500m). In contrast, dissolved Si in near surface waters of the northern shelf and the southernmost station is exceptionally high. These results indicate a continental supply of dissolved Si from the Ganga–Brahmaputra river system (G–B) and submarine groundwater discharge (SGD) to the shelf region, and an intrusion of high salinity waters from the Arabian Sea in the southern bay. The δ30Si values of ∼1.34±0.10‰ for deep/bottom waters of the BoB (depth >1500m) are similar to those reported for the deep Southern Ocean and indicate the dominant control of water mass mixing. The dissolved Si concentrations in the bottom waters of the BoB are generally higher than those of the water mass endmembers, which suggest the need for an additional source of Si; in situ particle dissolution and/or benthic release in the central bay seem to be the potential candidate.The annual Si budget in the top ∼100m of the BoB seems to suggest that meso-scale eddies frequently occurring during non-monsoon periods can supply at the most ∼2.6gSi/m2/year, which is about 33% of the Si requirement to support new production in the bay. The supply of dissolved Si (∼1.3±0.5×1011mol/year) from the G–B river system and SGD has been calculated based on the distributions of dissolved Si concentration and δ30Si in the northern shelf waters. A comparison of this supply with the reported Si flux upstream of the estuarine zone indicates about 40% removal of dissolved Si in the G–B estuary. The mass balance of Si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess Si in the bay.

Impact of diurnal forcing on intraseasonal sea surface temperature oscillations in the Bay of Bengal

AbstractThe diurnal cycle is an important mode of sea surface temperature (SST) variability in tropical oceans, influencing air‐sea interaction and climate variability. Upper ocean mixing mechanisms are significant at diurnal time scales controlling the intraseasonal variability (ISV) of SST. Sensitivity experiments using an Ocean General Circulation Model (OGCM) for the summer monsoon of the year 2007 show that incorporation of diurnal cycle in the model atmospheric forcings improves the SST simulation at both intraseasonal and shorter time scales in the Bay of Bengal (BoB). The increase in SST‐ISV amplitudes with diurnal forcing is ∼0.05°C in the southern bay while it is ∼0.02°C in the northern bay. Increased intraseasonal warming with diurnal forcing results from the increase in mixed layer heat gain from insolation, due to shoaling of the daytime mixed layer. Amplified intraseasonal cooling is dominantly controlled by the strengthening of subsurface processes owing to the nocturnal deepening of mixed layer. In the southern bay, intraseasonal variability is mainly determined by the diurnal cycle in insolation, while in the northern bay, diurnal cycle in insolation and winds have comparable contributions. Temperature inversions (TI) develop in the northern bay in the absence of diurnal variability in wind stress. In the northern bay, SST‐ISV amplification is not as large as that in the southern bay due to the weaker diurnal variability of mixed layer depth (MLD) limited by salinity stratification. Diurnal variability of model MLD is not sufficient to create large modifications in mixed layer heat budget and SST‐ISV in the northern bay.

Phytoplankton size structure in the southern Bay of Bengal modified by the Summer Monsoon Current and associated eddies: Implications on the vertical biogenic flux

The present study combines field and satellite observations to investigate how hydrographical transformations influence phytoplankton size structure in the southern Bay of Bengal during the peak Southwest Monsoon/Summer Monsoon (July–August). The intrusion of the Summer Monsoon Current (SMC) into the Bay of Bengal and associated changes in sea surface chemistry, traceable eastward up to 90°E along 8°N, seems to influence biology of the region significantly. Both in situ and satellite (MODIS) data revealed low surface chlorophyll except in the area influenced by the SMC. During the study period, two well-developed cyclonic eddies (north) and an anti-cyclonic eddy (south), closely linked to the main eastward flow of the SMC, were sampled. Considering the capping effect of the low-saline surface water that is characteristic of the Bay of Bengal, the impact of the cyclonic eddy, estimated in terms of enhanced nutrients and chlorophyll, was mostly restricted to the subsurface waters (below 20m depth). Conversely, the anti-cyclonic eddy aided by the SMC was characterized by considerably higher nutrient concentration and chlorophyll in the upper water column (upper 60m), which was contrary to the general characteristic of such eddies. Albeit smaller phytoplankton predominated the southern Bay of Bengal (60–95% of the total chlorophyll), the contribution of large phytoplankton was double in the regions influenced by the SMC and associated eddies. Multivariate analysis revealed the extent to which SMC-associated eddies spatially influence phytoplankton community structure. The study presents the first direct quantification of the size structure of phytoplankton from the southern Bay of Bengal and demonstrates that the SMC-associated hydrographical ramifications significantly increase the phytoplankton biomass contributed by larger phytoplankton and thereby influence the vertical opal and organic carbon flux in the region.