Spring Intermonsoon Research Articles

Variability in tidal harmonics of the coastal and estuarine waters of Goa during winter monsoon and spring inter-monsoon

Harmonic analysis was carried out with simultaneously observed tide elevation in the coastal and estuarine regions of Goa. The tide elevations were consistent along the coastal stretch of 80 km; however, significant variations were obtained inside the estuarine channels. In both estuaries, the increase in tide elevation was observed from mouth to head, also reflected in the major harmonic constituents. M2 was found to be the principal constituent, which was consistent in the coastal region. The maximum M2 amplitude of 66.27 cm was recorded at Pontemol (upstream of Zuari estuary) during spring inter-monsoon. The increase of M4 tide in both the estuarine channels confirms the non-linearity of the system. The phase lag of MS4 from the forcing tides of M2 and S2 indicated flood dominance of the system. The tidal current velocities were well-reflected by the tidal ellipse. Our datasets do not reflect significant changes in tidal dynamics in the two seasons. Instead, it was noticed that a small shift in bottom topography could lead to a considerable difference in the tidal dynamics of the system, which is reflected in the estimated form number and tidal skewness values. The study puts up an effort to compare the earlier harmonics with the present ones to infer the decadal evolution of the astronomical constituents in the estuarine stretch of Goa. Furthermore, the association between the larger wave with high water could indicate about the flood-risk of this region.

Copepod carcasses in the western Bay of Bengal and associated ecology.

Although a portion of all copepods in the natural environment are physiologically inactive or dead, there is little data on this aspect in Indian seas. Using the classical neutral red live staining technique, the dead percentage of copepods in the western Bay of Bengal (BoB) is determined for the first time in this study, which takes into account hydrography and zooplankton samples collected on April 2015 (Spring Intermonsoon [SIM]) from 6 inshore and 8 offshore locations in the western BoB. The offshore parts of the research area were infested with a persistent and extensive warm-core eddy/gyre during the sampling time, as demonstrated by satellite data of sea surface temperature, mean sea level anomaly, and surface currents. As a result, the mixed layer depth in the offshore zone was higher, whereas nutrients, chlorophyll a, and turbidity were significantly lower than inshore. Copepods dominated the zooplankton community (> 80%), with abundances ranging from 112 to 2580 No m-3 in the study area. Calanoids (particularly Paracalanidae) dominated nearshore waters (35-65%), whereas cyclopoids (notably Corycaeidae and Oncaeidae) dominated offshore (49-71%). There were 48 species of copepods identified in total, and all of them had dead individuals (carcasses). Offshore waters had a larger percentage of copepod carcasses (27-39%) than inshore seas (18-28%). The larger percentage of carcasses is possibly related to a higher top-down feeding pressure on cyclopoids in offshore waters due to extreme oligotrophy and the dominance of the microbial food web.

Optical characterisation and classification of water types in the southern South China Sea and Straits of Malacca

We describe the characteristics of surface water spectral reflectance variability and identify reflectance-based optical water types that are present in the southern South China Sea (SSCS) and the Straits of Malacca (SoM). Simultaneous measurements of in-water hyperspectral remote sensing reflectance (Rrs), particulate backscattering coefficient (bbp) and in-water constituents were conducted at 473 stations from May 2009 to March 2019 during different monsoon seasons at both coastal and oceanic waters. Empirical orthogonal function (EOF) and hierarchical cluster analysis (HCA) were performed to determine the spectral variability of Rrs and group the homogenous optical water type regimes. The results indicate that monsoons and regional hydrodynamic conditions have great influence on the optical properties. The SSCS exhibited strong seasonal patterns with maximum values of all the bio-optical parameters during the northeast monsoon and minimum values during the spring inter-monsoon. In contrast, the SoM waters are subjected to strong influence of freshwater discharges, thus resulting in optically complex water regimes. The EOF analyses clearly revealed two dominant modes of optical variability, with each mode corresponding to the optical conditions of SSCS and SoM, respectively. The correlation between EOF amplitude factors and optical parameters indicated that the Rrs variability in SoM was primarily driven by the particulate backscattering and secondarily by light-absorbing components, while it was only minimally influenced by the absorption coefficients in SSCS. Hierarchical clustering revealed the presence of five distinct optical water types, which varied considerably according to the concentration of biogeochemical and optical properties. Classes 1 and 2, which were only found in SSCS waters, were defined by high magnitudes within the blue spectral region with very low concentrations of in-water constituents, indicating clear waters. Class 3 represents the optically transitional waters between coastal and open oceans, characterizing by a well-defined Rrs peak at the blue-end of the spectrum with no single dominant constituent. Classes 4 and 5 are most representative of waters influenced by riverine plumes, particularly in the SoM, with the highest Rrs magnitudes in the green and a well-defined absorption peak in the blue spectral region. Our findings confirmed that the performance of global Chl-a algorithms varied with the optical conditions present in the study area with the retrieval uncertainties increasing substantially from Class 1 to Class 5 water types. This finding will contribute significantly to the development of regional class-specific bio-optical models.

Variations of Colored Dissolved Organic Matter in the Mandovi Estuary, Goa, During Spring Inter-Monsoon: A Comparison With COVID-19 Outbreak Imposed Lockdown Period

Colored dissolved organic matter (CDOM) is one of the important fractions of dissolved organic matter (DOM) that controls the availability of light in water and plays a crucial role in the cycling of carbon. High CDOM absorption in the Mandovi Estuary (Goa) during spring inter-monsoon (SIM) is largely driven by both in-situ production and anthropogenic activities. Here we have presented the CDOM variation in the estuary during SIM of 2014–2018 and compared it with that of 2020 when the COVID-19 outbreak imposed lockdown was implemented. During 2020, low CDOM absorption was observed at the mid-stream of the estuary as compared to the previous years, which could be attributed to low autochthonous production and less input from anthropogenic activities. On the other hand, high CDOM observed at the mouth during 2020 is linked to autochthonous production, as seen from the high concentrations of chlorophyll a. High CDOM in the upstream region could be due to both autochthonous production and terrestrially derived organic matter. Sentinel-2 satellite data was also used to look at the variations of CDOM in the study region which is consistent with in-situ observations. Apart from this, the concentration of nutrients (NO3–, NH4+, and SiO44–) in 2020 was also low compared to the previous reports. Hence, our study clearly showed the impact of anthropogenic activities on CDOM build-up and nutrients, as the COVID-19 imposed lockdown drastically controlled such activities in the estuary.

Variations of Colored Dissolved Organic Matter in the Mandovi Estuary, Goa, During Spring Inter-Monsoon: A Comparison With COVID-19 Outbreak Imposed Lockdown Period

Colored dissolved organic matter (CDOM) is one of the important fractions of dissolved organic matter (DOM) that controls the availability of light in water and plays a crucial role in the cycling of carbon. High CDOM absorption in the Mandovi Estuary (Goa) during spring inter-monsoon (SIM) is largely driven by both in-situ production and anthropogenic activities. Here we presented the CDOM variation in the estuary during SIM of 2014 to 2018 and compared it with that of 2020 when the COVID-19 outbreak imposed lockdown was implemented. During 2020, low CDOM absorption was observed at the mid-stream of the estuary as compared to the previous years, which could be attributed to low autochthonous production and less input from anthropogenic activities. On the other hand, high CDOM observed at the mouth during 2020 is linked to the autochthonous production, as seen from the high concentrations of chlorophyll a. High CDOM in the upstream region could be due to both autochthonous production and terrestrially derived organic matter. Sentinel- 2 satellite data was also used to look at the variations of CDOM in the study region which is consistent with in-situ observations. Apart from this, the concentration of nutrients (NO3-, NH4+ and SiO44-) in 2020 was also low compared to the previous reports. Hence, our study clearly showed the impact of anthropogenic activities on CDOM build-up and nutrients, as the COVID-19 imposed lockdown drastically controlled such activities in the estuary.

Seasonal variability of the Arabian Sea intermediate circulation and its impact on seasonal changes of the upper oxygen minimum zone

Abstract. Oxygen minimum zones (OMZs) in the open ocean occur below the surface in regions of weak ventilation and high biological productivity with associated sinking organic matter. Very low levels of dissolved oxygen alter biogeochemical cycles and significantly affect marine life. One of the most intense though poorly understood OMZs in the world ocean is located in the Arabian Sea between 300 and 1000 m of depth. An improved understanding of the physical processes that have an impact on the OMZ in the Arabian Sea is expected to increase the reliability of assessments of its future development. This study uses reanalysis velocity fields from the ocean model HYCOM (Hybrid Coordinate Ocean Model), which are verified with observational data, to investigate advective pathways of Lagrangian particles into the Arabian Sea OMZ at intermediate depths between 200 and 800 m. In the eastern basin, the vertical expansion of the OMZ is strongest during the winter monsoon, revealing a core thickness 1000 m deep and oxygen values below 5 µmol kg−1. The minimum oxygen concentration might be favoured by a maximum water mass advection that follows the main advective pathway of Lagrangian particles along the perimeter of the basin into the eastern basin of the Arabian Sea during the winter monsoon. These water masses pass regions of high primary production and respiration, contributing to the transport of low-oxygenated water into the eastern part of the OMZ. The maximum oxygen concentration in the western basin of the Arabian Sea in May coincides with a maximum southward water mass advection in the western basin during the spring intermonsoon, supplying the western core of the OMZ with high-oxygenated water. The maximum oxygen concentration in the eastern basin of the Arabian Sea in May might be associated with the northward inflow of water across 10∘ N into the Arabian Sea, which is highest during the spring intermonsoon. The Red Sea outflow of advective particles into the western and eastern basin starts during the summer monsoon associated with the northeastward current during the summer monsoon. On the other hand, waters from the Persian Gulf are advected with little variation on seasonal timescales. As the weak seasonal cycle of oxygen concentration in the eastern and western basin can be explained by seasonally changing advection of water masses at intermediate depths into the Arabian Sea OMZ (ASOMZ), the simplified backward-trajectory approach seems to be a good method for prediction of the seasonality of advective pathways of Lagrangian particles into the ASOMZ.

Occurrence of enhanced herbivory in the microbial food web of a tropical estuary during southwest monsoon

Tropical monsoonal estuaries have a significant role in global fishery as they act as rich feeding and breeding grounds for many commercial species. Food web dynamic studies in these coastal ecosystems are mainly focused on the classic food chain which assumes that microbial food webs contribute more towards the recycling of organic carbon and nutrients rather than production. But recently it is observed that phyto-microbial food web which is largely dependent on pico-autotrophs can contribute significantly towards the higher trophic level. Based on these observations we have analysed phyto-microbial pathway of food web in a highly productive tropical monsoonal estuary. We assessed the grazer-prey interaction in the microbial food web of Cochin estuary during spring intermonsoon and southwest monsoon seasons. Our results show that during spring intermonsoon the food web is dependent more on heterotrophic bacteria, but in southwest monsoon the dependency of grazers switches over to autotrophic picoplankton. We also analysed the growth rate, grazing rate and carbon contribution of autotrophic picoplankton experimentally during both seasons and found that autotrophic picoplankton show higher growth rate during southwest monsoon. The higher grazing rate and clearance rate of autotrophic picoplankton recorded during southwest monsoon indicate the efficient consumption of autotrophic picoplankton crop during the season. We could also observe a change in autotrophic picoplankton community structure between the seasons with the dominance of picocyanobacteria during spring intermonsoon and that of picoeukaryotes during southwest monsoon. Thus we infer that high freshwater influx into the estuary during monsoon results in high turbid and low saline (mesohaline to oligohaline) environment which increases the growth rate of low light adapted autotrophic picoplankton populations and in turn enhances herbivory (phyto-microbial pathway) in the microbial food web. Apart from protists, autotrophic picoplankton were found to be widely consumed by many larval forms of mesozooplankton, gastropods, bivalves, rotifers and large sized filter feeders either as aggregates or as unicellular forms. Thus monsoon enhanced herbivory can have a significant role in the growth and survival of larval forms recruited into the estuary during monsoon season and in turn the fishery.

Influence of eddies on phytoplankton composition in the Bay of Bengal

Several cyclonic (CE) and anticyclonic (ACE) eddies form in the Bay of Bengal (BoB) and they expect to contain different proportion of nutrients due to divergence and convergence of waters respectively and may have impact on phytoplankton composition. In order to test this hypothesis, one CE and two ACE (north ACE (ACEN) and south ACE (ACES)) were sampled in the BoB during spring intermonsoon. Higher concentration of dissolved inorganic nitrate and lower dissolved inorganic phosphorus was observed in the upper 25 m of water column in the CE and contrasting to that was observed in the ACE regions. Silicate concentrations were above the limiting levels for diatoms growth in the study region. High dissolved organic nitrogen (DON) and phosphorus (DOP) were observed in the upper 10 m in the CE and 80 m in the ACEs. Due to lack of nitrate in the study region DON seems to support phytoplankton growth in the BoB. The phytoplankton groups were estimated using marker pigment composition and dominant groups were evaluated using pigment ratios. Occurrence of autotrophic dinoflagellates were observed only in the ACES, which was formed in the central eastern BoB. Premnesiophytes, chrysophytes were found only in ACEN which was formed in the northeastern shelf region. In addition to this, prochlorophytes and chlorophytes were also found in both ACEN and ACES. The concentration of fucoxanthin, marker pigment for diatoms, was low and confined to the subsurface waters in the CE and ACEN regions due to unavailability of nitrate and their inability to uptake DON. This study suggested that eddies bring different composition of phytoplankton from the region of their origin to the open sea resulting in variations in their diversity. This may have impact on food web dynamics in the BoB. More than 50 eddies form each year in the BoB and their impact on the taxonomic and size composition of phytoplankton and fluxes of carbon to depth needs further studies.

Influence of environmental variables on the distribution and community structure of mesozooplankton in the coastal waters of the eastern Arabian Sea

The spatio-temporal variation of mesozooplankton in response to environmental variables was studied for the coastal waters of southeastern Arabian Sea during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) of 2014. Overall 84 mesozooplankton taxa belonging to 17 taxonomic groups were identified. The copepod group dominated species richness (58 species) of the mesozooplankton community. Spatial variation of mesozooplankton biomass and total abundance changed significantly in relation to the distribution of sub-surface water (0.5 m) dissolved organic carbon (DOC). Carnivorous and omnivorous mesozooplankton abundance increased substantially with an increasing DOC level. Factor analysis and Mantel test revealed that the distribution of mesozooplankton community, biomass and abundance had significant positive correlation with the distribution of chlorophyll a (Chl-a) and DOC during both the seasons. Distribution of Chl-a had a positive influence on herbivorous mesozooplankton abundance, while DOC had a positive relationship with omnivorous and carnivorous mesozooplankton abundance. Our study indicates that the distribution of Chl-a and DOC influences the spatial variation of mesozooplankton community and abundance in the coastal waters of eastern Arabian Sea during the inter-monsoon seasons (SIM and FIM).

Biological Calcification Rate and Species‐Specific Contributions of Coccolithophores to Total Calcite Inventory in the Eastern Indian Ocean

AbstractCoccolithophores are calcifying protists that have a significant role in marine biological pump through calcite production (CP). We determined the biological calcification rate and ecologically key species (e.g., Gephyrocapsa oceanica) contribution to total calcite stocks in the eastern Indian Ocean (EIO) during spring intermonsoon; this region was quite undersampled compared to other oceanic regions. Our results indicated that the numerically dominant species are of great importance to cell calcite even with low cellular calcite. CP rate in EIO ranged from 0.148 to 85.017 μmol C·m−3·day−1 in the entire data set, which was lower than those in the global ocean. Coccolithophores contributed 8.5% of total phytoplankton carbon fixation. CP acted as a function of light irradiance in the euphotic zone, while a decoupling between CP and primary productivity was found in most of the bottom of euphotic zone. To some degree, this decoupling indicated calcification was more nitrate dependent and less light dependent than photosynthesis. CP/primary productivity ratios were highly variable in our database and were possibly influenced by photoacclimation across various coccolithophore species. As roughly estimated, the turnover rate of coccolith calcite took values around 0.02–0.05 day−1. The association between particulate organic carbon and calcite stocks implied a potential increasing ballast as depth increase. The current profiles of calcification and associated biogeochemical elements obtained from this field study helps our understanding about characteristics of biological pump in the EIO and are valuable for estimating calcification rate in the region using satellite data and numerical modeling.

Copepods size structure in various phases of a cold-core eddy - Normalised Abundance Size Spectra (NASS) approach.

The present study addresses the copepods size structure in different phases of a cold-core eddy based on a 17 day time-series measurement (21st February to 8th March 2016) from a coastal site in the western Bay of Bengal during the Spring Intermonsoon (SIM). Copepod size measured using FlowCAM and microscope was converted into biovolumes. Based on the salinity and temperature in discrete depth layers, sampling days were grouped into 3 Phases, pre-eddy (21–24 February), Core-eddy (28 February – 6 March) and transition (25–27 February and 7–8 March). The core-eddy phase was characterised as cool, less oxygenated and nutrient-rich in subsurface layers than pre-eddy and transition. Copepod abundance and total biovolume were relatively high in core-eddy phase compared to other phases. Copepods biovolumes were arranged in 15 size classes. Contribution of each size classes in total biovolume differed between the eddy phases. In total biovolume, smaller copepods (biovolume size class ≤ log2 (−3) mm−3) contribution was high (76.84%) in pre-eddy, but larger copepods proportion was high in core-eddy phase. Normalised Abundance Size Spectra (NASS) slope varied from −2.45 to −1.02. Similar to the taxonomic diversity, size diversity also high in core-eddy phase. Relatively flatter slope, high intercept and diversity in core-eddy phase suggest that copepods production and trophic efficiency was substantially increased. In short, copepods taxonomy and their size structure explain, pre-eddy contains the smaller coastal forms and core-eddy have a mixture of smaller coastal, larger oceanic and Oxygen Minimum Zone (OMZ) tolerant forms. Larger oceanic and OMZ tolerant forms in core-eddy phase indicate their entry through the transverse of the offshore formed cold-core eddy in the study site. Slope, intercept of NASS and size diversity captured the copepod species composition and production differences between the eddy phases in terms of abundance. This study suggests plankton size spectra and size diversity can be used as a tool to predict the plankton food web response in the varying environmental conditions. This study is the first of its kind explaining about copepods size structure and size spectrum changes with the hydrographical characteristics in Indian waters.

Optical characteristics of colored dissolved organic matter during blooms of Trichodesmium in the coastal waters off Goa.

Trichodesmium, a marine cyanobacterium, plays a significant role in the global nitrogen cycle due to its nitrogen fixing ability. Large patches of Trichodesmium blooms were observed in the coastal waters, off Goa during spring intermonsoon (SIM) of 2014-2018. Zeaxanthin was the dominant pigment in the bloom region. Here, we present the spectral absorption and fluorescence characteristics of colored dissolved organic matter (CDOM) during these blooms. CDOM concentration was much higher in the bloom patches as compared with nonbloom regions. During the bloom spectral CDOM absorption had distinct peaks in the UV region due to the presence of UV-absorbing/screening compounds, mycosporine-like amino acids (MAAs) and in the visible region due to phycobiliproteins (PBPs). The spectral fluorescence signatures by the traditional peak picking method exhibited three peaks, one was protein-like, and the other two were humic-like. Apart from these, Trichodesmium exhibited strong protein-like fluorescence with 370/460 nm (Ex/Em), which is a signature of cyanobacteria. A parallel factor analysis (PARAFAC) on the fluorescence excitation-emission matrix (EEM) of Trichodesmium dataset fitted a 3-component model of which one was protein-like, and two were humic-like. The fluorescence index (FI) values during Trichodesmium bloom was very high (~ 3) compared with the typical range of 1.2-1.8 observed for the natural waters. Bloom degradation experiments proved that increase in tryptophan fluorescence enhances the CDOM absorption. Our study indicates that Trichodesmium blooms provide a rich source of organic matter in the coastal waters and long-term monitoring of these blooms is essential for understanding the health of ecosystem.

Phytoplankton growth and microzooplankton grazing in the central and northern South China Sea in the spring intermonsoon season of 2017

Phytoplankton growth rates and mortality rates were experimentally examined at 21 stations during the 2017 spring intermonsoon (April to early May) in the northern and central South China Sea (SCS) using the dilution technique, with emphasis on a comparison between the northern and central SCS areas which had different environmental factors. There had been higher temperature but lower nutrients and chlorophyll a concentrations in the central SCS than those in the northern SCS. The mean rates of phytoplankton growth (μ0) and microzooplankton grazing (m) were (0.88±0.33) d−1 and (0.55±0.22) d−1 in the central SCS, and both higher than those in the northern SCS with the values of μ0 ((0.81±0.16) d−1) and m ((0.30±0.09) d−1), respectively. Phytoplankton growth and microzooplankton grazing rates were significantly coupled in both areas. The microzooplankton grazing impact (m/μ0) on phytoplankton was also higher in the central SCS (0.63±0.12) than that in the northern SCS (0.37±0.06). The microzooplankton abundance was significantly correlated with temperature in the surface. Temperature might more effectively promote the microzooplankton grazing rate than phytoplankton growth rate, which might contribute to higher m and m/μ0 in the central SCS. Compared with temperature, nutrients mainly affected the growth rate of phytoplankton. In the nutrient enrichment treatment, the phytoplankton growth rate (μn) was higher than μ0 in the central SCS, suggesting phytoplankton growth in the central SCS was nutrient limited. The ratio of μ0/μn was significantly correlated with nutrients concentrations in the both areas, indicating the limitation of nutrients was related to the concentrations of background nutrients in the study stations.

Influence of environmental parameters on bio-optical characteristics of colored dissolved organic matter in a complex tropical coastal and estuarine region

Dissolved organic matter (DOM) is an important source of carbon in aquatic ecosystems, and colored DOM (CDOM), which is smaller than 0.2 μm and interacts with ultraviolet (UV) and visible light, affects the spectral quality and quantity of light in water. In this study, the spatial and temporal variations of CDOM with changes in environmental conditions were investigated from March 2014 to May 2017 in the coastal waters and two estuaries (Zuari and Mandovi) of Goa, western India, and the major sources and sinks controlling the optical properties of these waters were identified. The CDOM absorption in the estuaries was two times higher than that of the coastal waters. It was also determined that the CDOM absorption at 412 nm (ag412) in the coastal and estuarine waters significantly varied between seasons. The ag412 was found to be higher in the coastal waters during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) than during the northeast monsoon (NEM). The high absorption during the SIM was of autochthonous origin, while terrigenous DOM was the primary contributor mainly during the FIM. The photobleaching of CDOM was highest during the SIM, resulting in the predominance of low-molecular-weight DOM in the coastal waters. This photobleaching of DOM also resulted in deeper UV light penetration, as indicated by the diffuse attenuation coefficient Kd at 350 nm. In the Mandovi and Zuari estuaries, higher levels of CDOM were observed during the southwest monsoon (SWM) and SIM than the FIM and NEM. The terrigenous DOM contribution was higher in the estuaries during the SWM, while phytoplankton contributed to a higher level of CDOM during the SIM. CDOM exhibited non-conservative mixing behavior in the study region, as it decreased in estuaries with lower salinities and increased at salinities between 20 and 31. Considering the importance of CDOM in the carbon cycle, this study highlights the various sources and sinks of CDOM controlling the optical properties and biogeochemical processes of coastal and estuarine waters.

High Levels of Isoprene in the Marine Boundary Layer of the Arabian Sea during Spring Inter-Monsoon: Role of Phytoplankton Blooms

The Arabian Sea possess an intense oxygen minimum zone (OMZ) due to high primary productivity. These important biogeochemical aspects of the Arabian Sea have led us to investigate air-sea exchanges...

First record of epibiotic parasitic dinoflagellate Ellobiopsis chattoni on copepod Bestiolina similis from northeastern Arabian Sea

Parasitic dinoflagellate Ellobiopsis chattoni was observed on the body surface of copepod Bestiolina similis collected from northeastern Arabian Sea during early spring intermonsoon. Each host was infested with a single parasite, with females being more severely affected. The study was first of its kind from northeastern Arabian Sea. With the present report, the biogeographic boundary of this species can be extended to the northeastern Arabian Sea.

Near reef abundance and on–offshore distribution of tuna larvae around Minicoy Island in Lakshadweep Sea, India

ABSTRACTNear reef abundance and coast–offshore distribution of tuna larvae around Minicoy Island were studied based on three cruises carried out on board FORV Sagar Sampada during 2014–15. The samples were collected by oblique tows using bongo twin nets (300 micron) for three seasons (Fall inter-monsoon (FIM), Spring inter-monsoon (SIM) and Summer monsoon (SM)) in six on–offshore stations ranging from 1 to 20 km off-shore of the Minicoy. Results indicated that the tuna larvae aggregate in the near reef waters and total tuna larvae abundance (all the species combined) shows a decreasing trend towards off-shore. Tuna larvae are found to be more abundant during SM and FIM. Comparison of the species-wise variation in tuna larvae for one season (SM) indicated that the larvae of Euthunnus affinis (32.5%) dominated among the tuna species, followed by Auxis spp. (31.6%), Thunnus spp. (3.9%) and Katsuwonus pelamis (2.7%). A pattern of highest larvae abundance in the near reef areas was observed except for Thunnus larvae. GAM indicated that mean seawater temperature and mean seawater salinity are the major environmental factors that significantly influence the larvae abundance, particularly for Thunnus followed by Katsuwonus pelamis, E. affinis and Auxis spp. The study confirms that the environmental parameters show high variation between stations and among transects as a result of the complex oceanographic characteristics around the islands. Further studies with better understanding of the larvae dynamics and their association with the environmental parameters including physical processes are essential to elucidate these relationships more clearly.

The large-bodied copepods off Masirah Island, Oman: An investigation of Southwest Monsoon onset and die-off

Abstract Masirah Island, situated off the southern coast of Oman in the Arabian Sea, is surrounded by upwelled water during the Southwest Monsoon (SWM) and was used as a base for time-series studies of the copepod community for a decade beginning in 2006, covering the Spring Intermonsoon (SIM), April–May, and including June, and the SWM period, August–October. Subeucalanus pileatus and Calanoides natalis were the two most abundant copepod species (33.9% and 32.7% respectively of the total abundances). These two species shifted their dominance during the two main seasonal periods. During the SWM, C. natalis was the dominant species occurring mainly at sea surface temperatures (SST) between 18 and 27 °C while S. pileatus dominated during the SIM occurring at warmer SSTs, frequently between 25 and 28 °C. Results from generalized linear models indicated that the season and the SST were the best predicted variables affecting the abundances of C. natalis and S. pileatus. Calanoides carinatus was also affected by wind speed and wind direction, meaning that strong SWM winds and low temperatures (seasonal upwelling) favor the occurrence of this species. Upwelling favorable winds lasted longer in 2015/2016 than in 2009/2010; peak winds shifted from June in 2009–2011 to July in 2012–2016. The SST was higher in 2016 than in the previous five years. Time-series of C. natalis life stages showed three large peaks of early copepodites from June through September 2016 during the SWM season, in contrast to only one peak observed at the beginning of the SWM season from previous years. This result suggests that C. natalis was able to capitalize on the longer SWM in 2016 by continuing to reproduce throughout September. Diapausing C. natalis reached the surface in mid-to-late June as stage CV, ruling out any reproduction by this species at its overwintering depth. Unfavorable SST at the end of the SWM corresponded to a downward migration by C. natalis in 2015. Overall, the physical forcing of this upwelling region appears to be changing and C. natalis seems to be adapting to these changing conditions by continuing to reproduce at the surface as long as SST is favorable.

Seasonal spreading and transport of buoyant plumes in the shelf off Kochi, South west coast of India- A modeling approach

We investigated the seasonal spreading and transport of buoyant plume in the shelf off Kochi using Finite Volume Community Ocean Model (FVCOM). The modelled river plume typically consisted of an offshore bulge and a coastal current. The spreading of the bulge extended up to a distance of 19 km from inlet during the summer monsoon to <10 km in the spring inter-monsoon. The Kelvin number varied between 0.1 and 0.9 which revealed that the plume exhibited both the features of small and large scale plumes, resulting in a highly complicated plume pattern. During the southwest monsoon the plume fringe twisted towards the south, while during the northeast monsoon it twisted towards north according to the reversal of monsoonal winds. The fresh water transport with respect to coastal currents varied in accordance with seasonal river discharge such that the value peaked in the wet season and dropped in the dry season. During the non-realistic (no wind) condition the plume initiated barotropic and baroclinic flow, after which it was acted upon by earth’s rotation so that the plume propagated in the direction of Coriolis force (towards north), as geostrophic currents. The model run ‘with wind’ and ‘without wind’ condition revealed that in the shelf off Kochi the plume is transported in accordance with monsoonal winds/currents by nullifying the effect of earth’s rotation. The categorization of plume influenced area and realization of the direction of plume transport can be used for interpreting the dynamically and potentially active zones in the shelf off Kochi.

Subsurface chlorophyll maxima in the North Eastern Arabian sea: Simulation on impact of warming

Abstract Stratified tropical oceanic systems are in general observed with Subsurface Chlorophyll Maxima (SCM), which was identified as light adaptation of shade-loving picophytoplankton groups. Apart from the light adaptation strategies of phytoplankton, the physical properties of water masses have significant role to hold the phytoplankton in particular layers. The present study gives theoretical explanation on the influence of fluid properties of water on the settling velocities of micro-nano phytoplankton groups, which contribute the SCM. The data and samples were collected from North Eastern Arabian Sea (NEAS) during March (Spring Inter Monsoon), by means of Bio-Argo floats, CTD and remote sensing. The analysis gives indications to the possible strengthening of SCM in the scenario of warming and enhanced stratification. The study conducted simulations using basic stoke’s equations on most abundant species of the area and found that the fluid density has a significant role in the settling of non-motile, suspended phytoplankton groups, irrespective of their cellular density. The simulations show strong decelerations at the same depths, in the upper part of the pycnocline but with varying settling velocity. A numerical expression derived based on curve fitting and multiple regression analysis substantiates the influence of vertical density on SCM. The sensitivity analysis (Global sensitivity Analysis) indicates warming trend in NEAS strengthening the stratification, which in turn influences the concentration in SCM and is capable of altering the primary production.