Southeastern Arabian Sea Research Articles

Anthropogenic marine litter: An approach to environmental quality for India's southeastern Arabian Sea coast

Anthropogenic marine litter (AML), mainly plastic, is a global concern that is persistent and widespread. To prevent and mitigate this threat, we need to understand the magnitude and source of AML. There is limited knowledge about AML pollution on the Indian Coast. In this context, the present study examined the distribution, abundance, typology, and beach quality based on AML along 22 beaches on the southeastern coast of the Arabian Sea. A total of 4911 AML items were classified into 9 categories, weighing 16.79 kg, and retrieved from a total area of 8000 m2. The mean abundance and weight of AML in the current study were 0.45 ± 0.34 items/m2 and 1.53 ± 0.92 g/m2, respectively. Thottapally showed the most abundant AML among the studied beaches with 0.96 items/m2, followed by Azheekkal with 0.73 items/m2. Plastic, being the most common item, accounts for 77.6 % of all items and has a mean density of 0.35 items/m2 comprising hard plastic (22 %), thermocol (13 %), food wrappers (7 %), cigarette butts (7 %), plastic rope (6 %), and plastic cutlery (6 %). Hazardous anthropogenic litter (HAL) was maximum at Thottapally (17.71 %; 85 out of 480 items collected). Based on the cleanliness of beaches, they are graded “moderately clean” (63 %) by the General Index (GI), “clean” (54 %), and “moderately clean” (40 %) as calculated by the Clean Coast Index (CCI). Hazardous Anthropogenic Beach Litter Index (HABLI) classifies 72 % of beaches as “moderately safe”, while the Environmental Status Index (ESI) rates 68 % of beaches as “mediocre”. Besides, model simulations demonstrated the pathways of AML propagation, which correlate to the littoral and coastal current flow patterns over the region. Land-based activities were the crucial factors influencing AML distribution. The study highlighted the need for effective regional litter management strategies, policy instruments for the litter impact pathways, economic, regulatory, and behavioural management tools, which were also discussed.

Diet composition and feeding habits of flat needlefish Ablennes hians (Valenciennes, 1846) (Beloniformes: Belonidae) in the Southeastern Arabian Sea

Diet composition and feeding habits of flat needlefish Ablennes hians (Valenciennes, 1846) (Beloniformes: Belonidae) in the Southeastern Arabian Sea

Nutritional variability in selected deep-sea fish species from the southeastern Arabian Sea: A comparative proximate analysis

Nutritional variability in selected deep-sea fish species from the southeastern Arabian Sea: A comparative proximate analysis

Assessing the geographical distribution of carnivorous and planktivorous fishes in the western Bay of Bengal

There is a clear distinction in the feeding habits of fishes between the northeastern and southeastern Arabian Sea (west coast of India) — the north is carnivorous, and the south is planktivorous —, but such a distinction is not available for the western Bay of Bengal (BoB; comprising the east coast of India and the coasts of Sri Lanka and Bangladesh). We address this question by using publicly available landing data of finfishes for the four Indian states (Tamil Nadu, Andhra Pradesh, Odisha, and West Bengal) located along the western boundary of the BoB. Our analysis shows that the southwestern BoB is rich in fisheries and is more planktivorous compared to the northwestern BoB, which is more carnivorous. North–south variability exists even within the group of fishes such as anchovies and carangids. The meagre landing information for Sri Lanka and Bangladesh suggests that they show planktivorous and carnivorous characteristics of the southwestern and northwestern BoB, respectively. We show that monthly data and clarity on where the fish were caught are necessary to link the climatic conditions to the fishery along the western BoB. The landings on the Arabian-Sea and Gulf-of-Mannar part of the Tamil Nadu coast must be separated from the landings on its BoB coast. This separation can not be done with existing landing data and should be the first step in analysing fisheries data from the western BoB. The challenges in delineating the distribution of finfishes using available data illustrated here deliver guidance for future data collection and analysis at higher spatial and temporal scales for sustainable fisheries resource management in the region.

Length-weight relation and relative condition factor of six deep-sea fishes from the south-eastern Arabian Sea

Marine biology,marine ecology,Phytoplankton, marine algae, zooplankton, nekton, benthos, corals and coral reefs, ecology, zoogeography and systematics, primary production and experimental work on ecosystem including energy flow at different trophic levels, population dynamics and theoretical models, recycling of matter, influence of environmental factors on metabolic activity, marine microbiology, mariculture and experimental work on propagation of cultivable species, biology of fishes, marine reptiles, sea birds and mammals, physical, chemical, geological and meteorological features of the environments related to marine biology, marine environmental damage caused by pollution and engineering works, underwater observations, conservation of living resources, modifications in methodology and techniques and development of new apparatus related to marine sciences

Coupled physical-biogeochemical simulations of upwelling, ecological response to fresh water

An eddy resolving 1/12⁰ coupled physical biogeochemical ocean model configured for the Indian Ocean is used to study the upwelling off the west coast of India, the dynamics of the south eastern Arabian Sea and the associated bio-physical-chemical mechanisms. The model reproduces well the climatological features and local and remotely forced dynamical processes of the upwelling system in physical variables as well as biogeochemical parameters. Modelling experiments show that fresh water from rainfall and river runoff alter the phytoplankton concentration as well as the distribution of the phytoplankton size classes. It is seen that rainfall influences the nanoplankton concentrations while river runoff influences larger diatom concentrations. Fresh water due to rainfall shoals the mixed layer (ML) and thermocline restricting upwelling due to enhanced stratification resulting in reduced nutrient and thereby reduced chlorophyll, essentially nanoplankton. Further a thinner and warmer ML modifies thermodynamics and can consequently influence zooplankton by altering mesozooplankton grazing. River runoff additionally influences by supplying nutrient causing a reduction in diatom and increase in nanoplankton concentrations thus increasing the preferred prey of zooplankton in the SEAS. Our simulations indicate phosphorous limitation in the region. The biophysical controls linking ecological responses to fresh water like zooplankton food preferences are detailed with discussions on implications to food-web dynamics of the region.

Sea Surface Salinity Anomaly in the Bay of Bengal during the 2010 Extremely Negative IOD Event

Based on Soil Moisture and Ocean Salinity (SMOS) data and the Ocean Reanalysis System 5 (ORAS5) dataset, positive salinity anomalies exceeding 2 psu in the northern Bay of Bengal (BoB) and negative salinity anomalies with the peak of the freshening anomalies reaching −2 psu around Sri Lanka were observed in autumn 2010. Here, an analysis of the anomalous salt budget revealed that anomalous horizontal advection contributed most to the variability in salinity in the BoB. With the development of La Niña and negative Indian Ocean dipole (nIOD) in summer and autumn, the strong summer monsoon current and Wyrtki jet combined with the anomalous basin-scale cyclonic circulation led to more high-salinity water entering the northern BoB. In addition, more freshwater was transported southward along the eastern coast of India by east Indian coastal current (EICC) in autumn, resulting in extremely negative salinity anomalies around Sri Lanka and positive salinity anomalies in the northern BoB. Moreover, the freshwater around Sri Lanka was carried farther into the southeastern Arabian Sea by the west Indian coastal current (WICC) in November, which affected the salinity stratification in winter and then influenced the variation of the Arabian Sea Mini Warm Pool (ASMWP) in the following spring. The ASMWP could affect the Indian summer monsoon (ISM) through its influence on the monsoon onset vortex (MOV) over the southeast Arabian Sea (SEAS).

Squat lobsters of the genera Raymunida Macpherson & Machordom, 2000 and Munidopsis Whiteaves, 1874 (Decapoda: Anomura: Galatheoidea) from the Indian Exclusive Economic Zone, with descriptions of three new species

ABSTRACT We report four squat lobster species of the genera Raymunida Macpherson and Machordom, 2000 (Munididae) and Munidopsis Whiteaves, 1874 (Munidopsidae) from the Indian Exclusive Economic Zone. The genus Raymunida is recorded for the first time from Indian waters, including a new species R. shraddhanandi sp. nov. and R. vittata Macpherson, 2009 based on material from the Andaman Sea. Raymunida shraddhanandi sp. nov. differs from its closest congener R. formasanus Lin, Chan and Chu, 2004 in the armatures of the anterior branchial carapace region, antennal peduncle and third maxilliped merus, and the length of the fourth pereopod. Two new species of the genus Munidopsis, referred to the Anoplonotus group, are described from the south-western Bay of Bengal and the south-eastern Arabian Sea. Munidopsis bengala sp. nov. and M. kadal sp. nov. are distinguished from all of their allies – M. bruta Macpherson, 2007, M. shulerae Vázquez-Bader, Gracia and Lemaitre, 2014, and M. truculenta Macpherson and Segonzac, 2005 – by the unarmed dorsodistal margin of the third maxilliped merus and the relative length of the antennal peduncle, respectively. http://www.zoobank.org/urn:lsid:zoobank.org:pub:5C724556-6532-41F2-9A6E-8424CAE0F2C6

Recurrent dinoflagellate blooms in the South-eastern Arabian Sea - A preliminary assessment with focus on Ornithocercus magnificus blooms along the nearshore waters off Dakshina Kannada, South-west coast of India

Frequent bloom incidences of the dinoflagellate Ornithocercus magnificus Stein, 1883 along the surface waters off South-eastern Arabian Seas (SEAS) have been reported since 2015 and specifically during the years 2015, 2019 and 2020. All these blooming incidences coincided with the strong El Nino phases in the Indian Ocean. The present study addressed the bloom (1.65 x 106 cells l-1) of this species in the nearshore waters (7 m depth) off Dakshina Kannada (Surathkal and Chitrapur) on 11.02.2016 which occurred during pre-monsoon period and corresponding to the super El Nino year 2015/2016. A detailed evaluation of the physico-chemical characteristics and phytoplankton abundance as well as community structure was made, concurrent to the three phases viz., pre-bloom, bloom and post-bloom periods in this region. The study revealed that the bloom occurred in high saline (35.0±0.91 PSU), well oxygenated (7.414±0.823 mg l-1) and nitrogen limited waters (oligotrophic conditions) and had a positive correlation with oceanic nino index (ONI; rs= 0.790, p<0.001). Some of the O. magnificus (3%) harboured ectosymbionts probably cyanobacteria in their cingulum in response to the beginning of a stratified oligotrophic condition in this region. The water quality was fair during the bloom period with no conspicuous discolouration of the surface waters. The present study also attempted to evaluate the influence of the increasing frequency of the O. magnificus blooms on the oilsardine fishery along the SEAS. Keywords: Cyanobacteria, Dinoflagellate, Ectosymbionts, Nitrogen limited waters, Pre-monsoon season, SEAS, Sea surface temperature (SST), Oilsardine

Spatial and seasonal trends of trace metals in the surficial sediments from off Kochi - Geochemistry and environmental implications.

Fifty four sediment samples representing pre and post-monsoon seasons were collected along a transect from off Kochi, lying between the latitudes 9°57'59.5″-9°54'30.4″ and longitudes 76°11'7.04″-75°38'50.3″ of the South eastern Arabian Sea. The present study investigates the levels of trace metals (Zn, Cd, Pb, Cu, Fe, Ni, Mn, Co and Cr), total organic carbon (TOC), total inorganic carbon (TIC), elemental composition and grain size to assess the extent of environmental pollution and to discuss the distribution of these trace metals in the surficial sediments. Sediment pollution assessment was done using the Contamination factor (C.F), Geoaccumulation Index (Igeo), Enrichment Factor (EF), and Pollution Load Index (PLI). The majority of trace metals analysed in this study exhibited the highest concentrations at stations 1, 2 and 3 where the land-based anthropogenic input was found to be maximum.

Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd ∕ Ca records and benthic assemblage analyses

Abstract. We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cdw records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate–bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cdw variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate–bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cdw more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cdw at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cdw in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediate-water Cdw records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cdw by monsoon-induced changes in upper-water stratification and surface primary productivity.

Dynamics of trace metals in sediments of a seasonally hypoxic coastal zone in the southeastern Arabian Sea

This study examined the effect of water column hypoxia on the distribution and geochemical fractionation of trace metals in the seasonally hypoxic coastal environment in the southeastern Arabian Sea. Water and surface sediments were collected fortnightly from the Alappuzha mud bank between April and August 2016, which covered the pre-upwelling and upwelling seasons. The water column was warm and well-oxygenated during April–May. During June–August, the incidence of cold and hypoxic water indicated strong coastal upwelling prevailed in the entire study domain. The Fe and Mn content in sediments gradually decreased, because of the reductive dissolution and subsequent release of metals under hypoxia. The concentration of metals such as Ni, Zn and V decreased substantially under oxygen deficiency, whereas Cr showed marked enrichment in sediments. Although the geochemical forms of trace metals displayed the dominance of residual fractions (inert), the reactive non-residual metal forms (exchangeable, Fe/Mn-(oxy)hydroxide, and organic matter/sulphide bound) showed considerable variability under hypoxia. The shift from Fe/Mn-(oxy)hydroxide bound to organic matter and sulphide bound was evident during hypoxia. Cr exhibited a strong affinity towards organic matter and sulphide, and Pb and Zn showed relatively high association towards the Fe/Mn-(oxy)hydroxide phase. Even with such a phase shift induced by the hypoxic conditions, the concentrations of these metals remained within the normal background levels, indicating the pristine nature of the mud bank environment.

Tracing impact of El Niño Southern Oscillation on coastal hydrology using coral 87Sr/86Sr record from Lakshadweep, South-Eastern Arabian Sea

El Niño Southern Oscillation (ENSO) is one of the dominant climate modes influencing global precipitation and temperature. ENSO has a large impact on the monsoonal precipitations over the Indian subcontinent and thereby influences hydrological conditions. Due to dearth of long-term instrumental records of the hydrological parameters on sufficient spatial resolution, it is difficult to assess the impact of ENSO on regional hydrology. Though several geochemical proxies have been used to reconstruct past ENSO events through tracing the changes in past hydrological and climatic parameters, however, such reconstructions are often complicated by the influence of multiple processes and/or factors and their nonlinear relation with the proxy records. In this study, Sr isotope composition (87Sr/86Sr) was measured in Porites coral from the Lakshadweep, south-eastern Arabian Sea to reconstruct past ENSO events and to trace its regional hydrological impacts. The high precision measurements of 87Sr/86Sr in Lakshadweep coral show resolvable variations ranging from 0.709080 to 0.709210. The 87Sr/86Sr record shows an inverse relation with Niño 3.4 record; lower values matched with El Niño years and higher values with La Niña years. Our investigation reveals that ENSO driven precipitation changes impacted submarine groundwater discharge (SGD) to the Minicoy Atoll and resulted in 87Sr/86Sr variations of the Minicoy Atoll water. Therefore, deviation from the average seawater 87Sr/86Sr value can be quantified in terms of SGD contribution to the Minicoy Atoll. Our estimates based on binary mixing between seawater and SGD 87Sr/86Sr suggest a significant supply of SGD, maximum up to ~20 % of the total volume of the Minicoy Atoll during La Niña years due to higher rainfall compared to El Niño years. This finding highlights potential application of coral 87Sr/86Sr record as an alternate proxy to reconstruct past ENSO events and to trace its quantitative impact on regional hydrology, chemical and nutrient fluxes to coastal oceans via SGD.

Landings of IUCN Red Listed finfishes at Chetlat Island of Lakshadweep, southeastern Arabian Sea

The Lakshadweep Islands are well-known for their abundant fishery resources. Present study primarily focused on the systematic representation of IUCN (International Union for Conservation of Nature) Red Listed marine finfish landings of Chetlat Island of Lakshadweep archipelago (India). Monthly collections were carried out from September 2019 to February 2020 from the study area. A list of finfishes along with their scientific name, common name, family, and present conservation status was prepared. As per the IUCN Red List, out of 41 fish species identified, one species is ‘Endangered’ (EN), two species are ‘Near Threatened’ (NT), four species are ‘Vulnerable’ (VU), one species ‘Data Deficient’ (DD), 29 species ‘Least Concern’ (LC), and four species are ‘Not Evaluated’ (NE) categories. Information on the conservation status of fishes plays a significant role in fisheries science since it forms the basis for managing marine fishery resources.

Distinct environmental parameters influence the abundance of living benthic foraminifera morphogroups in the southeastern Arabian Sea.

The ambient environmental parameters have a great bearing on the morphology of living flora and fauna. In this study, we tested this hypothesis on one of the most dominant groups of living unicellular marine microorganism, benthic foraminifera, in the dynamic region of the southeastern Arabian Sea. The living benthic foraminifera from 43 surface samples collected between 25 and 2980m of water depth were segregated into eight morphogroups (tapered/cylindrical, flattened-ovoid, biconvex, planoconvex, flattened-tapered, spherical, rounded-trochospiral, and rounded-planispiral). We report that the high organic carbon availability is combined with deficiency of oxygen results in benthic foraminifera with low surface area to volume ratio and mostly consists of tapered/cylindrical, flattened-ovoid forms, with a preference for infaunal habitat. However, the tests of the living benthic foraminifera thriving in the oxygen-rich bottom waters have a high surface area to volume ratio, commonly reported as epifaunal, consisting of biconvex and planoconvex forms. Additionally, we also report that the abundance of other morphogroups, namely flattened-tapered, spherical, rounded-trochospiral, and rounded-planispiral, is also controlled by the distinct environmental parameters. We suggest that the living benthic foraminifera are an excellent indicator of the ambient environmental parameters and can be used to reconstruct paleoenvironments.

A Case Study on the Dynamics of Phytoplankton Blooms Caused by Tropical Cyclones in the Southeastern Arabian Sea

A phytoplankton bloom was observed in the southeastern Arabian Sea after the passage of the anti-S-type track Tropical cyclone ARB 06 in 1998, which lasted for 22 days. The dynamic mechanism of the phytoplankton bloom was investigated with high-resolution satellite remote sensing data, reanalysis data and buoy data. The results show that the double thermocline structure hindered the uplift of deep nutrients to the surface before the tropical cyclone arrival. During and after the passage of the cyclone, the shallower thermocline disappeared, the deeper thermocline changed from 45.7 m to 76.5 m, the mixed layer deepened, and the upper oceanic stratification weakened. Additionally, a weak ocean eddy pair was enhanced gradually after the passage of cyclone. The relative vorticity of the ocean eddy pair was calculated. In the oscillation with a period of two days, the vorticity of the cyclonic eddy was stronger than that of the anticyclonic eddy, which enhanced a strong upwelling at a depth of more than 55 m for more than 10 days. The vertical current shear generated by the oscillation not only enhanced the vertical mixing, but also plays an important role in the distribution of phytoplankton blooms. The phytoplankton bloom was triggered by the nutrient below the thermocline into the sea surface, where a strong upwelling and weakened oceanic stratification occurred after the passage of cyclone. This study offers new insights on the mechanism of phytoplankton bloom induced by tropical cyclones and will be helpful for evaluating tropical cyclone-induced biological responses in future.

Relative dominance of wind stress curl and Ekman transport on coastal upwelling during summer monsoon in the southeastern Arabian Sea

Coastal upwelling in the south eastern Arabian Sea (SEAS) that occurs and intensifies during the summer monsoon season (June–September) is assessed in terms of alongshore wind stress and its curl using ECMWF reanalysis (ERA 5) winds for the period 2016–2020. Critical role of wind stress curl and its spatio-temporal variability, with advancing monsoon in modulating upwelling along the coast, is studied using Ekman pumping and vertical velocities integrated from coast to offshore boundary of positive curl. From the analysis it is observed that the Ekman transport dominates the southern part of the region while Ekman pumping is stronger for the northern section of the study area. This study also helps to delineate the offshore extent of upwelling throughout the monsoon season with a strong yet narrow zone hugging the coast during July/August (peak monsoon) while a broad and diffused upwelling zone for the rest of the season. This spatio-temporal variability in coastal upwelling is dominated by wind stress curl and its significance was not yet explored well for the SEAS region. From the analysis, 2016 was observed to be strong upwelling, while 2019 was the weakest upwelling season as inferred from data on wind, sea surface temperature, sea level anomaly and chlorophyll-a concentration.

Upper layer characteristics of the South Eastern Arabian Sea associated with an unusual low saline pool during fag end of southwest monsoon

Upper layer characteristics of the South Eastern Arabian Sea associated with an unusual low saline pool during fag end of southwest monsoon

Marine heatwaves in the Arabian Sea

Abstract. Marine heatwaves (MHWs) are prolonged warm sea condition events that can have a destructive impact on marine ecosystems. The documentation of MHWs and assessment of their impacts is largely confined to a few regional seas or to global mean studies. The north Indian Ocean received almost no attention in this regard despite the fact that this ocean basin, particularly the Arabian Sea, has been warming at the most rapid pace among the other tropical basins in recent decades. This study shows the characteristics of MHW events for the Arabian Sea during 1982–2019. Our analysis shows that the duration (frequency) of MHWs exhibits a rapidly increasing trend of ∼20 d per decade (1.5–2 events per decade) in the northern Arabian Sea and the southeastern Arabian Sea close to the west coast of India, which is a multifold increase in MHW days (frequency) from the 80s. Notably, since the beginning of the satellite record, the years 2010 and 2016 have exhibited the maximum number of heatwave days when more than 75 % of days of the pre-monsoon and summer monsoon season experience heatwaves. The accelerated trend of the heatwave days is found to be driven by the rapid rise in the mean sea surface temperature (SST) of the Arabian Sea in the recent decade. Moreover, longer heatwave days are also associated with dominant climate modes. Among them, the Indian Ocean Basin Mode via the decaying phase of El Niño is the most influential mode contributing to more than 70 %–80 % of observed heatwave days in this basin. Further analysis of the most prolonged observed heatwave during April–June 2010 indicates that surface heat flux associated with the weaker latent heat loss and the shallow mixed layer was the primary cause of this event. Further, we note that the pre-monsoon cyclonic storms in the Arabian Sea often contribute to the waning of such heatwaves in the basin.

The puzzle of the anchovy–sardine inverse fishery at the south‐eastern coast of the Arabian Sea and climate variability

AbstractAn out‐of‐phase pattern of fishery between anchovy and sardine exists in upwelling systems of different geographical locations. We examined the inverse relationship of anchovy (Stolephorus sp., Engraulidae) and sardine (Sardinella longiceps, Clupeidae) fishery along the coast of the south‐eastern Arabian Sea (SEAS) by analysing the annual landing data of Kerala, Karnataka, and Goa states. Landings of anchovy and sardine showed wide fluctuations with distinct episodes of abundance and decline. We selected anchovy period (1984–1996) and sardine period (2000–2013) for close examination according to the environmental data availability. The study reveals that the observed inverse fishery along the coast of SEAS is well connected to regional environmental parameters and global climate variability. During the abundant anchovy period, the anomalies of surface currents were observed towards the north and also towards the coast during the spawning periods of anchovy (March–May, October–December). The pattern of surface current anomalies is reversed during the abundant sardine period. Anomalies of surface current is directed towards offshore during the early life stage of sardine (June–September), but it reversed towards the coast during the abundant sardine period. Similarly, an opposite upwelling pattern is noted in both species’ spawning seasons during the anchovy and sardine abundant periods. During these periods, the spatial distribution of global surface air temperature and wind anomalies showed contrasting spatial patterns and hence they are related to various climate indices in sardine and anchovy abundant periods in the SEAS.