Algal blooms have been documented along the west and east coasts of India. A review of bloom occurrences in Indian waters from 1908 to 2009 points out that a total of 101 cases have been reported. A ...comparison of the bloom cases reported before and after the 1950s reveals that there is an increase in the number of bloom occurrences. The reports of algal blooms indicate their predominance along the west coast of India especially the southern part. Majority of the blooms reported along the west coast of India are caused by dinoflagellates, whereas diatom blooms prevail along the east coast. There have been 39 causative species responsible for blooms, of which
Noctiluca scintillans
and
Trichodesmium erythraeum
are the most common. Reporting of massive fish mortality in Indian waters has been associated with the blooming of
Cochlodinium polykrikoides
,
Karenia brevis
,
Karenia mikimotoi
,
N. scintillans
,
T. erythraeum
,
Trichodesmium
thiebautii
and
Chattonella marina
. Most of the blooms occurred during withdrawal of the south-west monsoon and pre-monsoon period. In Indian waters, this process is mainly influenced by seasonal upwelling and monsoonal forcing that causes high riverine discharge resulting in nutrient-enriched waters that provides a competitive edge for blooming of phytoplankton species.
•Field data used to understand interannual variability in biophysical coupling in the SO.•High N:Si value at SAZ in Summer was associated with winter/spring diatom bloom.•Sea ice melting played an ...important role in interannual variability of CHL at AZ.•Frontal zones are characterized by strong interannual variability.
To understand the role of physical processes and their interannual variability on the dissolved nutrient concentration and phytoplankton biomass distribution, field data collected in the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2009–2011 are used. In the subtropical zone, macronutrients were limited (N:P < 1, N:Si < 1, Si:P ≈ 1) and the phytoplankton biomass variability was mainly governed by the mesoscale eddy activity associated with the Agulhas Return Current. High nutrient low chlorophyll condition prevailed in the sub-Antarctic zone and further south. A South-North gradient of the upper layer dissolved SiO2 was higher than that of NO3. The sub-Antarctic zone was characterized by the highest N:Si ratio (>4) and it was associated with the enhanced draw down of silicate due to the winter/spring diatom blooms in the region. The chlorophyll-a (CHL) concentration in the Polar Frontal Zone was low (∼0.2 mg m–3) in 2009 and 2010 but it was high (0.5 mg m–3) in 2011. This increase in CHL in 2011 was due to the supply of dissolved iron from the strong winds and subsequent mixing during the winter of 2010. Further, the increased CHL values in the Antarctic zone (0.5 mg m–3) in 2011 compared to 2009 and 2010 could be due to the increased sea ice melting associated with positive Southern Annular Mode. The increased phytoplankton biomass in the summer of 2011 coincides with an increase in nitrate utilization (N:P≈13) compared to 2009 and 2010. Observations showed that ISSO frontal zones are characterized by inter-annual variability in terms of nutrient utilization and phytoplankton biomass production.
Harmful algal blooms (HABs) have been documented along the coasts of India and the ill effects felt by society at large. Most of these reports are from the Arabian Sea, west coast of India, whereas ...its counterpart, the Bay of Bengal (BOB), has remained unexplored in this context. The unique characteristic features of the BOB, such as large amount of riverine fresh water discharges, monsoonal clouds, rainfall, and weak surface winds make the area strongly stratified. In this study, 19 potentially harmful species which accounted for approximately 14% of the total identified species (134) of dinoflagellates were encountered in surface waters of the BOB during November 2003 to September 2006. The variations in species abundance could be attributed to the seasonal variations in the stratification observed in the BOB. The presence of frequently occurring HAB species in low abundance (≤40 cell L
− 1
) in stratified waters of the BOB may not be a growth issue. However, they may play a significant role in the development of pelagic seed banks, which can serve as inocula for blooms if coupled with local physical processes like eddies and cyclones. The predominance of
Ceratium furca
and
Noctiluca scintillans
, frequently occurring HAB species during cyclone-prone seasons, point out their candidature for HABs.
Microplastic pollution in marine waters around the globe is increasing exponentially. This is the first comprehensive review which focuses on microplastics as a source and vector for metals, ...antibiotics, toxic chemicals, pathogenic bacteria (Vibrio cholerae), and Harmful Algal Bloom (HAB)-forming dinoflagellates across the continents through ballast water. Microplastics in ballast waters serve as ‘hotspots’ for the development and spread of multiple drug-resistant human pathogens through co-selection mechanisms. Microplastic inoculation at distant countries through ballast water may pose a serious threat to human health due to higher incidences of bacterial disease outbreaks and HABs. The 2017 ballast water management convention lacks a provision for on-board treatment of microplastic-contaminated ballast water. We conclude that there is a pressing need to include microplastics in the ballast water management convention as a hazardous material. Efficient on-board ballast water treatment strategies and effective limits for microplastics in ballast waters need to be developed.
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•Metals, chemicals and antibiotics adsorb on microplastics in contaminated waters.•V. cholerae and HAB-producing species form biofilms on microplastics.•Ballast water transports microplastics and ‘plastisphere’ communities across continents.•Disease outbreaks and Harmful Algal Blooms can occur in the recipient country.•Ballast water convention should include microplastics as a hazardous material.
Phytoplankton composition plays a major role in biogeochemical cycles of the ocean. The intensity of carbon fixation and export is strongly dependent on the phytoplankton community. Yet, the ...contribution of different types of phytoplankton to the total production on various communities is still poorly understood in the Indian Ocean sector of Southern Ocean (SO). Therefore the variability of chlorophyll-
(
) and diatoms in the frontal ecosystems of the Indian sector of SO have been investigated along with the sea surface temperature (SST), sea surface wind (SSW), photosynthetically active radiation (PAR), and nutrients datasets for the period of 1998–2012. Combined analysis of
, model and satellite observations indicate that the variability of
and diatoms were primarily influenced by light and wind. The
was higher at the sub-Antarctic front (SAF) followed by the sub-tropical front (STF) and the polar front (PF). The diatom concentration was higher at the SAF followed by the PF and STF. Maximum concentration of
and diatoms commonly observed at the SAF region are probably due to the moderate PAR, SST and wind. Dominance of diatoms at the PF may be attributed to their adaptability for low light conditions. The results from this study in the frontal ecosystems would help to understand the biogeochemical cycle of the Indian sector of the SO.
Ballast water transport is considered as one of the major vectors for dispersal of microplastics around the global oceans. In this commentary, a simple, inexpensive solution has been proposed to ...reduce microplastic pollution and its mobility via ballast water. A screening chamber (with stainless steel three layered mesh) is proposed to be attached to the existing Ballast Water Treatment Systems (BWTSs) in cargo ships to filter back-flushed sea water from BWTSs. The three layered screens (500, 300 and 100 μm) will not only avoid clogging and easy separation of different size groups of microplastic particles but also help in smooth discharge of water to the sea. This technique is expected to remove a large number of microplastic particles (ranging from 0.0015 to 1020 million) from a single voyage. The proposed chamber may help to collect 0.0003–204 metric tons of particles/day, depending upon the geographical location of ballast intake in the global ocean. These estimations were made by considering a daily turnover of 0.033 billion tonnes of ballast water globally. This proposed screening chamber attached to the existing BWTSs in cargo ships, along with other region-specific ocean cleaning initiatives, will help in mitigating microplastic pollution in the global ocean.
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•A three layered screen chamber in BWTS will mitigate MP pollution in the marine environment.•The proposed technique will remove approximately 0.0015–1020 million microplastic particles in a single voyage.•An estimated 0.0003–204 metric tons of MPs (globally) could be removed per day.•This technique, along with region-specific ocean cleaning programmes, will reduce MP pollution.
A simple technique to mitigate microplastic pollution in the global ocean.
Upper layer diapycnal mixing in the Subtropical Front (STF) was estimated using microstructure shear profiles collected from the Indian sector of the Southern Ocean (ISSO) during the austral summer ...of 2012. Observations were made in the northern and southern boundary of the highly mesoscale turbulent STF, which is characterized by the presence of the dynamic Agulhas Return Current. During the observational period, the STF was populated with alternating cyclonic and anticyclonic eddies. In this mesoscale turbulent region, the average eddy diffusivity at the base of the euphotic zone was 5.5 × 10−5 m2 s−1. The average diapycnal nitrogen flux at the base of the euphotic zone, calculated using direct turbulence measurements, and nitrate (NO3) and nitrite (NO2) concentrations, was 6.4 × 10−5 μ mol m−2 s−1. The satellite-derived primary production in the STF was ~1000 mg C m−2 day−1. The observed diapycnal nutrient flux could only sustain <1% of the production observed in the region. Analysis of satellite-derived ocean currents, sea level anomalies, and thermohaline distribution further shows that despite the study area is a highly mesoscale turbulent region, the primary supply of nutrients is a result of advection (vertical or zonal) rather than vertical mixing.
•Enhanced surface Chl‑a and nutrients are noted in Subtropical front (Indian Ocean).•Inferred diapycnal nutrient flux at the base of euphotic zone.•Contribution of diapycnal nutrient flux to the primary production was negligible.•Major sources of surface nutrients are horizontal/vertical advection.
Phytoplankton, the primary producers in all aquatic systems, plays an important role in key biogeochemical processes that are linked to higher trophic levels and climate variability. The present ...study deals with the phytoplankton community structure in the Indian Ocean, particularly in the higher latitudes with respect to environmental variables to understand the region specific dominant community and its governing environmental settings. The study areas were selected along the latitudinal transect between 3oN and 53oS (northern Indian Ocean to Indian Ocean sector of Southern Ocean). The surface water phytoplankton community based on microscopy coupled with diagnostic pigment indices showed marked variation in community structure from tropical to polar latitudes of the Indian Ocean. The Prokaryotic diagnostic pigment (ProkDP) dominated in the Equatorial and South Equatorial regions, the Flagellate diagnostic pigment (FlagDP) in the North Equatorial region (NER), Southern Tropical Indian Ocean and Sub-tropical Front (STF) region whereas, the Diatom diagnostic pigment (DiatDP) dominated only at the Polar Front (PF) region. The influence of a suite of environmental variables - temperature, nutrients, salinity and mixed layer depth (MLD), on the dominant phytoplankton groups at the STF and PF was observed. This understanding of community dominance from this poorly explored area with respect to influencing factors is very vital baseline information to design the perturbation experiments in future work to understand the phytoplankton process studies of each region.
The juncture of the Agulhas Return Front (ARF) and Subtropical Front (STF) in the Indian Ocean sector of Southern Ocean (SO) is characterized by high mesoscale turbulence, which results in sporadic, ...short lived phytoplankton proliferation. The biota, mainly the phytoplankton community from such a complex hydrodynamic region and its response to the mesoscale turbulence, are areas of interest for investigation. Hence, during the sixth Indian expedition to SO, a two-day time series was occupied at the ARF and STF merged region (40°S 58°30′E) from 13 to 15 January, 2012. The vertical profiles of phytoplankton (based on pigment indices) indicated variation in the percentage contribution of phytoplankton functional groups (Micro, Nano and Pico). Though the overall community structure was dominated by nanoplankton, as exhibited by pigment indices and CHEMTAX analysis, drastic shifts in the community were observed at 120m depth at six hourly intervals. The oscillation between Flagellates (nanoplankton) to prokaryotes (picoplankton) and then to diatoms (microplankton) at this depth in three consecutive observations coincided with the significant variations in phosphate and nitrate concentrations, along with increase in abundance of the grazer community (ciliates and heterotrophic dinoflagellates). From the present study, it is evident that the flagellate group is the ideal one to survive in such a complex regime. However, the observed small interval oscillation in the phytoplankton community could be a coupled effect of bottom-up (vertical advection that alters the nutrient flux), and top-down (increased abundance of microzooplankton) factors.
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