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Response of phytoplankton biomass to nutrient stoichiometry in coastal waters of the western Bay of Bengal
Abstract
The hydro-biological parameters such as thermohaline structure, nutrients, and chlorophyll-a (chl-a) during pre-southwest monsoon (PRSWM) and post-southwest monsoon (POSWM) periods of 2017, was investigated and compared along a coastal transect in the western Bay of Bengal (BoB). The impact of cold-core and warm-core eddies on chl-a in the very near coastal waters of the western BoB was analyzed. During PRSWM period, the water column was recharged with phytoplankton growth-promoting nutrients entrained by cold-core eddies that resulted in an increment in chl-a concentration. A three-times increase in chl-a concentration was observed due to the cold-core eddy, whereas the presence of warm-core eddy reduced the chl-a concentration by 87%. However, during POSWM period, despite having higher nitrate concentration and adequate light in the water column, the surface chl-a was substantially lesser than PRSWM. The molar ratio of ambient inorganic macronutrients during POSWM period played a significant role resulting in lower chl-a concentration. Nitrogen to phosphate (N:P) and nitrogen to silicate (N:Si) ratios were less than the Redfield values. The chl-a concentration off Godavari, a biological hotspot, was the lowest during POSWM period. This study highlights phosphate limitation off Godavari during the high river discharge period.
... Poleward flowing EICC could also favor the coastal upwelling during this season (Gomes et al., 2000;Shetye et al., 1993), but this was suppressed in the subsurface (50 m) due to warmer gyre. Restriction of coastal upwelling near Godavari embayment by warm waters and EICC was reported by Mathew et al., 2021. The riverine influx from the major rivers leads to the low salinity in the Northwestern BoB (Rao & Sivakumar, 2003;Shetye et al., 1996;Vinayachandran et al., 2002). ...
... Overall, spatial nitrate concentration was high in VD transects and minimal in the MN. Similar observations like low nitrate concertation in the entire western Bay of Bengal and more severe in the low saline influenced northern regions were reported (Jyothibabu et al., 2008;Krishna et al., 2016;Mathew et al., 2021;Sarma et al., 2013). ...
... The distribution of Chlorophyll-a in a marine environment is governed by various factors such as solar irradiation, currents, freshwater input, eddies etc. Phytoplankton biomass is highly sensitive to variations in the temperature, salinity, nutrients, turbidity etc. (Madhu et al., 2006;Gomes et al., 2000;Jyothibabu et al., 2008;Prasanna Kumar et al., 2002;Bandyopadhyay et al., 2017;Acharyya et al., 2012;Mathew et al., 2021;Sarma et al., 2020). Upwellingassociated nutrients enhance phytoplankton biomass (Gomes et al., 2000;Prasanna Kumar et al., 2002;Madhu et al., 2006;Paul et al., 2008;Sarma et al., 2020). ...
This study investigates phytoplankton functional group variations in the western Bay of Bengal (WBoB) during the Spring Intermonsoon. Samples were collected from four cross-shore transects: Mahanadi (MN), Vamsadhara (VD), Godavari (GD), and Krishna (KS). East India Coastal Current and warm gyre influenced the southern transects (KS, GD), VD was experiencing moderate upwelling and MN was characterized by low salinity and oligotrophic conditions due to freshwater input. In response to hydrography, phytoplankton biomass and functional types differed within and between the transects. Chlorophyll-a (Chl-a) was spatially high in VD and low in MN. The subsurface Chlorophyll-a maxima (SSCM) was prominent and shallow in the MN and VD, compared to the southern transects. Total diagnostic pigments concentration was high in VD, followed by GD, KS and MN. Phytoplankton functional groups and each groups contribution to Chl-a was calculated through CHEmical Taxonomy (CHEMTAX). Diatoms and cyanophytes were the dominant functional types in the surface layers. Progressive shift from diatoms in the nearshore region to cyanophytes in the offshore was observed. The low saline and low-nutrient conditions were conducible for the growth of cyanophytes, while nutrient-rich optimum light layer of SSCM and upper layer of VD were favorable for diatoms. Cryptophytes contribution to Chl-a was higher in southern transects compared to the north. Prymnesiophytes and prasinophytes were high in the subsurface and deep layers could be due to their adaptions to light and nutrients. The present study highlights the significance of physical processes associated hydrography in structuring the phytoplankton functional types.
... Though two coastal stations on the east and west coasts were established and measured for a few decades Shenoy et al., 2011Shenoy et al., , 2012, these measurements are biased by the tide and coarse temporal resolution. Establishing and maintaining measurements with high temporal resolution in the coastal waters is crucial to address this gap (Mathew et al., 2021;Samanta et al., 2023;Baliarsingh et al., 2023). A network of coastal stations, particularly moored buoys equipped with sensors for measuring biogeochemical parameters and traditional physical and atmospheric measurements, with real-time data transmission capabilities, is essential for addressing this lacuna. ...
... The coastal observatory was deployed in the coastal waters off Kochi (9 • 51.154′N; 76 • 11.955′E) at a bathymetry of 20 m. The deployment location was chosen to address the estuarine-land-atmosphere interaction with the coastal ocean and its role in biogeochemical cycles (Mathew et al., 2021). The first measurement was carried out from 25 th May, 2022 (tail end of the pre-monsoon) to 20 th August, 2022 (peak southwest monsoon). ...
... The cyclonic eddies that bring the subsurface nutrients to the surface through upwelling have positive chlorophyll anomalies at the centre of eddies. Results signifies that the presence of cyclonic eddies are responsible for the increase of sea surface chlorophyll anomalies in the BoB (Sridevi et al., 2019;Mathew et al., 2021;Siswanto et al., 2022) and reduction of chlorophyll is evident due to the presence of anticyclonic eddies in the BoB (Mathew et al., 2021;Peter et al., 2022). Cyclonic eddies that are associated with the upward movement of thermocline, which is associated with the upward velocities can carry nutrients from subsurface to surface of ocean (Falkowski et al., 1991;Prasanna Kumar et al., 2002, 2004, 2009Vidya and Kumar, 2013). ...
... The cyclonic eddies that bring the subsurface nutrients to the surface through upwelling have positive chlorophyll anomalies at the centre of eddies. Results signifies that the presence of cyclonic eddies are responsible for the increase of sea surface chlorophyll anomalies in the BoB (Sridevi et al., 2019;Mathew et al., 2021;Siswanto et al., 2022) and reduction of chlorophyll is evident due to the presence of anticyclonic eddies in the BoB (Mathew et al., 2021;Peter et al., 2022). Cyclonic eddies that are associated with the upward movement of thermocline, which is associated with the upward velocities can carry nutrients from subsurface to surface of ocean (Falkowski et al., 1991;Prasanna Kumar et al., 2002, 2004, 2009Vidya and Kumar, 2013). ...
Mesoscale eddies are the important phenomenon of world oceans that play a prominent role in influencing the near‐surface atmosphere and transport of heat. Ocean and atmosphere responses due to the presence of oceanic mesoscale eddies are not well studied in the Northern Indian Ocean (NIO) region. The present study analysed the ocean and atmospheric response due to the presence of mesoscale eddies using a composite analysis. Generally, it is assumed that the anti‐cyclonic eddies are associated with the positive anomalies of sea surface temperature (SST), and cyclonic eddies are accompanied by negative anomalies. However, for the two marginal seas in the NIO, that is, the Bay of Bengal (BoB) and Arabian Sea (AS), the composite picture of SST anomalies associated with mesoscale eddies showed an abnormal response of SST. This unusual behaviour of cold SST near anti‐cyclonic eddies and warm SST near cyclonic eddies in BoB and AS is analysed with the mean composites of precipitation, outgoing longwave radiation (OLR), and turbulent fluxes. The mean composites over all anti‐cyclonic eddies reveal the presence of enhanced precipitation and convection (low OLR), whereas over cyclonic eddies, suppressed convection and negative precipitation anomalies are evident. The composites of latent heat flux (LHF), precipitation and OLR near surface cold anti‐cyclonic eddies indicate the loss of heat from the ocean to the atmosphere, intense precipitation along with cloudy conditions are responsible for the cooling of the ocean. Along with this, stronger winds are also evident during cloudy conditions could play a role in anomalous cooling of SST observed near anti‐cyclonic through intense surface mixing. Similarly, vice versa is observed near the surface warm cyclonic eddies. The overlying atmosphere and abnormal response of SST near oceanic eddies are essential for understanding the significant role of oceanic mesoscale eddies in influencing the large‐scale circulation of the atmosphere and climate.
... Despite the high nutrient inputs characteristic of the Mo season, their effective utilization by phytoplankton is hindered (Devassy and Goes 1989). Conversely, the PoM period witnessed the lowest PP, potentially owing to insufficient N proportions in adjacent waters, which is critical for optimal phytoplankton production (Mathew et al. 2021). ...
Tropical mangroves are subjected to extreme environmental conditions yet remain highly productive ecosystems. The Mandovi and Zuari estuaries of Goa on the western coast of India are lined by mangroves and are a rich source of fisheries that supports the coastal livelihood. Despite their ecological and economic significance, the nutrient and organic matter dynamics in tropical mangrove ecosystems remain poorly studied. The hydrographic conditions in the mangrove and adjacent estuarine waters are dictated by monsoonal precipitation, freshwater input, tides, and influx of seawater, resulting in subsequent changes in their physicochemical parameters and organic matter concentrations. The impact of these changes on the phytoplankton biomass and primary productivity in estuarine waters was investigated during the pre-monsoon, monsoon and post-monsoon seasons. Dissolved inorganic nutrients, such as nitrite, nitrate, ammonia, phosphate and silicate, displayed wide seasonal variability and were higher during the low tide than those in high tide. Isotopic and elemental ratios of carbon and nitrogen revealed that primary productivity in the estuaries was enhanced by the outwelling of mangrove-derived organic matter. The highest chlorophyll a and primary productivity were observed during the pre-monsoon season, with Zuari exhibiting higher levels compared with Mandovi. Multivariate statistical analyses confirmed that primary productivity was influenced mainly by temperature, salinity, pH, dissolved oxygen and organic matter. Results from our study emphasizes the role of outwelling of organic matter from mangroves and their influence on primary productivity in adjacent estuarine waters.
... Nutrients are crucial parameters that regulate the autotrophic potential of marine ecosystems (Mathew et al. 2021). These nutrients essentially include nitrogen, phosphorus, and silica. ...
Oceanic water columns, their overall health, and the ongoing biogeochemical processes in these oceanic masses are integral in regulating several global phenomena and providing ecosystem services to humankind. Ever since the potential of oceanic water bodies and the significance of all the biotic life forms in this hydrosphere have been realized by the global scientific community, measuring and monitoring these water masses have become imperative.
The Western Pacific Ocean (WPO) is one of the most active eddy regions in the world, where a variety of ocean processes are frequently observed, but little research has been conducted on the phytoplankton communities and their photosynthetic physiological status within the eddies in this region. The bio-optical parameters of phytoplankton communities and their physiological status within the warm core and cold core eddies of the WPO during the winter of 2021 were investigated based on fast repetition rate fluorometry (FRRF). In this paper, environmental factors, phytoplankton community parameters, chlorophyll a (Chl a), and various bio-optical parameters were investigated for two opposite types of eddies at the WPO. The results show the maximum [Fv/Fm, 0.18 to 0.26 (warm eddy), 0.14 to 0.28 (cold eddy)] and effective photosynthetic efficiency [Fq'/Fm', 0.11 to 0.23 (warm eddy), 0.10 to 0.27 (cold eddy)] of the DCM for both warm and cold eddies, the electron transport rates ETRRCII [0.002–6.18 mol e− mol RCII−1 s−1 (warm eddy), 0.002–4.94 mol e− mol RCII−1 s−1 (cold eddy)] and the primary production potential PPmax [0.68–118.19 mg C (mg Chl a)−1 day−1 (warm eddy), 2.47–243.49 mg C (mg Chl a)−1 day−1 (cold eddy)] for different types eddies. In warm eddy, temperature and Chl a concentrations had significant effects on Fv/Fm and Fq'/Fm', while in cold eddy the correlation of Fv/Fm and Fq'/Fm' with temperature was not significant, and Fv/Fm was significantly negatively correlated with DIP only. Light was the main variable affecting the electron transport capacity and primary production potential of the phytoplankton community in the eddies, while larger cyanobacteria and dinoflagellates contributed significantly to the primary production potential of the cold eddy. In addition, both eddies centers had higher primary production potentials, with the cold eddy had a higher primary production potential than the warm eddy, based on microscopic analysis this phenomenon may be due to differences in electron transfer rates between phytoplankton communities.
Understanding the mechanism of harmful algal bloom formation is vital for effectively preventing algal bloom outbreaks in coastal environments. Karenia spp. blooms in the East China Sea show a significant correlation with nutrient regimes. However, the impact of key components of nutrients, especially dissolved organic nitrogen (DON), on the blooms of Karenia spp. is not clear. Quantitative research is still lacking. In this study, the cruise observations, field mesocosm-flask culture experiments, and a multinitrogen-tri-phytoplankton-detritus model (NTPD) are combined to reveal the quantitative influence of nutrient regimes on the shift of Prorocentrum donghaiense and Karenia spp. in the East China Sea. It has a synchronism rhythm of diatom-P. donghaiense-Karenia spp.-diatom loop in the field culture experiment, which is consistent with the results of the cruise observation. The results showed that the processes of terrigenous DON (TeDON) and dissolved inorganic nitrogen (DIN: NO −3 -N, NH +4 -N) absorption promoted P. donghaiense to become the dominant algae in the community; whereas the processes of DON from P. donghaiense absorption promoted Karenia spp. to become the dominant algae in ambient DIN exhaustion. In addition, the three-dimensional fluorescence components of humus C, tyrosine and fulvic acid can indicate the processes of growth and extinction of P. donghaiense and Karenia spp., respectively. This study infers that P. donghaiense and Karenia spp. regime shift mechanism associated with the nutrient regime in coastal waters, which provides a scientific basis for the environmental management of coastal ecosystem health.
The occurrences of phytoplankton bloom have recently increased in some coastal bays in Indonesia. Harmful algal blooms are now a significant problem for several coastal bays, which causes problems and economic losses to the fishing industry, harm to aquatic ecosystems, fish mortalities, and other living organisms. The negative impact also affects the marine environment, tourism, and consumer health. The most frequent algal bloom occurrences are three coastal bays: Jakarta Bay, Lampung Bay, and Ambon Bay. In particular, those three bays are the most infected areas of harmful algal bloom in this country. Samples were taken using a 20 um net plankton and directly preserved with Lugol’s solution before being identified. Some bloom maker species were identified as toxic, mainly belonging to dinoflagellates, such as Pyrodinium sp, Aleandrium sp, Gymnodinium sp , and Cochlodinium sp . And the species that most frequently cause a bloom in those coastal waters are Skeletonema, Chaetoceros, Trichodesmium , and Noctiluca . The nutrients, mainly nitrate and phosphate, are the main driving factor resulting from anthropogenic activities on the land that make the coastal waters eutrophic. This paper discusses the recent distribution, causes, and impact of harmful algal blooms in the three coastal bays in this country.
The concentration and composition of nutrients, such as N, P, and Si, respond to biogeochemical processes and in turn, impact the phytoplanktons’ community structure and primary production. In this study, historical data was systematically analyzed to identify long-term variations in nutrient trends, red tide frequency, phytoplankton community abundance, and dominant species succession in the southern Yellow Sea (SYS). Results showed that N/P concentration ratios dramatically increased as a function of increasing dissolved inorganic nitrogen concentrations, and Si/N concentration ratios were generally larger than 1, indicating that N limitation morphed to P limitation and potentially to Si limitation, which impacted the phytoplankton community. Furthermore, inter-annual trends over the past 50 years show that phytoplankton community abundance has been higher in spring and summer, relative to autumn and winter. Moreover, with respect to red tide frequency, diatom abundance gradually decreased, while that of dinoflagellates gradually increased. Dominant species succession showed that the phytoplankton community exhibited an evident tendency to transform from diatoms to dinoflagellates. These research results clearly depict the presence of an important correlation between the phytoplankton community and nutrient structure in the SYS.
The first disaster caused by harmful algal blooms in Lampung Bay was reported in 1991, where mass mortality of cultivated shrimp occurred in the brackish water ponds due to a Trichodesmium bloom. After this incident, the phenomenon reoccured in the following years continuously. Around nine species bloom makers in this bay are namely Pyrodinium sp., Noctiluca sp., Phaeocystis sp., Dinophysis sp., Trichodesmium sp., Ceratium sp., Prorocentrum sp., Pseudonitzhia sp., and Cochlodinium sp. The most frequent causative species, such as green Noctiluca and Trichodesmium , co-occurring during blooms and causing fish mortalities in the fish farming floating nets (KJA). Two species are known as the most potentially harmful species, namely Pyrodinium sp. and Cochlodinium sp. Cochlodinium blooms happened at the end of 2012, and since then, this species has continuously reappeared in the following years. The outbreak of Cochlodinium sp. still appeared in 2017 and 2018, but no fish-killing occurred. Phytoplankton bloom events occur at specific locations, mainly at fish farming floating nets on the west side of the bay, next to Hurun Cove. This paper discusses the occurrence of algal blooms in Lampung Bay and the triggering factors for increasing phytoplankton populations that cause harmful algal blooms.
The study presents long-term variability in satellite retrieved phytoplankton size classes (PSC) at two coastal sites, off Gopalpur and Visakhapatnam, in the north-western Bay of Bengal. The abundance-based models by Brewin et al. (2010) (B10) and Sahay et al. (2017) (S17), for retrieval of PSC (micro, nano, and picophytoplankton), from satellite data, were validated. Both the models performed well in the retrieval of nano and microphytoplankton. However, B10 performed poorly in retrieving picophytoplankton. The statistical analysis indicated better performance of the S17 model and hence was applied to Moderate Resolution Imaging Spectroradiometer onboard Aqua satellite (MODISA) data to understand the temporal (at monthly climatology) and spatial variability (from nearshore to offshore). The spatial distribution indicated nearshore dominance of micro and offshore dominance of picophytoplankton. In nearshore waters off Gopalpur, microphytoplankton dominated throughout the year except for months of south-west monsoon (June and July) where the dominance of picophytoplankton was observed. All PSC exhibited similar distribution at an annual scale with a primary peak during pre-monsoon (March and April) and a secondary peak during post-monsoon (September - November). However, microphytoplankton concentration during post-monsoon was higher off Gopalpur in comparison to Visakhapatnam. The higher microphytoplankton concentration during pre-monsoon was attributed to recurrent phytoplankton blooms. Whereas, post-monsoon increment could be attributed to enhanced phytoplankton growth by availing nutrients sourced from monsoonal precipitation induced terrigenous influx. The outcome of the present study recommends the use of the S17 model for satellite retrieval of PSC from the north-western Bay of Bengal.
This Special Report on the Ocean and Cryosphere1 in a Changing Climate (SROCC) was prepared following an IPCC Panel decision in 2016 to prepare three Special Reports during the Sixth Assessment Cycle2 . By assessing new scientific literature3 , the SROCC4 responds to government and observer organization proposals. The SROCC follows the other two Special Reports on Global Warming of 1.5°C (SR1.5) and on Climate Change and Land (SRCCL)5 and the Intergovernmental Science Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment Report on Biodiversity and Ecosystem Services. This Summary for Policymakers (SPM) compiles key findings of the report and is structured in three parts: SPM.A: Observed changes and impacts, SPM.B: Projected changes and risks, and SPM.C: Implementing Responses to Ocean and Cryosphere Change. To assist navigation of the SPM, icons indicate where content can be found. Confidence in key findings is reported using IPCC calibrated language66 and the underlying scientific basis for each key finding is indicated by references to sections of the underlying report.
The Bay of Bengal provides many ecosystem services that contribute to human well-being, such as nutrient cycling, fish stocks, water quality, biodiversity, raw materials, and climate regulation. Valuation of the benefits provided by ecosystem services can aid in designing more efficient policies for the protection and in reaching the environmental objectives for the Bay. The most commonly valued services for marine ecosystems are the cultural services, followed by provisioning services, and then regulating and supporting services respectively. Based on landings, herring-like fishes, Indo-pacific mackerels and Hilsa dominated reported landings data, while tuna and shrimp dominated in terms of landed value.
One of the strongest oxygen minimum zones (OMZ) in the world oceans occurs in the Bay of Bengal (BoB) with no systematic trends in dissolved oxygen (DO) since past half a century. An evaluation is made here to examine the influence of cyclonic (CE) and anticyclonic eddies (ACE) on OMZ in the BoB. Cyclonic eddy (CE) injected nutrients to photic zone leading to increase in integrated Chl-a to upper 100 m compared to ACE and Transition zone (TZ) region resulting in decrease in DO in the OMZ. In contrast, increase in DO concentrations, by 3–4 times, was observed between 100 and 200 m depth in the ACE compared to TZ region associated with vertical and horizontal supply of DO. This study suggested that CE intensify OMZ through increase in organic matter production and its subsequent decomposition whereas ACEs ventilate DO and weaken the OMZ in the BoB.
An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being ~600 μmol m−2 s−1 on sunny days and declining to ~50 μmol m−2 s−1 on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41–80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR.
An ecosystem disruptive bloom of red Noctiluca scintillans (hereafter Noctiluca) was observed in coastal waters of the north-western Bay of Bengal during April 2014. Based on the principle of phytoplankton group/species specific remote sensing reflectance (Rrs), a technique of detecting green Noctiluca and diatom was developed earlier using Rrs at 443, 488, and 531 nm of Moderate Imaging Spectroradiometer-Aqua (MODIS). This was appropriately modified to detect bloom of red Noctiluca in coastal waters of the Bay of Bengal. Additional Rrs data at longer wavelengths viz. 667 and 678 nm were included in the existing algorithm, and the spectral shapes were accounted to detect the bloom of red Noctiluca. The classification scheme discriminates red Noctiluca from the green form of the same species and diatom. Phytoplankton group/species products were generated using the modified approach and validated with the reported events of red and green Noctiluca blooms in the Indian coastal waters. The present study also highlights two specific results based on MODIS retrieved time-series phytoplankton group/species image analysis: first, the observation of coexistence of diatom, red, and green Noctiluca in coastal waters of the north-western Bay of Bengal, and the second, phytoplankton community shift resulting in red/green Noctiluca proliferation following diatom.
The Bay of Bengal (BoB) is strongly density stratified due to large
freshwater input from various rivers and heavy precipitation. This strong vertical
stratification, along with physical processes, regulates the transport and vertical
exchange of surface and subsurface water, concentrating nutrients and intensifying the
oxygen minimum zone (OMZ). Here, we use basinwide measurements to describe the
spatial distributions of nutrients, oxygen, and phytoplankton within the BoB during
the 2013 northeast monsoon (November–December). By the time riverine water
reaches the interior bay, it is depleted in the nutrients nitrate and phosphate, but not
silicate. Layering of freshwater in the northern BoB depresses isopycnals, leading to
a deepening of the nutricline and oxycline. Oxygen concentrations in the OMZ are
lowest in the north (<5 μM). Weak along-isopycnal nutrient gradients reflect along-
isopycnal stirring between ventilated surface water and deep nutrient-replenished
water. Picoplankton dominate the phytoplankton population in the north, presumably
outcompeting larger phytoplankton species due to their low nutrient requirements.
Micro- and nanoplankton numbers are enhanced in regions with deeper mixed layers
and weaker stratification, where nutrient replenishment from subsurface waters is
more feasible. These are also the regions where marine mammals were sighted. Physical
processes and the temperature-salinity structure in the BoB directly influence the OMZ
and the depth of the oxycline and nutricline, thereby affecting the phytoplankton and
marine mammal communities.
Elemental ratio of nutrients and its influence on chlorophyll a distribution was studied along the central coast of Bay of Bengal using multivariate statistical methods. High chlorophyll concentration was observed during summer (1.81 μg l⁻¹) and premonsoon (1.95 μgl⁻¹), however, it was high in top 20 m during premonsoon season in tandem with high nitrate (N) and silicate (Si) concentration. N:P (phosphate) ratio was less than Redfield ratio (16:1) during all seasons, indicating the Bay of Bengal as nitrate limited and confirmed the results of Principal ComponentAnalysis (PCA) with positive loading and multiple regression analysis showing negative correlation between this ratio and chlorophyll concentration during all seasons. Whereas, N:Si ratio was < 1 and Si:P ratio > 7 in top 20 m during all seasons explained the deficiency of phosphorus and enrichment of silicate in the central Bay of Bengal. Regression analysis between Si:P and N:Si ratios with chlorophyll showed negative correlation during premonsoon and summer respectively. Thus, the present results confirmed that nutrient molar ratios such as N:P<16; Si:P>7 and N:Si<1 was indicative of a potential N and Si limitation and are the primary limiting nutrients in the central Bay of Bengal in determining chlorophyll concentration.
In order to understand the phytoplankton community structure and its relationship with the environmental variables in the near shore waters of Kalpakkam, east coast of India, observations were carried out during 2008-2009. Phytoplankton population was comprised of 219 species, and the density was higher during the southwest monsoon (SWM) and inter-monsoon seasons than that of north east monsoon (NEM) season. The nutrient status on a temporal and spatial scale indicated the impact of point sources carrying anthropogenic runoff. Comparison of ambient nutrient ratios with the Redfield ratio (N/P/Si = 16:1:16) showed a clear temporal variation in the factors that regulate the phytoplankton growth. SWM and inter-monsoon season was evident to have an acute N-limitation of algal growth (~76 %) whereas P-limitation was encountered during the NEM season (~75 %). Interestingly, a sizable population of cyanobacteria (Trichodesmium erythraeum) were noticed during NEM season when there was an exponential increase in nitrogen concentration, probably due to nitrogen fixation. No significant impact of temperature on phytoplankton proliferation was observed in situ during the study period.
Upper oceanographic and surface meteorological time-series observations from a moored buoy located at 9.98°N, 88°E in the south-western Bay of Bengal (BoB) were used to quantify variability in upper ocean, forced by a tropical cyclone (TC) Jal during November 2010. Before the passage of TC Jal, salinity and temperature profiles showed a typical BoB post-monsoon structure with relatively warm (30 °C) and low-saline (32.8 psu) waters in the upper 30-to 40-m layer, and relatively cooler and higher salinity (35 psu) waters below. After the passage of cyclone, an abrupt increase of 1 psu (decrease of 1 °C) in salinity (temperature) in the near-surface layers (up to 40-m depth) was observed from buoy measure-ments, which persisted up to 10–12 days during the relaxation stage of cyclone. Mixed layer heat budget analysis showed that vertical processes are the dominant contributors towards the observed cooling. The net surface heat flux and horizontal advection together contributed approximately 33 % of observed cooling, during TC Jal forced stage. Analysis showed the existence of strong inertial oscillation in the thermocline region and currents with periodicity of ∼2.8 days. During the relaxa-tion stage of the cyclone, upward movement of thermocline in near-inertial frequencies played significant role in mixed layer temperature and salinity variability, by much freer turbulent exchange between the mixed layer and thermocline.
The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. Despite the contrasting chlorophyll and primary productivity pattern, sediment trap data shows that annual fluxes of organic carbon reach comparable rates in both the basins. The traditional mechanisms of nutrient supply to the upper ocean waters cannot account for this. We propose eddy pumping as a possible mechanism of vertical transfer of nutrients across the halocline to the oligotrophic euphotic zone during summer monsoon when upper ocean is highly stratified. This would induce rapid biological uptake and in turn significantly increase biological production. In the northern Bay, riverine input acts as an additional source of nutrients and augments the subsurface nutrient injection to the euphotic zone by eddy pumping. Notwithstanding this, the lower than expected primary production in the north suggests the possible role of riverine sediment in limiting the sunlight for photosynthesis.
This study focuses on the improved estimation of mesoscale surface ocean circulation obtained by merging TOPEX/Poseidon (T/P) and ERS-1 and -2 altimeter measurements between October 1992 and May 1998. Once carefully intercalibrated and homogenized, these data are merged through an advanced global objective analysis method that allows us to correct for residual long wavelength errors and uses realistic correlation scales of ocean dynamics. The high-resolution (0.25°×0.25°) merged T/P+ERS-1 and -2 sea level anomaly maps provide more homogeneous and reduced mapping errors than either individual data set and more realistic sea level and geostrophic velocity statistics than T/P data alone. Furthermore, the merged T/P+ERS-1 and -2 maps yield eddy kinetic energy (EKE) levels 30% higher than maps of T/P alone. They also permit realistic global estimates of east and north components of EKE and their seasonal variations, to study EKE sources better. A comparison of velocity statistics with World Ocean Circulation Experiment surface drifters in the North Atlantic shows very good agreement. Comparison with contemporary current meter data in various oceanic regimes also produces comparable levels of energy and similar ratios of northward and eastward energy, showing that the maps are suitable to studying anisotropy. The T/P+ERS zonal and meridional components of the mapped currents usually present comparable rms variability, even though the variability in the Atlantic is more isotropic than that in the Pacific, which exhibits strong zonal changes. The EKE map presents a very detailed description, presumably never before achieved at a global scale. Pronounced seasonal changes of the EKE are found in many regions, notably the northeastern Pacific, the northeastern and northwestern Atlantic, the tropical oceans, and the zonally extended bands centered near 20°S in the Indian and western Pacific Occans and at 20°N in the northwestern Pacific.
This paper presents evidence of a seasonal shift from P to N as the nutrient limiting the accumulation of algal biomass in Chesapeake Bay. Following the winter/spring maximum in freshwater runoff, (1) the ratio of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN/PO4) was greater than the N/P of algal biomass; (2) alkaline phosphatase activity was high; (3) phosphate turnover times were short; (4) ammonium turnover times were long; and (5) growth rates of phytoplankton were stimulated by additions of phosphate but not by additions of ammonium or silicate. During the period of low runoff in summer, all indicators reversed, and N limited algal growth rates. Silicate concentrations also showed evidence of biological depletion in spring, which may have limited diatom abundance. Due to the concordance of all indicators at large and small scales, we argue that phytoplankton growth rates exert primary control over biomass accumulation. We conclude that P and Si limit the accumulation of algal biomass along the major axis of Chesapeake Bay in spring, whereas N limits algal accumulation in summer, similar to the conclusions of D'Elia et al. (1986; Can. J. Fish. Aquat. Sci. 43: 397-406) for the Patuxent subestuary. Controlling eutrophication of the Bay and its subestuaries will require basin-specific management practices for both N and P reductions in influent waters. Such management efforts will provide ecosystem tests of nutrient limitation on a scale similar to those successfully conducted in lakes.
Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, riv
Marine diatoms require dissolved silicate to form an external shell, and their growth becomes Si-limited when the atomic ratio
of silicate to dissolved inorganic nitrogen (Si:DIN) approaches 1:1, also known as the “Redfield ratio.” Fundamental changes
in the diatom-to-zooplankton-to-higher trophic level food web should occur when this ratio falls below 1:1 and the proportion
of diatoms in the phytoplankton community is reduced. We quantitatively substantiate these predictions by using a variety
of data from the Mississippi River continental shelf, a system in which the Si:DIN loading ratio has declined from around
3:1 to 1:1 during this century because of land-use practices in the watershed. We suggest that, on this shelf, when the Si:DIN
ratio in the river decreases to less than 1:1, then (i) copepod abundance changes from >75% to <30% of the total mesozooplankton, (ii) zooplankton fecal pellets become a minor component of the in situ primary production consumed, and (iii) bottom-water oxygen consumption rates become less dependent on relatively fast-sinking (diatom-rich) organic matter packaged
mostly as zooplankton fecal pellets. This coastal ecosystem appears to be a pelagic food web dynamically poised to be either
a food web composed of diatoms and copepods or one with potentially disruptive harmful algal blooms. The system is directed
between these two ecosystem states by Mississippi River water quality, which is determined by land-use practices far inland.
The evidence of phytoplankton bloom in response to the cyclone Sidr in the central Bay of Bengal (BoB) basin has been reported. Satellite observations of surface chlorophyll-a concentration (Chl-a) from MODIS-Aqua, sea surface temperature (SST) from MODIS-Aqua and TMI were studied for before, during and after cyclone periods. The mixed-layer depth (MLD) derived from the nearest Argo float that was closest to the cyclone track along with the float temperature and salinity (T/S) profiles near the cyclone track were studied to further complement the satellite observations. Nearly 900 km long phytoplankton bloom was observed along the cyclone path. Along the path of cyclone Sidr, the average increase in Chl-a due to Sidr during the cyclone and after cyclone period was about 285 per cent and 150 per cent respectively, from the before-cyclone period. The mean decrease in SST from before cyclone period was about 1.75°C and 1.27°C between during and after cyclone period respectively, with maximum cooling of 3.2°C observed during cyclone period. The Argo float observations indicate deepening of MLD by 15 m, during the cyclone and shoaling of MLD after the cyclone by about 35 m. Upward Ekman pumping due to cyclonic winds caused upwelling of nutrient rich subsurface waters, which led to the bloom in the BoB.
The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations.
Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high
productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events,
such as heavy rainfall and cyclone Sidr, on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll a (Chl a) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234mm) on
4–5 October 2007 caused an increase in Chl a concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl a, by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr. These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ~40% of fishery production
during October–December 2007 compared to that in the same period in 2006.
KeywordsChlorophyll–cyclone–primary production–short-term variations–rainfall–atmospheric disturbances; extreme events
This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to Highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially disconnected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.
Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological
productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian
Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast
of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized
by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection
of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand,
is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and
increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea
remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing
during winter resulted in one-and-a-half times higher production in 1997 than in 1995.
We examine the global distribution of dissolved inorganic nitrogen (DIN) and particulate nitrogen (PN) export to coastal systems
and the effect of human activities and natural processes on that export. The analysis is based on DIN and PN models that were
combined with spatially explicit global databases. The model results indicate the widely uneven geographic distribution of
human activities and rates of nitrogen input to coastal systems at the watershed, latitudinal, and regional-continental scales.
Future projections in a business-as-usual scenario indicate that DIN export rates increase from approximately 21 Tg N yr−1 in 1990 to 47 Tg N yr−1 by 2050. Increased DIN inputs to coastal systems in most world regions are predicted by 2050. The largest increases are predicted
for Southern and Eastern Asia, associated with predicted large increases in population, increased fertilizer use to grow food
to meet the dietary demands of that population, and increased industrialization. Results of an alternative scenario for North
America and Europe in 2050 indicate that reductions in the human consumption of animal protein could reduce fertilizer use
and result in substantial decreases in DIN export rates by rivers. In another scenario for 2050, future air pollution control
in Europe that would reduce atmospheric deposition of nitrogen oxides in watersheds is predicted to decrease DIN export by
rivers, particularly from Baltic and North Atlantic watersheds. Results of a newly developed global PN river export model
indicate that total global PN and DIN export by rivers in 1990 are similar, even though the global distribution of the two
differ considerably.
We have examined the seasonality of phytoplankton in the western and northern Bay of Bengal using shipboard data collected during three seasons as well as ocean colour imagery from OCTS and SeaWiFS. Seasonal changes in the hydrography of the bay observed during these seasons gave rise to striking differences in biomass and primary productivity. Heavy fresh water influx from rivers and the resulting vertical stratification impeded vertical transfer of nutrients. Although such a nutrient regime resulted in an oligotrophic environment, chlorophyll a and primary production were substantially enhanced by physical processes that could erode the strong halocline. In March–April (pre-southwest monsoon), the poleward flowing East India Coastal Current brought to the surface, nutrient laden cooler waters that enriched the coastal region, but highest biomass (Chl a, 53 mg m−2) and productivity (4.5 g C m−2 d−1) were in the region of an eddylike structure along the coast and in the region between 13° and 16°N lat. Its appearance in satellite images of two consecutive years suggests the structure to be an annual feature. Wind driven coastal upwelling and increased river runoff during the following season, the southwest monsoon (July–August), increased phytoplankton biomass dramatically (92 mg m−2) but productivity averaged only 0.3 g C m−2 d−1 suggesting light limitation due to intense cloud cover. With a reduction in cloud cover and enhanced irradiance during the following season, the northeast monsoon (January–February), primary production increased especially in the northern part of the bay where phytoplankton appeared to benefit from both improved light conditions and nutrient inputs from estuarine mechanisms and river runoff.
The signature of cold-core eddies and their role in altering the biological productivity of the Bay of Bengal was examined using two recent sets of hydrographic data collected along the central and western Bay of Bengal during fall (14 September–12 October, 2002) and spring (12 April–7 May, 2003) intermonsoons under the Bay of Bengal process studies (BOBPS) programme. Based on the thermohaline structure and the satellite-derived sea-level anomaly maps nine cyclonic eddies were identified. Out of this, four cyclonic eddies—two each along the central Bay and along the western boundary—occurred during fall intermonsoon 2002, while five occurred—three along the central Bay and two along the western boundary—during spring intermonsoon. The eddy depressed the temperature, which varied from 3 °C to 7 °C at 120 m depth. Maximum depression of temperature was associated with spring-time eddies in the northern Bay, where subsurface stability was low. The reduced water column stability in spring leads to greater eddy-pumping, thereby cooling the water to a greater extent. However, the cyclonic eddies were unable to break the stratification of the top 20 m layer, thereby curtailing their effects below this depth during both seasons. Eddy-pumping not only cooled the water column but also enhanced the nutrient concentrations. This in turn increased the biological productivity of the Bay to times. In addition, the subsurface chlorophyll maximum (SCM), which is generally located between 40 and 70 m in fall and 60 and 90 m in spring intermonsoons, shallowed under the influence of the eddies and also enhanced the chlorophyll concentration in the SCM to more than double. Thus, eddy-pumping of nutrients controls the biological productivity of the Bay of Bengal during both the seasons. In the fall intermonsoon, however, the riverine input of nutrients and sediments in the northern Bay also plays a role in altering the biological productivity. This has an overall implication to the basin-wide new production and export flux and, at least partly, resolves the reason for the comparable annual fluxes of organic carbon between the high-productivity Arabian Sea and low-productivity Bay of Bengal.
The seasonal dynamics of nutrient ratios and abundance of phytoplankton cells from the central (CB) and western (WB) Bay of Bengal (BOB) were studied during the fall intermonsoon (FIM; September–October 2002) and spring intermonsoon (SpIM; April–May 2003). The nutrient molar ratios of macronutrients such as nitrate to phosphate (N:P), nitrate to silicate (N:Si) and silicate to phosphate (Si:P) in the top 120 m were calculated for both FIM and SpIM. During both the seasons, the N:P ratios along the CB and WB were lower than 16, indicating nitrate deficiency. Whereas, along both transects the N:Si ratio was <1 and Si:P >3 in the top 20 and 40 m during FIM and SpIM, respectively, indicating Si enrichment. Relatively greater nutrient concentrations along the WB than the CB appear to contribute to higher phytoplankton abundance. The preponderance of diatoms in the Bay could be attributed to rapid utilization of available nutrients in particular during FIM thus resulting in low N:Si ratios in the water column. Among diatoms, pennales were predominantly controlled by nutrients and their ratios. While, apart from nutrients, physical stratification, light and eddies also seem to influence the distribution and abundance of centrales.
We present an analysis of extensive nutrient data sets from two river-dominated coastal ecosystems, the northern Adriatic Sea and the northern Gulf of Mexico, demonstrating significant changes in surface nutrient ratios over a period of 30 years. The silicon:nitrogen ratios have decreased, indicating increased potential for silicon limitation. The nitrogen:phosphorus and the silicon:phosphorus ratios have also changed substantially, and the coastal nutrient structures have become more balanced and potentially less limiting for phytoplankton growth. It is likely that net phytoplankton productivity increased under these conditions and was accompanied by increasing bottom water hypoxia and major changes in community species composition. These findings support the hypothesis that increasing coastal eutrophication to date may be associated with stoichiometric nutrient balance, due to increasing potential for silicon limitation and decreasing potential for nitrogen and phosphorus limitation. On a worldwide basis, coastal ecosystems adjacent to rivers influenced by anthropogenic nutrient loads may experience similar alterations.
The first special volume of Limnology and Oceanography, published in 1972, focused on whether phosphorus (P) or carbon (C) is the major agent causing eutrophication in aquatic ecosystems. Only slight mention was made that estuaries may behave differently from lakes and that nitrogen (N) may cause eutrophication in estuaries. In the following decade, an understanding of eutrophication in estuaries proceeded in relative isolation from the community of scientists studying lakes. National water quality policy in the United States was directed almost solely toward P control for both lakes and estuaries, and similarly, European nations tended to focus on P control in lakes. Although bioassay data indicated N control of eutrophication in estuaries as early as the 1970s, this body of knowledge was treated with skepticism by many freshwater scientists and water-quality managers, because bioassay data in lakes often did not properly indicate the importance of P relative to C in those ecosystems. Hence, the bioassay data in estuaries had little influence on water-quality management. Over the past two decades, a strong consensus has evolved among the scientific community that N is the primary cause of eutrophication in many coastal ecosystems. The development of this consensus was based in part on data from whole-ecosystem studies and on a growing body of evidence that presented convincing mechanistic reasons why the controls of eutrophication in lakes and coastal marine ecosystems may differ. Even though N is probably the major cause of eutrophication in most coastal systems in the temperate zone, optimal management of coastal eutrophication suggests controlling both N and P, in part because P can limit primary production in some systems. In addition, excess P in estuaries can interact with the availability of N and silica (Si) to adversely affect ecological structure. Reduction of P to upstream freshwater ecosystems can also benefit coastal marine ecosystems through mechanisms such as increased Si fluxes.
Recent measurements of chlorophyll, primary productivity (PP) and nutrients along the central Bay of Bengal (BOB) during summer, fall and spring intermonsoons showed that the northern bay becomes less productive compared to the south in summer and fall intermonsoon, in spite of the nutrient input to the upper ocean by way of river influx as well as eddypumping. Along the western boundary also, highest PP in the northern bay did not occur during summer or in the fall intermonsoon, but occurred in the spring intermonsoon. The reason for this was explored using diffuse attenuation (Kd(490)) and photosynthetically active radiation (PAR) which indicates the influence of the river flux in curtailing the downward penetration of solar radiation and cloud cover respectively. During summer and fall intermonsoon, biological productivity in the northern BOB is severely limited by the reduced downward penetration of solar radiation due to the large quantities of sediment brought by the adjoining rivers. Though the cloud cover reduces PAR in the northern BOB, this has only a secondary effect in comparison to the light limitation due to turbidity, which showed an order of magnitude increase in the northern Bay
[1] Satellite derived chlorophyll a imageries are used to present a phytoplankton bloom in the Bay of Bengal during the northeast monsoon (November-February) and the mechanisms that can upwell nutrients to sustain the bloom are investigated using sea level anomalies and winds. OCTS and SeaWiFS chlorophyll a images show that there is a phytoplankton bloom in the southwestern part of the bay during November-January. The chlorophyll a concentration of the bloom can be as high as 2 mg m(-3) compared to near zero value before the bloom. Open ocean upwelling driven by Ekman pumping causes the bloom. The cyclones which are common during this period lead to localized intense blooms in the western Bay of Bengal. The offshore extent and the intensity of the bloom varies from year to year. The bloom was absent during 1997 due to weak Ekman pumping.
Hydrographic data collected during March-April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal of about 10 degrees N carrying warmer waters of southern origin. The inshore side of the current was markEd. by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 x 10 sup(6)m sup(3)/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans:a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport inducEd. by the curl of wind stress, which has a well-defined annual cycle due to the monsoon and which is conductive to the formation of an anticyclonic gyre only during the months of January-May. The pattern of circulation due to barochlinic transport inducEd. by the wind stress, however, is not known at present, and this makes it difficult to conclude unequivocally that e wind stress curl over the bay is the sole mechanism to force the gyre.
Time series of temperature and salinity in the upper ocean, measured at 17 degrees 30'N, 89 degrees E in the northern Bay of Bengal, from 27 July to 6 August 1999 captured an event of upper layer freshening. Initially, the upper layer that is homogeneous in both temperature and salinity was about 30 m deep. Subsequently, the arrival of a freshwater plume caused the depth of the mixed layer to decrease to about 10 m and the salinity in the surface layer by about 4 psu. The plume led to the formation of a new halocline and hence a barrier layer within the upper 30 m of the water column. The ensuing ocean-atmosphere interaction was restricted to the new thinner mixed layer. The cooling that was restricted to the mixed layer led to an inversion in temperature amounting to 0.5 degrees C just below the mixed layer. The source of the plume is traced to freshwater from river discharge and rainfall that was advected by Ekman flow as a 15 m thick layer. This study suggests that wind-driven circulation is crucial in determining the path of freshwater in the Bay of Bengal. The fresh water affects the sea surface temperature and ocean- atmosphere coupling through the dependence of the depth of the mixed layer on salinity.
In this exciting and innovative textbook, two leading oceanographers bring together the fundamental physics and biology of the coastal ocean in a quantitative but accessible way for undergraduate and graduate students. Shelf sea processes are comprehensively explained from first principles using an integrated approach to oceanography that helps build a clear understanding of how shelf sea physics underpins key biological processes in these environmentally sensitive regions. Using many observational and model examples, worked problems and software tools, the authors explain the range of physical controls on primary biological production and shelf sea ecosystems. Boxes throughout the book present extra detail for each topic and non-mathematical summary points are provided for physics sections, allowing students to develop an intuitive understanding. The book is fully supported by extensive online materials, including worked solutions to end-of-chapter exercises, additional homework/exam problems with solutions and simple MATLAB and FORTRAN models for running simulations.
To understand the influence of river discharge on phytoplankton composition along western coastal Bay of Bengal (BoB), surface water samples were collected during peak discharge period. River discharge from the Ganges influences northwest (NW) coastal BoB whereas peninsular rivers (Godavari and Krishna) discharge to the southwest (SW) coastal Bay. River discharge from the Ganges is an order of magnitude higher than penin-sular river resulting in low saline, less suspended matter and lower nutrients concentrations in the NW and contrasting to that was observed in the SW.~50%of the phytoplankton were composed of Thalassiosira spp., Nitzschia spp., Microcystis spp., Amphiprora spp. and Thalassionema spp. in the SW whereas Thalassiosira spp., Nitzschia spp., Chaetoceros spp., Merismopedia spp. and Peridinium spp. in the NW. Significant variability in phytoplankton composition was observed from coast to offshore. Our study revealed that river discharge and associated physico-chemical characteristics governed the phytoplankton community along western coastal BoB.
One of the significant aspects of estuarine water quality assessment is to discern the effect of tide on the environmental parameters. The variability of different physical, chemical, and biological parameters, in response to semi-diurnal tides, over a tidal cycle was investigated at contrasting salinity regimes of Mahanadi estuary. This estuary is formed at the mouth of Mahanadi River (the third largest river in peninsular India) that meets the Bay of Bengal on the east coast of India. Significant variability was observed in salinity, phosphate concentration, picophytoplankton biomass between upper and lower estuary irrespective of tides. Salinity and ammonium concentration varied significantly between high and low tide, irrespective of sites. A low salinity regime was observed in the upper estuary. Dissolved oxygen levels were higher at the lower estuary and relatively decreased at the upper estuary during high tide with increased tidal amplitude. The biochemical oxygen demand levels < 4 mg.l⁻¹ signified estuarine water quality in between fairly-clean and moderately polluted. Ammonium and phosphate distribution suggested anthropogenic influx at the lower/middle part of the estuary. Higher silicate content in upper estuary regardless of tides indicated prominent riverine source. Inorganic macronutrient molar ratios indicated a potential nitrogen limiting condition. Smaller phytoplankton size classes contributed predominantly to total phytoplankton biomass, irrespective of tides. This study highlighted the significance of continuous sampling over the tidal cycle to understand the effect of tides on estuarine water quality, which will aid in developing forecast models.
This paper introduces the cumulative impact of cyclonic (cold-core) and anticyclonic (warm-core) eddies on the distribution of phytoplankton biomass in the Bay of Bengal (BoB). Estimations of the ocean surface eddy features in the BoB using satellite-derived monthly, seasonal, and multi-year (2012-2016) datasets showed significant dominance of warm-core features over the cold-core ones. Multi-year monthly data of the BoB evidenced strikingly higher geographical area (%) for warm cores (av. 16.5 ± 5.5) compared to the cold-core features (av. 2 ± 1.2). The highest warm-core extent (%) was in May (av. 25.8 ± 1.4), followed by April (av. 22.6 ± 1.4), February (av. 19.5 ± 0.3) and March (av. 18.3 ± 0.9). Contrastingly, a relatively higher extent (%) of cold cores occurred in November (av. 3.9 ± 2.2), October (av. 3.78 ± 2.0), and December (av. 3.5 ± 2.1). The analysis also showed that the extreme oligotrophy of the study domain during the Pre-Monsoon (March-May) was caused by a robust, warm gyre. Upwelling and winter convection, which enhances phytoplankton biomass in the Arabian Sea, are either very weak or absent in the BoB. Similarly, nutrient inputs through river influx into the BoB are insufficient to enhance phytoplankton stock beyond the shelf waters. In this scenario, we showed here that the significantly larger spread of warm-core over the cold-core eddies is a simple ecological indicator of the basin-scale low phytoplankton stock in the BoB.
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.
Several anticyclonic (ACE) and cyclonic (CE) eddies constitute the circulation in the Bay of Bengal (BoB) and are associated with the downwelling and upwelling processes leading to oligotrophic and eutrophic conditions respectively. In this study, the nitrogen (N2) fixation rates and controlling factors are estimated through deck incubation experiments in the BoB using enriched N2 gas dissolution method. We observed measurable concentrations of dissolved inorganic nitrogen (DIN) and phosphate in the CE and close to detection limits in the ACE in the mixed layer. Photic zone integrated N2 fixation rates ranged between 53.3 and 194.1 μmol m⁻² d⁻¹ with lower rates in the ACE (91 ± 18 μmol m⁻² d⁻¹) than CE (162 ± 28 μmol m⁻² d⁻¹) and no eddy regions (NE; 138 ± 27 μmol m⁻² d⁻¹). The photic zone integrated N2 fixation rates are linearly correlated with photic zone integrated chlorophyll a and the mean phosphate concentrations in the photic zone suggesting that phosphate is controlling the N2 fixation in the BoB. The observed high N:P ratio (25 ± 3) also indicate that severe phosphate limitation in the BoB. This is further confirmed from increase in N2 fixation rates by 1.2 to 8 times due to artificial increase in phosphate from that rates at in situ phosphate concentrations. This study suggests that though the conditions are conducive for N2 fixation in the BoB, the removal of dissolved phosphate within the estuaries opened to the BoB, provide weaker inputs from subsurface due to stratification and less input from atmospheric dust may limit N2 fixation in the BoB.
In order to examine the role of eddies on total and size-fractionated primary production, two cyclonic (CE), one anticyclonic (ACE) and no-eddy (NE) regions were sampled in the BoB during pre-summer monsoon (June 2019). The upper ocean is strongly stratified due to freshwater discharge from major rivers. Low (high) nutrients concentrations were noticed in the ACE (CE) regions due to convergence (divergence). Prominent existence of deep Chlorophyll-a maximum (DCM) was observed and its depth is governed by mixing associated with eddies. High concentration of zeaxanthin and fucoxanthin was observed in the upper 50 m and DCM respectively. The dominant contribution of picoplankton biomass (40–80%) to total phytoplankton biomass was observed in the photic zone whereas micro and nanoplankton contributed between 10 and 30%. The photic zone integrated total primary production was higher in the CE and NE than ACE regions associating with higher nutrients in the former than latter region. The primary production by microphytoplankton was higher in the CE and NE than ACE regions. Higher picophytoplankton production was observed at depth below 10 m from surface (10–80%) than nano and microphytoplankton (1–30%). The microphytoplankton production was higher in the CE (164 ± 16 mgC m⁻² d⁻¹) than ACE (60 ± 26 mgC m⁻² d⁻¹) due to availability of nutrients in the former region resulted from upwelling of subsurface waters. The photic zone integrated total and microphytoplankton primary production displayed linear relation with nutrients (nitrate and phosphate) and inverse relation was observed with picophytoplankton suggesting that the availability of nutrients due to eddy-driven mixing determined the contribution of primary production by different size classes. This study suggests that eddy-driven nutrients increased contribution of primary production by microphytoplankton leading to enhanced export production under CE in the BoB and therefore, these regions can be considered as an efficient regions of carbon sequestration for the atmospheric CO2. More than 30 eddies form every year with life time of 3-6 months and its impact on sinking carbon fluxes and atmospheric CO2 sequestration in the BoB needs evaluation using numerical modelling.
Clear thermal inversion was observed with cold surface waters (< 24°C) overlying the warm (> 26°C) subsurface water in the coastal waters of the northwestern Bay of Bengal during winter (January 2015). Simultaneously, preponderance of the cyanobacteria Trichodesmium erythraeum was observed dominating the phytoplankton community with > 90% of total population, reaching maximum density of 9.8 × 105 filaments/L. Further, the Trichodesmium predominance was associated with low water temperature (< 24°C).
An intense bloom of red Noctiluca scintillans (NS) occurred off the Rushikulya estuarine region along the east coast of India, an important site for mass nesting events of the vulnerable Olive Ridley sea turtle. At its peak, densities of NS were 3.3 × 105 cells-l− 1, with low relative abundance of other phytoplankton. The peak bloom coincided with high abundance of gelatinous planktivores which may have facilitated bloom development by their grazing on other zooplankton, particularly copepods. Ammonium concentrations increased by approximately 4-fold in the later stages of bloom, coincident with stable NS abundance and chlorophyll concentrations in the nano- and microplankton. This increase likely was attributable to release of intracellular ammonium accumulated through NS grazing. Dissolved oxygen concentrations decreased in sub-surface waters to near hypoxia. Micro-phytoplankton increasingly dominated chlorophyll-a biomass as the bloom declined, with diminishing picoplankton abundance likely the result of high predation by the ciliate Mesodinium rubrum. Together, these data illustrate factors that can disrupt ecosystem balance in this critically important Indian coastal region.
The potential fishing zone (PFZ) advisories received from Indian National Center for Ocean Information Services (INCOIS), Hyderabad during November, 2003 to February, 2005 were validated along Ganjam coast. Feedback was collected for both the PFZ and Non-PFZ locations in the prescribed format of INCOIS. The feedback contains the date of fish catch, time of hauling, latitude, longitude, name of the ship or boat, type of net used, depth of catch, distance away from the coast, direction, catch in kg, major catch in kg and major variety. The catch per unit effort (CPUE) was computed by dividing total fish catch by number of hours of actual fishing per haul. The CPUE data were then compared for both PFZ and NON-PFZ area. The results revealed that the CPUE was more in boats operated in PFZ areas than non-PFZ areas in all the sectors of Ganjam coast. In Rushikulya base, the CPUE was more in February 2005 followed by December 2004 indicating. At Gopalpur and Sonepur sector, CPUE was maximum during January 2005 indicating. The average catch per unit vessel in PFZ areas showed similar trend in all the stations with maximum at Gopalpur. The search time analysis for boats operated in notified and non-notified areas revealed that the search time was less where the PFZ advisories were utilized. The study confirms that satellite based PFZ advisories helps significantly in reduction of search time, fuel cost and yields better catch.
1] Mesoscale cyclonic eddies (cold core) are though known to enhance the phytoplankton biomass in the Bay of Bengal, their contribution to the carbon export to the deep ocean are yet to be quantified. Using biogenic flux data obtained from the sediment traps, located at 17 27 0 N, 89 13 0 E (northern Bay of Bengal trap: NBBT) and 13 07 0 N, 84 24 0 E (central Bay of Bengal trap: CBBT), we explored the variability of biogenic flux in response to cyclonic eddies. Temporal variation of the biogenic flux at NBBT (1994–1998) and CBBT (1993–1996) showed four distinct peaks with no well-defined seasonal pattern. The inverse relationship between the high flux events and sea surface height anomaly (SSHA) along with the dominant periodicity of 10–15 weeks from the Chi-square analysis of SSHA confirmed the role of eddies in the observed higher biogenic fluxes. During the eddy events, enhanced opal flux suggested eddy-driven new production, while lower carbonate to opal ratio along with high organic carbon indicated large export via diatoms. Finally, we show that 42% of the total organic carbon collected at mid-depth ($1000 m) in the Bay of Bengal is contributed by eddy-driven processes via diatom bloom making it a regional sink for CO 2 . Citation: Vidya, P. J., and S. Prasanna Kumar (2013), Role of mesoscale eddies on the variability of biogenic flux in the northern and central Bay of Bengal, J. Geophys. Res. Oceans, 118, doi:10.1002/jgrc.20423.
A diagnostic algorithm for determining the upper layer current in the tropical Indian Ocean (TIO) is presented in this study. The algorithm is based on quasi-linear and steady-state physics. The current in the upper layer consists of three components, namely geostrophic component, wind-driven (Ekman) component and buoyancy component. Each of these components is estimated from different satellite data. The geostrophic component is estimated from merged altimeter product. The Ekman component is estimated from ocean surface winds derived from Oceansat-2 Scatterometer (OSCAT), whereas the buoyancy component is estimated from sea surface temperature (SST) derived from a combination of infrared and microwave radiometers. The product derived for the year 2010 is compared with a similar product, known as Ocean Surface Current Analysis Real-Time (OSCAR), computed by the same algorithm, but with somewhat different data sets. Afterwards, the product is compared with currents measured by several buoys at different locations in the TIO and it is observed that the current variability is better captured by the data generated using the present algorithm.
New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about 15 x 106 tons) contribute about 7 x 109 tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about 13.5 x 109 tons annually; bedload and flood discharges may account for an additional 1-2 x 109 tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.-Authors
The Bay of Bengal is a semi-enclosed tropical ocean basin that is highly influenced by monsoonal winds and receives large volumes of fresh water from both river discharges and rainfall. Tropical cyclones are a major hazard in coastal regions, both in terms of loss of life and economic damage. The extensive coastal belt of India is very vulnerable to these tropical cyclones. Such cyclones originate in the Bay of Bengal and the Arabian Sea during the spring (April-May) and fall (October-November). The surface circulation in the Bay of Bengal undergoes seasonal reversal and forced by remote effects from the equatorial Indian Ocean in addition to monsoon winds and fresh water inputs. These circulation features had a strong influence on the primary' productivity of the Bay of Bengal. Coastal upwelling along the southern part of the west coast of India turns the coastal waters into a region of high biological productivity. The open ocean upwelling, wind driven mixing and lateral advection makes the open ocean waters of the central Arabian Sea more productive (Prasanna Kumar et al. 2001). Kumar, P et al., (2002) reviewed the reasons for less production in the Bay of Bengal compared to Arabian Sea and attributed to the presence of strong stratified surface layer, warmer SST and weak winds. The weaker winds over the Bay are unable to erode the strong stratified surface layer, there by restricting the turbulent wind driven vertical mixing. This inhibits the injection of nutrients from below mixed layer to surface layer. The storm surge activity in the Bay of Bengal is one of the mechanism for the nutrient pulsing into the well-lit surface layer, when the observed patches of high chlorophyll and productivity in coastal regions of the Bay do not bear any obvious relation to the upwelling. Also the presence of gyres and physical processes leads to high chlorophyll in the western Bay of Bengal (Gomes et al. 2000)
We present here an analysis of the stoichiometry of dissolved nutrients in 10 large world rivers, Amazon, Changjiang, Huanghe, Mackenzie, Mississippi, Po, Rhine, Seine, Yukon and Zaire, and in two river-dominated coastal ecosystems prone to eutrophication, the northern Adriatic Sea and the northern Gulf of Mexico. Our analysis suggests that proportions of dissolved silica (Si), nitrogen (N) and phosphorous (P) in rivers carrying nutrients of anthropogenic origin, as well as in the coastal waters strongly influenced by those rivers, have changed historically in a way that now closely approximates the Redfield ratio (Si:N:P=16:16:1). It is likely that coastal phytoplankton productivity has increased under these favourable nutrient conditions and was accompanied by an increasing incidence of noxious phytoplankton blooms and bottom water hypoxia.
Chemical oceanographic studies in the North Indian Ocean have revealed several interesting and unique features. These are caused by the diverse conditions prevailing in the area which include immense river runoff in the northeast (Bay of Bengal) and a large excess of evaporation over precipitation and runoff in the northwest (Arabian Sea, Persian Gulf and Red Sea), resulting in the formation of several low- and high-salinity water masses. The occurrence of coastal upwelling seasonally makes the region highly fertile, and the existence of Asian landmass, forming the northern boundary, prevents quick renewal of subsurface layers. Consequently, dissolved oxygen gets severely depleted below the thermocline and reducing conditions prevail at intermediate depths (ca. 150–1200m) resulting in the reduction of nitrate (denitrification). The North Indian Ocean may contribute up to 10% of the global marine denitrification. The “denitrified” nitrogen, when combined with the rate of photosynthetic production reaching below the euphotic zone, gives the average residence time of water between 75 and 1200m as 43–51 years. The inorganic nutrient concentrations in the subsurface layers are very high in close proximity of the euphotic zone. The two-layered circulation leads to an active recycling of nutrients. The presence of organic fractions of nitrogen and phosphorus in significant concentrations in the deep water suggest that oxidation of organic matter is incomplete even great depths. The relationships between the apparent oxygen utilization (AOU) and nutrients and the stoichiometric composition of organic matter, deduced from the oxidative ratios and by analysis of plankton, are not very different from other oceanic areas.
Daily variations in nutrients were monitored for 15 months (September 2007–November 2008) in the Godavari estuary, Andhra Pradesh, India, at two fixed locations. River discharge has significant influence on nutrients loading to the estuary, which peaks during June–August (peak discharge period; monsoon) whereas exchanges at the sediment–water interface, groundwater and rainwater contribute significantly during other period. Despite significant amount of nutrients brought by discharge to the study region, phytoplankton biomass, in terms of chlorophyll-a (Chl a), did not increase significantly due to high suspended load and shallow photic depth. Nutrients showed downward gradient towards downstream of the estuary from upstream due to dilution by nutrient poor seawater and biological uptake. The N:P ratios were higher than Redfield ratio in both upstream and downstream of the estuary during no discharge period suggesting PO4 to be a limiting nutrient for phytoplankton production, at levels <0.10 μmol L−1. On the other hand, Si:N ratios were always more than unity during entire study period at both the stations indicating that Si(OH)4 is not a limiting nutrient. Our results suggest that suspended matter limits phytoplankton biomass during peak discharge period whereas PO4 during no discharge period.