In-situ datasets of important physical and bio-chemical parameters in the continental shelf of the northern Bay of Bengal

Data equipped with this article were collected from Northern Bay of Bengal (NBoB) wrapping both the eastern and western coast for CTD and sediment samples and only the eastern coast for water sampling. In-situ data of physical parameters, heavy metals, elements, Total Organic Carbon (TOC), nutrients, chlorophyll-a and phaeopigment were sampled across the shallow continental shelf. These data were assembled from 15 CTD points, 76 water samples, and 10 surface sediment samples adjacent to Bangladesh coast. Vertical CTD profiles were collected for Temperature ( °C), Salinity (PSU), Density (kg m−3), Turbidity (NTU), Fluorescence (mg m−3), and Dissolved Oxygen (DO, mg/l). Heavy metals (mg/l) of water column enlisted as Calcium (Ca), Cadmium (Cd), Copper (Cu), Cobalt (Co), Iron (Fe), Manganese (Mn), Magnesium (Mg), Nickel (Ni), Lead (Pb), and Zinc (Zn). Total Organic Carbon (TOC) was measured as Non-Purgeable Organic Carbon (NPOC) in ppm. Measurements of Chlorophyll - a, Nitrate, Nitrite, Phosphate, Ammonia, Silica and Phaeopigment were taken from 76 water sampling points. The survey was conducted with the assistance of a fishing vessel ‘Agro food-4 ‘of ‘Sea Resource Ltd.’ lengthening a fishing period from January to February (in winter), 2016. SBE 19 plus V2 CTD machine was deployed for sampling of vertical physical features, Niskin sampler of HYDRO-BIOS consisting of a non-metallic interior was used to collect water sample. Sediment was collected by Van Veen Grab sampler with built-in messenger. Water samples were analyzed following the standard procedure in the laboratory to access in-situ data. The shallow coastal and offshore regions of Bangladesh support for vast biological resources to its adjacent inhabitants. Therefore, understanding the influence of physico-chemical properties on other biological resources in coastal ecosystem is a crucial one to investigate. However, the shelf region of the BoB has a lack of in-situ baseline or reference data to compare with in terms of ocean biogeochemistry. Thus, these datasets can be utilized for further reference and also in validating other remotely-sensed physico-chemical parameters in this region.

(Mg), Nickel (Ni), Lead (Pb), and Zinc (Zn). Total Organic Carbon (TOC) was measured as Non-Purgeable Organic Carbon (NPOC) in ppm. Measurements of Chlorophyll -a, Nitrate, Nitrite, Phosphate, Ammonia, Silica and Phaeopigment were taken from 76 water sampling points. The survey was conducted with the assistance of a fishing vessel 'Agro food-4 'of 'Sea Resource Ltd.' lengthening a fishing period from January to February (in winter), 2016. SBE 19 plus V2 CTD machine was deployed for sampling of vertical physical features, Niskin sampler of HYDRO-BIOS consisting of a non-metallic interior was used to collect water sample. Sediment was collected by Van Veen Grab sampler with built-in messenger. Water samples were analyzed following the standard procedure in the laboratory to access in-situ data. The shallow coastal and offshore regions of Bangladesh support for vast biological resources to its adjacent inhabitants. Therefore, understanding the influence of physico-chemical properties on other biological resources in coastal ecosystem is a crucial one to investigate. However, the shelf region of the BoB has a lack of in-situ baseline or reference data to compare with in terms of ocean biogeochemistry. Thus, these datasets can be utilized for further reference and also in validating other remotely-sensed physico-chemical parameters in this region.  In-situ data from CTD casting in the field for temperature, salinity, density, turbidity, fluorescence and DO. In-situ laboratory measurement of elements from sediment sample and metals and TOC in water sample. Laboratory measurement of Nitrate, Nitrite, Phosphate, Ammonia, Silica, Chlorophyll -a, and Phaeopigment. Instruments: i) CTD Machine, ii) Grab sampler iii) 'Niskin' water sampler, iv) CHNS element analyzer, v) TOC analyzer, vi) Flame AAS vii) Spectrophotometer. Model of the instruments used: i) Sea-Bird Electronics CTD Machine (SBE 19 plus V2), ii) Van Veen grab sampler, iii) 2.5 L model, HYDRO-BIOS Apparatebau GmbH, Altenholz -Germany, iv) VarioMicro V1. 6

Value of the Data
• CTD data is key to investigate physical processes in shallower coastal BoB. Productivity can be perceived by in-situ chlorophyll-a, whereas, phaeopigment could be used as an indicator of phytoplankton bloom. Nutrients are inherent for productive zone detection and metals are prime to assess water quality as well as element cycling. In addition, major elements of sediment are crucial to identify the cycle path through the water column, atmosphere, and sediment. • Overall information from the spatial distribution of the obtained nutrients and chlorophyll would result in decision making to the fishing community in the nearby fishing ground. Entirely, these distinct in-situ data can assist to perform ecosystem and biogeochemical modeling. Moreover, to validate remotely sensed data in this region. • Physical parameters can be further related to detect mixed, barrier and isothermal layer thickness. Availability of limiting nutrients, such as nitrate, phosphate, and silica would implement in estimating phytoplankton growth, abundance, and biodiversity. Silica, would act as a tracer for the biogeochemical cycle, upwelling and carbon sequestration. Aquatic ecosystem's health can be indicated by the presence of heavy metals in the seawater.

Data Description
One of the largest bays in the world, the Bay of Bengal (BoB), is located at the northeastern wing of the Indian ocean. This bay ensembles higher magnitudes of changes in the aspect of thermodynamic parameters, compared to its neighboring Arabian Sea or the northern Indian Ocean [1] . Yet, a lack of collaborative research and research vessel, as well as paucity and unavailability of observational data [2] , restricts through and continuous monitoring of these physico-chemical dynamics. The input of a large quantity of freshwater from major rivers [3] have a dominant influence on physico-chemical properties, therefore, needs to analyze with in-situ data of this region. Large fluctuations in physico-chemical and biological parameters have accelerated from land derived nutrients to the complex coastal ocean over the past decades [4] . Significant stress on coastal habitats has been exerted by multitude of physical, chemical, and biological processes [5][6][7][8] . Partial CTD data of this manuscript was used in Hossain et al., 2018 [9] . Contributing to the pre-existing data gaps to study important physico-chemical processes, this article presents data of Temperature ( °C), Salinity (PSU), Density (kg m −3 ), Turbidity (NTU), Fluorescence (mg m −3 ), DO (mg/l), Carbon (%), Hydrogen (%), Nitrogen (%), Sulfur (%), C:N ratio, C:H ratio, Total Organic Carbon (ppm), Calcium (mg/l), Cadmium (mg/l), Copper (mg/l), Cobalt (mg/l), Iron (mg/l), Manganese (mg/l), Magnesium (mg/l), Nickel (mg/l), Lead (mg/l), and Zinc (mg/l) ( Tables 1 , 2 , and 4 & Fig. 2 ). Placed adjacent to one of the largest deltaic systems in the world, with more than 1.7 billion tons of sediment flowing downstream yearly [10] , makes the NBoB an interesting place for studying nutrients distribution. Recent studies propounded that the BoB might be more productive than previously estimated [11] . A better understanding of such discoveries is only possible by more in-situ datasets in the respective locations. Keeping that in mind, we present Nitrate (mg/L), Nitrite (mg/L), Phosphate (mg/L), Ammonia (mg/L), Table 1 Outline of physical parameter's data from CTD cast, including date of sampling, coordinates, tidal context, 'mean' value, median and 'standard deviation' (SD) for each station.

Coordinates
Temperature (   Silica (mg/L), Chlorophyll -a (mg/m 3 ) and phaeopigment (mg/m 3 ) in this article ( Table 5 & Fig. 4 ). Though the study area covers the continental shelf region of the northern BoB, it is focused on the northeastern part of the BoB, which contains a total of 15, 76 and 10 points sequentially for CTD, water and sediment samples that spanned between 18 January 2016 to 11 February 2016 ( Fig. 1 ). Sea-Bird CTD (SBE 19 plus V2) was maneuvered for taking profiles of physical parameters. Water samples, each with a volume of 500 ml was collected from each sampling point using the 'Niskin' Water Sampler, which is fully made of PVC and its interior is free from metal parts to avoid contamination with built-in messenger. Additionally, we deployed Van Veen grab sampler to extract sediment from the bottom of each station. Finally, concentrations of individual parameters from water and sediment samples were further analyzed at CARS (center for Advanced Research in Science) at the University of Dhaka using TOC analyzer (TOC -VCPH), Flame Atomic Absorption Spectrometry (Flame AAS) and CHNS element analyzer. Concentrations of individual nutrients were extracted from the obtained water sample and were further analyzed at oceanography lab at the University of Dhaka using DR60 0 0 TM UV VIS Spectrophotometer with built-in RFID Technology (Hach Company, USA). This model's scanning capability covers from UV to visible Spectrum, and includes 250 methods that were pre-programmed for water testing. Accordingly, these concentration data and physico-chemical parameter's values are represented in tabular format, comma-separated value ( * .csv) and ( * .txt) file.

Data collection
In-situ data were gathered on-board, during a cruise that lasted for 25 days from 18 January to 11 February (categorized in winter) at the time of fishing operation of 'Agro Food-4 of 'Sea Resource Ltd.' having a dimension of a fishing-trawler 42:11:13 for length: wide: height accordingly. Data for physical properties were collected on-board by CTD measurement. Preserved sample for nutrients, chlorophyll, TOC, heavy metals, and elements were analyzed further in the laboratory after transportation. The depth of each station was determined by SIMRAD sonar. 'Niskin' water sampler having 2.5 L capacity of HYDRO-BIOS with built-in messenger was operated manually for water sample collection which had a non-metallic interior to avoid contamination with metals. Although the sampling depths varied with site conditions, most samples were attained at 1 m, 2 m, 3 m, 5 m, and some samples at 10 m, 15 m, 20 m, 25 m with the maximum depth at 35 m. Sediment samples were mostly varied from 40 m to 60 m contour lines. Polypropylene bottles were used for sample collection. However, it was cleaned earlier  figure, (a), is for Sediment with purple diamond while, the "plus" signs represent points for CTD profiles and (b) are water samples illustrated with circles marked orange accordingly located near the northern Bay of Bengal. Bathymetry data were taken from Esri, GEBCO, NOAA, National Geographic, Garmin. Deep-green parts of the map represent a deeper sea while shallow seas are yellowish-green. with 1:1 dilute hydrochloric acid, rinsed with Milli Q water and finally dried up. Collected water samples were labeled in bottles and kept in ice for preservation until those were transported to the laboratory for detailed biochemical analysis.

Lab procedures 2.2.1. CTD data collection
Temperature was measured from 15 different sampling stations during the cruise from insitu observations by a Sea-Bird Electronics CTD Machine (SBE 19 plus V2). SBE 19plus CTD can measure with an accuracy of temperature ( ± 0.005 °C). The resolution of temperature from this device is 0.0 0 01. Temperature was measured in ( °C). SBE 19 plus V2 CTD has a resolution of conductivity 0.0 0 0 05 (most oceanic water: resolves 0.4 ppm in salinity), 0.0 0 0 01 (freshwater: resolves 0.1 ppm in salinity). Salinity was measured in PSU (Practical Salinity Unit). Salinity and Density ( б -t, kg m −3 ) data were derived using SBE data processing software (SBE Data Processing Win-32). Fluorescence and turbidity were measured by WET Labs ECO-FLNTU sensors attached with the CTD machine. Fluorescence was measured in mg/m 3 and turbidity was measured in NTU. For safety instruction, SBE 19 plus V2 CTD was submerged for priming of the pump below the sea surface for each profile before its sensor started reading. For this reason, in-situ measurement and parameters were obtained from 1 m depth instead of the sea surface. For all parameters priming data were removed using processing software, but the first scan count of priming was determined by another software Seasave V7. To communicate CTD with PC and for downloading data Seaterm V2 software was used. The unprocessed data was in a hex file (.hex) and the data was taken in ASCII format after processing. The mean value and standard deviation of each vertical physical parameter's profiles are shown in ( Table 1 ) as a tabular form. The statistical distribution of data has been manifested in ( Fig. 2 ).

Element analysis (CHNS)
Sediment samples were dried first and the analysis was carried out with CHNS element analyzer at CARS (center for Advanced Research in Science), University of Dhaka, Bangladesh. Analyzer VarioMicro V1.6.1, GmbH, Germany model was used to perform the measurement. Assay specification for analysis was: Sample pan: Tin boat, Combustion temperature: 1150 °C, Reduction temperature: 850 °C, Gas flow rate: Helium -200 mL/min, Oxygen −14 mL/min, Time for analysis: 800 s/sample, Detector: Thermal conductivity detector (TCD). All values are in percent relative to dry weight. Measured particulars have been conveyed in ( Table 2 ).

TOC measurement
Liquid samples were used for analysis. Assessments were carried out with TOC analyzer, Shimadzu, Japan (Model: TOC -VCPH). The system was TOC-regular sense, instrument option: TOC/ASI/IC unit, catalyst: regular sensitivity. Zero water required for blank preparation that refers to a standard solution with zero concentration of TC (Total Carbon) or IC (Inorganic Carbon). Zero water also refers to the water used to prepare standard solutions. Standard solution for TC: weighted 2.125 g of potassium hydrogen phthalate was dried at 105-120 °C for about 1 hour and then cooled in a desiccator. Therefore, it dissolved in zero water to leveled off up to 1 L, further, the required concentration of standard solutions was prepared with this stock solution. Standard solution for IC: previously dried (using silica gel desiccator) 3.5 g sodium hydrogen carbonate and 4.41 g sodium carbonate (dried for 1 hour at 280-290 °C and cooled in a desiccator) was dissolved in zero water (final volume 1 L) and required concentrations were prepared. Preparation of standard curve is shown in ( Fig. 3 ).

Metal analysis
Preserved water samples were kept in room temperature until metal content analysis by Flame AAS (Atomic Absorption Spectroscopy). Calcium (wavelength 422.7 nm), Cadmium (wavelength 228.8 nm), Copper (wavelength 324.8 nm), Cobalt (wavelength 240.7 nm), Iron (wavelength 248.3 nm), Manganese (wavelength 279.5 nm), Magnesium (wavelength 285.2 nm), Nickel (wavelength 232.0 nm), Lead (wavelength 283.3 nm) and Zinc (wavelength 213.9 nm) specific hollow cathode lamp was used to analyze the samples. The instrument has a minimum detection limit of 0.06 mg/L for Ca, 0.01 mg/L for Cd, 0.03 mg/L for Cu, 0.05 mg/L for Co, 0.04 mg/L for Fe, 0.02 mg/L for Mn, 0.01 mg/L for Mg, 0.06 mg/L for Ni, 0.20 mg/L for Pb, and 0.01 mg/L for Zn in the flame method. Samples were aspirated through a nebulizer and the absorbance was measured with a blank as reference. A calibration curve was obtained using standard samples. Standard curve concentrations for each metal are shown in ( Table 3 ). The correlation coefficient was found for Ca -0.998, Cd -0.998, Cu -0.999, Co -0.998, for Fe -0.996, Mn -0.998, Mg -0.999, Pb -0.999, and Zn -0.996. The sample had to be diluted many folds to keep the results in the analytical range. Analyzed data for metal concentrations have been shown in ( Table 4 ) with a summary of sampling points.

Chlorophyll and nutrient
The method used for measurement of chlorophyll-a and phaeopigment was Double extraction hot and cold treatment spectrophotometric method using 90% ethanol as a solvent and 2 M of HCl [12] . This method was picked out due to the double extraction of pigment. Besides, hot and cold treatment stimulate better extraction so that, little or no pigment left over the filter  Filtration of the water sample was done with the help of a vacuum filtration device using circle shaped Whatman GF/F filter paper (47 mm diameter, 45μ pore size) on the flat circular surface of the filter holder. Care was taken so that the filter paper fits well with the rim of the sample holding cup keeping no gap. The water sample was gently blended by reversing and up-righting the sample bottle 3 times. Immediately measured 500 ml of the mixed sample water and was gently poured inside the sample holding cup (volume taken of sample water depends on the density of the phytoplankton population present in the sample and should be judged by eye observation). Then filtration was done connecting the vacuum suction pipe with the port of the filter holder. After finishing the filtration, using a Millipore pincer the filter paper was rolled and placed inside the bottom of a test tube vertically. For an acceptable mean value of chlorophyll-a concentration, it was replicated 3 times. Placed the 3 test tubes in a still made heavy test tube rack. Then 4 ml hot ethanol (kept in the water bath before the filtration) was added to each test tube containing rolled filter paper and was placed on the rack in a water bath for 3 min. After the period was over the rack was placed under the flow of tap water to cool down carefully (so that no tap water enters the test tubes). Another 3 sets of fresh test tubes were taken to transfer the liquid those keeping the filter papers intact in the first set of test tubes. Further, the process was repeated of hot and cold treatment by adding a 2nd volume of hot ethanol in the test tube containing the filter paper. After cooling in the tap water with the help of a bent needle the rolled filter paper was brought out near the mouth of the test tube and using the pincer squeezed out the absorbed pigment sucked by the filter itself. Then added the liquid to the first set of extracted samples and added some 90% hot ethanol to adjust volume up to 10 ml. This 10 ml of the extracted sample was filtered again with the help of a syringe and 25 mm circle of Whatman GF/F filter paper and it was transferred into another set of 3 test tubes. DR60 0 0 TM UV VIS Spectrophotometric reading was taken using a cuvette having a path length of 2.5 cm with the filtered 10 ml sample. Reading was taken first at 665 and   ( continued on next page ) 750 nm OD and then added 33 μL of 2 M HCl with the help of a micropipette and was gently mixed. Waited for 1 min and again reading was taken at a wavelength of 665 nm and 750 nm OD. All the OD measurements were taken against a blank using 90% ethanol. Determination of biological and physiochemical parameters including Ammonia (NH 3 − N ), Nitrate (NO 3 − N ), Nitrite (NO 2 − ), Phosphate (PO 4 3 − ), Silica (SiO 2 ), were performed using standardized methods of seawater analysis [13] . Finally, the remaining nutrients were measured, adopting the photometric methods (HACH, USA).

Calculation: chlorophyll-a
Eb = (Eb 665 -Eb 750 ), Ea = (Ea 665 -Ea 750 ), Eb/Ea value must be 1.6 or very closer to this for a more accurate concentration of chlorophyll-a. Chl a ( μg / l ) = 29 . 6 ( Eb − Ea ) × v /V × l, Phaeopigment ( μg / l ) = [ ( 20 . 8 Ea × v /V × l ) − chl a ] , Where, Eb = OD before adding 2 M HCl, Ea = OD after adding 2 M HCl, v = Extracted volume of the pigment in ml, V = Filtered volume of sample water in liter, l = Path length of the cuvette used to measure OD in cm. Three replicates were taken for each sample water, as n = 3, so the mean value of it was the Chl-a and phaeopigment concentration of that sample water.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.