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Plankton distribution in relation to biophysical conditions along Bangladesh coast and estimation of primary productivity in the Bay of Bengal.

Abstract: Plankton dynamics from surface water along Bangladesh coast was studied seasonally for a period of April 2012 – November 2014 in relation to environmental variables like temperature, pH, salinity, conductivity, total dissolved solids, turbidity, secchi disk depth, attenuation coefficient and chlorophyll concentrations. Beside of this, annual euphotic depth and integrated primary productivity of the Bay of Bengal was estimated for the period of January 2010 – December 2014 using chlorophyll based models to identify the best model which one is mostly suited with the Bay.

Considering the total coastal area of Bangladesh, in total of 107 phytoplankton and 28 zooplankton species had been identified from 15 different sampling stations through the study. These numbers of plankton were recorded within four studied zones (western, central, eastern and south-eastern) along Bangladesh coast during summer (May 2012 and April 2014) and winter (January 2013 and December 2014) season. Most of phytoplankters were marine diatoms and some are dinoflagellates and flagellates, but no coccolithophores were recorded from there during the study. On the basis of the study, a total of 35 diatom, 8 dinoflagellate and 9 copepode species were newly recorded from the coastal area of Bangladesh. Phyto- and zooplankton density ranged from approximately 34,000 – 240,000 and 1,800 – 20,000 cells/L, respectively. The highest abundance for both phyto- and zooplankton was found at the south-eastern zone of Bangladesh coast. Seasonal changes in abundance and diversity of phytoplankton significantly differed showing maximum diversity in summer with high diversity index (2.76 in April 2014) and minimum in winter (0.326 in January 2013). The species evenness varied from 0.137 (May 2012) to 0.991 (April 2014), which signifies minimum variation in percentage contribution of individual species to total phytoplankton population in summer and maximum in winter.

The highest attenuation coefficient (K) value was calculated as 0.59 in the south-eastern coastal zone of Bangladesh. Correlation studies of total cell count to biophysical parameters indicated significant negative relation with salinity, conductivity and TDS, but strong positive relation with chlorophyll a concentration, whereas phyto- and zooplankton had a strong correlation with each other along Bangladesh coast. Multivariate procedures like ordination by principal component analysis and multi dimensional scaling of phytoplankton population based on their occurrence data and magnitude of abundance indicated that some genera (Coscinodiscus concinnus, C. concinniformis, Gyrosigma acuminatum, Odontella aurita, Rhizosolenia styliformis, Thalassiosira lineata, etc.) have specific preference for water temperature and salinity and flourished maximally in particular season(s), while other genera (Chaetoceros similis, C. denicus, Haslea trompii, Dytilum brightwelli, Ceratium trichoceros, C. longipes, C. symmetricum, C. candelabrum, C. fusus, etc.) appeared in wide range of temperature and salinity gradient.

The annual euphotic depth (Zeu) estimated from chlorophyll (chl) and photosynthetically available radiation (PAR) was ranged from 34-52 m and 41-77 m, respectively with 2-17 m SD value between them in the Bay of Bengal. Along Bangladesh coast, it was found <9 m and <12 m, respectively during 2010-2013, but in 2014 it was increased to 12-21 m for Zeu(chl) and 11- 19 m for Zeu(PAR) in an amazing manner with 2-5 m SD value. On the other hand, three chlorophyll based primary productivity (PP) models were used to estimate IPP in the BoB for 2010-2014 using MODIS data. They were the widely accepted model Vertically Generalized Production Model (VGPM), a revised form of VGPM and a new type of model which uses chlorophyll as input variable along with sea surface temperature (SST) proposed by Behrenfeld and Falkowski (BFM), Kameda and Ishizaka (KIM) and Ishizaka et al. (IEM), respectively. It was found that the degree of agreement among the results from three models was low, but the difference appeared relatively small. Among the three models, the output from IEM was differed the most from that estimated by the other two models. The 2010-2014 mean value of IPP produced from IEM was the highest (335.54 mgC/m2/d), while the IPP from BFM was the lowest (271.2 mgC/m2/d), and that of KIM was in the middle (328.45 mgC/m2/d) for the BoB

where IPP ranged 50-650 mgC/m2/d. The annual IPP during 2010-2014 in the coastal areas of Bangladesh was estimated as the highest from KIM (2300-2700 mgC/m2/d) and the lowest from IEM (1200-1400 mgC/m2/d), while the moderate IPP by BFM was 2000-2300 mgC/m2/d. The results revealed that the production along the coast of Bangladesh was higher than any other coastal areas of the Bay of Bengal.

The VGPM was verified by the primary production data of the Bay of Bengal, Bangladesh. In situ measurements on cloudy days indicated that the use of satellite data from sunny days only overestimated primary production. The VGPM with open sea parameters including maximum primary production per unit of chl in the vertical profile (PBopt) explained only 33% of the variability of IPP. Formulations of the open water PBopt showed no correlation with in situ PBopt. Adjustment of chl a and SST dependent PBopt improved the estimation of IPP to 41% of the variation. IPP calculated with light dependency and the adjusted PBopt based on IEM explained 67% of the variation. The IEM showed the best fit for chlorophyll based IPP estimation in the Bay of Bengal, whereas the KIM was found as the best fitted model for coastal areas of the Bay.

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