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The phytoplankton community structure exhibits seasonal and spatial variations in response to the environmental conditions, which aids in understanding the ecosystem's health. Given this, four samplings were conducted between October 2013 to April 2015, encompassing the monsoon, post-monsoon, and pre-monsoon seasons, from the Haldia port ecosystem of India. The samples were collected from the flowing estuary and an extended semi-enclosed dock. We hypothesized that the seasonal phytoplankton community (diversity, abundance, and carbon biomass) response will differ based on the environmental and hydrographical characteristics of the study site. Picophytoplankton and nano-microphytoplankton dominated the phytoplankton community in terms of numbers and biomass, respectively. Bacillariophytes dominated the nano-microphytoplankton abundance and total biomass, except during the monsoon when Dinophytes contributed (inner-zone). The dominance of Bacillariophytes and Chlorophytes in the outer-zone with picophytoplankton and Dinophytes in the inner-zone indicated group-specific hydrographic preferences that supported the hypothesis. The positive correlation of the picophytoplankton abundance (pre-monsoon) with Secchi disc depth and the negative correlation of diatoms (post-monsoon) with temperature signify the seasonal role of light and temperature, respectively. The highest nano-microphytoplankton species diversity (Shannon-Wiener's index) during the pre-monsoon (inner-zone) with more rare species indicated the probable influence of stable waters with increased water transparency. However, the community was unevenly distributed in the estuary due to the high abundance of the diatom, Aulacoseira granulata. Although harmful algal blooms were not detected, the higher temperature and nutrient concentrations could have favoured potentially harmful species (Pseudonitzschia delicatissima, Dinophysis acuta) during the monsoon. The system ranged from oligo- to mesotrophic state with moderate pollution levels (Carlson's Trophic State Index and Shannon's Index), indicating a reduction of the nutrient accumulation effects by the existing water renewal frequency. This study recommends incorporating qualitative and quantitative phytoplankton assessment in ecological monitoring of the stable coastal sites to prevent future harmful algal episodes.

The phytoplankton community structure exhibits seasonal and spatial variations in response to the environmental conditions, which aids in understanding the ecosystem's health. Given this, four samplings were conducted between October 2013 to April 2015, encompassing the monsoon, post-monsoon, and pre-monsoon seasons, from the Haldia port ecosystem of India. The samples were collected from the flowing estuary and an extended semi-enclosed dock. We hypothesized that the seasonal phytoplankton community (diversity, abundance, and carbon biomass) response will differ based on the environmental and hydrographical characteristics of the study site. Picophytoplankton and nano-microphytoplankton dominated the phytoplankton community in terms of numbers and biomass, respectively. Bacillariophytes dominated the nano-microphytoplankton abundance and total biomass, except during the monsoon when Dinophytes contributed (inner-zone). The dominance of Bacillariophytes and Chlorophytes in the outer-zone with picophytoplankton and Dinophytes in the inner-zone indicated group-specific hydrographic preferences that supported the hypothesis. The positive correlation of the picophytoplankton abundance (pre-monsoon) with Secchi disc depth and the negative correlation of diatoms (post-monsoon) with temperature signify the seasonal role of light and temperature, respectively. The highest nano-microphytoplankton species diversity (Shannon-Wiener's index) during the pre-monsoon (inner-zone) with more rare species indicated the probable influence of stable waters with increased water transparency. However, the community was unevenly distributed in the estuary due to the high abundance of the diatom, Aulacoseira granulata. Although harmful algal blooms were not detected, the higher temperature and nutrient concentrations could have favoured potentially harmful species (Pseudonitzschia delicatissima, Dinophysis acuta) during the monsoon. The system ranged from oligo- to mesotrophic state with moderate pollution levels (Carlson's Trophic State Index and Shannon's Index), indicating a reduction of the nutrient accumulation effects by the existing water renewal frequency. This study recommends incorporating qualitative and quantitative phytoplankton assessment in ecological monitoring of the stable coastal sites to prevent future harmful algal episodes.

Seasonal and spatial phytoplankton distribution in relation to environmental factors was investigated in New Mangalore Port, a major port along the west coast of India. A well-mixed water column characterized the non-monsoon seasons, whereas it was weakly stratified during monsoon. Water quality index (TRIX) scores indicated good water quality except during pre-monsoon (inner zone surface) and monsoon (near bottom waters). Surface abundance of tychopelagic diatoms (Paralia sulcata, Melosira nummuloides, Cylindrotheca closterium, and Nitzschia sigma) was higher during non-monsoon seasons. Certain centric diatoms, e.g., Leptocylindrus danicus, P. sulcata, and Rhizosolenia imbricata, dominated during pre-monsoon (inner zone) and positively correlated with TRIX. High Skeletonema costatum and dinoflagellate abundance during the monsoon season coincided with high nutrient concentrations. Five potential toxic and fourteen harmful/bloom forming algal species were encountered at abundances below the level that can be considered as harmful to the ecosystem. In addition to a baseline database, this study highlights the potential use of certain diatom species as indicators of hydrography and water quality for monitoring dynamic coastal marine ecosystems.