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    Seasonality in autotrophic mesoplankton in a coastal upwelling-mud bank environment along the southwest coast of India and its ecological implications

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    Date
    2017
    Author
    Karnan, C.
    Jyothibabu, R.
    ManojKumar, T.M.
    Balachandran, K.K.
    Arunpandi, N.
    Jagadeesan, L.
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    Abstract
    Mesoplankton refers to both autotrophic and heterotrophic�plankton�with a body size between 200 � 20,000��m. Here, we applied a FlowCAM to identify the�autotrophs�present in the mesoplankton size class in a�coastal environment�along the southwest coast of India (off Alappuzha, Kerala), which is characterized by intense coastal�upwelling�and mud bank formation during the Southwest�Monsoon. 18 time series sampling (weekly/biweekly) sessions were carried out spanning over the Pre-Southwest Monsoon (April) to the Late-Southwest Monsoon (September) period in 2014. The study showed that during the Pre-Southwest Monsoon when�nitrate�level was relatively low in the study area, the mesoplankton community was entirely contributed by�zooplankton, mostly consisting of copepods. During this time, the only autotrophic mesoplankton found in the water column, that too inconsistently, was�Trichodesmium�erythraeum.�However, the entire scenario changed with the onset of the Southwest Monsoon due to hydrographical transformation and�nutrient enrichment�caused by the coastal upwelling. Especially during the Peak (July) and Late-Southwest Monsoon (August), the mesoplankton composition changed with a significant dominance of larger�diatoms�such as�Fragilaria�and�Coscinodiscus. The autotrophic mesoplankton abundance was noticeably low during the Pre-Southwest Monsoon (av. 3145 � 2396 individual m?3�and av. 2045 � 1907 individual m?3�in M1 and M2, respectively), as compared to the Southwest Monsoon (av. 30436 � 5983 individual m?3�and av. 32346 � 11664 individual m?3�in M1 and M2, respectively). Similar was the seasonal trend in the autotrophic mesoplankton biomass, which increased from a low Pre-Southwest Monsoon value (av. 8.45 � 7.1�mgC�m?3and av. 4 � 3.7�mgC�m?3�in M1 and M2, respectively) to a significantly high value during the Southwest Monsoon (av. 117.04 � 40.2mgC m?3�and av. 136.9 � 68.1�mgC�m?3�in M1 and M2, respectively). The FlowCAM analysis results also showed that the dominant copepods present in the study region during the Southwest Monsoon (Temora turbinata,�Oithona similis,�Pseudodiaptomus serricaudatus�and�Centropages tenuiremis) were smaller in size than those present during the Pre-Southwest Monsoon period (Acartia�danae,�A. erythraea, and�Centropages orsini). The ecological implication of the present observation is that the smaller-sized copepods grazing on larger�phytoplankton�during the Southwest Monsoon (upwelling) period cause sloppy feeding and lead to inefficient grazing of the larger diatom stock, eventually facilitating frequent phytoplankton�blooms�along the southwest coast of India.
    URI
    https://doi.org/10.1016/J.CSR.2017.08.006
    http://drs.nio.org/drs/handle/2264/7813
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