Estuarine, Coastal and Shelf Science 246 (2020) 107017 Available online 28 September 2020 0272-7714/© 2020 Elsevier Ltd. All rights reserved. Occurrence of enhanced herbivory in the microbial food web of a tropical estuary during southwest monsoon P.M. Sooria a, * , N. Nandini Menon a , R. Ranith a , M. Nair b , A. Anjusha b , A. Shivaprasad c , K. Ajith Joseph a , Saramma A. V d a Nansen Environmental Research Centre (India),KUFOS Amenity Centre,Madavana, Kochi, India b CSIR-National Institute of Oceanography, Regional Centre, Kochi, India c Department of Science and Technology, Govt. of India, New Delhi, 110016, India d Cochin University of Science and Technology, Kochi, India A R T I C L E INFO Keywords: Herbivory Monsoon Autotrophic picoplankton Cochin estuary ABSTRACT Tropical monsoonal estuaries have a signifcant role in global fshery as they act as rich feeding and breeding grounds for many commercial species. Food web dynamic studies in these coastal ecosystems are mainly focused on the classic food chain which assumes that microbial food webs contribute more towards the recycling of organic carbon and nutrients rather than production. But recently it is observed that phyto-microbial food web which is largely dependent on pico-autotrophs can contribute signifcantly towards the higher trophic level. Based on these observations we have analysed phyto-microbial pathway of food web in a highly productive tropical monsoonal estuary. We assessed the grazer-prey interaction in the microbial food web of Cochin estuary during spring intermonsoon and southwest monsoon seasons. Our results show that during spring intermonsoon the food web is dependent more on heterotrophic bacteria, but in southwest monsoon the dependency of grazers switches over to autotrophic picoplankton. We also analysed the growth rate, grazing rate and carbon contri- bution of autotrophic picoplankton experimentally during both seasons and found that autotrophic picoplankton show higher growth rate during southwest monsoon. The higher grazing rate and clearance rate of autotrophic picoplankton recorded during southwest monsoon indicate the effcient consumption of autotrophic pico- plankton crop during the season. We could also observe a change in autotrophic picoplankton community structure between the seasons with the dominance of picocyanobacteria during spring intermonsoon and that of picoeukaryotes during southwest monsoon. Thus we infer that high freshwater infux into the estuary during monsoon results in high turbid and low saline (mesohaline to oligohaline) environment which increases the growth rate of low light adapted autotrophic picoplankton populations and in turn enhances herbivory (phyto- microbial pathway) in the microbial food web. Apart from protists, autotrophic picoplankton were found to be widely consumed by many larval forms of mesozooplankton, gastropods, bivalves, rotifers and large sized flter feeders either as aggregates or as unicellular forms. Thus monsoon enhanced herbivory can have a signifcant role in the growth and survival of larval forms recruited into the estuary during monsoon season and in turn the fshery. 1. Introduction Studies on monsoon associated pelagic food web dynamics have high socio-economic impact as they give direct insights in to the global fshery production (Sreeush et al., 2018). Around 70% of fsh landing that come from the west coast of Indian subcontinent is due to the high primary production associated with monsoon induced upwelling (Jyo- thibabu et al., 2015; Sreeush et al., 2018). Most of the hypotheses proposed to explain the high production during monsoon emphasise the variability in the linear food chain as it has got a prominent and direct impact on the higher trophic levels (L´ evy et al., 2007; Roxy et al., 2016; Praveen et al., 2016; Ezhilarasan et al., 2018; Sreeush et al., 2018). Nevertheless, a few studies indicate that the role of microbial food web in the secondary and tertiary production of Indian coast cannot be ignored (Madhupratap et al., 1996; Nair et al., 1999; Brown et al., 1999). Both experimental and feld studies reveal that grazers in the * Corresponding author. Nansen Environmental Research Centre, India. E-mail address: soorya.mohan1@gmail.com (P.M. Sooria). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: http://www.elsevier.com/locate/ecss https://doi.org/10.1016/j.ecss.2020.107017 Received 10 May 2020; Received in revised form 7 September 2020; Accepted 16 September 2020