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The largest continental shelf Oxygen Minimum Zone (OMZ) in the world is formed along the Indian western shelf in the eastern Arabian Sea during the Southwest Monsoon [(SWM); June–September], which is a natural pollution event associated with the coastal upwelling. This study examines the composition, abundance, and distribution of copepods during the Northeast Monsoon [(NEM); November to February] and SWM in 50 m depth zones along the Indian western shelf in the eastern Arabian Sea. The NEM was characterised by warm, stratified, and low-salinity waters in the southeast Arabian Sea and cold, high-salinity, and well-mixed waters in the northeastern Arabian Sea. During the SWM, cold and Dissolved Oxygen (DO) deficient waters (<22 μM/0.5 ml L−1), which are the signs of coastal upwelling, were evident all along the study zone, but with more intensity off Kochi, Mangalore, and Goa in the south than off Mumbai and Okha in the north. The zooplankton total biomass and abundance showed seasonality with a general decrease during the SWM (av. 3.68 ± 1.29 ml m−3 and av. 5711 ± 3096 Ind. m−3, respectively) compared to the NEM (av. 7.37 ± 2.17 ml m−3 and av. 14,473 ± 4966 Ind. m−3, respectively). At the same time, the abundance of Polychaeta and Siphonophora showed an increase during the SWM (av. 1187 ± 1055 Ind. m−3 and av. 169 ± 119 Ind. m−3, respectively), probably a result of the DO deficient waters associated with upwelling. Two striking seasonal features in Copepoda community were evident in this study: (a) a compositional shift from Cyclopoida dominant during the NEM to Calanoida dominant during the SWM, and (b) the coastal OMZ along the Indian western shelf during the SWM was dominated by Calanoida, which include oceanic OMZ species such as Pleuromamma indica , Lucicutia flavicornis , L.paraclausii , Eucalanus elongatus , Subeucalanus pileatus , S.subcrassus , and Clausocalanus furcatus. This forms a clear imprint for the extension of the oceanic OMZ into nearshore waters during the SWM due to coastal upwelling. • Considered Copepoda as an indicator of coastal hypoxia along the Indian western shelf. • A compositional shift of Copepoda was evident during different seasons. • Many Copepoda of oceanic OMZ were found in the coastal hypoxic zones. • The study shows the biological continuity of the coastal and oceanic OMZ. [ABSTRACT FROM AUTHOR]

This study presents how flood water transforms the hydrobiology of the nearshore waters close to estuarine inlets along the southwest coast of India, which are susceptible to significant freshwater discharge during the Southwest Monsoon (SWM) and depression events during the rest of the year. In the current study, coastal hydrography off Kochi that was acquired just before and during the flood event in August 2018 was compared with normal SWM conditions that prevailed in August 2014 and 2015. The SWM hydrography of August 2014 and 2015 revealed upwelling waters in the subsurface and a low saline lens in the top layer with a high concentration of chlorophyll- a (>3 mg m−3). The impact of floodwater was found in August 2018, even at a distance of 7 km from the Kochi inlet, as the low saline plume that was 3 m thick before the flood, expanded to 8 m thick and occupied almost the entire water column during the flood. It was evident that the flood water decreased the total chlorophyll- a concentration and microplankton abundance in the study zone. During the typical SWM in August 2014 and 2015, the nano-size fraction of phytoplankton dominated the chlorophyll- a (>70%), but the micro-size fraction dominated (50%) during the August 2018 flood time. All these suggest that the dilution effect of the flood could noticeably decline the phytoplankton biomass and abundance even at 7 km out to sea from the Kochi inlets, although it is unknown how this affects the higher trophic levels. • The impact of floodwater on the coastal phytoplankton community has been addressed. • The Southeastern Arabian coastal waters have been addressed. • The results showed that flood water transforms the hydrobiology of the nearshore waters. • Flood water dilution decreased chlorophyll-a concentration and microplankton abundance. • Micro-size fraction and their mean biovolume increased during the flood condition. [ABSTRACT FROM AUTHOR]

• Phytoplankton size responses in the Eastern Arabian Sea are being investigated. • Distinctive size responses exist to varying nutrient levels of winter convection and coastal upwelling. • Winter convection promoted nanophytoplankton. • Coastal upwelling favoured micro and mesophytoplanton. • These can be attributed to their differences in nutrient entrainment mechanisms. Even though the seasonal pattern of phytoplankton biomass distribution in the seas around India is reasonably well understood, there is relatively little information on phytoplankton size classes. The current study shows how phytoplankton of different size classes respond to nutrient enrichment caused by vertical convective mixing during the Northeast Monsoon [(NEM) November-February] and coastal upwelling during the Southwest Monsoon [(SWM) June-September] in the Eastern Arabian Sea (EAS). In the Northeastern Arabian Sea (NEAS), phytoplankton biomass [Chlorophyll-a (Chl-a)] and primary production (PP) were moderate and comparable during both the NEM (Chl-a av. 21 mg m−2 and PP av. 601 mg C m−2 d-1) and the SWM (Chl-a av. 27 mg m−2 and PP av. 516 mg C m−2 d-1). The Southeastern Arabian Sea (SEAS) was oligotrophic (Chl-a av. 12 mg m−2 and PP av. 197 mg C m−2 d-1) during the NEM due to strong surface stratification, but due to coastal upwelling, it turned into a very productive system along the shelf waters during the SWM (Chl-a av. 45 mg m−2 and PP av. 1201 mg C m−2 d-1). Interestingly, the subsurface Chl-a maximum (SCM) was almost absent in the NEAS during both seasons. Similarly, SCM was absent in the entire coastal upwelling zone of the EAS during the SWM. In these areas and situations, Chl-a was found to accumulate in the nutrient-rich surface layers of the shallow euphotic zones. The nutrient enrichment of the coastal upwelling along the shelf waters in the EAS favoured the growth of larger micro -and meso -phytoplankton during the SWM [av. 43.8 mg m−2 (av. 65.9% total Chl-a)], whereas the entire NEAS favoured the growth of nano-phytoplankton during the NEM [av. 13.9 mg m−2 (av. 64.8% total Chl-a)]. Differences in the physical processes enabling the entrainment of nutrients (Nitrate (NO 3), Phosphate (PO 4) and Silicate (SiO 4)] into the euphotic layer could explain the observed differences in the phytoplankton size fractions. The convective mixing in the NEAS during the NEM erodes the bottom of the mixed layer (50–120 m), resulting in only moderate enrichment of NO 3 (av. 0.62 ± 0.45 µM) and SiO 4 (av. 3.01 ± 0.83 µM), and low level of PO 4 (av. 0.49 ± 0.13 µM) in the mixed layer (av. 60 m). This caused N -limitation in the NEAS, which favoured the dominance of nano-phytoplankton. Alternatively, coastal upwelling during the SWM drives deeper subsurface waters (75–150 m) into the shallow surface mixed layer (av. 30 m) in shelf water, causing significant enrichment of NO 3 (av. 11.02 ± 3.55 µM), SiO 4 (av. 18.34 ± 11.37 µM) and PO 4 (av. 1.15 ± 0.21 µM). The enhanced nutrients in the coastal upwelling zones favoured the dominance of larger micro and meso -phytoplankton. However, during the SWM, the oceanic waters of the entire EAS showed low nutrient concentration compared to the shelf waters, which favoured the dominance of nanophytoplankton. In conclusion, our work reveals the large contribution of nano-phytoplankton in Chl-a biomass across the entire EAS, underlining their ecological relevance in both N-limited and N-enriched environments. [ABSTRACT FROM AUTHOR]