Research papersInfluence of short-term hydrographic variations during the north-east monsoon on picophytoplankton community structure in the eastern Arabian Sea
Introduction
The Arabian Sea (AS) constitutes the north-western part of the Indian Ocean and its semi-enclosed feature leads to an unusual climate, hydrography, and biogeochemical processes (Naqvi et al., 2003). As a result of the semiannual reversal of monsoon winds, seasonal variation of water column characteristics is also observed (Qasim, 1982, Banse, 1987). During the south west monsoon (SWM), coastal upwelling is a common phenomenon (Banse, 1968, Banse and McClain, 1986, Shetye et al., 1994) whereas during the north-east monsoon (NEM) convective mixing is prominent in the northern AS (Madhupratap et al., 1996) with decreasing intensity of mixing towards the southern AS (Prasanna Kumar et al., 2000). Additionally, during this period downwelling is also observed (Rao et al., 2008). Monsoonal forcing results in seasonal variations in the mixed layer depth, flux of nutrients to the upper mixed layer and thereby on pelagic food web structure and production (Madhupratap et al., 1996, Morrison et al., 1998, Prasanna Kumar et al., 2000, Wiggert et al., 2000, Shankar et al., 2005).
In the AS, phytoplankton biomass and primary productivity are high during the SWM and the NEM (Marra et al., 1998, Prasanna Kumar et al., 2000). During the NEM, interannual variations in the phytoplankton biomass and primary production are significant along the eastern (Bhattathiri et al., 1996, Sawant and Madhupratap, 1996, Parab et al., 2006, Ahmed et al., 2016) and the western AS (Campbell et al., 1998, Brown et al., 1999, Garrison et al., 2000). Only a few of the above studies have dealt with the smaller sized phytoplankton groups in the eastern AS (Roy et al., 2015, Ahmed et al., 2016).
During the last decades, the importance of picophytoplankton (PP; < 3 µm) has been demonstrated in the marine environment as major contributors to the phytoplankton biomass (Worden et al., 2004, Richardson and Jackson, 2007). PP comprises of three major groups, Prochlorococcus (PRO), Synechococcus (SYN) and picoeukaryotes (PEUK). PRO dominates the oligotrophic waters and is encountered down to 150 m depth (Chisholm et al., 1988, Partensky et al., 1999, Johnson et al., 2006, Fuller et al., 2006). SYN is abundant in mesotrophic waters with a shallower vertical distribution than that of PRO (Bouman et al., 2006, Zwirglmaier et al., 2007, Flombaum et al., 2013). PEUK, along with SYN dominate the nutrient-rich coastal ecosystems (Blanchot et al., 2001, Jiao et al., 2005, Sharples et al., 2009). Although limited information is available on the PP community from the eastern AS, short-term variability which can impart an in-depth understanding of the PP dynamics still remains unknown.
In this study, we focused on the short-term vertical variations (every 3 h for 9 days) in the PP community structure at a fixed location over the continental slope in the eastern AS, off Goa, India. The objective of this study was to assess the influence of different hydrographic conditions such as stratification and mixing on the PP distribution pattern. We hypothesize that in the eastern AS, the PP community structure and carbon biomass varies in response to the short-term hydrographic variability resulting from coastal advection and vertical water column mixing. This study will also serve as a basis for understanding the response of phytoplankton community as a whole with chlorophyll biomass as the proxy.
Section snippets
Sampling
Sampling was conducted for 9 days (D) at an interval of 3 h at one fixed location over the continental slope of the eastern AS (15° 18′ 46″N, 72° 41′ 53″E) (Fig. 1), onboard the ocean research vessel ORV Sindhu Sankalp (SSK-27) during the early NEM season (18–26 November 2011). Temperature, salinity and dissolved oxygen (DO) data profiles were taken from precalibrated CTD. Mixed layer (ML) depth was derived from the sigma-t criteria, as the depth at which a change from the surface sigma-t of
Environmental variables
Based on the ML depth, the sampling period was divided into two phases: phase I (from D1 to D5) was characterized by a shallow ML (25.58 ± 4.85 m) with high temperature (29.39 ± 0.2 °C) and low salinity (34.34 ± 2.8) water mass, and phase II (D6 to D9) was characterized by a deep ML (51.44 ± 5.50 m) with lower temperature (28.62 ± 0.2 °C) and higher salinity (35.15 ± 0.43) water mass (Fig. 3a and b). DO concentration ranged from 3.9 to 6.1 mg dm−3 in the ML (Fig. 3c). Nutrient concentrations within
Variations in picophytoplankton community structure
There is a scarcity of information on the short-term variability in PP (Jacquet et al., 2002), especially in the AS where monsoonal winds influence the hydrography. Hence a high resolution study at a fixed station over the continental slope of the eastern AS was carried out to better understand the impact of short-term variability in hydrography and environmental conditions on the PP community dynamics. During this study, the dynamic hydrography was well reflected in the PP community structure.
Acknowledgements
The authors would like to thank the Director of Council of Scientific and Industrial Research (CSIR)-National Institute of Oceanography for his support. The authors wish to acknowledge Mr. Sundar D., chief scientist of the SSK-27 cruise and Mr. Michael G.S., for providing CTD data. We thank Dr. V.V.S.S. Sarma for providing chlorophyll a (HPLC derived) data. We thank Dr. Shankar D. for the support and encouragement. We thank Mr. S. Subha Anand for providing nutrient data and our project team
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