Dynamic responses of picophytoplankton to physicochemical variation in the eastern Indian Ocean

Abstract

Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus ( Pro ), Synechococcus ( Syn ), and picoeukaryotes ( PEuks ). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks . Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a ) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.

The Horizontal Distribution of Siliceous Planktonic Radiolarian Community in the Eastern Indian Ocean

“…These environmental variables play an important role in circulatory changes of EIO in the spring season. The nutrients (silicate, phosphate and nitrate) may be depleted throughout the EIO, except in coastal areas, when coastal upwelling cause nutrients to elevate [31]. In this study, the Chlorophyll a concentration in the surface layer ranged from 0.005 to 0.89 µg/L, which is similar to the remote sensing Chlorophyll-a data and high chlorophyll a of the same period can be seen in this region (Figure 4; Supplemantry Figure S1).…”

“…At the depth below 100-200 m, it is characterized by high salinity and low temperature as shown in Figure 4. BOB is located in the north which has the characteristics of intrusive river runoff with low salinity and high temperature in the EIO [31], Sumatra is open coastal zone with conjunction of the BOB, characterized by low salinity and oxygenated water and the equator area is characterized by warm water and mixed layer of Chlorophyll a [30].…”

Biogeographic variations of picophytoplankton in three contrasting seas: the Bay of Bengal, South China Sea and Western Pacific Ocean

“…Pro were present in high abundances at low nutrient concentrations (Fig. 4), consistent with adaptation to growth under low nutrient availability (Olson et al 1990, Zubkov et al 2000, van den Engh et al 2017, Wei et al 2019b. It is usually believed that elevated nutrient concentrations and heavy metals induced by coastal currents or upwellings could be toxic to Pro growth (Mann et al 2002, Gaulke et al 2010, Lee et al 2014.…”

“…A large data set has been accumulated on picophytoplankton populations in the world oceans (e.g. Campbell et al 1997, Zhang et al 2013, Chen et al 2015, Wei et al 2019b, but a caveat to their investigation is that the abundances and distributions of different picophytoplankton populations were measured from just 1 oceanic or coastal ecosystem, i.e. previous estimates could not address large-scale spatial variations in picophytoplankton populations.…”

“…However, the 3 seas still have contrasting differences in structural and functional components (Qu et al 2005, Cheung et al 2008, Wei et al 2019b. Importantly, the variability in picophytoplankton abundance and distribution is intimately related to these physical processes, as well as environmental factors (e.g.…”