Contrasting Photophysiological Characteristics of Phytoplankton Assemblages in the Northern South China Sea

Abstract

The growth of phytoplankton and thus marine primary productivity depend on photophysiological performance of phytoplankton cells that respond to changing environmental conditions. The South China Sea (SCS) is the largest marginal sea of the western Pacific and plays important roles in modulating regional climate and carbon budget. However, little has been documented on photophysiological characteristics of phytoplankton in the SCS. For the first time, we investigated photophysiological characteristics of phytoplankton assemblages in the northern South China Sea (NSCS) using a real-time in-situ active chlorophyll a fluorometry, covering 4.0 × 105 km2. The functional absorption cross section of photosystem II (PSII) in darkness (σPSII) or under ambient light (σPSII’) (A2 quanta-1) increased from the surface to deeper waters at all the stations during the survey period (29 July to 23 August 2012). While the maximum (Fv/Fm, measured in darkness) or effective (Fq’/Fm’, measured under ambient light) photochemical efficiency of PSII appeared to increase with increasing depth at most stations, it showed inverse relationship with depth in river plume areas. The functional absorption cross section of PSII changes could be attributed to light-adapted genotypic feature due to niche-partition and the alteration of photochemical efficiency of PSII could be attributed to photo-acclimation. The chlorophyll a fluorometry can be taken as an analog to estimate primary productivity, since areas of higher photochemical efficiency of PSII coincided with those of higher primary productivity reported previously in the NSCS.

Physiological and Ecological Responses of Photosynthetic Processes to Oceanic Properties and Phytoplankton Communities in the Oligotrophic Western Pacific Ocean

“…In contrast, the strong effects of excess irradiance pressure and limitation by nutrients in the surface inhibited the F v /F m , F q ′/F m ′, and F q ′/F v ′ ( Figure 4 ; Schuback et al, 2017 ; Wei et al, 2019b ). Consistent with previous observations ( Melrose et al, 2006 ; Claquin et al, 2008 ; Jin et al, 2016 ; Xie et al, 2018 , etc. ), variability in temperature exerted an evident influence on F v /F m and σ PSII ( p < 0.05; Figure 7 ).…”

“…With average DIN/DIP ratio notably less than the 16:1 Redfield ratio (Figure 2 and Table 2), the growth of WPO phytoplankton communities are significantly limited by nutrient availability (Saito et al, 2002). As a physiological consequence of nutrient limitation (Oxborough et al, 2012;Jin et al, 2016;Zhu et al, 2016), F v /F m and F q /F m were considerably low throughout the Z eu and among stations (Figures 4, 6). From a photophysiological point of view, the photochemical efficiency in natural phytoplankton assemblages is indirectly affected by the nutrient level (Moore et al, 2006;Rabouille and Claquin, 2016;Schuback et al, 2016).…”

“…Thereafter, the ecologically relevant rates of carbon fixation can be converted by FRRF-derived ETR RCII through a conversion factor, i.e., the effective electron requirement for carbon fixation ( Melrose et al, 2006 ; Zhu et al, 2016 ; Schuback et al, 2017 ; Morelle and Claquin, 2018 ). Additionally, the applicability of FRRF-based measurements to estimate marine primary production, alone or in combination with other techniques, are potentially limited since the light absorption characteristics and electron transport vary significantly in response to environmental constraints or combinations thereof and changes in species taxonomy and physiology ( Lawrenz et al, 2013 ; Jin et al, 2016 ; Schuback et al, 2017 ; Xie et al, 2018 ; Wei et al, 2019b ; Zhu et al, 2019 ). As such, more recent studies have sought to better characterize the extent and nature of variation between these photosynthetic processes and environmental/biological variables ( Moore et al, 2003 ; Suggett et al, 2009 ; Schuback et al, 2017 , etc.).…”

Fast Repetition Rate Fluorometry (FRRF) Derived Phytoplankton Primary Productivity in the Bay of Bengal

“…The maximum value equals 0.65 when all functional RCII are operating at maximum efficiency. Most often, F v /F m ranges from 0.65 in highly-productive regions to <0.3 in oligotrophic gyres (Falkowski and Kolber, 1995; Behrenfeld et al, 1996; Jin et al, 2016). In the BOB, most observed values of F v /F m (0.11~0.37) were only about half of the values expected for nutrient replete phytoplankton (Figure 4), hence indicating a biophysical consequences of nutrient limitation for phytoplankton assemblages.…”

“…Their increase at subsurface further confirms the inhibition of surface supersaturating irradiance and the presence of slow-relaxing NPQ components (Figure 4A). Likewise, low temperature usually has limited the ETR RCII due to low ribulose-1, 5-bisphosphate carboxylase/oxygenase activity (van de Poll and Buma, 2009; Jin et al, 2016). Accordingly, the negative correlation between these ChlF parameters and nutrients is likely attributable to the changes of temperature and light environment in vertical profiles (Figure 8 and Table 1).…”