Table_1_Diel Patterns of Variable Fluorescence and Carbon Fixation of Picocyanobacteria Prochlorococcus-Dominated Phytoplankton in the South China Sea Basin.docx

<p>The various photosynthetic apparatus and light utilization strategies of phytoplankton are among the critical factors that regulate the distribution of phytoplankton and primary productivity in the ocean. Active chlorophyll fluorescence has been a powerful technique for assessing the nutritional status of phytoplankton by studying the dynamics of photosynthesis. Further studies of the energetic stoichiometry between light absorption and carbon fixation have enhanced understanding of the ways phytoplankton adapt to their niches. To explore the ecophysiology of a Prochlorococcus-dominated phytoplankton assemblage, we conducted studies of the diel patterns of variable fluorescence and carbon fixation by phytoplankton in the oligotrophic South China Sea (SCS) basin in June 2017. We found that phytoplankton photosynthetic performance at stations SEATS and SS1 were characterized by a nocturnal decrease, dawn maximum, and midday decrease of the maximum quantum yield of PSII (Fv(′)/Fm(′), which has been denoted as both F_v/F_m and Fv′/Fm′) in the nutrient-depleted surface layer. That these diel patterns of Fv(′)/Fm(′) were similar to those in the tropical Pacific Ocean suggests macro-nutrient and potentially micro-nutrient stress. However, the fact that variations were larger in the central basin than at the basin's edge implied variability in the degree of nutrient limitation in the basin. The estimated molar ratio of gross O₂ production to net production of carbon (GOP:NPC) of 4.9:1 was similar to ratios reported across the world's oceans. The narrow range of the GOP:NPC ratios is consistent with the assumption that there is a common strategy for photosynthetic energy allocation by phytoplankton. That photo-inactivated photosystems or nonphotochemical quenching rather than GOP accounted for most of the radiation absorbed by phytoplankton explains why the maximum quantum yield of carbon fixation was rather low in the oligotrophic SCS.</p

Patchiness of phytoplankton and primary production in Liaodong Bay, China

<div><p>A comprehensive study of water quality, phytoplankton biomass, and photosynthetic rates in Liaodong Bay, China, during June and July of 2013 revealed two large patches of high biomass and production with dimensions on the order of 10 km. Nutrient concentrations were above growth-rate-saturating concentrations throughout the bay, with the possible exception of phosphate at some stations. The presence of the patches therefore appeared to reflect the distribution of water temperature and variation of light penetration restricted by water turbidity. There was no patch of high phytoplankton biomass or production in a third, linear patch of water with characteristics suitable for rapid phytoplankton growth; the absence of a bloom in that patch likely reflected the fact that the width of the patch was less than the critical size required to overcome losses of phytoplankton to turbulent diffusion. The bottom waters of virtually all of the eastern half of the bay were below the depth of the mixed layer, and the lowest bottom water oxygen concentrations, 3–5 mg L^–1, were found in that part of the bay. The water column in much of the remainder of the bay was within the mixed layer, and oxygen concentrations in both surface and bottom waters exceeded 5 mg L^–1.</p></div

Contour maps of photosynthetic rates (Panel A, in mg C m^–3 h^–1), concentrations of Chl <i>a</i> (Panel B, in mg m^–3) and assimilation numbers (Panel C, in mg C mg^–1 Chl <i>a</i> h^–1), dissolved inorganic nitrogen (DIN, Panel D, in μM), silicate (SiO₃-Si, Panel E, in μM), and phosphate (PO₄-P, Panel F, in μM).

<p>Triangles in F denote stations with phosphate concentrations less than 25 nM.</p

Study area, sampling stations (● and △) and ¹⁴C incubation stations (△) in Liaodong Bay, China.

<p>Study area, sampling stations (● and △) and ¹⁴C incubation stations (△) in Liaodong Bay, China.</p

Contour maps and/or corresponding location maps of temperature (Panels A and B, in °C), Secchi-disk depth (Panels C and D, in meters), Secchi-disk depth associated with temperature (Panel E), and dissolved oxygen (DO, Panel F, in mg L^–1).

<p>Contour maps and/or corresponding location maps of temperature (Panels A and B, in °C), Secchi-disk depth (Panels C and D, in meters), Secchi-disk depth associated with temperature (Panel E), and dissolved oxygen (DO, Panel F, in mg L^–1).</p

Relationship between DO concentrations and salinity (Panel A) and location of stations in three groups with different DO concentrations (Panel B).

<p>In Panels A and B, circle dots denote stations with high DO of more than 13 mg L^–1 at a salinity of 1 to low DO concentrations of 3–5 mg L^–1 at salinities of 16–26; Triangles and plus signs denote stations with DO concentrations of roughly 7–9 mg L^–1 and 9–11 mg L^–1, respectively, in both cases at salinities of 20–28.</p