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Photosynthesis is the most important bioenergy conversion process on Earth. Capturing instantaneous changes in in situ photosynthesis in open ocean ecosystems remains a major challenge. In this study, fast repetition rate fluorometry (FRRF), which can obtain nondestructive, real-time and in situ estimates of photosynthetic parameters, was used for the first time to continuously observe the spatial variation in in situ photosynthetic parameters in the eastern Indian Ocean (EIO). We further formulated new insights regarding abiotic and biotic factors of potential importance in determining photosynthetic performance. First, we found that the distributions of micro/nano- and picophytoplankton were opposite under the control of nutrient concentrations. Micro/nanophytoplankton had higher cell abundances in the nearshore and upwelling regions, while picophytoplankton had higher abundances in the open ocean, and Prochlorococcus was the dominant group. Second, based on the FRRF technology, we obtained the high-precision and high-density vertical profile map of photosynthetic parameters in the euphotic layer. It was observed that values of the maximum photochemical efficiency (Fv/Fm; 0.14–0.55, unitless) and the functional absorption cross-section of PSII (σ PSII ; 1.71–4.90 nm2 RCII−1) increased with increasing depth, while high values of the photosynthetic electron transfer rates (ETR RCII ; 0.0019–17.0292 mol e− mol RCII−1 s−1) and the nonphotochemical quenching (NPQ NSV ; 0.35–7.26, unitless) occurred in the shallow 50 m layer, and the values decreased as the depth increased. Finally, we discussed limiting factors that regulated the distribution of photosynthetic parameters and concluded that optical properties varied significantly with changes in the ocean physico-chemical parameters and taxonomic composition of phytoplankton assemblages in the EIO. Picophytoplankton (especially cyanobacteria), rather than the micro/nanophytoplankton community, was the dominant factor influencing photosynthesis. Among abiotic factors, photosynthetically active radiation (PAR) was the proximal limiting factor affecting photosynthetic efficiency, followed by temperature and dissolved inorganic nitrogen (DIN). Consequently, phytoplankton photosynthetic parameters exhibited great variability, allowing rapid responses to environmental condition changes. In this study, we established the basis for detecting future changes in primary production in this oligotrophic area. • In situ photosynthetic parameters in the eastern Indian Ocean are presented. • Fv/Fm increases with increasing seawater depth. • Picophytoplankton is the dominant factor influencing photosynthesis. • Photosynthetically active radiation is the proximal limiting factor affecting photosynthesis. [ABSTRACT FROM AUTHOR]

Abstract Statistical analysis of rainfall data from 2005 to 2015 showed that atmospheric deposition supplied large amount of dissolved inorganic nitrogen (38–155 mg·m−2·month−1) in N-deficient South China Sea and Eastern Indian Ocean. To understand marine ecosystem responses to increasing nutrient disturbances, we implemented field mesocosm experiments to study phytoplankton community structure and biodiversity responses to nutrient treatments with nitrate, phosphate and iron across tropical seas. Our results showed that DIN supply would change phytoplankton community structure and stimulated the regime shift from cyanobacteria to diatoms (relative dominance R > 0). Phytoplankton communities were dominated by diatoms (relative abundance >50%) accompanied by high chlorophyll a content with 1.58–39.27 μg·L−1 in DIN-added cultures, whereas cyanobacteria dominated communities (relative abundance >60%) with low biomass of 0.12–0.18 μg·L−1 in undisturbed cultures. Simultaneously increased DIN loading from atmospheric deposition would decrease ecological diversity of tropical seas owing to species competition and succession (Shannon diversity H′ decreased to <1). Graphical abstract Unlabelled Image Highlights • Wet deposition supplied large amount of DIN in two tropical seas. • Regime shift from cyanobacteria to diatoms was stimulated following DIN supply. • The study indicated that phytoplankton diversity of two tropical seas would be reduced following exogenous nutrients supply. • Management practices should be advocated to protect ecological diversity. [ABSTRACT FROM AUTHOR]

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A Great Shearwater Ardenna gravis, observed and photographed off Amsterdam Island, on 2 February 2017, was a rare but not exclusive sighting of this species from the eastern Indian Ocean. We summarize the occurrence of this species in the southern Indian Ocean based on surveys conducted during the period 1985-2017. [ABSTRACT FROM AUTHOR]