Interannual variability in particulate organic matter distribution and its carbon stable isotope signatures from the western Indian shelf waters.

Particulate organic carbon and nitrogen (POC, PN, collectively particulate organic matter, POM) and the stable isotopic signature of POC (δ13CPOC) are important to delineate its sources and recycling in shelf water. The present study provides insights into the factors responsible for spatial and interannual variability in POM and δ13CPOC values along the western Indian shelf waters (8° N -21° N) during the southwest (SW) monsoon (August) 2017 and 2018. The dominance of phytoplankton-derived POM with a negligible terrestrial influence was evident from the positive correlation between POC and TChla contents, ratios of C: N, and δ13CPOC signatures. Prominent upwelling signatures [cold nutrient-rich water, higher POM, total Chlorophylla (TChla), and δ13CPOC values] were noted in the south (8-12° N), whereas low nutrient warm waters (lower values of POM, TChla, and δ13CPOC) were prevalent in the north (13-21° N). Phytoplankton biomass was significantly higher and matured in 2017 due to an early and stronger upwelling in the south. In 2018, delayed and weak upwelling (evident from Ekman offshore transport and pumping velocity) resulted in the late development of phytoplankton bloom and lower POM. Furthermore, considerably lower nutrient supply within the mixed layers in 2018 compared to 2017 was partially attributed to the enhanced spatial expansion of low salinity waters closer to the surface. In the north, in 2018, higher wind speeds enhanced vertical mixing resulting in increased nutrient supply and TChla compared to 2017. We conclude that monsoon wind speed in the northern shelf and strength as well as the timing of the upwelling, including freshwater flux in the south, can be the key factors in modulating the interannual variability in POM distribution and δ13CPOC signature in the western Indian Shelf waters.

Impacts of Zn and Cu enrichment under ocean acidification scenario on a phytoplankton community from tropical upwelling system.

Increasing dissolution of CO2 in the surface ocean is rapidly decreasing its pH and changing carbon chemistry which is further affecting marine biota in several ways. Phytoplankton response studies under the combination of elevated CO2 and trace metals are rare. We have conducted two consecutive onboard incubation experiments (R. V. Sindhu Sadhana; August 2017) in the eastern Arabian Sea (SW coast of India) during an upwelling event. A nutrient enriched diatom bloom was initiated onboard and grown under ambient (≈400 µatm, A-CO2) and high CO2 levels (≈1000 µatm; H-CO2) with different zinc (Zn; 1 nM) and copper (Cu) concentrations (1 nM, 2 nM and 8 nM). Phytoplankton community composition and the dominant genera were different during these two experiments. CO2 enrichment alone did not show any significant growth stimulating impact on the experimental community except enhanced cell density in the first experiment. Addition of Zn at A-CO2 level revealed no noticeable responses; whereas, the same treatment under H-CO2 level significantly reduced cell number. Considerably high protein content under H-CO2+Zn treatment was possibly counteracting Zn toxicity which also caused slower growth rate. Cu addition did not show any noticeable impact on growth and biomass production except increased protein content as well as decreased carbohydrate: protein ratio. This can be attributed to relatively higher protein synthesis than carbohydrate to alleviate oxidative stress generated by Cu. The centric diatom Chaetoceros and toxin producing pennate diatom Pseudo-nitzschia showed no significant response to either CO2 or Zn enrichment. Large centric diatom Leptocylindrus and Skeletonema responded positively to Zn addition in both CO2 levels. The former species showed the most sensitive response at the highest Cu and H-CO2 treatment; whereas, the pennate diatoms Nitzschia and Pseudo-nitzschia (toxigenic diatom) showed higher resilience under elevated CO2 and Cu levels. This observation indicated that in future ocean, increasing CO2 concentrations and trace metal pollution may potentially alter phytoplankton community structure and may facilitate toxigenic diatom bloom in the coastal waters.