- Dona Paula, Goa, India.
- +91-0832- 2450327
- iiosc2020[at]nio[dot]org
| Abstract Submission No. | ABS-2022-05-0253 |
| Title of Abstract | Nitrogen limitation and excess phosphate linked to nutricline variability and their stoichiometry in the eastern Arabian Sea |
| Authors | Sherin C.K.*, G.V.M. Gupta, Sudheesh V., Yudhistir Reddy, N.V.H.K.Chari, Kausar F. Bepari, Ch.V Ramu, Anil Kumar Vijayan |
| Organisation | Centre for Marine Living Resources and Ecology. |
| Address | Centre for Marine Living Resources and Ecology, LNG Road, Puthuvype. Kochi, Kerala, India Pincode: 682508 Mobile: 9676027815 E-mail: sherinsci@gmail.com |
| Country | India |
| Presentation | Oral |
| Abstract | Repeat measurements (nine times) of nutrients along seven to ten coast-offshore transects of eastern Arabian Sea (EAS) in 2018-19 revealed nitrogen as the major limiting nutrient controlling the primary production in the EAS. The EAS experienced upwelling at varying extent during the summer monsoon (SM), and the winter convection in the north EAS primarily controlled the depth of nitracline (3 µM NO3-). The nitracline was found positively aligned with the upper boundary of thermocline (depth of 26ᵿC or D26), with the strongest relationship in the south EAS (R2=0.91; p<0.001) due to its relatively strong stratified and stable conditions compared to that in the north EAS (R2=0.63; p<0.001). A very unstable water column in the north during the winter convection resulted in their poor correlation, whereas they were tightly coupled during the summer monsoon upwelling along the entire EAS. Mixed layer nutrient availability varied with the nitracline/thermocline and relatively faster exhaustion of nitrogen than phosphorus during the nutrient enrichment events viz. upwelling and convective mixing yielded low N/P ratios. The N/P ratios of nutricline waters upwelled/entrained to the mixed layers were found significantly low (6-14) especially at the north EAS where perennial denitrifying zone persisted in the intermediate depths. With continued uptake of nutrients following Redfield ratio, the system during oligotrophic inter-monsoons become nitrogen limited with surplus (excess) phosphate (P*), resulting in lowest N/P ratios (1.5±0.7) in the mixed layer. Higher nutrient supply through entrainment and upwelling from the relatively shallow nitracline led to higher P* in the north EAS (0.43±0.10 µM) and throughout EAS (0.40±0.14 µM) during the summer monsoon, respectively. This excess phosphate led to a dominance of diazotrophy under calm environment during the inter-monsoon seasons which reduced the P* to almost half (0.22±0.12 µM) compared to the nutrient enrichment seasons and counterbalance to some extent the N-limiting condition in the surface waters. The N-limiting condition can be severe in a global warming scenario, so more observations and extensive experiments are needed to ascertain the capability of diazotrophs to counteract the N-limiting situations by utilising the excess phosphate. |