Dissolved nutrient dynamics along the southwest coastal waters of India during northeast monsoon: a case study

ABSTRACT

Dissolved nutrients, Chl-a and primary productivity were measured from seven transects along the coastal waters of the southeastern Arabian Sea during northeast monsoon. Ten major estuaries were chosen to study the influence of estuarine discharge on the nutrient dynamics in the coastal waters. The mean water discharge of the estuaries in the north (64.8 ± 18 × 10⁵ m³ d⁻¹) was found to be higher than those in the south (30.6 ± 21.4 × 10⁵ m³ d⁻¹), whereas the nutrient concentrations were found to be higher in the estuaries of the south. The results from the offshore waters were discussed in accordance with the depth contour classification, that is, shelf (depth ≤ 30 m) and slope waters (depth ≥ 30 m). Our results suggest that the estuarine discharge plays a major role in the nutrient distribution in near shore shelf waters, whereas in shelf and slope waters, it was mainly controlled by in situ biological processes. The inorganic form of N to P ratios were found to be higher than Redfield ratio in slope waters when compared with shelf waters, suggesting that PO₄³⁻ (<0.15 µmol L⁻¹) is a limiting nutrient for primary production. The multivariate statistical analysis revealed that the nutrient dynamics in the coastal waters was controlled by both biological and physical processes.

KEYWORDS:

• Primary production
• nutrients
• dissolved organic nitrogen (DON)
• dissolved organic phosphorus (DOP)
• coastal waters
• northeast monsoon

Acknowledgements

The authors thank the Secretary, Ministry of Earth Sciences, Government of India, and Director, ICMAM-PD, Ministry of Earth Sciences, Government of India, for the financial support and facilities during the study period. Thanks are also extended to Dr. R. Kirubakaran, NIOT, for providing the facility to use the Scintillation Counter for ¹⁴C sample analysis. This study is a part of MoES-ICMAM’s comprehensive project ‘Ecosystem Modeling for Southwest Coast of India’.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

G. Durga Rao http://orcid.org/0000-0002-2235-0437

Vishnu Vardhan Kanuri http://orcid.org/0000-0001-7152-5486