Decoding Seasonal C:N:P Ratios and Their Predictors in the Northern Indian Ocean: Implications for Nutrient Stoichiometry in a Changing Climate

Abstract Phytoplankton productivity, abundance, and growth in marine ecosystems are influenced by nutrient availability and elemental ratios. It is now well established that the classical Redfield ratio (106C:16N:1P) is not universal; instead, nutrient ratios vary with environmental conditions and follow strong latitudinal patterns. However, these observations are biased toward specific regions and seasons. Here, we analyze a large data set on carbon:nitrogen:phosphorus (C:N:P) ratios in particulate organic matter, as well as in dissolved inorganic and organic pools of the northern Indian Ocean. On average, the particulate organic C:P (∼130 39) and N:P (∼19 4) ratios in the surface ocean were higher than the Redfield ratio but consistently lower than those observed in the Pacific and Atlantic oceans. In contrast, the particulate organic C:N ratios (∼7 1) across tropical oceans reflect the Redfield value. We found seasonal variations, with higher particulate organic C:P, N:P, and C:N values during summer and winter compared to spring and autumn. High particulate organic C:P, N:P, and C:N ratios in the mixed layer during nutrient‐enrichment periods, along with increased inorganic N:P ratios, indicate that phytoplankton acclimate to ambient N:P ratios. Differential remineralization of nitrogen‐ and phosphorus‐rich biomolecules relative to carbon‐rich compounds regulates these ratios at greater depths. We found that inorganic nutrient ratios and temperature play key roles in shaping planktonic composition in the northern Indian Ocean. Generalized Additive Model reveals relationships between particulate C:P and N:P ratios with temperature and nutrients, indicating that future warming may lower these ratios, with implications for the biological pump under climate change scenarios.