Vertical Distribution of Phosphorous Fractions and Bioavailability of the Nutrient in the Southern Indian Ocean

Abstract

The study evaluated concentration, vertical distribution, and bioavailability of phosphorus fractions in the Indian Ocean core sediments. Sediment samples were collected from two sites in the Indian Ocean during January–February 2010 using gravity corer. Phosphorus fractions in sediments were analyzed by a modified sequential extraction procedure (SEDEX). The vertical profile revealed silt and clay as dominant grain size fractions in both cores. The enriched levels of organic carbon were observed in samples due to adsorption on fine grained sediments. Loosely bound P was the most dominant fraction, and its higher concentration in sediments was due to increased productivity and subsequent inputs of biogenic components. Detritus from marine organisms and lower oxygen concentrations (depleted Eh values) enhanced the production of authigenic P. The lower iron bound P fraction at specific depths was attributed to the nature of Fe minerals present within sediments. Decreased level of organic phosphate fraction was due to mineralization. The presence of Detrital P, the diagenetically stable form of P, might be due to inputs from the Himalayan, peninsular Indian, and Sri Lankan regions. The biogenic form was the dominant phosphate fraction throughout the cores suggesting the dissolution of fish debris. Estimated bioavailable phosphorous (BAP) ranged from 82.2 to 98.7% in Core I and from 90.21 to 92.98% in Core II, suggesting the fact that a major portion of the phosphate was bioavailable and hence the alterations in environmental conditions can release the BAP to water column resulting in nutrient enrichment. Graphical Abstract

Phosphorus speciation in coastal sediment of Osaka Bay: relation to anthropogenic phosphorus loading

“…Previous studies (Yang et al 2019, Liao et al 2020) suggested that higher Ex-P values occur near river mouths and surface sediments. However, our selected core sample layers were slightly deeper (37 cm-42 cm, see figure 3), and contained Ex-P, which is easily available in the water column (Mullungal et al 2022). Thus, the low values of Ex-P can be attributed to the fact that they represent the residues remaining following their utilization by primary producers.…”

“…The SEDEX method extracts different P species from sediment based on their solubility and reactivity. A sequential extraction procedure was applied to estimate five species of P from sediment: (1) Exchangeable P (Ex-P) or loosely bound P, an important inorganic P species, is absorbed by sediment, can easily be released into the water column, and is used by phytoplankton (Mullungal et al 2022). ( 2) Iron-bound P (Fe-P), an inorganic P species, is produced by co-precipitation with Fe/Mn oxides or hydroxides (Zhou et al 2016).…”