Spatio-temporal variations of phosphorus (P) fractions in surface sediments of the southern Caspian Sea

Sediments act as a sink and a secondary source of contaminants, accounting for a central part of coastal and marine biogeochemical cycles. Phosphorus (P) is a macronutrient that governs primary productivity and phytoplankton growth, but excess P influx results in algae bloom and deteriorates aquatic ecosystems. This study assesses seasonal fluctuations, spatial distribution and fractions of P in the sediments of the southern Caspian Sea. In this study, at eight sampling points, composite samples of the surface (from 0 to 10 cm) seabed sediments were collected at 10 and 30 m water depths. The sampling campaigns were carried out in the four seasons and a total of 64 sediment samples were obtained. Total organic matter (TOM), total P (TP) concentration and particle size distribution were determined. Then, P was fractionated using a four-step sequential procedure to quantify the loosely bound P (LP), the reductant soluble P (FeP), the metallic oxide-bound P (AlP) and the calcium carbonate (CaCO₃) bound P (CaP). The inorganic P (IP) pool refers to the sum of LP, FeP, AlP and CaP, while the organic P (OP) was calculated by subtracting IP from TP.

The results show that seasonal fluctuations of mean TP were statistically insignificant (p-value > 0.05). Still, the highest levels were recorded in autumn (1555 mg kg^-1), followed by winter (1405 mg kg^-1), spring (1378 mg kg^-1) and summer (1130 mg kg^-1). These minor temporal variations in P levels are associated with the seasonal differences in the amount of runoff and the intensity of rivers discharging into the Caspian Sea, and thereby their sediment load and the physicochemical characteristics. The large riverine influx resulted in TP contamination hotspots in the river deltas of Anzali wetland, Babolrood and Sefidrud (northern Iran), where high loadings of suspended particles are discharged into the sea. The spatial TP distribution is thus site-specific and uneven. The main P fraction was CaP, reflecting the phosphate (PO₄^3-) strong affinity for, and association with, calcium-bearing minerals. Only a minor fraction of P was determined as LP. The fraction of the mud-size particles was the main explanatory factor for the spatial distribution of overall low levels of non-residual (or bioavailable) P forms (i.e., LP, FeP and AlP) during spring and summer, while the sand fraction had the greatest explanatory value for the distribution of residual (non-bioavailable) P form (CaP) during autumn and winter. This study demonstrates that P bioavailability in sediments is mostly controlled by the physicochemical characteristics of the sediment material, which again is steered by seawater chemistry. A low content of bioavailable P fractions could therefore be related to the relatively low content of fine-grained (i.e. below 63 µm) particles in sediments of the southern Caspian Sea.