A 15-Year Record of Organic–Inorganic Phosphorus Variability in Eastern Mediterranean Wet Deposition

Wet deposition is a major mechanism of phosphorus (P) deposition to the ultra-oligotrophic Eastern Mediterranean, yet long-term constraints on the relative roles of dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) remain limited. In this study, 15-year observations of DIP and DOP variability were conducted at an Eastern Mediterranean regional background site, focusing on the temporal variability, drivers, and deposition. Wet deposition samples were collected on an event basis and analyzed for DIP using a colorimetric molybdate-reactive method. Total dissolved phosphorus (TDP) was determined following oxidative digestion, and DOP was defined by subtracting DIP from TDP. Deposition fluxes were calculated by coupling concentration measurements with precipitation depth, enabling assessment of both concentration-driven and rainfall-driven variability.Across the 15-year period, both DIP and DOP exhibited pronounced event-to-event variability typical of atmospheric deposition in the region. Preliminary results show that DIP was frequently enhanced during dust outbreak episodes consistent with mineral dust influence, indicating efficient wet scavenging of particulate and soluble inorganic P associated with crustal minerals. In contrast, DOP was more frequently associated with air masses bearing marine and continental/anthropogenic impacts. At the interannual scale, variability in both concentrations and fluxes tracked changes in rainfall intensity and event frequency, as well as the occurrence of dust-transport episodes. To better constrain sources and processes, deposition chemistry was evaluated in tandem with air-mass back trajectories, and, where available, supporting aerosol and meteorological data. The results indicate that dust-driven wet deposition delivers episodic pulses of bioavailable DIP. DOP supplies a more sustained, compositionally diverse pool. This pool may become bioavailable following photochemical and microbial transformation after deposition. Overall, the 15-year record show that the organic fraction is significant and that the annual DIP:DOP partitioning can change depending on the transport pathways and rainfall distribution. This has direct implications for regional external nutrient inputs and their future projections in response to changes in dust emissions and hydroclimate. AcknowledgmentsThis work has been supported by the HFRI grant # 4050 BIOCAN.