Effect of stormwater and land use on biogeochemical transformations of dissolved and particulate phosphorus in freshwater systems.

Climate change increases the frequency of storm events, which flush land pollutants, including phosphorus (P), into freshwater bodies; nonetheless, understanding the biogeochemical transformation of P in dissolved and particulate forms during storm events is still challenging. We collected surface water samples during storm and dry weather flow from five study sites along a pH gradient for measurement of P species, including: dissolved inorganic P (DIP) and organic P (DOP); colloidal organic and inorganic P (COP and CIP); and particulate organic and inorganic P (POP and PIP). In dry weather flow, DIP levels rise due to low concentrations of suspended particulate matter (SPM) (11-23 mg L) and desorption from the particulate matter. In contrast, during stormwater flow, high SPM concentrations (25-65 mg L) lead to the removal of DIP from the system through adsorption onto fine sediments. This inference is supported by the negative correlation between the P partitioning coefficient (K) and both SPM (r = -0.2) and pH (r = -0.4). Additionally, the correlation between benthic chlorophyll-a, DIP and PIP suggests that P uptake from different fractions influences the geochemical transformation of P during stormwater flow. This study shows that stormwater flow modifies the P species shift between the particulate and dissolved phases, with urban and agricultural land uses influencing tributaries and mainstreams more than forested areas. These findings emphasize the relevance of stormwater dynamics and land use in freshwater P control.