Changes in riverine dissolved organic matter caused by gypsum‐induced flocculation

Gypsum (CaSO₄·2H₂O) is increasingly used to bind P to soil on agricultural fields, which mitigates eutrophication caused by runoff of excess PO₄ ^3- fertilizers into adjacent aquatic environments. Gypsum also binds dissolved organic matter (DOM) to soil particles. Gypsum that gets into fresh water after field applications may result in enhanced particle formation by DOM flocculation and alter C transfer in rivers draining agricultural catchments. We tested the potential effects of gypsum additions on DOM cycling by adding concentrated gypsum solution into river water before subjecting it to controlled mixing to increase particle collisions and flocculation. Gypsum addition increased the amount of suspended particulate matter in river water three to four times higher than in controls without gypsum. The flocs contained a relatively high amount of minerogenic particles. Gypsum-induced flocculation removed colored dissolved organic matter which, together with removal of minerogenic particles, may result in increased water clarity. Gypsum addition and the associated changes in the DOM pool did not affect microbial growth or DOM processing, suggesting that flocculation did not target the labile fraction of the DOM pool. While acknowledging that the responses detected in our study might depend on the region, we propose that the changes in riverine DOM cycling caused by gypsum application results in either no changes or slightly positive changes to the water quality of the rivers and should not be considered an obstacle for eutrophication prevention using gypsum applications.