Turbidity and dissolved organic matter as significant predictors of spatio-temporal dynamics of phosphorus in a large river-floodplain system

Abstract

Phosphorus (P) inputs are increasing in river‐floodplain systems, but the factors whichinfluence the dynamics of this nutrient are not clear. To assess P dynamics in this kindof river, the main channel of the Middle Paraná River, 3 anabranches, 9 secondarychannels, and 20 lakes (7 permanently connected and 13 temporarily connected tothe fluvial system) were sampled. Multiple linear regressions were applied to explainspatio‐temporal patterns of P through commonly measured limnological variables.Particulate P increased during the sediment peak (evaluated through turbidity). Solublereactive P (SRP) was positively associated with dissolved organic matter (DOM,mainly the chromophoric fraction), which increased during high waters in the fluvialsystem but was highly variable in each kind of aquatic environment. In temporarilyconnected lakes, vegetated zones dominated by emergent macrophytes displayedthe highest SRP and chromophoric DOM concentrations. The flood and sedimentpeak positively affected P load in the river due to the increase in dissolved and particulatefractions, respectively. In addition, particle‐bound alkaline phosphatase activitywas positively associated with SRP concentration and load. These results suggest thatthe sediment peak incorporates particulate P in the system while the floodplain is a Psource during floods through exportation of the dissolved fraction. Dissolved P couldbe largely exported associated with DOM, which stimulates phosphatase biosynthesisby decreasing P bioavailability. The effect of aquatic macrophytes on P dynamicsseems to be influenced by DOM exudation. According to these considerations,DOM should be taken into account to analyse P dynamics in river‐floodplain systems.