Linking phosphorus entrapment and release potential in fluvial biofilms to carbon and light availability in natural environments

In the global phosphorus (P) cycle, aquatic ecosystems play a crucial role, as there, long-term retention of P through sedimentation and subsequent burial of P containing minerals takes place. This study zooms into a rather overlooked section of the aquatic part of the global P cycle: The P entrapment pathways of benthic microbial biofilms. Benthic microbial biofilms are able to entrap P in its various forms, biochemically transform it, and contribute to internal loading via the release of P. The importance and the dynamics of P entrapment and P release in fluvial benthic microbial biofilms are, as of now, not completely understood.

For this field study, we performed a longitudinal sampling campaign along a 25 kilometer stretch of a third order Central European river, with the aim of investigating P entrapment patterns of benthic microbial biofilms. We distinguished between extracellular P entrapment and intracellular P entrapment and recorded metabolic characteristics of the sampled biofilm, as well as environmental variables.

We found that the ratio of intracellular P to extracellular P differed greatly between sampling sites. High values for this ratio (on average, 26.7) were related to relatively pristine sampling sites with rather recalcitrant allochthonous carbon inputs. Further downstream, at sampling sites exposed to anthropogenic disturbances, the ratio declined sharply (on average below, 1). These biofilms were subject to P-rich wastewater treatment plant effluent and labile dissolved organic matter of rather autochthonous origin. Measurements of the equilibrium P concentration, as a measure for P release potential from the sediment, showed that sites with benthic biofilms with a higher share of extracellular P have a highly increased P release potential from the sediment. We further found distinguishable carbon use metabolic profiles of the biofilms between different sampling sites, though a higher carbon use functional diversity did not necessarily contribute to a higher overall P entrapment in the biofilm.

Our results show clear patterns of benthic biofilm P entrapment along the sampled river stretch. These patterns seem to be connected to the changing environmental variables along the sampled river stretch. Furthermore, the P release potential from the sediment was highly correlated with an increased share of extracellular P in the biofilms.