Leaves stimulate aquatic phosphorus uptake by dark-grown but not by light-grown microbial communities in sediments: A laboratory study

It is a widely discussed topic in restoration ecology to use organic carbon (OC) management to increase nutrient uptake in chronically loaded agricultural streams. However, although nitrate uptake has been shown to correlated closely with the availability of dissolved OC, this relationship is less clear for phosphate. Likewise, the role of particulate OC (POC) on stimulating nutrient uptake has to be clarified. We aimed at investigating the effects of leaf additions on the net uptake of soluble reactive phosphorus and nitrate-nitrogen from the water column by benthic biofilms in combined laboratory flume and batch experiments.Fine sediments were colonised under nutrient-enriched conditions in the dark (dark-grown heterotrophic biofilms) and under a 12/12-hr dark/light cycle (light-grown photoautotrophic biofilms) in flumes. In each light treatment, half of the biofilms were grown under strong OC-limitation, the other half under near-optimal C:N:P ratios. For the uptake experiments, the colonised sediments were incubated in nutrient-enriched water to which (1) fresh alder leaves, (2) pre-leached leaves, or (3) no leaves were added. Net nutrient uptake (or release) rates were determined via the change in nutrient concentrations in the water column over time.Net phosphorus uptake by dark-grown biofilms was significantly increased by the addition of leaves. Despite their high initial nutrient leaching, fresh leaves showed stronger stimulating effects on the net P uptake than leached leaves over the entire experiment. Furthermore, biofilms grown under near-optimum C:N:P ratios responded stronger to the POC supply than biofilms grown under C-limited conditions. In contrast to dark-grown biofilms, leaf additions had no significant effects on the net P uptake by light-grown biofilms. We also found no effects of leaf additions on net nitrate uptake.Our study shows that an increased POC supply in nutrient-impacted streams may boost heterotrophic phosphate uptake. However, the stimulating effect of the POC addition depends on a variety of POC-dependent (e.g. respiration, heterotrophic assimilation, denitrification) and independent (e.g. adsorption, photoautotrophic assimilation) processes, making nutrient control via POC management challenging. We suggest application of management approaches that focus on restoration of riparian areas and wetlands to increase the amount of natural, complex POC with a moderate degradability, thus keeping the heterotrophic metabolism low but potentially stimulating in-stream N and P retention.