The nutrient uptake bioassay (NUB): A method to estimate the nutrient uptake capacity of biofilms for the functional assessment of river ecosystems

River ecosystems host a large biodiversity and provide essential ecosystem services but are threatened by multiple anthropogenic activities that degrade their structure and functioning. Although both structure and functioning are key components of ecological status, river monitoring programmes rely almost exclusively on structural indicators, such as community structure, or water quality, neglecting functional attributes. Scientists have pointed towards some promising functions, such as organic matter decomposition, as potential indicators of river functional status. However, it may not be the best indicator to assess the ecosystem functioning of river ecosystem subjected to certain impacts, such as nutrient inputs. Additionally, managers have seldom used functional indicators, probably because of the lack of simple, routine protocols. Here we present the nutrient uptake bioassay (NUB), a simple and straightforward method to measure nutrient uptake by river biofilm that could be developed as a functional indicator of river ecological status. The NUB consists in deploying biofilm carriers in the river for a specific period, allowing biofilm colonization. Biofilm carriers are then incubated in the field in a nutrient-enriched standard solution for one hour and finally the remaining nutrient concentration in the standard medium is measured. Nutrient uptake is calculated from the difference between the initial and final nutrient concentrations. Chlorophyll in the biofilm carriers can also be measured to calculate biofilm accrual rates. The NUB has been developed based on a mesocosm and a field experiments, which showed that the NUB provides additional, non-redundant information to current biomonitoring techniques. Overall, the NUB is a cheap, robust and reliable method that can be applied by most river monitoring practitioners and can be adapted to most river types and situations. As nutrient uptake is linked to the self-purification capacity of rivers, the NUB results provide information about an important ecosystem service. Therefore, we suggest that the NUB can be developed as part of the monitoring toolbox available for managers to improve the assessment of river ecological status and to diagnose the causes of ecosystem impairment.