The impacts of biofouling on automated phosphorus analysers during long-term deployment

In-situ nutrient analysers are a promising tool for improving the temporal resolution of data and filling knowledge gaps in drivers of harmful algal blooms. There are significant challenges however regarding instrument biofouling and data drift, which remain largely unquantified and unresolved. In this study the effects of biofouling on data consistency and accuracy is quantified on automated wet chemical analysers during long-term monitoring. In 2019 three fractions of phosphorus (P); total phosphorus (TP), total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP), were measured in-situ at four sites in Southern Ontario, Canada. The analysers were exposed to a wide range of P concentrations and biofouling extremes over an 8-month period. They were calibrated using chemical standards both in the field and the lab, and validated with fortnightly grab samples, and the representativeness of real-time data under a range of biofouling conditions were analysed. Results show that analysers biofouling during long-term deployment can desensitize instrument measurements, with greatest impacts on instruments operating in highly turbid environments. Temporal changes in calibration curves suggest that equilibrium P concentrations (EPC₀) of sediments accumulating inside filters can elicit a rapid exchange of dissolved P (SRP, TDP) with the water sample. Data drift increases the further from the EPC₀ an instrument is required to analyse, and thus this study demonstrates that for in-situ P monitoring, unless filters are frequently replaced or renovated, in-situ probes should ideally be dedicated to a specific waterbody type defined by similar EPC₀ values. It is recommended that in order to ensure accuracy in in-situ monitoring of TP, TDP and SRP during long-term deployment, preliminary site trials should be conducted to ascertain sediment EPC₀; the extent of biofouling should be monitored; and/or frequent grab samples taken for post-deployment validation. The findings apply to any in-situ phosphorus monitoring techniques for SRP or TDP.