Toward Best Practices For Assessing Near Surface Sensor Fouling: Potential Correction Approaches Using Underway Ferry Measurements

Data from long term deployments of sensors on autonomous, mobile and cabled observation platforms suffer potential quality issues associated with bio-fouling. This issue is of particular concern for optical sensors, such as fluorescence and/or absorbance-based instruments for which light emitting/receiving surfaces are prone to fouling due constant contact with the marine environment. Here we examine signal quality for backscatter, chlorophyll and CDOM fluorescence from a single triplet instrument installed in a ferry box system (nominal depth of 3m) operated by Ocean Networks Canada. The time series consists of 22 months of 8-10 daily transits across the productive waters of the Strait of Georgia, British Columbia, Canada (Nanaimo on Vancouver Island and Vancouver on mainland BC). Instruments were cleaned every 2 weeks since all three instruments experienced significant signal attenuation during that period throughout the year. We experimented with a variety of pre- and post-cleaning measurements in an effort to develop ‘correction factors’ with which to account for the effects of fouling. We found that CDOM fluorescence was especially sensitive to fouling and that correction factors derived from measurements of the fluorescence of standardized solutions successfully accounted for fouling. Similar results were found for chlorophyll fluorescence. Here we present results from our measurements and assess the efficacy of each of these approaches using comparisons against additional instruments less prone to signal attenuation over short periods.