Assessing an In-Situ Total Algae Sensor Fluorometer for Performance in Freshwater Estuaries

In-situ fluorometry sensors offer a new means of estimating total algal biomass by measuring the fluorescence of photosynthetic pigments like chlorophyll-a. Researchers and water quality monitors, such as the National Estuarine Research Reserves (NERRs), are broadly deploying these new total algae sensors to augment current grab sample spectrophotometry techniques with high frequency sensor data. However, field performance assessments for these new sensors are limited, with potential for site-specific environmental parameters to interfere with chlorophyll-a fluorescence measurements. In this assessment, I examine the effectiveness and applicability of the YSI Total Algae probe for use in Old Woman Creek National Estuarine Research Reserve and other NERRs across the country. I also attempt to quantify and correct for interferences with sensor readings due to attenuation effects from water temperature and turbidity using controlled lab tests of deionized water and live water samples from Old Woman Creek. I then developed multiple correction models based on these lab tests, with the end goal of creating a strong correlational model between paired field measurements of in-situ fluorometric chlorophyll-a and ex-situ spectrophotometric chlorophyll-a collected over a 3 year period. Correction models were tested against uncorrected sensor values using simple linear regression analyses and Akaike Information Criterion (AIC) tests. I was unable to develop a strong correction model for temperature and turbidity, with the best full dataset correction model (r2=0.441) barely improving upon the uncorrected model (r2=0.440). However, additional models created with a "best-case" dataset that subset the full dataset to include only measurements with similar conditions to the live sample lab tests resulted in significant model improvements in corrected (r2=0.663) and uncorrected (r2=0.655) models. The lack of improvement between corrected and uncorrected models could be due to multiple compounding factors, including incident light refraction in turbidity tests, a chlorophyll-a concentration effect in temperature tests, and an algal colonial behavior effect throughout lab and field measurements. The improvement of models between the full dataset and best-case dataset highlights the importance of site-specific calibration and consideration of dynamic characteristics in coastal estuaries. In-situ fluorometry sensors are powerful tools for high frequency total algae sampling, but researchers and water quality monitors must be cognizant of interference effects and the site-specific limitations of these new methods.