Abstract
Traditional laboratory methods exist for the measurement of phytoplankton abundance and distribution, but they are costly and time-consuming – the analyses require skilled technicians and microscopists, and it can take weeks to months for results to be available. This approach also requires discrete samples to be collected in the field from a specific time and location. Optical measurements made by in situ sensors can provide instantaneous results, and data can be collected at high temporal and spatial resolution. Due to the increasing affordability and reliability of commercially available optical sensors, they are increasingly being used to measure water quality. Measuring the fraction of light absorbed at specific wavelengths and subsequently released at longer wavelengths (that is, fluorescence) is diagnostic of different compounds. Because phytoplankton contain different kinds of pigments that each have a unique fluorescence spectrum, spectral fluorometers have been designed to detect phytoplankton abundance and species composition. One promising instrument is the bbe FluroProbe (bbe Moldanke, GmBH, Germany), which is designed for the instantaneous analysis of total chlorophyll and algal class determination. Use of this sensor would not only reduce the costs associated with traditional multiday laboratory processes, but it would provide real-time information at a high resolution (~1 data point every 4 seconds). The objective of this study was to compare the use of an in situ fluorometer – the bbe FluoroProbe (owned by the USGS) – to traditional laboratory-based approaches used to describe the phytoplankton community during high-speed mapping surveys in the Sacramento - San Joaquin River Delta (California, USA) in 2018. I hypothesized that the data generated by the FluoroProbe would be significantly correlated (p < 0.05) to data collected using traditional laboratory approaches, including total chlorophyll-a concentration determined by extraction and cell biovolume and density by taxonomic group determined by phytoplankton enumeration. A significant correlation would indicate that the fluoroprobe can be used to reduce the need for discrete sample collection, lab analysis, and overall sampling expense. Additionally, this study will improve our fundamental understanding and confidence in our ability to make phytoplankton measurements with the in situ FluoroProbe and across time and space.
During three multi-day Delta-wide mapping surveys in 2018, comprising a total of nine days (May 15-17, July 24-26, and October 17-19) discrete samples were collected from approximately 10 stations a day (n = 87) and were analyzed in the laboratory for total chlorophyll-a concentration and phytoplankton enumeration (cell density and biovolume by phytoplankton taxonomy). Sampling stations ranged from riverine to estuarine habitats, providing a rich data set of phytoplankton taxonomic data to compare against the FluoroProbe. A correlation analysis was conducted on bbe FluoroProbe and discrete sample data collected.
The FluoroProbe’s total chlorophyll output and the total chlorophyll-a concentration determined by traditional lab methods were significantly correlated (r = 0.81, df = 85, p < 0.05). Total phytoplankton biovolume (µm3/L) and total phytoplankton density (cells/L) were both significantly correlated to the Fluoroprobes total chlorophyll concertation (µg/L) (biovolume r = 0.363, df = 85, p < 0.05, density r = 0.420, df = 85, p < 0.05). There was generally no clear trend in the data by season, except for the July 2018 phytoplankton cell count data which showed higher values than any other months. Seasonal variability may not solely explain the variations in community composition. Environmental, biological, and hydrological fluctuations may be altering pigment content, cell counts, cell sizes, and species type. In summation, the FluoroProbe requires further testing, but it has the potential to provide continuous measurements in an aquatic habitats and the ability to monitor rapid changes as they relate to phytoplankton abundances.