Phytoplankton Pigments Absorption
Chlorophyll a is the primary molecule responsible for photosynthesis. That means that chlorophyll a is found in every single photosynthesizing organism (Wetzel, 2001). In the visible region of the spectrum of light, chlorophyll a absorbs light with wavelengths of blue (475 nm) and red (650 nm) (Speer, 1997). As all phytoplankton have chlorophyll a, a chlorophyll sensor can be used to detect these organisms in situ. It only provides a rough estimate of biomass (Schuz, 2007). Since, Chlorophyll sensors rely on fluorescence to estimate phytoplankton levels based on chlorophyll concentrations in a sample of water (YSI Incorporated-Tech note).
Fluorescence means that when the chlorophyll is exposed to a high-energy wavelength (approximately 470 nm), the electron goes to an excited position and when it returns to the electronic ground state from this excited state it emits a lower energy light (650-700 nm). This returned light can then be measured to determine how much chlorophyll is in the water, which in turn estimates the phytoplankton concentration (YSI Incorporated-Tech note).
Measuring phytoplankton biomass is important because they play some of the biggest roles in climate control, oxygen supply and food production. Photosynthetic production uses up carbon dioxide, which helps regulate CO2 levels in the atmosphere, and produces oxygen for other organisms to live (Schmidt, 2000). They are also called primary producers, and make up the bottom of the food web (Lindsey & Scott, 2010). Strong relationships exist between phytoplankton and zooplankton, which feed on phytoplankton (Karabin, 1985). And simple statistics revealed a positive correlation between zooplankton biomass and chlorophyll a concentration and between zooplankton abundance and phytoplankton biomass (Ryszard & Katarzyna, 2008).
Other optical parameter important is turbidity, or the presence of suspended particles in the water, which affects the amount of light that reaches into the water (EPA, 2015). The more sediment and other particles in the water, the less light will be able to penetrate. With less light available, photosynthetic production will decrease (EPA, 2015).
Theory of Operation of the equipment ECO FLNTU
The Environmental Characterization Optics (ECO) combination fluorometer and turbidity sensor allows the user to measure chlorophyll fluorescence at 470 nm and turbidity at 700 nm within the same volume (Wet Labs, 2010). The fluorometer monitors chlorophyll concentration by directly measuring the amount of chlorophyll a fluorescence emission, using blue LEDs as the excitation source. Fluoresced red light is received by a detector positioned (Wet Labs, 2010). Turbidity is measured simultaneously by detecting the scattered light from a red (700 nm) LED to the same detector used for fluorescence (Wet Labs, 2010).
Data from the ECO fluorometer and turbidity sensor, whether digital or analog, which are used to transmit information, usually through electric signals. They represents raw output from the sensor. Applying linear scaling constants, this data can be expressed in meaningful forms of chlorophyll fluorescence and NTUs (Wet Labs, 2010).
Typical technical problems that need to be considered when interpreting the data
Since, active fluorescence can thus be measured by a fluorometer, assuming that the measured yield of fluorescence is proportional to the abundance of photosynthesising phytoplankton, relative values of fluorescence can be used to quantify primary biomass. However, yield is affected by several factors, including the intensity of sunlight each cell is exposed to (Behrenfeld & Boss, 2006). The decrease of the fluorescence signal in surface waters during daylight is called fluorescence quenching (Marra, 1997). If uncorrected, quenched fluorescence yields will generate values under-representative of phytoplankton abundance at the surface (Müller et al., 2001).
Behrenfeld, M. J. & Boss, E. 2006. Beam attenuation and chlorophyll concentration as alternative optical indices of phytoplankton biomass. Journal of marine research, 64: 431-451.
EPA. 2012. Turbidity. In water: monitoring & assessment. Retrieved Feb 20, 2016, from http://water.epa.gov/type/rsl/monitoring/vms55.cfm
Karabin, A. 1985. Pelagic zooplankton (Rotatoria+Crustacea) variation in the process of lake eutrophication. II. Modifying effect of biotic agents. Polish journal of ecology, 33: 617–644.
Lindsey, R. & Scott, M. 2010. What are phytoplankton?. In NASA earth observatory. Retrieved Feb 20, 2016, from http://earthobservatory.nasa.gov/Features/Phytoplankton/
Marra, J. 1997. Analysis of diel variability in chlorophyll fluorescence. Journal of marine research, 55:767-784
Müller, P., Li, X. P. & Niyogi, K. K. 2001. Non-photochemical quenching. A response to excess light energy. Plant physiology. 125: 1558-1566.
Ryszard, G. & Katarzyna K.-M. 2008. Interactions between phytoplankton and zooplankton in the hypertrophic Swarzedzkie lake in western Poland. Journal of plankton research, 30: 33-42.
Schmidt, L. J. 2000. Polynyas, CO2, and diatoms in the southern ocean. In NASA earth observatory. Retrieved Feb 20, 2016, from http://earthobservatory.nasa.gov/Features/Polynyas/
Schulz, K. 2007. Phytoplankton measuring and culture techniques. In phytoplankton ecology (lecture). Retrieved Feb 20, 2016, from http://www.esf.edu/efb/schulz/phytotechniques.doc
Speer, B.R. 1997.Photosynthetic pigments. Retrieved Feb 20, 2016, from http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html
Wet Labs, 2010. Eco FLNTU, combination fluorometer and turbidity sensor- User’s guide. 21p.
Wetzel, R. G. 2001. Limnology: lake and river ecosystems (3rd ed.). San Diego, CA: Academic Press.
YSI Incorporated-Tech note. The basics of chlorophyll measurement. In YSI Environmental Tech Note. Retrieved Feb 20, 2016, from http://www.ysi.com/media/pdfs/T606-The-Basics-of-Chlorophyll-Measurement.pdf