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Remote Sensing

Varied wavelengths enhance monitoring of phytoplankton by laser-induced fluorescence

Varied wavelengths enhance monitoring of phytoplankton by laser-induced fluorescenceAll dissolved organic matter fluoresces under UV excitation, but only fresh DOM fluoresces under green light, providing a way to track degradation.
5 March 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0605

To investigate how climatic changes impact phytoplankton communities, it is necessary to develop fast new methods to study such communities over large distances. The most important parameters to monitor are the chlorophyll-a concentration and the amount of dissolved organic matter (DOM) produced by phytoplankton cells. It is important to detect not only total DOM, but also ‘new’ or ‘fresh’ DOM, which is produced by living phytoplankton cells and in the initial stage of degradation.

Optical remote sensing methods that employ upwelling radiation spectra from the seawater subsurface layer can only be used for measuring total colored DOM (CDOM).1 Compared with such absorption spectra, DOM fluorescence spectra contain more detailed information, and results2,3 suggest that they may be used for DOM composition and source investigations. In the referenced works, radiation in the UV-to-blue wavelength range induced DOM fluorescence, and the researchers did not differentiate between old and new DOM. In our work we investigate the difference between fluorescence of the same DOM induced by radiation at green and UV wavelengths.4

We assumed that almost all DOM components, both old and fresh, fluoresce under UV or blue excitation. As such, this technique cannot be used to separate fresh DOM from old. On the other hand, green light has insufficient energy to induce the fluorescence of the simpler molecules of old DOM, but the energy is still enough to cause the long-chain molecules of fresh DOM to fluoresce. Thus, we expected that green excitation would allow us to detect fresh DOM fluorescence on the background of total DOM fluorescence.

Figure 1. (a, b) Laser induced fluorescence spectra of seawater samples before boiling. (c, d) Laser induced fluorescence spectra immediately after boiling. (e, f) Temporal dependence of dissolved organic matter (DOM) and chlorophyll-a fluorescence during the DOM degradation process. The figures on the left (a, c, e) show the 355nm (UV) excitation; the 532nm (green) excitation is on the right (b, d, f).

To examine our assumption, we simulated phytoplankton cell death and resulting degradation processes in the laboratory by boiling seawater samples collected during an algal bloom. In the two weeks after boiling, we repeatedly measured Ichl355, the maximum of the chlorophyll-a fluorescence induced by laser radiation at 355nm; Ichl532, the same, but induced at 532nm; Q355, the integral parameter of DOM fluorescence at 355nm and the shaded area in Figure 1(a,c); and Q532, the same, but at 532nm and the shaded area in Figure 1(b,d).

Figure 1 presents the results of the experiment. The spectral intensity values are normalized by the Raman scattering intensity of water. The laser-induced fluorescence (LIF) spectra of the seawater samples before boiling are shown in Figure 1(a, b) for the UV and green excitations. In both cases, the fluorescence of chlorophyll-a is detected. Figure 1(c, d) presents the UV and green LIF spectra directly after boiling. The chlorophyll-a fluorescence is absent, and in both cases, the integral parameters of the DOM fluorescence have increased. The temporal dependence of the measured parameters is shown in Figure 1(e, f). The parameter values are plotted as percentages of their own maximum along the y axis. The first point in Figure 1(e, f) corresponds to the preboiling, initial seawater sample, which was measured 2h after it was collected. This corresponds to the LIF spectra in Figure 1(a, b). The following points correspond to measurements taken after boiling the seawater samples. The second point agrees with the LIF spectra in Figure 1(c, d). The water temperature was about 23° C.

After boiling, the chlorophyll-a fluorescence intensity at both wavelengths fell to zero due to phytoplankton cell death. At the same time, both Q355 and Q532 approximately doubled because phytoplankton cell decay products generated new DOM. Subsequently, Q532 decreased smoothly with time, while Q355 remained constant within experimental error. Such behavior may be explained by the fact that the samples included a lot of fresh DOM directly after boiling, and the DOM molecular structure simplified as it degraded. This supports our hypothesis that UV and blue radiation induce total DOM fluorescence (old and fresh) nonselectively, but it is possible to detect fresh DOM fluorescence by 532nm laser excitation.

The DOM production and degradation processes that take place in nature are very different from those in our laboratory. These experiments should thus be repeated under natural conditions, although it can be very difficult to select for a single process in natural waters. Nevertheless, with the results obtained we can determine the main features of the DOM fluorescence dynamic under various excitation wavelengths. The ability to detect fresh DOM detection will allow us to use LIF spectroscopy to investigate DOM production processes in phytoplankton communities.

Pavel Salyuk
Laser Optics and Spectroscopy, V. I. Ilichev Pacific Oceanological Institute,
Maritime Institute of Physics and Technology, Maritime State University
Vladivostok, Russia
Oleg Bukin, Sergey Golik, Irina Lastovskaya
Laser Optics and Spectroscopy,
V. I. Ilichev Pacific Oceanological Institute
Vladivostok, Russia 
Andrey Pavlov
Institute for Automatic and Control Processes
Vladivostok, Russia 
Elena Baulo
Far Eastern State Technical Fish University
Vladivostok, Russia