Ryohei Tsuda

Spatial changes of phytoplanktonic size spectra in Lake Biwa

The degree of eutrophication in Lake Biwa was examined from the characteristics of spatial distribution in size spectra computed by a size spectra analyser with an optical fiber sensor. Two types of size spectra could be recognised: (1) size distribution only presenting a peak in a size less than about 20 μm in equivalent diameter and (2) size distribution with two peaks in size spectra.The second pattern with two peaks was observed in the size spectra from the south basin, the sites near the central part and the southern part of the north basin, and seemed to be associated with advanced eutrophic conditions. In addition, the degree of eutrophication along the east side of the south basin was examined from changes in size spectra and from hydrological conditions. We inferred that the eutrophic condition around the central sites of the north basin is influenced by the water current from the gyre.

In-situ spectral absorption profiler using optical fibers

Phytoplankton spectral absorption of light energy is one of the important factors to estimate primary production of the ocean. We have developed a system which measures in situ spectral absorption of suspended matter, mainly phytoplankton, by a modified opal glass technique. The system was designed to provide sub meter-scale profile of phytoplankton distribution without water sampling. The system consists of a halogen lamp as a white light source, a multi-channel spectrometer and submergible optical sensor unit. The collimating lens and diffuser constitute this unit. The optical fiber cables are employed to transmit the light between the sensor unit and the equipment on shipboard. The received light was measured by spectrometer which ranges from 400 nm to 800 nm with 1024-channel temperature stabilized linear photodiode array. Our preliminary experiment with the cultured phytoplankton (diatoms, flagellate and green algae) has shown that minimum detectable absorption coefficient is 0.01 m-1.