Michio Kumagai

Fatty acid composition as an indicator of physiological condition of the cyanobacterium Microcystis aeruginosa

Abstract The present study examined the fatty acid composition of Microcystis aeruginosa grown in a batch culture and that of Microcystis-dominated plankton collected in an experimental enclosure in a shallow, eutrophic embayment of Lake Biwa (Akanoi Bay). In pure culture, we detected 16 : 0, 18 : 2ω6, 18 : 3ω3, 18 : 3ω6, and 18 : 4ω3 acids as major fatty acids of M. aeruginosa, with trace amounts of C20 polyunsaturated fatty acids. In both pure culture and the field enclosure, the ratio of total fatty acid weight to dry weight decreased with decreasing availability of dissolved inorganic nitrogen. The ω3/ω6 ratios of C18 polyunsaturated fatty acids [(18 : 3ω3 + 18 : 4ω3)/(18 : 2ω6 + 18 : 3ω6)] varied greatly (range, 2–5) in response to the changes in physical and chemical conditions for Microcystis growth. Most notably, the ω3/ω6 fatty acid ratios were significantly positively correlated with the growth rate of cells in a batch culture. We suggest that the fatty acid composition is a useful indicator of the physiological state of Microcystis in freshwater lakes.

Automated monitoring of six cyanobacterial taxa from Lake Biwa by image processing

Lake Biwa, Japan, is a significant freshwater source supplying both industry and 14 million people. Over recent years, its water quality has suffered due to an increasing occurrence of algal blooms, comprising three main cyanobacteria genera: Microcystis, Anabaena and Planktothrix. Manually identifying algal species and quantifying their numbers is both labour intensive and costly. In this paper we present an image processing system for automatically detecting and classifying six cyanobacteria taxa, and report preliminary classification results. We viewed water samples at high magnification, and transferred images of cyanobacteria to the computer for processing. After image segmentation, we extracted properties such as shape, spectral, and textual fearures. Classification was implemented by a hierarchy of 2class classifiers. Out of a total of 244 images examined, seven were misclassified, indicating an error rate of approximately 3%. Although preliminary, the results suggest that accurate classification of algal specimens can be achieved via automated image processing.

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.

Observation of dissolved oxygen depletion at the bottom of Lake Biwa using the autonomous underwater vehicle “Tantan”

Lake Biwa is the largest lake in Japan with a surfaee area of670 km, maximum depth of 104 m, anda mean depth of 41 m. lt is loeated about 34°58-35°31 N; 135°521360 17E in the Shiga Prefeeture at the eenter o f the Japanese arehipelago. Beginning in the 1960s, the lake began to show signs of eutrophieation and oxygen depletion in the hypolimnion before winter vertieal mixing (NAKA 1973). Over the past 30 years the average air temperature of Shiga Prefeeture has inereased by about 1-1.2 oe, and Lake Biwa bottom water temperature has inereased by 1.4 oe (HAYAMI & FUJIWARA 1999); mean annual preeipitation (mainly winter snow) over the same area has deereased sinee 1980 (KUMAGAI et al. 2006). These climatie changes have likely eontributed to the weakening of winter vertieal mixing and dissolved oxygen (DO) depletion at the bottom o f the lake. Lake Biwa is the souree of drinking water for 14 million people in the Kansai area; therefore, preserving the quality of the lake water quality and its ecosystem is a serious eoncern. With the help of a highly specialized research vessel and underwater robots as well as the implementation of a routine monitoring program, an advanced monitoring system of the lake has been developed (KUMAGAI et al. 2002). Because monitoring ehanges in the deeper (80-90 m depth) areas of the lake is diffieult, an underwater robot has recently been used because it can maintain a constant distance from the lake bottom (URA 2002, FoNG & STACEY 2003). In this study, high-resolution digital images and environmental sensors of the underwater robot were used to evaluate DO depletion at the bottom of the lake and its environmental effects.

Abstracts from the Japanese Journal of Limnology

The Japanese Journal of Limnology is another official publication of the Japanese Society of Limnology. The original papers in the journal were peer-reviewed by a few authorized referees, and appeared in Japanese with English abstracts.