Analysis of long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa
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ASIA/OCEANIA REPORT
Analysis of long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa Naoyuki Kishimoto • Satoshi Ichise Kei Suzuki • Chihiro Yamamoto
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Received: 31 January 2012 / Accepted: 10 September 2012 / Published online: 30 September 2012 Ó The Japanese Society of Limnology 2012
Abstract The long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa was analyzed using periodic phytoplankton census data from January 1979 to December 2009. Population densities obtained from census data were transformed into biovolumes, and phytoplankton species were categorized into three size fractions: net phytoplankton (C4,000 lm3 cell-1, Cca. 20 lm in diameter), large nanophytoplankton (100–4,000 lm3 cell-1, ca. 6–20 lm in diameter), and small nanophytoplankton (\100 lm3 cell-1, \ca. 6 lm in diameter). Although the annual total biovolume gradually decreased over time, the total biovolumes in winter and spring were found to increase. Furthermore, a decrease in the biovolume of net phytoplankton and an increase in that of small nanophytoplankton were observed. Because of succession in the phytoplankton community, the average cell volume of the phytoplankton community decreased from 269 lm3 cell-1 in the 1980s to 56 lm3 cell-1 in the 2000s. Lake warming accompanied with the intensification of thermal stratification and the Handling Editor: Maiko Kagami.
Electronic supplementary material The online version of this article (doi:10.1007/s10201-012-0390-8) contains supplementary material, which is available to authorized users. N. Kishimoto (&) Faculty of Science and Technology, Ryukoku University, Otsu 520-2194, Japan e-mail: [email protected] S. Ichise Lake Biwa Environmental Research Institute, Otsu, Shiga 520-0022, Japan K. Suzuki C. Yamamoto Graduate School of Science and Technology, Ryukoku University, Otsu 520-2194, Japan
augmentation of wind speed were observed at Lake Biwa over the study period. Serial analysis correcting for autocorrelation revealed that oligotrophication in the epilimnion, induced by lake warming and limitation of light available for phytoplankton growth by wind-induced water mixing, was a potential factor in the succession of the phytoplankton community. Keywords Biovolume Cell size Lake Biwa Microalgae Phytoplankton Succession
Introduction Phytoplankton are the main producers in lake ecosystems. The succession of a lake phytoplankton community is accompanied by variations in internal production; the mechanism of this succession should be determined in order to effectively manage organic pollution in the lake. The succession is often induced by environmental changes, including anthropogenic environmental changes (Bradbury et al. 2004; Matzinger et al. 2006; Søndergaard and Jeppesen 2007) and climatic variations (Blenckner 2005; Matzinger et al. 2006). Eutrophication is an example of anthropogenic influences on lake ecosystems, and it alters lake phytoplankton communities dramatically (Schindler 1978, 2006; Ko¨hler and Hoeg 2
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