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The dynamics of microbial and herbivorous food webs in a lake with special reference to intermittent nutrient supply from bottom intrusion
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Shin-ichi Nakano1, 2*, Yuji Tomaru3, 4, Toshiya Katano1
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1 Center for Marine Environmental Studies, EhimeUniversity, Bunkyo-cho 3, Matsuyama 790-8577, Ehime, Japan
2 Present address:Laboratory of Aquatic Food Web Dynamics, Faculty of Agriculture, Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Ehime, Japan
3 Faculty of Agriculture, EhimeUniversity, Tarumi 3-5-7, Matsuyama 790-8566, Ehime, Japan
4 Present address:National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Maruishi 2-17-5, Ohno-cho, Saeki-gun 739-0452, Hiroshima, Japan
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*Corresponding author, e-mail:
Key words: Autotrophic picoplankton, Bacteria, Diatoms, Flagellates, Nutrients, Physical event
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Abstract
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Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water (bottom intrusion).No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001.These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton (diatoms and dinoflagellates) roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton (APP) cell density, which reached maxima in August of every year at very similar levels (4.0-5.0 x 105 cells ml-1).
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Results
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The maximum and minimum water temperatures recorded in the present study differed each year(Fig. 2).
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Fig. 2: Changes in water temperatures at depths of 2 m and 60 m in UchiumiBay, from 1999 to 2001.
The water temperature at 2 m (surface water temperature) showed a similar seasonal pattern in each year, although there are gaps in the automatic data record due to some troubles, especially in the summer of 1999 (Fig. 2). By contrast, the seasonal pattern of water temperature at 60 m (bottom water temperature) was different from year to year (Fig. 2).