Seasonal changes in abundance of Microcystis-grazing protists and their feeding properties in a hypereutrophic pond. Yuichiro Nishibe\ Shin-ichi Nakano\ Path ...
1 1-15 I Seasonal changes in abundance of Microcystis -grazing protists and their feeding properties in a hypereutrophic pond Yuichiro Nishibe\ Shin-ichi Nakano\ Path mala IM. Manage' and Zen ichiro Kawabata" 1CenterforMarineEnviromnental Studies. Ehime University. Matsuyama. 790-8577Japan "CenterforEcologicalResearch.KyotoUniversity.Otsuka509-3.Kami tanakamiHirano-choAza.Otsu520-2113, Shiga Japan
A bstract
We identified Penardochlcimys sp. (Testate amoebae) and Polytomella sp (Phytoflagellata) as grazers of the notorious bloom-forming cyanobacterium Microcystis and examined their seasonal abundance and feeding from April 1999 to March 2000 in a hypereutrophic pond. Abundance of two protists remained high from early summer to fall during Microcystis blooms and became undetectable during winter. Judging from the food vacuole contents of Penardochlcimys sp. and Polytomella sp., the former showed selective grazing on Microcystis, and the latter consumed not only Microcystis but also other phytoplankters. We suggest that grazing by Penardochlcimys sp. and Polytomella sp. is important factors for the decline of Microcystis biomass.
I n t r o d u c t io n
Cyanobacterial blooms have been reported in many lakes and reservoirs as a common symptom of eutrophication, and Microcystis often dominates in the blooms. In temperate region. Microcystis blooms occur during summer and fall, and the biomass is considerably high during the periods. However, the loss process of Microcystis biomass has not yet been fully understood. Despite of large biomass. Microcystis is inedible prey for many metazooplankters because of its large colon} size (Fulton and Pearl 1987a, b), toxic effects (Fulton and Pearl 1987b) and low nutritional value (Hanazato and Yasuno 1987). Hence, it has been considered that grazing impact on Microcystis by metazooplankton is small (Hanazato 1989). Previous studies have reported that the some protists including flagellates, ciliates and sarcodine can graze on Microcystis (Cole and Wynne 1974. Reynolds 1981, Yamamoto 198 L Takamura and Yasuno 1983, Yamamoto and Suzuki 1984, Sugiura et. al. 1992, Zhang and Watanabe 1996). Thus, these protists may play important roles for the decline of Microcystis biomass. Unfortunately, those studies were conducted in laboratory experiments using isolated strains or in natural environments within limited seasons. Thus, the seasonal abundance of the protists and their grazing impact on Microcystis in natural environment has not yet been fully investigated. We newly found that Penardochlcimys sp. (Testate amoebae) and Polytomella sp. (Phytoflagellata) as grazers of Microcystis by microscopic observation of food vacuole contents in a hypereutrophic pond in fall 1998. The final objective of our research is to estimate the grazing impact on Microcystis by Penardochlcimys sp. and Polytomella sp. In this study, we investigated seasonal changes in abundance of Penardochlcimys sp. and Polytomella sp,. and their feeding properties in a hypereutrophic pond.
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Study
s it e
Fuaiike pond is an impoundment located in Sancho, Matsuyama City, Ehime prefecture, Japan, It has a surface area of ca. 7400 m2, and its maximum depth was about 1.5 m. The pond is hypereutrophic due to anthropogenic loading from the watershed and the cyanobacterium Microcystis spp. formed heavy blooms from early summer to fall every year. M a t e r ia l s
and m ethods
Water samples were collected from the surface with a 15-liter bucket, at the near-shore station, once a week from April to September 1999, January to March 2000, and twice a week from October to November 1999. All samples were taken at around the same time of the day (09:30 to 10:30 h). For counting the cell density of phytoplankton and protists, two 100 ml water samples were fixed with acidified Lugol’s solution to a final concentration of 1 %. Each fixed sample was concentrated by natural sedimentation, and phytoplankton and protists cells were enumerated for each species in haematocytometer under a microscope. After phytoplankton enumeration, concentrated samples were slightly sonicated to disperse Microcystis colonies, and cells of the cyanobacterium were counted as descried above. For analysis of food vacuole contents of Penardochlctmys sp., 100 ml water sample were fixed with glutaraldehvde to a final concentration of 1 %. Fixed samples were concentrated by reverse filtration, and about 0.05 ml of the concentrated sample containing Penardochlamys sp. was mounted on a glass slide. The food vacuole contents of Penardochlamys sp. were observed under a microscope at x 400 magnification, and all phytoplankton in food vacuoles were identified to species and counted. We tried to observe the food vacuole contents of Polytomella sp. using same sample, but the cell of the flagellate was broken by the fixatives. Hence, we used unfixed sample to observe the food vacuole contents of Polytomella sp. About 0.05 ml of the unfixed sample containing Polytomella sp. was mounted on a glass slide and mixed with 10 % methyicellulose solution to slow down swimming flagellates. The food vacuole contents of Polytomella sp. were observed under a microscope at x 400 magnification within one to four hours after sampling, and all phytoplankton in the food vacuoles were identified to species and enumerated. R esults
a n d d is c u s s io n
Microcystis spp. predominated in phytoplankton from May to November, and their heavy blooms were observed during the periods. There were several sharp peaks of cell densities of Microcystis spp. from May to October. Penardochlamys sp. was detected from 26 May to 30 November when Microcystis spp. were abundant. The cell densities of Penardochlamys sp. largely fluctuated between 1.4 and 347.5 cells ml'1 with several sharp peaks (26 May, 14 July, 1 September, 16 October and 26 October). Although some peaks of the cell densities of Penardochlamys sp. coincided with those of Microcystis spp., we did not have a significant correlation (p>0.05, n=36) between the abundance of Penardochlamys sp. and that of Microcystis spp. We examined 1397 cells of Penardochlamys sp. for food vacuole contents, and their 96.3 % contained phytoplankton cells in their food vacuoles. In all cases, phytoplankters in the food vacuole of Penardochlamys sp. consisted of only Microcystis spp,, and other phytoplankters were not detected. These results suggest that Penardochlamys sp. had selective grazing on Microcystis. The average numbers of Microcystis cells contained in food vacuoles o f Penardochlamys sp. were 6.00 ± 3.26 cells. Cell densities of Polytomella sp. was abundant from 2 June to 8 December. The
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Nishibe et al., Seasonal changes and feeding properties of Microcystis-grazing protists
changing pattern of cell densities of Polytomellci sp. was similar to that o f Microcystis spp., and we found a significant positive correlation (p
