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Volume 62, Issue 3
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Takeshi YOSHIMURA, Isao KUDO2001 Volume 62 Issue 3 Pages 205-217
Published: October 31, 2001
Released on J-STAGE: June 12, 2009
JOURNAL FREE ACCESSNutrient concentrations and alkaline phosphatase activity were measured in Lake Ohnuma from June 1995 to March 1997. Sufficient amounts of dissolved inorganic nitrogen remained throughout the year, whereas soluble reactive phosphorus concentrations were extremely low (close to the detection limit). The N:P ratio in the standing stock of nutrients ranged 74-3000 (by atoms), far richer in nitrogen than the Redfield ratio. Chlorophyll a concentrations were high (5-22μg L-1) during icefree seasons, but the elemental composition of phytoplankton exhibited a moderate phosphorus de-ficiency. A high seasonal variability in alkaline phosphatase activity suggested that not only was phytoplankton growth highly limited by phosphorus, but that the degree of this limitation varied. Alkaline phosphatase activity served as a simple and an effective indicator for judging the phosphorus status of phytoplankton.View full abstractDownload PDF (900K) -
Hiroshi UEDA, Hidetoshi KIMURA2001 Volume 62 Issue 3 Pages 219-227
Published: October 31, 2001
Released on J-STAGE: June 12, 2009
JOURNAL FREE ACCESSPlankton and limnological conditions were investigated in six spring waters in the basin of the Shigenobu River, Shikoku Island, Japan, four times from June to October 2000. Nutrient concentrations in the spring waters were as high as those seen in meso-or eutrophic lakes. In contrast, mean chlorophyll-a concentrations in four of the surveyed waters were less than lag L-1. These low values are equivalent to those present in oligotrophic lakes. When the phytoplankton density was low, phytoplankton consisted mainly of diatoms, usually Fragilaria, but when it was high, Chlamydomonadaceae and Chrysophyceae (Dinobryon) as well as diatoms were dominant. The major components of microzooplankton were usually protozoans and rotifers. Their mean abundances were 4.6-47.9 and 2.8-30.3 inds. L-1, respectively, which were less than those observed in oligotrophic lakes. Tropocyclops prasinus was the only planktonic species among copepods, and occurred only in spring waters with high phytoplankton densities; others are considered benthic or groundwater species. In one spring water, cercaria stage larvae of Trematoda was the most abundant zooplankton (maximum 299.1 inds. L-1). Microzooplankters were most abundant when the water level was lowest, and rotifers were significantly more abundant when the water level was low. Since the low water level is considered to be due to a decrease in the welling of groundwater, these results suggest that the densities of plankton in the spring waters are limited by replacement by groundwater.View full abstractDownload PDF (621K) -
Ho-Dong PARK, Atsushi YOKOYAMA, Tokio OKINO2001 Volume 62 Issue 3 Pages 229-248
Published: October 31, 2001
Released on J-STAGE: June 12, 2009
JOURNAL FREE ACCESSThe fate of microcystin in the aquatic ecosystem has been investigated by many researchers throughout the world. Possible pathways of microcystins in Lake Suwa were studied from five per-spectives: production, adsorption, physiochemical decomposition, bioaccumulation and biodegradation. Temporal variability in the concentration of microcystin was investigated during the warm seasons of eight years (1991-1998) in the lake. High concentrations of intracellular microcystin were found during the exponential growth phase of the blooms, whereas concentrations of extracellular microcystin were highest at the end of the blooms. However, concentrations of extracellular microcystin remained very low compared to the levels of intracellular microcystin. The relatively high percentages of microcystin in filtered lake water at the end of blooms suggests that the release of microcystin from cells occurs during senescence and the decomposition period of Microcystis cells.
Freshwater unionid bivalves, Anodonta woodiana, Cristaria plicata and Unio douglasiae, bioaccumulated the microcystins into mussels in the lake. Microcystins were detected in the hepatopancreas (53% of total microcystin in the mussel), gill and muscle (34%), gonad (6%) and gut (7%) of U. douglasiae. The maximum concentration of microcystin in the hepatopancreas of bivalves, was 12.6 (A. woodiana), 297 (C. plicata), and 420 (U. douglasiae) μg g-1 dry weight, respectively. A bacterium, a new species or even a new genus capable of degrading microcystins-RR, -YR and -LR, was isolated from the lake. When the bacterium was added to microcystins present in a culture medium, the microcystins were degraded thoroughly in a few days. The degradation rate was strongly dependent on temperature the maximum rate was at 30°C.
The presence of a cyanobacterial toxin can be a potential threat, and therefore water treatment re-quires more attention. A complete understanding of the mechanism of microcystin degradation in the ecosystems requires more intensive study, including a quantitative enumeration of microcystin-degrading bacteria. This should be done in conjunction with a study on the microbial ecological mechanism of the degradation of cyanobacteria itself.View full abstractDownload PDF (1529K) -
Masamichi YAMAMOTO, Tokio OKINO2001 Volume 62 Issue 3 Pages 249-259
Published: October 31, 2001
Released on J-STAGE: June 12, 2009
JOURNAL FREE ACCESSChanges in the fish community in Lake Suwa after World War II were analyzed based on fishery statistics data describing the amount of fish landed on an annual basis reported by the Lake Suwa Fishery Cooperative. The total amount of fish landed in Lake Suwa increased from 1946, reached a peak in 1970, and then decreased and recorded its lowest amount in 1999. Resource species caught in the fisheries were basically categorized into fish, shellfish and shrimp. During the period of increasing catches in Lake Suwa prior to 1970, pond smelt Hyposesus transpacificus nipponensis and crucian carp Carassius carassius were the major species, although there were some years when shellfish species accounted for a ratio of 1/2 to 1/3 of the total catch. It should be especially noted that diverse fish species were caught during the years when the fishery catch was increasing. During the period after 1970, most of the species caught were smelt fish, and the catch of shellfish decreased drastically while that of crucian carp was constantly decreasing. Fish species other than smelt, crucian carp and common carp were rarely caught in recent years, and the catch of common carp was catching up with the amount of crucian carp, which was the 2nd highest catch following smelt. The recent annual catch from Lake Suwa was about 1/4 of that in the 1970s, and the catch of pond smelt has decreased to 1/10 of that in the 1970s. One of the reasons for the decrease in the amount of fish landed since 1970 may be a decrease in the aquatic plant area along the coastline due to the reclamation and dredging of the littoral zone. The decline in fishing due to the aging of fishermen and the reduction in fish prices could be other reasons. As there are some situations that cannot be explained only by the decline in the fishing conation of fishermen, the reduction of the fish catch should be discussed also in the context of the influence of changes in the aquatic environment of Lake Suwa. Regarding changes in the composition of fish fauna, alien fish species such as largemouth bass and bluegill as well as ukigori (floating goby) are also increasing in recent years. Such changes should be carefully observed in future.View full abstractDownload PDF (807K) -
Tsuneo NAKAJIMA, Yasuhiro FUJIOKA, Masayoshi MAEHATA, Taisuke OHTSUKA, ...2001 Volume 62 Issue 3 Pages 261-270
Published: October 31, 2001
Released on J-STAGE: June 12, 2009
JOURNAL FREE ACCESSFish distributions were surveyed in the Konan area surrounding the southern part of Lake Biwa from March 1998 to November 2000. From 879 localities, fishes of 16 families, 42 genera, and 55 species or subspecies were collected. Native fishes such as Zacco platypus, Zacco sp., Zacco temmincki, Gnathopogon elongatus elongates, Pseudogobio e. esocinus, Carassisus auratus langsdorfii, Plecoglossus altiuelis altivelis, and Rhinogobius sp. were collected at many localities confirming that they were widely distributed. The introduced fishes Lepomis macrochirus and Micropterus salmoides were also found in a large number of localities. The distribution patterns of individual species are related to geomorphology. The A-type pattern is represented by Zacco platypus, which is distributed in both delta and alluvial fan areas. The B-type pattern is represented by Zacco sp. and Gnathopogon e. elongatus, which are distributed only in the alluvial fan area. The C-type pattern is represented by Zacco ternmincki, which is distributed in hilly areas. The D-type pattern is represented by Lepomis macrochirus and Micropterus salmoides, which are restricted to the delta area. The fishes that exhibit the B-type pattern were originally distributed in both the delta and alluvial fan areas, but they seem to have retreated from the delta area following the introduction of Lepomis macrochirus and Micropterus salmoides.View full abstractDownload PDF (1083K)
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