Norihiko Yoshida

Progress of eutrophication and change of chironomid fauna in Lake Yamanakako, Japan

Benthic macroinvertebrate distribution was studied on April 9, 1994, and March 2, 2003, in Lake Yamanakako. The average density of the benthic community for the entire lake was 3168 ind. m−2, comprising principally oligochaetes (41.0%) and chironomids (59.0%) in 1994. In 2003, the benthic community for the entire lake was 1847 ind. m−2, principally consisting of oligochaetes (69.9%) and chironomids (30.1%). In 1994, the larval density of Propsilocerus akamusi was 3.5 times that of Chironomus nipponensis and in 2003 the figure was 5.7 times. However, the larval biomass of P. akamusi was 2.1 times greater than that of C. nipponensis in 1994 and 2.8 times greater in 2003. The larval density of Tanypodinae decreased drastically, by about 12-fold, from 1994 to 2003. P. akamusi larvae were particularly abundant at the lake center in 1994, but they inhabited the entire lake bottom in 2003. P. akamusi density was closely related to water depth and ignition loss. C. nipponensis larvae also showed the widest distribution pattern in 2003, whereas their larvae had inhabited the northeastern parts and the lake center in 1994. Recently, the number of C. nipponensis larvae in Lake Yamanakako is tending to decrease, whereas that of P. akamusi larvae is increasing, suggesting ongoing eutrophication.

Long-term dynamics of freshwater red tide in shallow lake in central Japan

ObjectiveThe aim of this study is to clarify the long-term dynamics of the red tide occurring in Lake Kawaguchi.MethodsThe measurement of environmental factors and water sampling were carried out monthly at a fixed station in Lake Kawaguchi’s center basin from April 1993 to March 2004. On June 26, 1995, the horizontal distribution ofPeridinium bipes was investigated using a plastic pipe, obtaining 0∼1-m layers of water column samples at 68 locations across the entire lake.ResultsP. bipes showed an explosive growth and formed a freshwater red tide in the early summer of 1995, when the nutrient level was higher than those in the other years, particularly the phosphate concentration in the surface layer. The dissolved total phosphorus (DTP) concentration was sufficient forP. bipes growth in that year. In the study of its horizontal distribution,P. bipes was found at all the locations. The numbers of cells per milliliter ranged from 67 to 5360, averaging 1094±987 cells/ml, with particularly high densities along the northern shore. Since then,P. bipes has annually averaged about 25 cells/ml in Lake Kawaguchi.ConclusionWe observed that the red tide caused byP. bipes correlates with a high DTP concentration in Lake Kawaguchi.

BATHYMETRIC DISTRIBUTION OF BENTHIC MACROINVERTEBRATES IN DEEP LAKE MOTOSU, FUJI FIVE LAKES, JAPAN

The distribution of benthic macroinvertebrates was studied in deep Lake Motosu (oligotrophic lake), Japan. Bathymetrical sampling surveys were carried out using a standard Ekman grab at each of 16 stations in the lake on 8 March 2013. The average densities of the benthic community for all the stations were 2185 ind./m2, comprised principally of Tubifex tubifex (Muller) 23.4%, Asellus hilgendorfi Bovallius 4.9% and chironomids 65.8%. T. tubifex and chironomids inhabited the entire lake bottom, with higher densities in the shallower region (25 m>) than in the deeper region (ca. 120 m). On the other hand, the density of A. hilgendorfi increased with water depth and was highest in the deeper regions (>70 m). The densities of chironomid larvae were lower in the deeper region, with fewer chironomid larvae located in the center of the lake (637 ind./m2). The most abundant species, Micropsectra yunoprima Sasa, exhibited the widest distribution from 10.0 m to 121.0 m (mean density 926 ind./m2), followed by Procladius choreusi (Meigen) from 10 m to 74.1 m (mean density 365 ind./m2). The results of this study were compared to previous data by Kitagawa (1973) on chironomid larvae. The densities of these chironomid larvae increased throughout the whole lake in 2013. The water quality has not undergone a major change since the 1970’s. In this study, the number of M. yunoprima larvae has shown a tendency to increase, especially in shallower regions; where they are widely distributed, suggesting the continuation of favorable bottom condition for larvae since the 1970’s.

Long-term dynamics of benthic macroinvertebrates from a small shallow lake in central Japan

Few reports of benthic macroinvertebrates in Lake Shibire (Japan) have been documented since KURIYAGAWA & Aso {1979), but recently HIRABAYASHI & YOSHIZAWA (1993) reported the horizontal distribution of benthic macroinvertebrates and found that oligochaetes, chaoborids and chironomids accounted for 75.5%, 15.5% and 9.0% of total macroinvertebrate density, respectively. According to their report, the most abundant chironomid species were Chironomus nipponensis and Propsilocerus akamusi, both common in eutrophic and mesotrophic lakes in Japan. The present study covers seasonal trends over 11 years (1994-2004) in density ofbenthic macroinvertebrates, especially the two principal chironomid species, in the lake center o f Lake Shibire. The study also considered macroinvertebrate density in connection with their predators.

Long-term change of DOM contents of shallow lake water in central Japan

Experiments on the microbial loop have shown that dissolved organic matter (DOM) is the main component o f soluble detritus. In the past, materials such as algal exudates, decomposed microbes, and animal excreta were believed to be the main sources of DOM. Now, many studies report DOM (WETZEL 2001), but information on changes in the composition DOM in relation with the dynamics of ecosystems is stilllimited. Long-term monitoring of DOM associated with the total lake condition is likely to provide important information for this research field. Lake Shibire is a fairly shallow lake in Yamanashi Prefecture, in central Japan. In this study, we report the results o f monitoring water quality and the analysis ofDOM over a period of 10 years. During the research period a major event occurred: the immigration of a nonindigenous fish, Micropterus sa/moides (largemouth bass). The effects ofthis event on DOM contents are also discussed in this paper.

Long-term dynamics of phytoplankton from a small shallow lake in central Japan

Lake Shibire is a small shallow lake located on the top of a mountain. Although only two camping lodges stand around this lake, several tens of thousands of people visit there every year for fishing, camping and trekking. There are few limnological studies about this lake except for a series of surveys carried out by KURIYAGAWA (1981). In this paper, long-term dynamics of phytoplankton was studied in relation to the lakes environment.

Change in Fish Species and its Artificial Impact on Lake Kawaguchi.

This lake was mainly stocked with Carassius cuvieri and Hypomesus by the Kawaguchiko Fishermens Association until the 1980s. However, since Micropterus salmoides salmoides were confirmed to exist in the lake in 1973, by early 1985 Hypomesus were no longer caught. This condition continued for a short period ; then, in the 1990s, the fish species composition showed a great change in the lake. Micropterus salmoides salmoides and Salmo (Oncorhynchus) mykiss became the main catch. As a result, 370, 000 anglers visited Lake Kawaguchi in 1996, against 35, 000 in 1978. The total number of anglers who visit the lake is increasing rapidly. At present, Lake Kawaguchi has become the most famous lake for fly fishing in Japan. In order to respond to the demand of so many anglers, the Kawaguchiko Fishermens Association stocks the lake with many species of carnivorous fish. Moreover, they annually stock an abundance of small plankton-feeding fish which in turn become the food of carnivorous fish. Owing to the above circumstances, Lake Kawaguchi has become like an artificial fishing pond, with disturbing effects on the lake ecosystem. The ecological impact on other aquatic organisms has become a cause for concern.