Effects of environmental variables on the distribution of benthic macroinvertebrates inhabiting sediments were studied at 25 sites along the shoreline of Lake Takkobu in the Kushiro wetland of northern Japan in summer 2003. During the last decade, the lake's status has undergone a drastic shift from clear water dominated by submerged macrophytes to turbid water dominated by phytoplankton. The canonical correspondence analysis showed that four environmental variables explained the significant variation in the macroinvertebrate species composition: submerged plant biomass, bottom sediment organic matter content (OMC), distance from the mouth of the Takkobu River, and bottom-layer pH. Five species of Chironomidae [Chironomus sp. (except plumosus group), Psectrocladius sp., Corynoneura sp., Parachironomus sp. arcuatus group, and Zavreliella sp.] occurred in sites with relatively lower pH and a high submerged plant biomass, whereas three species of Tubificidae (Tubifex tubifex, Aulodrilus limnobius and Aulodrilus sp.) and two of Chironomidae (Nanocladius sp. and Monodiamesa sp.) occurred in sites with high pH and little vegetation. The three Tubificidae species also preferred organic-rich sediments. Irrespective of aquatic vegetation, Sphaerium sp. (Bivalvia) and Monodiamesa sp. (Chironomidae) occurred in low-OMC sites, whereas Tanypus sp. (Chironomidae) preferred high-OMC sites. The number of macroinvertebrate taxa showed the highest correlation with the number of submerged plants, suggesting that macroinvertebrate species richness was related mostly to submerged plant species diversity in this lake. The quantity and species richness of submerged plants and OMC are thus important determinants of the community structure of macroinvertebrates inhabiting sediments in Lake Takkobu.
The effect of salmon carcasses on the growth of a freshwater amphipod, Jesogammarus jesoensis (Schellenberg) (Gammaridea, Anisogammaridae), was examined experimentally. Growth rate was determined by rearing juvenile amphipods in four food treatments (‘leaves’, ‘salmon’, ‘leaves and salmon’ and ‘leaves with salmon leachate’) for 20 days. Mass loss and oxygen consumption of the leaves were also measured in the three treatments with leaves. The oxygen consumption rate of leaves was lower in the ‘leaves’ treatment than in either the ‘leaves and salmon’ or ‘leaves with salmon leachate’ treatments, indicating that microbial activity on leaves was enhanced by the presence of salmon carcasses. Mass loss of leaves did not differ between the three treatments with leaves. The growth rate of the amphipods did not differ between the ‘leaves’ and ‘salmon’ treatments, or between the ‘leaves and salmon’ and ‘leaves with salmon leachate’ treatments. Growth rates in the two latter treatments were higher than rates in the ‘leaves’ treatment, but not higher than the rates in the ‘salmon’ treatment. Therefore, it appears likely that consuming leaves fertilized with nutrients from salmon carcasses facilitates growth in this amphipod.
We reviewed the Japanese species of Anisocentropus McLachlan (Trichoptera, Calamoceratidae) and confirmed 3 species, Anisocentropus (Anisocentropus) kawamurai (Iwata, 1927), A. (A.) pallidus (Martynov, 1935) and A. (A.) magnificus Ulmer, 1907. The last species is recorded from Japan for the first time. For clear comparisons, adult and immature stages of the three species are described based on associations established by rearing larvae. The male, female and pupa of A. (A.) kawamurai, female and pupa of A. (A.) pallidus and female and immature stages of A. (A.) magnificus are newly described. Anisocentropus (A.) minutus (Martynov, 1930), known from southeastern Asia, is synonymized with A. (A.) kawamurai (Iwata, 1927).
Abundance, biomass and retention of salmon carcasses (Oncorhynchus keta and O. masou masou) were studied in a spring-fed tributary, the Naibetsu River, Chitose River system, Hokkaido, northern Japan, in 1998-2003, where almost all adult salmon were caught for a salmon enhancement program by a fish trap at the lower reach every fall. The abundance and biomass fluctuated seasonally, with two peaks in fall and winter, and annually. The total number of carcasses in fall was significantly correlated with the highest water level at lower reach in fall, indicating that the adult salmon swam over the trap at flood. Carcass abundance in winter may depend on the population size spawned in the wild at the upper reach, since the trap was removed in this season. Since the large wood debris and gravel were absent or scarce, many carcasses were lost from the study site within 20 days and a few carcasses remained over 31 days. Carcass removal by some terrestrial animals from the study site was detected.
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