Features of Lake Takkobu ecosystem assessed by natural abundance of 13C and 15N

Stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) of particulate organic carbon (POM), sediments and macrophytes, unionid mussels, crustaceans and fish were measured in Lake Takkobu, the eastern part of Kushiro moor, where eutrophication has rapidly proceeded over this last decade. Nowadays the northern part of the lake has a heavy Anabaena bloom in summer, while the vegetation of submerged and floating-leaved plants has remained in the southern part. Seasonally, POM with high δ¹³C and low δ¹⁵N has appeared in summer, whereas the opposite is the case in spring and autumn. Horizontally, the summer POM with both higher δ¹³C and lower δ¹⁵N is distributed in the north, while the opposite obtains in the south. One cause of this phenomenon is the difference in water quality, with low pH and high DIC concentrations in the south and vice versa in the north; another cause is that nitrogen-fixing Anabaena bloom only in the north in summer. The means of δ¹³C of Anadonta woodiana were similar to those of POM in the south as well as those in the north, when the summer POM data were excluded. Thus, it is concluded that the unionid mussels feed mostly on POM. The δ¹⁵N of the unionid mussels, however, were 6.6 - 8.2 ‰ higher than those of POM, suggesting that it is doubtful whether A. woodiana should be regarded as the primary consumer in this lake. Most fish and macroinvertebrate species that inhabit this lake did not have δ¹³C similar to A. woodiana, indicating that they did not depend on the same primary producer as the unionid mussels did. The means of δ¹³C of five respective species, i.e. A. woodiana, Pacifastacus leniusculus (exotic crayfish), Palaemon pauciden (shrimp), Pungitius pungitius and Gymnogobius laevis were significantly higher in the north than those in the south, suggesting that these five species should be considered resident. The means of δ¹⁵N of P. pungitius, G. laevi, Tribolodon ezoe and Phoxinus percnurus sachlinensis were significantly higher in the south than those in the north. However, the means of δ¹³C of the latter two species did not differ between the north and the south, and the mean values of both δ¹³C and δ¹⁵N of Hypomesus olidus also showed no significant differences between north and south, due to the migratory habits of this species.