Responses of Manila clam growth and its food sources to global warming in a subarctic lagoon in Japan

摘要

Akkeshi Lake is a subarctic shallow brackish lagoon located in Hokkaido, Japan. The Manila clam, Rudi-tapes philippinarum, is cultured in sandy sediments at the shallow, intertidal flat near the mouth of the lake. To quantitatively evaluate the effects of environmental factors such as water temperature and food availability on the growth of the Manila clam and to estimate the responses of Manila clam growth and food availability to global warming in Akkeshi Lake, we developed a numerical model by coupling a three-dimensional ecosystem model with a bioenergetics model for the growth of the Manila clam. We ran the model under two different conditions: the present condition and the global warming condition. For the global warming condition, water temperature was increased by 2 ℃ at the open boundary for the entire computational period. The growth of the Manila clam was limited by water temperature and food availability. The Manila clam grew up to 1.33 g dry weight ind.~(-1) at the lake mouth (station A) for 5 years, whereas it grew up to 1.00 g dry weight ind.~(-1) at the lake center (station B). The difference in the biomass of the Manila clam between two stations was due to the difference in food availability. Under the global warming condition, the water temperature limitation for the Manila clam was relaxed with a water temperature increase. The Manila clam grew up to 1.55 g dry weight ind.~(-1) at station A and 1.10 g dry weight ind.~(-1) at station B. While the growth of the Manila clam was improved in the lake under the global warming condition, its food sources, especially phytoplankton, decreased because of ingestion increases of grazers.