Abstract
Bottom environmental changes during the past 100 years in Kitanada Bay, Southwest Japan were reconstructed, based on the data on grain size, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) concentrations, ratio of total organic carbon to total nitrogen (C/N) and to total sulfur (C/S) in surface sediments and sediments cores. The grain size distribution pattern in surface sediment at the present study site showed predominance of coarser sediments around the bay mouth and more finer towards the bay interior, suggesting that tidal current is more swift around the bay mouth and its velocity decreases toward bay interior. The present results indicate a stagnant hydrodynamic condition in inner Kitanada Bay. High levels of TOC, TN and TS concentrations around fish farming cages in the north suggest the use of large quantities of organic substances for fish farming. Benthic environment has become highly reductive due to the supply of organic substances in combination with stagnant hydrodynamic environment. TOC and TN concentrations in sediment cores (33∼79cm) showed an increase from 1970’s when fish farming started. This result suggests that Kitanada Bay has become eutrophic due to fish farming. At the same time, grain size profiles show a decrease in grain size toward the top, while it is reverse in the southern and bay mouth area. This result suggests a transformation in the hydrodynamic environment by the resistance perpetrated on tidal current by fish cages. In the northern area of Kitanada Bay, hydrodynamic environment has become stagnant. In the southern and bay mouth area where no fish cages are observed, tidal current has become more agile. As the result, sea water exchange has become more active, and bottom environment has become less reductive.
