Benthic foraminifera have significantly changed their main constituents in the last three decades, in association with brackish environmental changes in two Lakes Shinjiko and Nakaumi. The Ammonia event, indicating a replacement of previously developed Haplophragmoides canariensis with recently developed Ammonia beccarii, was discovered at 3.0-4.0 cm sub-bottom depths of sediment cores obtained at 3.0-4.5 m water depths of Lake Nakaumi. These water depths are correlated with a halocline developed in this lake and are important to reconstruct the recent history of the relationships between the shallower and deeper parts of Lake Nakaumi. Thus, the discovery of the Ammonia event indicates that the sediment-water interface has changed not only in deeper area, but also in shallower area in recent years.
We suggest that the recent increase of Ammonia indicates a progressive loading of orgainc matter in the shallower area of Lake Nakaumi.