Investigating terrestrial and oceanic environmental conditions to identify possible factors influencing seaweed bed distribution in Tsushima Islands, Japan

Abstract

A decrease in seaweed beds has been a serious problem in the coastal areas of the Tsushima Islands, and other areas in Japan and worldwide. In the Tsushima Islands, certain coastal areas in the northeastern (NE) area contain seaweed beds, while their depletion is of serious concern in other areas. The influence of terrestrial and oceanic waters, and their relationship with seaweed bed distribution in the Tsushima Islands, Japan was examined to understand the marine environmental conditions. We conducted comprehensive investigations based on water sampling and their analyses in this study. Water qualities including total iron, dissolved iron, nitrate nitrogen, nitrite nitrogen and phosphate-phosphorus, and carbon stable isotope ratios of particulate organic matter (POM) in seawaters and river waters in the NE and southwestern (SW) areas of the Tsushima Islands were seasonally monitored. The seawater quality results showed no distinct differences between the NE and SW areas. This observation indicates that the materials, including iron supplied from terrestrial areas, were not the primary factors characterizing the seaweed bed distribution, despite the seaweed bed distribution in the NE area being larger than that in the SW area. Meanwhile, the seasonal variations in the δ13C values of POM in the NE area were more evident than those in the SW area. This implies the significant contribution of oceanic POM in the NE area, possibly due to the existence of island-induced current and cyclonic eddies, and its influence on the coastal water qualities. These environmental properties have been suggested to potentially influence the characteristics of seaweed bed distribution in the Tsushima Islands.