Journal of Oceanography, Vol. 64 (No. 6), pp. 815-830, 2008
Yukiyo Saitoh1, Kenshi Kuma1,2*, Yutaka Isoda1, Hiroshi Kuroda1, Hiromi Matsuura1, Taku Wagawa1, Hyoe Takata1, Naoto Kobayashi1, Seiya Nagao2,3 and Takeshi Nakatsuka2,4
1Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
2Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
3Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
4Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan
(Received 27 November 2007; in revised form 17 April 2008; accepted 24 April 2008)
Abstract: We report measurements of iron, nutrients, dissolved oxygen, humic-type fluorescence intensity and chlorophyll a concentrations in the coastal waters at the inflow (western) and outflow (eastern) ends of Tsugaru Strait (Japan) in June 2003 and 2004. Two different water masses (intensive eastward flow "subtropical Tsugaru Warm Current Water (TWCw)" and weak westward flow "subarctic Oyashio Water (OW)") were observed at the eastern end of the strait. TWCw at the southern part of the eastern strait was vertically homogeneous with a uniform concentrations of iron (0.7-1.1 nM for labile dissolved Fe and 14-20 nM for total dissolvable Fe in 2003) as well as other chemical, biological and physical components throughout the water column of 200 m due to strong vertical mixing in the strait. The degree of mixing in the Tsugaru Warm Current (TWC) is predominantly affected by diurnal tidal current, which is strong during the period of tropical tides and weak during the period of equinoctial ones. The especially strong vertical water mixing in 2003 is caused by large dissipation energy input due to the bottom friction of passage-flow through the strait and tidal current. At the northern part of the eastern strait, the fresh surface layer overlying the OW and the deep-bottom waters in 2003 contained large concentrations of dissolved iron, resulting from iron supplied from river runoff and shelf sediments, respectively. These results suggest that the most important mechanism for transporting iron in the strait is the strong vertical water mixing due to the tidal current, and that the iron sources in the coastal waters are the organic-associated, iron-rich freshwater input into the surface water.