Monthly field surveys conducted from April to November 2009 in a tidal front in the Seto Inland Sea, Japan provided a spatial and temporal dataset for investigating seasonal variations in nutrient supply and the formation of a chlorophyll a (Chl-a) maximum. The upward diffusive flux of nutrients is estimated from observational data but it accounted for less than 5% of the nutrients needed to support the primary production of phytoplankton in the front area of the stratification region when the density difference between the surface layer and bottom layer is greater than 0.5 kg m(-3). Instead of vertical diffusion, the lateral intrusion of water with high nutrient concentration from the mixed area represented the major nutrient supply in the front area. The depth of the lateral intrusion changed with the month: the surface layer in July became the middle layer in August. According to the calculation of numerical model, an anticlockwise circulation is intensified by removing river runoff (i.e., low precipitation) in this study area, and the change of lateral intrusion is likely caused by the change of anti-clockwise circulation along with the temporal variation in river runoff. Consequently, the Chl-a peak appeared in the vicinity of the surface front (up to 3 mu g L-1) in July, but was in the subsurface (up to 9 mu g L-1) in August. Diatom species were a relatively minor taxa of the phytoplankton community up to July, although a relatively high Si(OH)(4)-Si concentration (up to 20 mu mol L-1) was confirmed. In contrast, the subsurface Chl-a maximum (SCM) in August was mainly comprised of diatoms as evidenced by the reduction of both Si(OH)(4)-Si and SUN from the surface to subsurface layer (0-20 m depth). Therefore, the supply of both nutrients and the seed population necessary for the formation of the SCM results from the tidal frontal system and phytoplankton assemblages within the tidal front system should be varied on a monthly basis. (C) 2015 Elsevier Ltd. All rights reserved.