Different vertical distribution of diatoms in sea ice at the river mouth and off the eastern coast of Saroma-ko Lagoon, Hokkaido, Japan

Abstract

The composition and cell abundance of diatoms in sea ice near the eastern coast of Saroma-ko Lagoon, Hokkaido, Japan, and near the mouth of the Saromabetsu River were reported in late February and early March 2023. Off the lagoon coast, a diatom bloom (13.6 × 10⁶ cells l⁻¹) dominated by Detonula confervacea was found at the bottom 5 cm of sea ice. At the river mouth, however, the diatom cell abundance was highest in the top 5–10 cm layer (9.9 × 10⁶ cells l⁻¹) and decreased with the depth of the sea ice. The diatom community here was overwhelmingly dominated by arborescent colonies of marine diatom Nitzschia frigida throughout the ice (4.4 × 10⁵–9.9 × 10⁶ cells l⁻¹), accounting for 97–100% of the total cells, which were thought to be adapted to the bottom ice environment. The high cell abundance at the river mouth was associated with lower nutrient concentrations implying that N. frigida had consumed nutrients of river water trapped in sea ice. These results suggest that the composition and vertical distribution of diatom species in sea ice in the lagoon can differ at a small spatial scale.