Horizontal heterogeneity of microalgal biomass in the first-year sea ice of Saroma-ko Lagoon (Hokkaido, Japan)

The horizontal distribution of sea-ice microalgae in Saroma-ko Lagoon (Sea of Okhotsk, Hokkaido, Japan) was studied in relation to snow depth, ice thickness, salinity in the ice bottom and within-ice irradiance. Three scales of variation were considered: a 4-km transect extending from the Saromabetsu River towards the eastern inlet of the lagoon and two different sampling grids (10 X 10 m and 1 X 1 m). Diatoms, Detonula confervacea and three species of Navicula, dominated the algal assemblage. Along the transect, the taxonomic composition and percent contribution of species to the total cell volume varied. Chlorophyll a (chl a) concentration (2-119 mg m(-2)) was characterized by a quadratic trend, and variability in the residual stationary series showed structures of ca. 500, 100 and 70 m. Snow depth was quite uniform and ice thickness decreased as the ice-bottom salinity increased. Within-ice irradiance was minimum at mid-transect. The chl a trend was significantly related to ice thickness, which was controlled by ice-bottom salinity. Variance in the detrended chl a series was explained by ice-bottom salinity at the 70-m scale (68%). Analyses of data from the two grids did not show evidence of variation in chl a biomass at scales < 20 m. In Saroma-ko Lagoon, as in southeastern Hudson Bay, ice-bottom salinity explained the meso- and small-scale patchiness of ice algae. Salinity of underlying waters probably controlled the spatial co-heterogeneity of salinity and microalgal biomass in the ice bottom.