Dense sub-ice bloom of dinoflagellates in the Baltic Sea, potentially limited by high pH

The phytoplankton community, carbon assimilation, chlorophylla (Chl a), pH, light and attenuation and inorganic nutrientswere monitored under the ice in the coastal Gulf of Finland,Baltic Sea. Maximum ice and snow thickness was 40 and 15 cm,respectively. Freshwater influence had created a halocline 1–2m below the ice–water interface, and above this halocline,a dense bloom of dinoflagellates developed (max: >300 µgChl a L^–1). The photosynthetic uptake of carbon dioxideby this "red tide" increased the pH to a maximum of 9.0. Thesub-ice phytoplankton community was dominated by the dinoflagellateWoloszynskia halophila (max: 3.6 x 10⁷ cells L^–1). ThepH tolerance of this species was studied in a monoculture andthe results indicate that pH >8.5 limits growth of this speciesat ambient irradiance. This study shows that primary productivitymay raise the pH to growth limiting levels, even in marine,low-light environments where pH normally is not considered important.