Distribution and development of under-ice phytoplankton in 90-m deep water column of Lake Paijanne (Finland) during spring convection

Distribution and development of phytoplankton were studied in the deep and large Lake Paijanne from mid-winter until the disappearance of ice. Diatoms were an important part of the phytoplankton assemblage and, with cryptophytes and chrysophytes, made up 50-80% of the phytoplankton biomass. In mid-winter, chlorophyll a and phytoplankton biomass were uniformly distributed over the whole water column down to a depth of 90 m. Thus, most of the phytoplankton was in virtual darkness and there was negligible growth. Only motile cryptophytes were concentrated in the layers below the ice and were rare in deep water. After the disappearance of snow, convection developed, but at first cryptophytes were able to resist mixing. When convection turned from penetrative to predominantly horizontal, all phytoplankton were generally uniformly distributed in the water column. In spite of the full under-ice overturn with low average availability of light, the phytoplankton biomass doubled in April. The growth of cryptophytes was higher than that of diatoms, suggesting that motile species gained an advantage by being able to maintain themselves in the upper, illuminated layers. The results show that knowledge of the basic physical framework is essential for interpretation of under-ice phytoplankton results.