AULACOSEIRA SUBARCTICA IN KURILSKOYE LAKE, KAMCHATKA: A DEEP, OLIGOTROPHIC LAKE AND IMPORTANT PACIFIC SALMON NURSERY

Abstract

Kurilskoye Lake is a deep (max. depth 316 m), oligotrophic lake that has been affected by nearby volcanic eruptions and extensive fertilisation aimed at increasing primary productivity in order to support a sockeye salmon fishery. Aulacoseira subarctica has dominated the phytoplankton community since studies began in 1980. Maximum cell concentrations were relatively low (from 137 cells ml⁻¹ in 1987 to 3,710 cells ml⁻¹ in 1990) and, in most years, occurred between June and August. More intensive seasonal studies were started in 2000 and these showed that there was little or no net diatom cell increase in some years. Even when an increase did occur, it could take up to two months for the population to double, mainly because of limited light availability resulting from deep mixing. Phosphorus was the main limiting nutrient but silica concentrations were relatively low and limited diatom growth in one out of 19 years. Further phosphate increases are likely to cause more extensive silica limitation and a probable shift in phytoplankton community composition. The lake did not freeze every year but when it was ice covered for over 100 days A. subarctica did less well. Comparison with other A. subarctica populations showed that cell dimensions were similar but that the frequency of auxosporulation was low.