Light and Nutrient Limitation of Sea‐ice Microalgae (Hudson Bay, Canadian Arctic)1

ABSTRACT Changes in the biochemical composition of sea‐ice microalgae (southeastern Hudson Bay, Canadian Arctic) were used to assess the light and nutrient status of cells growing at the ire‐water interface. These changes allowed us to test the hypothesis that ire algae are limited by light at the beginning of their growth season and become periodically limited by nutrients as the season progresses. During the vernal growth season, three patterns of variation in cellular components were found in response to changes in environmental conditions. 1) Chlorophylls a and c, ATP, carbohydrate, and carbon followed the seasonal increase in under‐ice irradiance, which was mainly mused by melting of the snow cover. 2) Dissolved and biogenic silicon underwent periodic variations, which were coupled to the fortnightly neap‐spring cycle of tidal mixing. 3) Cellular contents of free amino acids, protein, and total nitrogen remained relatively constant during the season. An early decrease in intracellular chlorophylls a and c suggests that ire algae did respond to small changes in solar irradiance by changing the pigment composition of their photosynthetic units. Seasonal increases in ATP, carbohydrate, and total carbon indicate light limitation in April, followed in May by a period of excess irradiance and/or nutrients in short supply. The seasonal increase in ATP and the high values of the ratio free amino acids: protein show that neither phosphorus nor nitrogen limited algal growth at the ire‐water interface. In May, higher values of carbohydrate: protein, carbon: nitrogen, carbon: chlorophyll a, and also carbon: silicon and ATP: silicon indicate that the ice algae became silicon‐deficient in their natural environment. Following a period of light limitation, at the beginning of the season, ice‐algal growth became silicon‐limited, when in situ irradiance and the accumulated algal biomass were high and the tidally‐driven nutrient supply was not strong enough to satisfy algal nutrient requirements.