Cell viability, pigments and photosynthetic performance of Arctic phytoplankton in contrasting ice-covered and open-water conditions during the spring-summer transition

We examined phytoplankton biomass and community composition (mostly based on pigments) as well as cell viability with the cell digestion assay in surface waters of the Canadian Beaufort Sea during the spring-summer transition. Our aim was to understand phytoplankton responses to the large environmental changes (irradiance, temperature and nutrients) occurring during this period. Two categories of stations were visited in May and June 2008: ice-covered (IC), exposed to low irradiances, and open-water (OW), exposed to higher irradiances. We observed a large variation in the percentage of living cells (% LC) relative to the total community. No relationship was found between %LC and nitrate concentration (the nutrient potentially limiting in this environment). The in situ irradiance influenced the status of the cells at OW stations. Mean surface mixed layer irradiances >600 mu mol photons m(-2) s(-1) were associated with low cell viability and a decline in photosynthetic performance (Fv/Fm). For IC stations, % LC declined at temperatures above 0 degrees C, whereas for OW stations, it increased, suggesting that ice melting resulted in the release into surface waters of unhealthy cells from the bottom ice in one case, and that seasonal warming favored the communities present in open waters. A chlorophyll degradation pigment tentatively identified as pyropheophorbide a-'like' showed a significant negative relationship between its concentration (relative to chlorophyll a) and the % LC and Fv/Fm. Our results suggest that the melting conditions influence the distribution of this pigment and that it may be useful as a marker for low cell viability of ice algae being released into surface waters.