Response of coastal Antarctic phytoplankton to solar radiation and ammonium manipulation: An in situ mesocosm experiment

We tested the role of solar irradiance and ammonium inputs on phytoplankton bloom formation in Antarctic coastal waters (62°39.576′ S; 60°22.408′ W, Livingston Island, South Sethlands) through the combination of a large‐scale, in situ mesocosm experiment and a small‐scale experiment. Phytoplankton growth, nutrient use, and biomass development remained low at ambient irradiances and increased greatly (greater than thirtyfold) to yield large (up to 93 μ g chlorophyll a l −1 ) phytoplankton blooms in response to moderate shading. The phytoplankton communities tested were light limited when irradiance was reduced below 30% of the incident irradiance and stressed by high irradiance at the full ambient irradiance. Ammonium additions greatly stimulated phytoplankton growth, biomass, and stimulated the use of the large nitrate pool present in the Antarctic waters and lead to a decline in the specific UV absorption by mycosporine‐like amminoacids. The small‐scale experiment confirmed the role of UV irradiance in inhibiting phytoplankton growth and the capacity of ammonium inputs to overcome this inhibition. The alleviation of the high‐irradiance stress by ammonium additions provided evidence of a key role of ammonium inputs in allowing phytoplankton to resume growth and nutrient use. The results demonstrate that there is a narrow window of irradiance where phytoplankton growth is adequate and that coastal Antarctic phytoplankton communities, examined here, are either light limited or stressed by high irradiance at irradiances outside this range. More research is needed to analyze the interplay between light climate, ammonium, and bloom initiation in Antarctic coastal waters to test the generality of the results obtained.