Agusti S, Duarte CM, Llabres M, Agawin NSR, Kennedy H
mesocosm, nitrogen, irradiance, Antarctica, phytoplankton, 35 m3
We tested the role of solar irradiance and ammonium inputs on phytoplankton bloomformation in Antarctic coastal waters (62 39.5760 S; 60 22.4080 W, Livingston Island,South Sethlands) through the combination of a large-scale, in situ mesocosm experimentand a small-scale experiment. Phytoplankton growth, nutrient use, and biomassdevelopment remained low at ambient irradiances and increased greatly (greater thanthirtyfold) to yield large (up to 93 mg chlorophyll a l 1) phytoplankton blooms in responseto moderate shading. The phytoplankton communities tested were light limited whenirradiance was reduced below 30% of the incident irradiance and stressed by highirradiance at the full ambient irradiance. Ammonium additions greatly stimulatedphytoplankton growth, biomass, and stimulated the use of the large nitrate pool present inthe Antarctic waters and lead to a decline in the specific UV absorption by mycosporinelikeamminoacids. The small-scale experiment confirmed the role of UV irradiance ininhibiting phytoplankton growth and the capacity of ammonium inputs to overcome thisinhibition. The alleviation of the high-irradiance stress by ammonium additions providedevidence of a key role of ammonium inputs in allowing phytoplankton to resume growthand nutrient use. The results demonstrate that there is a narrow window of irradiancewhere phytoplankton growth is adequate and that coastal Antarctic phytoplanktoncommunities, examined here, are either light limited or stressed by high irradiance atirradiances outside this range. More research is needed to analyze the interplay betweenlight climate, ammonium, and bloom initiation in Antarctic coastal waters to test thegenerality of the results obtained.