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The response of Antarctic picophytoplankton to experimental light and nutrient manipulation was tested in a large-scale mesocosm experiment in an iron-rich coastal location in the Bransfield Strait (Johnson Cove) during the austral summer of 2000. The experiment consisted of 8 mesocosm units (25 m 3 ), shaded with screens to provide a light gradient (target irradiance 100, 50, 25 and 10% of the ambient light field). A set of 4 mesocosms encompassing the different light treatments was initially run with the ambient nutrient concentrations and in the remaining 4 mesocosms, ammo- nium (NH4Cl) was added together with phosphate (KH2PO4) and silicate (Na2SiF6) for the first 11 d of the experiment (Phase I). A second manipulation of light and nutrient (Phase II) was done to confirm the results obtained on Day 16 of the experiment, when the mesocosms shaded to 25% of the ambient light were exposed to full ambient light, and ammonium, was added to both of the mesocosms already receiving 100% ambient light. The importance of light availability was evident in the increased abundance of picophytoplankton with increased irradiance, the response being non linear, with the abundance at full ambient irradiance being comparable, or lower, than that at 50% of the ambient irradiance. There was, in addition, an interaction between light availability and nutrient supply as evidenced by the increased picophytoplankton abundance during the early part of the experiment, with increased nutrient availability in mesocosms exposed to 50 and 100% ambient light, and not in those shaded to 10 and 25% ambient light. The response of photosynthesis to irradi- ance (P-I curves) showed a strong response to nutrient additions, with extremely high specific photo- synthesis rates in the mesocosm with increased nutrient availability and exposed to full ambient light. The response was close to the theoretical maximum possible and provides ample evidence that nutrient additions, particularly ammonium, contribute to the optimum photosynthesis by picophyto- plankton in the iron-rich Antarctic coastal waters studied here.

Abundance of Antarctic picophytoplankton and their response to light and nutrient manipulation