Experimental induction of a large phytoplankton bloom in Antarctic coastalwaters

The experimental enclosure of an Antarctic planktonic community in a large (35 m(3)) mesocosm moored in Johnson's Dock (62 degrees 39.576' S, 60 degre es 22.408' W, Livingston Island, Bransfield Sector, Antarctica) was followe d by a large phytoplankton bloom. This bloom, dominated by the large diatom Thalassiosira antarctica, reached a biomass 1000-fold greater than in the ambient waters. The net growth rate of T. antarctica averaged 0.53 +/- 0.17 d(-1), with maximum net growth rates close to 1.0 d(-1), exceeding the pre dicted maximal population growth rates by 60 to 200 %. The gross primary pr oduction in the mesocosm (49 mmol C m(-3) d(-1)) was about 30 times greater than the concurrent gross production in the ambient waters, while sediment ation losses removed only between 2.1 to 13 % of the biomass d(-1) and cell mortality was negligible. The bloom development led to a decline of dissol ved inorganic nutrient concentrations to values several times lower than th ose in the ambient waters, indicating that the ambient nutrients were both available and sufficient to allow the development of the massive algal bloo m observed. Light-limitation of the phytoplankton community was likely the factor responsible for the low biomass and production in ambient waters rel ative to the mesocosm, as indicated by: (1) limited water transparency of a bout 1 m, which increased up to 6 m as a result of the sedimentation of the glacial flour in the mesocosm; (2) dense pigment packaging (14.7 +/- 3 pg chi a mum(-3)) inside the phytoplankton cells of the community in the ambie nt waters compared to much lower values (4.2 +/- 0.8 pg chi a mum(-3)) in t he mesocosm community; (3) a specific Light absorption of the phytoplankton community in the ambient waters 10 times higher (average specific PAR abso rption +/- SE = 0.018 +/- 0.0038 m(-1) mg chi a(-1)) than in the mesocosm c ommunity (0.0087 +/- 0.002 m(-2) mg(-1) chi a); and (4) a high apparent qua ntum yield of the phytoplankton in the ambient waters (0.091 mol O-2 [mol p hoton absorbed](-1)), 4-fold higher than that for the community developed i n the mesocosm.