MEPS 206:73-85 (2000)  -  doi:10.3354/meps206073

ABSTRACT: The experimental enclosure of an Antarctic planktonic community in a large (35 m³) mesocosm moored in Johnson¹s Dock (62°39.576¹ S, 60°22.408¹ W, Livingston Island, Bransfield Sector, Antarctica) was followed 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 predicted maximal population growth rates by 60 to 200%. The gross primary production 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 sedimentation 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 dissolved inorganic nutrient concentrations to values several times lower than those in the ambient waters, indicating that the ambient nutrients were both available and sufficient to allow the development of the massive algal bloom observed. Light-limitation of the phytoplankton community was likely the factor responsible for the low biomass and production in ambient waters relative to the mesocosm, as indicated by: (1) limited water transparency of about 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 chl a µm^-3) inside the phytoplankton cells of the community in the ambient waters compared to much lower values (4.2 ± 0.8 pg chl a µm^-3) in the mesocosm community; (3) a specific light absorption of the phytoplankton community in the ambient waters 10 times higher (average specific PAR absorption ± SE = 0.018 ± 0.0038 m^-1 mg chl a-1) than in the mesocosm community (0.0087 ± 0.002 m^-2 mg^-1 chl a); and (4) a high apparent quantum yield of the phytoplankton in the ambient waters (0.091 mol O₂ [mol photon absorbed]^-1), 4-fold higher than that for the community developed in the mesocosm.

KEY WORDS: Southern Ocean · Phytoplankton bloom · Mesocosms · Light limitation · Growth rates · Light absorption · Diatoms