Phytoplankton composition and abundance in relation to free-floating Antarctic icebergs

Free-drifting icebergs in the Weddell Sea are expected to affect the surrounding marine ecosystem. Sampling associated with iceberg C-18a, a large tabular, free-drifting iceberg in the NW Weddell Sea, carried out from 10 March to 7 April 2009, was designed to test the hypothesis that the icebergʹs presence modified phytoplankton composition and abundance. Areas that define a gradient of possible iceberg influence were sampled for phytoplankton: stations close (<1 km) and far (18 km) from iceberg C-18a, an area with numerous small icebergs, Iceberg Alley, and a control site 74 km away. Quantitative samples were obtained from Niskin bottles and counted with an inverted microscope for species abundance. Qualitative samples were collected with nets from the shipʹs seawater intake. Taxonomic determinations were performed with light and electron microscopy. Overall, diatoms dominated in the mixed layer (surface-∼40 m) and unidentified small flagellated and coccid cells at depth (∼100 m). Fragilariopsis nana, a diatom 2.4–15.5 μm in length, dominated numerically the phytoplankton and was most abundant at the control area. The icebergʹs effect on phytoplankton composition was consistent with the hypothesis that they facilitate phytoplankton communities enriched in diatoms, as found in other productive areas of Antarctica. Near the iceberg, diatoms were most abundant, principally at depth, while small flagellate concentration diminished. However, total phytoplankton abundance was lowest at Iceberg Alley in the area of highest meltwater contribution, as indicated by low mean temperature in the mixed layer, and highest at the control site. These results suggest that during austral fall, low growth or high zooplankton grazing could be counteracting the positive effect by icebergs on phytoplankton biomass, otherwise observed in summer months.