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20. Abundance, size structure and community composition of phytoplankton in the Southern Ocean in the austral summer 1999/2000

Author

Ishikawa,Akira, Wright,Simon W., van den Enden, Rick, Davidson,Andrew T., Marchant,Harvey J., and Faculty of Bioresources, Mie University/AustralianAntarctic Division/Australian Antarctic Division/Australian Antarctic Division/Australian Antarctic Division

Subjects
size structure, nanoplanktonic diatoms, phytoplankton, community composition, Southern Ocean
Abstract

The abundance, size structure and community composition of phytoplankton in the Southern Ocean were studied, using flow cytometry, microscopy and pigment profiles on two transects one latitudinal (N) and one longitudinal (W) during December 1999 and January 2000. In both transects, the concentration of autotrophic eukaryotes of 2-10μm equivalent spherical diameter (ESD) commonly exceeded those10μm ESD were detected by flow cytometry (however microscopy showed cells>10μm in length). Throughout transect N, chlorophyll α concentrations were generally. South of the Antarctic Polar Front (APF), chlorophyll α concentrations increased southward. CHEMTAX allocation of pigment data (italicized) showed that Diatoms contributed most chlorophyll with Haptophytes sub-dominant. North of the APF, chlorophyll α concentrations tended to increase northward. Here, Haptophytes contributed most chlorophyll, followed by Diatoms, Chlorophytes and Cyanobacteria, except at the northernmost stations where Cyanobacteria dominated. In transect W, chlorophyll α concentrations were also in most cases, but variable. Higher concentrations occasionally occurred in the west. In this transect, Diatoms contributed most (mean=61±15%) of the chlorophyll α, followed by Haptophytes. Nanodiatoms (particularly Fragilariopsis spp.) numerically dominated the diatom community. Fecal pellets composed of these nanodiatoms were observed in the Antarctic water, probably originating from heterotrophic dinoflagellates, implying a significant contribution of nanodiatoms to the microbial food web. However they contributed little to total chlorophyll α and diatom carbon biomass, particularly when chlorophyll and carbon concentrations were high.

Published
2002

21. Ocean acidification changes the structure of an Antarctic coastal protistan community.

Author

Hancock, Alyce M., Davidson, Andrew T., McKinlay, John, McMinn, Andrew, Schulz, Kai, and van den Enden, Rick L.

Subjects
OCEAN acidification, ECOSYSTEM dynamics, CARBON dioxide, ANTARCTIC environmental conditions, MICROSCOPY
Abstract

Antarctic near-shore waters are amongst of the most vulnerable in the world to ocean acidification. Microbes occupying these waters are critical drivers of ecosystem productivity, elemental cycling and ocean biogeochemistry, yet little is known about their sensitivity to ocean acidification. An unreplicated, six-level dose-response experiment was conducted using 650 L incubation tanks (minicosms) adjusted to fugacity of carbon dioxide (ƒCO2) from 343 to 11 641 μatm. The minicosms were filled with near-shore water from Prydz Bay, East Antarctica and the protistan composition and abundance was determined by microscopy analysis of samples collected during the 18 day incubation. No CO2-related change in the protistan community composition was observed during the initial 8 day acclimation period under low light. Thereafter, the response of protists to ƒCO2 were species-specific for both heterotrophic and autotrophic protists. The response by diatoms was related to cell size, large cells increasing in abundance with low to moderate ƒCO2 (634-953 μatm). Similarly, the abundance of Phaeocystis antarctica increased with increasing ƒCO2 peaking at a ƒCO2 of 634 μatm. Above this threshold the abundances of large diatoms and Phaeocystis antarctica fell dramatically, and small diatoms dominated, therefore culminating in a significant shift in the composition of the protistan community. The threshold CO2 level at which the composition changed agreed with that previously measured at this location, indicating it remains consistent among seasons. This suggests that near-shore microbial communities are likely to change significantly near the end of this century if anthropogenic CO2 release continues unabated, with profound ramifications for near-shore Antarctic ecosystems. [ABSTRACT FROM AUTHOR]

Published
2017
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