MEPS 168:229-244 (1998)  -  doi:10.3354/meps168229

Growth dynamics of Phaeocystis antarctica-dominated plankton assemblages from the Ross Sea

Walker O. Smith Jr1,*, Craig A. Carlson2, Hugh W. Ducklow3, Dennis A. Hansell2

1Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996, USA
2Bermuda Biological Station, Ferry Reach, St. George¹s GE01, Bermuda
3Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt., Virginia 23062, USA
*Present address: Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt., Virginia 23062, USA. E-mail:

ABSTRACT: Large-volume experiments were conducted using natural seawater assemblages collected in the southern Ross Sea during austral spring 1994 and summer 1995 to assess the carbon and nitrogen exchanges among phytoplankton, bacteria and dissolved organic carbon pools, and to compare the elemental partitioning in these experimental enclosures with those observed in situ. Large concentrations of particulate matter were produced in these enclosures, which were at all times dominated by the colonial haptophyte Phaeocystis antarctica. Particulate organic carbon concentrations exceeded 200 µmol l-1 at the end of the experiment. Bacterial carbon comprised only a small (<1%) fraction of the particulate carbon, but bacteria grew at 0.15 to 0.3 d-1 and were not limited by bacteriovores. Nutrient levels decreased concomitantly with POC increases, and nitrate was reduced to undetectable levels. Dissolved organic carbon (DOC) levels remained low (less than 50 µM) while nutrients were present, but increased dramatically (to more than 200 µM) after nitrate was depleted. Growth rates calculated from changes in particulate matter concentrations were slightly below the predicted maximum based on temperature. Field studies, however, showed no depletion of nitrate, similar levels of particulate organic carbon to those found during exponential growth, low levels of DOC, and relatively low levels of bacterial biomass. It appears that P. antarctica in the Ross Sea does not produce large amounts of DOC during nutrient-replete growth; furthermore, because macronutrients are rarely, if ever, depleted where P. antarctica is dominant in the Ross Sea, it is likely that much of the carbon generated during its growth remains in the particulate pool.

KEY WORDS: Carbon · Antarctica · Phaeocystis · Phytoplankton · Bacteria · Dissolved organic · Particulate organic · Nitrogen

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