Mathot, S. I. VIMS, College of William & Mary, sylviem@vims.edu
Smith, W. O. VIMS, College of William & Mary, wos@vims.edu

 
TEMPORAL CHANGES OF PHAEOCYSTIS SP. IN THE ROSS SEA: VARIATIONS IN COLONY SIZE, ABUNDANCE AND CARBON CONTENT
 
The Haptophyte Phaeocystis antarctica is one of the main species that develops in the Ross Sea as part of the massive, seasonal phytoplankton bloom, yet the carbon partitioning between the cells and its colonial matrix is poorly known. Furthermore, because temporal changes in chemical properties of Phaeocystis-dominated assemblages occur, changes within the colonies per se can be expected, but at present these changes are poorly documented. The quantitative role of mucilage to Phaeocystis colonies as well as its seasonal variations were assessed through microscopic observation of the colonies, their enumeration and size measurement as well as colonial cell counts. The phytoplankton biomass was overall very low in October-November, with chlorophyll concentrations evolving from 0.1 to 0.7 ug.l-1. During this pre-bloom period, Phaeocystis started to form small round colonies (diameter ~10-150 um). As the bloom progressed towards a maximum (up to 14 ug Chla.l-1) in December, phytoplankton assemblages were exclusively dominated by actively growing Phaeocystis colonies (diameter ranging from 50 to 300 um), which contributed up to 96% of the total autotrophic biomass. In late December- early January, the colonies were in a senescent stage, accompanied by a more general reduction of autotrophic biomass in the euphotic zone. In April, post-bloom conditions were encountered with extremely low phytoplankton biomass (ca. 0.1 ug Chla.l-1), and no Phaeocystis colonies were noticeable in the water samples. Phaeocystis colony carbon biomass was estimated on preserved water samples using both inverted and epifluorescence microscopy. According to visual observation, colonies were identified as spheres, rotational ellipsoids, cylinders, aggregates or 'sheets'. Colonies were measured, and colonial cells were enumerated for a large number of individual colonies. Colonial cells were also sized and their carbon content was estimated to be 13.15 pg per um3 according to Strathman's (1978) equation. A first attempt to assess Phaeocystis biomass through image analysis is also presented. Preliminary analysis of colonial volume (Vol) vs colonial cell number (cell #) suggests that the best fitting curve to the data points is a power relationship expressed as: Vol = alpha*[cell #]^beta, with alpha and beta varying along the course of the bloom. Overall, alpha increased by 2 orders of magnitude (from 8 to about 3800), whereas beta decreased from about 3 to 1.25. Implications of these variations are discussed, as the amount of extracellular carbon produced during Phaeocystis blooms is possibly substantial, and because of the cardinal role this organism plays in the carbon cycle. Data are presented which show the percentages of carbon attributable to extracellular mucilage change through the course of the bloom. Some preliminar calculations show that mucilage-related Phaeocystis carbon reaches an upper limit of 24% of the total colonial carbon (i.e. cells + mucus) during the growing phase of the bloom, whereas post-bloom values range between 7 and 40% of the total.
 
Day: Thursday, Feb. 4
Time: 02:45 - 03:00pm
Location: Sweeney Center
 
Code: SS31TH0245S