Composition and biomass of phytoplankton assemblages in coastal Antarctic waters: a comparison of chemotaxonomic and microscopic analyses

Abstract

We describe the distribution of phytoplanktonic community composition and biomass from the Western Antarctic Peninsula coast (between 64°and 68°S) using 2 analytical techniques: microscopy and HPLC of photosynthetic pigments. Phytoplankton biomass was estimated as chlorophyll a (chl a) by HPLC and chemotaxonomic quantification of microalgae biomass was performed by multiple regression analysis of pigment concentrations. For the estimation of chl a : diagnostic pigment ratios, it was found of primary importance to differentiate between phytoplankton assemblages within the study area. Three assemblages were differentiated according to their total standing stock and analyzed independently. Phytoplankton biomass was also estimated as carbon (C) concentration by microscopic analysis of cell abundance and biovolumes. Microscopy and chemotaxonomy give a high level of agreement for phytoplankton characterization, showing an on/offshore gradient, with high diatom and cryptophyte biomass in coastal waters, and a mixed assemblage with low biomass in open waters. This gradient was not observed in total cell abundance, indicating that the biomass gradient is controlled by cell size. Microscopy also showed shifts in diatom species throughout the area. C and chl a biomass estimates for the individual microalgae groups were strongly correlated for cryptophytes, chlorophytes and most diatoms, but did poorly for dinoflagellates, prymnesiophytes and chrysophytes. From this study, we conclude that both microscopy and chemotaxonomy can be used to accurately characterize phytoplankton assemblages, but some limitations are present in both techniques. Based on phytoplankton C concentrations, we estimated an average in situ growth rate of 0.28 d -1 . In situ cell C:chl a ratios had high variability (from 40 to 220) and were non-linearly related to sample growth rates. Significant differences were found among average C:chl a ratios of low (<1 µg chl a l -1 ) and high biomass communities (>1 µg chl a l -1 ), with values of 112 and 74 µg C µg -1 chl a, respectively. In addition, our results support the hypothesis that C quotas of diatoms and other microalgae do not differ greatly from each other, as previously believed.

Phytoplankton spatial distribution patterns along the western Antarctic Peninsula (Southern Ocean)

“…5, Table 1). The gradients in carbon biomass and chl a concentration were of the same magnitude as others previously reported for this area (Smith et al 1998, Garibotti et al 2003, showing phytoplankton standing stocks several times higher in coastal waters than in open waters (Table 1). Composition of the phytoplanktonic community and diatom populations also changed along the gradient, as indicated by shifts in the dominant algae groups and diatom species in regions differentiated within the area (Fig.…”

“…Phytoplanktonic cell abundance and carbon biomass data were from samples at the 50% PAR depth, presumed to be representative of the UML. This assumption is based on previous studies performed in the area that revealed a uniform vertical distribution of carotenoid pigments (diagnostic of different algae groups), suggesting no major phytoplankton composition variations with depth (Ross et al 2000, Garibotti et al 2003. The 50% PAR depth (5 to 30 m) was always within the UML.…”

“…Microscopic analyses were done as described in Garibotti et al (2003). Phytoplankton cells were identified and counted with an inverted microscope (Iroscope IS-PH) in settling chambers (Utermöhl 1958).…”

“…6). The unidentified phytoflagellate group might include chrysophytes and prymnesiophytes, and a small proportion of chlorophytes and cryptophytes (Garibotti et al 2003), but here they were pooled together, considering that they have similar ecological importance in the trophic chain. Nanoplanktonic cells (< 20 µm) dominated throughout most of the area, accounting for 98% of cell abundance and 41% of carbon concentration.…”

Influence of oceanographic features on spatial and interannual variability of phytoplankton in the Bransfield Strait, Antarctica

“…Therefore, the timing of the development and retreat of sea ice are considered as important factors in the temporal variability of phytoplankton communities in the AP (Arrigo et al, 1999;Garibotti et al, 2003Garibotti et al, , 2005Moline and Prézelin, 1996). Although the dynamics of sea ice are influenced by winds (Stammerjohn et al, 2003, and references therein), here we related these dynamics only with temperature.…”