Phytoplankton Community Structure and the Drawdown of Nutrients and CO ₂ in the Southern Ocean

Abstract

Data from recent oceanographic cruises show that phytoplankton community structure in the Ross Sea is related to mixed layer depth. Diatoms dominate in highly stratified waters, whereas Phaeocystis antarctica assemblages dominate where waters are more deeply mixed. The drawdown of both carbon dioxide (CO2) and nitrate per mole of phosphate and the rate of new production by diatoms are much lower than that measured for P. antarctica. Consequently, the capacity of the biological community to draw down atmospheric CO2 and transport it to the deep ocean could diminish dramatically if predicted increases in upper ocean stratification due to climate warming should occur.

Temperature effects on growth, colony development and carbon partitioning in three Phaeocystis species

“…Our data suggest that the geographical ranges of P. antarctica and P. pouchetii are broadly restricted by temperature, whereas the ability of P. globosa to bloom in disparate environments may depend more on other factors, such as nutrients, grazing, and irradiance. The highest growth rate for P. antarctica was observed at 4°C in our experiments, which is higher than the in situ water temperature from -1.86 to 0°C at which P. antarctica blooms develop in the Ross Sea (Arrigo et al 1999, van Hilst & Smith 2002, suggesting that P. antarctica growth is more strongly regulated in situ by light and nutrients than by temperature. However, growth of P. antarctica during spring may also be a function of its ability to maximize its growth and photosynthesis during periods of low irradiance induced by high solar angles, relatively deep vertical mixing, and the presence of ice (Moisan & Mitchell 1999, Kropuenske et al 2009).…”

Phytoplankton and iron limitation of photosynthetic efficiency in the Southern Ocean during late summer

“…A likely contributing factor to the biomass differences we observed is related to loss processes and the composition of the plankton community earlier in the season. In the Ross Sea, Phaeocystis antarctica tends to be important during the spring bloom (Arrigo et al, 1999;Caron et al, 2000); this species, which does not sink rapidly and, during its colonial life stage, may evade grazing by protozoa, was still present during our summertime sampling. Moreover, Caron et al (2000) reported that microzooplankton grazing rates were always low relative to phytoplankton growth rates during spring-summer 1996-1997 in the Ross Sea.…”

“…In high latitude regions, potential effects of low iron availability must be considered in the context of seasonal changes in light availability associated with incident solar radiation and deep winter mixing (e.g., Mitchell et al, 1991). For the Southern Ocean, in particular, uncertainty about what combination of factors regulates primary production has important implications for predicting the overall effects of marine processes in global carbon cycling (de Baar et al, 1995;Sarmiento and Le Quere, 1996;Arrigo et al, 1999). The role of iron in limiting Southern Ocean phytoplankton has been investigated chiefly via iron enrichment experiments in which properties such as chlorophyll, biomass, or physiological parameters are measured over time in control and enriched samples.…”

Southern Ocean phytoplankton physiology in a changing climate

“…Of course, these simplistic projections are made more complex by the biota and its physiological response to altered conditions, as any resultant shift in species composition will not only influence productivity, but also trophodynamics and biogeochemistry. If communities shift from diatom dominated to non-diatom dominated populations, an increase in primary production rates may occur, as nutrient utilisation efficiency and carbon fixation is generally lower in diatoms (Arrigo et al, 1999). However, smaller non-silicified cells may have lower sinking rates and therefore weaken the biological pump and reduce carbon sequestration.…”