Dale H. Robinson

Rapid and early export of Phaeocystis antarctica blooms in the Ross Sea, Antarctica.

The Southern Ocean is very important for the potential sequestration of carbon dioxide in the oceans and is expected to be vulnerable to changes in carbon export forced by anthropogenic climate warming. Annual phytoplankton blooms in seasonal ice zones are highly productive and are thought to contribute significantly to pCO2 drawdown in the Southern Ocean. Diatoms are assumed to be the most important phytoplankton class with respect to export production in the Southern Ocean; however, the colonial prymnesiophyte Phaeocystis antarctica regularly forms huge blooms in seasonal ice zones and coastal Antarctic waters. There is little evidence regarding the fate of carbon produced by P. antarctica in the Southern Ocean, although remineralization in the upper water column has been proposed to be the main pathway in polar waters. Here we present evidence for early and rapid carbon export from P. antarctica blooms to deep water and sediments in the Ross Sea. Carbon sequestration from P. antarctica blooms may influence the carbon cycle in the Southern Ocean, especially if projected climatic changes lead to an alteration in the structure of the phytoplankton community.

Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

Bio‐optical properties of the southwestern Ross Sea

The bio-optical properties of the southwestern Ross Sea were measured as part of the Antarctic research program Research on Atmospheric Variability and Atmospheric Response in the Ross Sea (ROAVERRS). The study area contained three distinct phytoplankton blooms, distinguishable by species composition. The largest in area was located to the north of the Ross Ice Shelf and was dominated by the prymnesiophyte Phaeocystis antarctica; chlorophyll a (Chl a) ranged from 0.45 to 8.2 mg m−3. Beam attenuation and particle absorption at 435 nm were as high as 3.4 m−1 and 0.35 m−1, respectively. A bloom of diatoms was more spatially restricted, located to the north and west of the P. antarctica bloom, with Chl a generally below 4 mg m−3. Neither diatoms nor P. antarctica exhibited evidence of the level of pigment packaging measured in waters near the Antarctic Peninsula during the Research on Antarctic Coastal Ecosystem Rates (RACER) program, possibly because of their smaller sizes. A much smaller cryptophyte bloom, located south of the Drygalski Ice Tongue, displayed a lower pigment-specific absorption spectra than did P. antarctica or diatoms, a sign of greater pigment packaging. Pigment-specific diffuse attenuation coefficients were consistent with the pigment-specific particle absorption coefficients (aph*), both being ∼3 times greater than similar measurements made during RACER. Spectral absorption by solutes determined through regression analysis of Kd against Chl a for the ROAVERRS data set was nearly identical to that measured during RACER. Total diffuse attenuation spectra at a given station could be reconstructed by summing the inherent optical properties of the major optical components (pure water, soluble material, detritus, phytoplankton) measured there. Differences in the absorption ratio of aph*(λ) at 490 nm to aph*(λ) at 555 nm among the three dominant phytoplankton taxa in the southwestern Ross Sea were responsible for most of the variability in the ratio of remote sensing reflectance (Rrs) at these same wavelengths. At a given concentration of Chl a, the ratio log [Rrs(490):Rrs(555)] was greatest in cryptophyte-dominated waters, which also possessed the lowest aph*(490):aph*(555) ratio, and lowest in P. antarctica–dominated waters. These bio-optical differences suggest that no simple empirical relationship between Chl a and log [Rrs(490):Rrs(555)] will apply to all three taxonomically distinct phytoplankton blooms in the southwestern Ross Sea.

Photophysiology in two major southern ocean phytoplankton taxa: photosynthesis and growth of Phaeocystis antarctica and Fragilariopsis cylindrus under different irradiance levels.

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125 µmol quanta/m2/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antarctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.

Modeling the heating and melting of sea ice through light absorption by microalgae

In sea ice of polar regions, high concentrations of microalgae are observed during the spring. Algal standing stocks may attain peak values of over 300 mg chl a m−2 in the congelation ice habitat. As of yet, the effect of additional heating of sea ice through conversion of solar radiation into heat by algae has not been investigated in detail. Local effects, such as a decrease in albedo, increasing melt rates, and a decrease of the physical strength of ice sheets may occur. To investigate the effects of microalgae on the thermal regime of sea ice, a time-dependent, one-dimensional thermodynamic model of sea ice was coupled to a bio-optical model. A spectral one-stream model was employed to determine spectral attenuation by snow, sea ice, and microalgae. Beers law was assumed to hold for every wavelength. Energy absorption was obtained by calculating the divergence of irradiance in every layer of the model (Δz = 1 cm). Changes in sea ice temperature profiles were calculated by solving the heat conduction equation with a finite difference scheme. Model results indicate that when algal biomass is concentrated at the bottom of congelation ice, melting of ice resulting from the additional conversion of solar radiation into heat may effectively destroy the algal habitat, thereby releasing algal biomass into the water column. An algal layer located in the top of the ice sheet induced a significant increase in sea ice temperature (ΔT > 0.3 K) for snow depths less than 5 cm and algal standing stocks higher than 150 mg chl a m−2. Furthermore, under these conditions, brine volume increased by 21% from 181 to 219 parts per thousand, which decreased the physical strength of the ice.

Strategies and rates of photoacclimation in two major Southern Ocean phytoplankton taxa: Phaeocystis antarctica (Haptophyta) and Fragilariopsis cylindrus (Bacillariophyceae)

We investigated rates and mechanisms of photoacclimation in cultures of Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) Willi Krieg, phytoplankton taxa that each dominate distinct areas of the Ross Sea, Antarctica. Both P. antarctica and F. cylindrus acclimated to increases in irradiance by reducing the effective size of the pigment antenna (σPSII) via xanthophyll‐cycle activity and reductions in chl. While enhanced photoprotection facilitated increases in specific growth rate and eventually led to higher light‐saturated photosynthetic rates (Pcellm) in P. antarctica, increases in those variables were much smaller in F. cylindrus. In response to a lower irradiance, relaxation of xanthophyll‐cycle activity led to an increase in σPSII in both taxa, which occurred much more slowly in F. cylindrus. A surprising increase in specific growth rate over the first 36 h of acclimation in P. antarctica may have facilitated the significant reductions in Pcellm observed in that taxon. In general, P. antarctica acclimated more quickly to changes in irradiance than F. cylindrus, exhibited a wider range in photosynthetic rates, but was more susceptible to photoinhibition. This acclimation strategy is consistent with growth in deeply mixed water columns with variations in irradiance that allow time for repair. In contrast, the slower acclimation rates, extensive photoprotection, and low photoinhibition exhibited by F. cylindrus suggest that it does not require the same period for repair as P. antarctica and is best suited for growth in habitats with relatively uniform irradiance, such as shallow mixed layers or sea ice.

PHOTOPHYSIOLOGY IN TWO SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHOTOSYNTHESIS OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) UNDER SIMULATED MIXED-LAYER IRRADIANCE1

In the Ross Sea, the prymnesiophyte Phaeocystis antarctica G. Karst. dominates deeply mixed water columns, while diatoms dominate shallower mixed layers. Understanding what controls the dynamics of these two phytoplankton taxa is essential because they dominate virtually all coastal polar waters, have different nutrient utilization characteristics, and support dissimilar food webs. We cultured two strains of P. antarctica and one strain of the diatom Fragilariopsis cylindrus (Grunow) Willi Krieg under three dynamic irradiance regimes that simulated different mixed‐layer depths and measured their photosynthetic characteristics, cellular pigment concentrations, and cellular carbon and nitrogen content. In both species, chl a–normalized maximum carbon uptake rate (Pm* ) and specific growth rate were highest in the deeply mixed treatment that had a dark period. In all irradiance treatments, both (Pm* ) and photosynthetic efficiency (α*) were greater for the two P. antarctica strains than for the F. cylindrus strain. In contrast, P. antarctica strains were more susceptible to photoinhibition (β*) than the F. cylindrus strain. When photosynthetic rates of each phytoplankton taxon were normalized by cellular particulate organic carbon (POC), the difference in the maximal photosynthetic rate () was generally reduced. In the dynamic irradiance treatment that simulated the shallowest mixed‐layer irradiance, all three phytoplankton had similar ; however, the diatom had a 2‐fold higher POC‐normalized photosynthetic efficiency (αC). Finally, we performed calculations using the measured POC‐normalized photosynthetic parameters to show that αC and can play a greater role than βC in determining the competitive outcome between P. antarctica and F. cylindrus in both shallow and deep mixed‐layer environments of the Ross Sea.