The Antarctic Circumpolar Current (ACC) is unique in that it has continually high concentrations of major plant nutrients but low phytoplankton biomass. This enigmatic phenomenon is the focus of significant speculation that trace nutrients, including Fe, may limit phytoplankton crop size. Global climatologies indicate that the ACC is a region with low surface temperatures, weak density stratification, little summertime surface solar irradiance, and strong wind stress. These physical phenomena act to limit growth rates of the phytoplankton community. Using a photo-physiological description of phytoplankton growth in a simple one-dimensional ecosystem model forced by observations or climatologies of mixing depth and surface irradiance, the authors make an evaluation of the potential for massive, nutrient-exhausting, phytoplankton blooms forming in the ACC. The ACC has persistent mixed layers in excess of 50 m. Literature values and model optimization indicate that the minimal aggregate specific loss rate and typical physical conditions of stratification and surface irradiance, the model predicts that phytoplankton in the ACC would not utilize >10% of the available macronutrients. Without a mechanism for increasing the strength of stratification, the authors predict that massive Fe additions to the Southern Ocean would fail to significantly mitigate the atmospheric CO{sub 2} derived from fossilmore » fuel.« less

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1991.36.8.1662

Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean

The multi-disciplinary program RACER (Research on Antarctic Coastal Ecosystem Rates) conducted eight surveys of a 69-station grid in a 100 × 250 km area in the southwestern Bransfield Strait from December 1986 to March 1987. Mean phytoplankton crop size in the upper 50 m during December, January, February and March was 291, 176, 58 and 50 mg Chl α m−2, respectively, and was inversely proportional to the increasing mean depth of the upper mixed layer (UML) (15, 17, 26 and 30 m, respectively). Massive mid-summer phytoplankton blooms (> 10 mg Chl α + phaeo m−3) were persistent nearshore where we observed shallow UMLs ( 20 m) with small density gradients (Δσt 0.05–0.20 from 0 to 75 m). Proximity to stabilizing meltwater and protection from intense Antarctic storm activity appear to be essential for the development of persistent massive blooms. A model of Antarctic phytoplankton growth based on mixing depth and pigment-specific light attenuation and in situ photosynthesis-irradiance relationships indicates that the depth of the UML (ZUML) can be used to predict the upper limit of the phytoplankton crop size. Observed phytoplankton biomass for diverse Southern Ocean ecosystems is discussed in relation to the mean light level of the UML, growth and loss rates of Antarctic phytoplankton, and the depth and duration of stratification required before a bloom ensues. Assuming nutrients do not limit the crop size, a best-fit to observations indicates specific loss rates must be approximately 0.3–0.35 day−1 and massive blooms occur only if ZUML <25 m. The grazing component of this predicted loss rate is higher than previously estimated. We conclude that grazing rates are greater than previously reported, or vertical flux rates of nutrients limit massive blooms.

Observations of modeling of the Antartic phytoplankton crop in relation to mixing depth

The influence of oceanographic processes on pelagic-benthic coupling in polar regions: A benthic perspective

Studies on phytoplankton were one component of the multi-disciplinary RACER program which had 69 stations within a 100 × 250-km rectangle in the southwestern Bransfield Strait and contiguous waters. Data were acquired during eight cruises between December 1986 and March 1987. All deep stations north of the continental shelf break were low in phytoplankton biomass ( 10 μM inorganic nitrogen) that nutrient depletion is not likely to have caused the rapid decline of the phytoplankton bloom. Grazing, sinking and advection all appear to be important mechanisms of massive bloom decline. Phytoplankton populations appeared to be low-light adapted, as they showed low Pmax values (1.1 mg C mg Chl a−1 h−1), low saturating ligh values (Ik ∼ 18 μEins m−2 s−1), high initial slope [α = 0.06 (mg C mg Chl a−1h−1/μEin m−2 s−1)] and a compensation point for net light-activated fixation of CO2 of ≈ 1.0 μEin m−2 s−1.

Spatial and temporal distribution of phytoplankton and primary production in the western Bransfield Strait region

Photoacclimation of antarctic marine diatoms to solar ultraviolet radiation

Fifteen oceanographic stations were occupied in the vicinity of Anvers Island, Antarctica, in January of 1985 and 1987. All stations showed high phytoplankton biomass (4.0 to 30 μg chl-a/liter) which was either uniformly distributed in the upper mixed layer or showed a pronounced sub-surface maximum at 4–5 m depth. As phosphate was less than 0.02 μm and nitrate about 2.0 μm in surface waters, it appears that nutrient limitation of phytoplankton growth may be of importance during such blooms. This view is supported by chemical measurements of the particulate material which showed high chl-a/ATP ratios (about 7.7), as well as high POC/ATP ratios (about 700). Microscopical analysis revealed a dominance of large-celled diatoms and the near absence of heterotrophic protozoans. Size fractionation studies showed that the nanoplankton accounted for only 28% of the total phytoplankton biomass. When phytoplankton biomass reaches the levels found at these stations, it appears that the cells are light-limited and hence dark-adapted, which results in the high chl-a/ATP ratios and the low assimilation values (0.49–1.64) obtained in our studies. Under such conditions greater than 50% of the total phytoplankton biomass is found below the 1% light level.

B. G. Mitchell

Papers

Phytoplankton blooms in the vicinity of Palmer Station, Antarctica