Antarctic Aquatic Ecosystems as Habitats for Phytoplankton

Abstract

Summary 1. The Southern Ocean is a large‐scale, relatively homogeneous upwelling ecosystem whose phytoplankton apparently grows suboptimally over much of its area. By contrast there is a wide variety of freshwater habitats in the Antarctic and in some of these phytoplankton growth efficiency is very high. The two habitats share similar temperature and irradiance regimes, but differ markedly in availability of inorganic nutrients, in grazing pressure and in the time‐ and space‐scales on which various physical processes act. 2. Concentrations of inorganic nutrients in the marine ecosystem have been represented as being in excess of phytoplankton requirements, but the ionic composition of some nutrient pools may not conform to phytoplankton preferences. 3. Nutrient‐limitation determines phytoplankton production in Antarctic lakes and gives rise to gross differences between lakes. 4. Irradiance in the water column varies greatly over the year in both marine and freshwater ecosystems. Most algae are shade‐adapted, with the ability to utilize low irradiance but with sub‐optimal response to high irradiance. However, local phytoplankton maxima may attain very high carbon fixation and growth rates. 5. Consistently low temperatures characterize both systems. Their effects on photo‐synthetic carbon uptake mirror shade‐adaptation. Division rates of marine phytoplankton may however be very much higher than predicted for ambient temperatures. 6. Vertical mixing is important in both ecosystems and influences the environment experienced by phytoplankton cells. This appears to have little effect on the average performance of phytoplankton in the strongly mixed surface water column of the Southern Ocean, where the mixed depth may exceed 100 m. This can be related partly to the shade‐adapted photosynthetic response. Euphotic depths range from 20 to 100 m. 7. Strong vertical mixing under ice‐free conditions in lakes may maximize photosynthetic efficiency, whilst distinct vertical stratification in permanently ice‐covered lakes gives rise to segregation of nutrient uptake and regeneration. 8. Physical removal of phytoplankton biomass by grazing is locally important in the Southern Ocean, in contrast to the estimated mean mesoscale impact of grazing. Vertical sedimentation losses appear important in the context of mixing depth and generation time, and may be modified by vertical circulation of water. 9. Loss of phytoplankton biomass from lakes during the ice‐free period is dominated by physical removal via the lake outflow. Grazing is generally unimportant, except where larvae of otherwise nektobenthic zooplankton hatch in synchrony with a phytoplankton maximum. Sedimentation is important under ice‐cover.

Factors regulating periphytic algal biomass

“…In addition, periphytic algae in the Antarctic lakes were able to produce a biomass as high as in the Swedish lakes, even though water temperatures seldom were above 4°C in these systems. This fact indicates that low temperature does not consistently limit biomass production of algae in Antarctic lakes, a notion strengthened by studiles on phytoplank.ton in Antarctic lakes (Priddle et al 1986). …”

Temperature responses of growth, photosynthesis, fatty acid and nitrate reductase in Antarctic and temperate Stichococcus

“…In general, the water temperature in Antarctic ranges from -1.8 to 5.0°C no matter in winter or in short summer (Wiencke and Dieck 1990). The freshwater Antarctic phytoplanktons are able to grow under the constant low temperatures from 0 to 5.0°C (Priddle et al 1986). Except psychrophiles, which could not survive above 15°C, many Antarctic phytoplanktons exhibit the ability to grow at temperatures above 20°C (Seaburg et al 1981;Hu et al 2008).…”

Nanoplankton assemblages in maritime Antarctic lakes: characterisation and molecular fingerprinting comparison

“…Different flagellated algae, including Chrysophyceae, Chlorophyceae, Prasinophyceae and Cryptophyceae, have also been reported to be very abundant in many other Antarctic lakes , Butler 1999a, Mataloni et al 2000, Izaguirre et al 2001, Marshall & Laybourn-Parry 2002, Bell & Laybourn-Parry 2003. The motility, pigment adaptation to low light intensity, and mixotrophic behaviour of some flagellates are adaptive advantages that allow them to dominate the plankton community in Antarctic ecosystems, subject to periodic freezing (Priddle et al 1986, Burch 1988.…”