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Thermokarst ponds are the most abundant type of water body in the arctic tundra, with millions occurring in the coastal plains of Alaska, Northwest Territories and Siberia. Because ice covers of at least 2 m in thickness are formed at these latitudes, tundra ponds freeze solid every winter As a result, the growing season is shortened to a range of 60 to 100 days, during which time the photoperiod is altered to a prolonged light phase. Tundra ponds are generally close to neutral in pH and low in ions, contain dissolved gases near saturation and are nutrient poor. In low arctic ponds there are two phytoplankton biomass and primary production peaks, whereas they may be only one in the high arctic. Nanoplanktonic flagellates of the Chrysophyceae and Cryptophyceae dominate the maxima. The mid-summer decline in phytoplankton in the low arctic can be attributed to a combination of phosphorus limitation and heavy grazing pressure. The cryptomonad Rhodomonas minuta Skuja is one of the most widespread phytoplankters in tundra ponds. Because of the altered photoperiods, many species do not form resting spores prior to ice formation but survive freezing in the vegetative state.

Thirty-nine subgeneric taxa of macroalgae have been collected from 83 sites in Georgian Bay, the North Channel and their drainage basin. There were 15 species in the Bay and Channel and 32 species in streams, rivers and impoundments in the basin. Only 8 of the Bay and Channel species were also found in the watershed. Cladophora glomerata was the most important species in Georgian Bay and the North Channel, having an estimated 640 × 103 m2 cover and 19 × 104 kg fresh weight standing crop. However, this species was largely concentrated on the southwestern shorelines of these water bodies. Its distribution along the northeastern shoreline appears to be limited by total ion and phosphorus levels. Chara globularis/vulgaris was the subdominant taxon in Georgian Bay and the North Channel with an estimated 70 × 103 m2 cover and 15 × 103 kg fresh weight biomass. This species was more widely distributed than C. glomerata. No other taxon contributed significantly to the standing crop including the frequently occurring Ulothrix zonata, Zygnema spp. and Spirogyra spp. The maximum benthic macroalgal biomass was estimated to be approximately 10% of the phytoplankton biomass.

Forty stream segments were sampled throughout the Cook Inlet drainage basin in June 1984. There was a great range in stream size, current velocity, temperature, shading, conductance and water color. This range in variables presumably has resulted in a diverse flora of 40 species of lotic macroalgae. The major divisions in terms of species numbers were the Chlorophyta (43%), Bacillariophyta (25%), Rhodophyta (13%) and Xanthophyta (13%). Filaments were the predominant form (60% of species), though mats were common (28% of species). Species numbers and abundance were significantly correlated to water temperature. Distribution was patchy in the basin, with total cover varying from less than 1% to 90% of the stream bottom. Lowland brown-water streams flowing through emergent wetlands tended to have the highest species diversity and abundance. In the drainage basin, the chrysophyte Hydrurus foetidus and the blue-green alga Phormidium retzii exhibited strongly developed dominance. This pattern of niche pre-emption appears to be typical of stream macroalgal communities in general.