The brown algae Desmarestia menziesii and D. anceps form dense underwater forests at the Western Antarctic Peninsula (WAP) in the upper to mid subtidal and play an important role in this coastal ecosystem. Seawater temperatures at the WAP increased by about 2 °C in the last 50 years and probably will rise further in future due to global warming. This may have consequences for the physiological performance and community interactions of these cold water adapted algae. In laboratory experiments with culture material we investigated the influence of increased temperatures on the photosynthetic response, growth and interspecific competition of both species. Increased temperature (5 °C) induced a higher growth rate of D. anceps and a higher optimum quantum yield of photosynthesis in both species compared to 0 °C. Neither interspecific competition nor interactions between competition and temperature were detected. The photosynthetic performance (rETRmax, α and Ek) of D. menziesii was significantly more often affected by temperature increases than of D. anceps. It was shown that a temperature increase from 0 °C to 5 °C was not harmful for both Desmarestia species in this set-up. Generally, growth of D. menziesii was very low possibly reflecting a dormancy state under continuous long-day conditions and thereby may have masked the influence of temperature on growth. In order to determine realistic survival thresholds of these key species of the Antarctic coastal ecosystem under global change scenarios, further multifactorial experiments are of great importance, including field material, different algal life stages and reproductive processes.