PARENT SESSION
Poster Session #31: Climate Change.
Wednesday, August 8, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Responses of phytoplankton to experimental greenhouse warming of alpine ponds in the Canadian Rocky Mountains.

STRECKER, ANGELA¹, COBB, TYLER¹, VINEBROOKE, ROLF¹, ¹

ABSTRACT- Early ecosystems responses to global warming are expected to involve rapidly reproducing organisms in alpine ponds as climate models predict more pronounced increases in temperature at higher elevations. We conducted a field mesocosm experiment using a replicated random-block design to determine the responses of phytoplankton to warmer temperatures in three fishless alpine ponds located in Banff National Park, Canada. Three blocks of four mesocosms (1000-L capacity) were each established next to, and received biota from, a permanent alpine pond. Two temperature treatment levels (ambient, 3°C-warmed) were achieved by placing greenhouse canopies over the mesocosms and varying their ventilation. After two months, elevated temperature did not have a significant effect on total phytoplankton biomass owing to compensatory shifts in species composition. Specifically, fast-growing unicellular and colonial species (Bitrichia, Eunotia, Pandorina, Synedra) replaced larger, slow-growing filamentous species (Bulbochaete, Desmidium, Mougeotia) and maintained phytoplankton abundance in the warmed mesocosms. Direct effects of temperature appeared to explain the observed phytoplankton response to the warming treatment, rather than indirect effects, because resource (light, nutrients) levels and herbivore biomass did not differ significantly between treatment levels. Our findings suggest that changes in phytoplankton species composition in shallow alpine lakes and ponds are the earliest indicators of the impacts of pronounced climate warming at high elevations.

KEY WORDS: global warming, alpine ponds, phytoplankton, species composition