Changes in algal net ecosystem production over the past 40 years in Arctic tundra ponds near Barrow, Alaska

Background/Question/Methods

Arctic environments have gained interest recently due to their sensitivity to climate change. Arctic regions are characterized by diverse freshwater ecosystems which are important to a variety of animals, plants and microorganisms. As warming has created longer growing seasons, extended ice-free periods as well as permafrost thaw, the production in freshwater ecosystems has the potential to be greatly impacted. Although an increasing number of studies have focused on the arctic regions, the rate of primary production and the potential of carbon up take/release in arctic freshwater ecosystems are still poorly understood. It is hypothesized that as climate change and warming occurs, increases in primary production will occur and ponds will become carbon sinks. Net ecosystem production (NEP) was determined on phytoplankton and periphyton samples collected weekly during the growing season (June – August) from arctic tundra ponds near Barrow, Alaska. Enrichment with ¹⁴C was used to determine both weekly photosynthetic rates and maximum rates of photosynthesis, which was measured at multiple temperatures (8, 14 and 20°C) and light levels. All data collected from these experiments was compared to data collected in the 1970s. Free water metabolism was determined using a YSI logger installed in the pond throughout the growing seasons (2010-2).

Results/Conclusions

While there were no significant difference among phytoplankton NEP throughout the growing season in 1970-2 as compared to 2011-2, NEP was slightly higher during the latter half of the season in 2011-2. Peaks in phytoplankton NEP around the 29^th week of the year in 2012 were observed and immediately preceed peaks in Daphnia abundance recorded in a related study. No significant difference was found in benthic NEP between 1972 and 2011-2 for the entire season. Comparisons of Photosythesis – Irradiance (P-I) curves showed that in 2012, as in the 1970s, phytoplankton reached higher rates of photosynthesis at warmer temperatures; however, these rates were substantially higher in 2012 as compared to 1973. P-I curves for periphyton were more variable, and like phytolankton, experienced photoinhibition at lower temperatures. Free water metabolism confirmed an increase in the production of O₂ via primary production towards the end of the growing season in 2010-2. As the growing season continues to increase in length due to climate change, these results suggest that algal primary production in arctic tundra ponds will increase and tundra ponds may become vital carbon sinks in northern latitudes.