Effects of warming on annual production and nutrient-use efficiency of aquatic mosses in a high Arctic lake

1. Primary production in Arctic and Antarctic lakes is typically dominated by benthic photosynthetic organisms. These include aquatic mosses that can be very abundant although they must be able to tolerate extreme seasonal climatic variation in irradiance and temperature. Climate change is expected to cause increased nutrient run-off and to prolong ice and snow cover on Arctic lakes. 2. Despite the substantial role that aquatic mosses play in Arctic and Antarctic lakes, our study is the first of its kind to study the in situ growth and nutrient-use efficiency of an Arctic aquatic moss, Drepanocladus trifarius, and how the net production of this species is coupled with a range of climatic factors undergoing change. Here, we verify the use of growth segments as a technique to record historic growth. We then used the method to reconstruct the growth of D. trifarius by comparing in situ measured growth with that determined from growth segments after one year's growth in a high Arctic lake. Finally, we determined nutrient-use efficiency, resorption efficiency and resorption proficiency by measuring the tissue nutrient content in different annual growth segments. 3. The observed annual mean production of D. trifarius was about 2-3 mg DW shoot(-1) year(-1) expressed as biomass increase and 10-30 mm shoot(-1) year(-1) expressed as length increase, while the age of most shoots was 7-10 years. About 90% of the variation in annual net production was explained by between-year variability in snow prolongation and net radiation in June. 4. The low tissue-N (1.05%DW) and tissue-P (0.081%DW) contents found in D. trifarius suggest that its growth in Lake Sommerfugleso is limited by the availability of N and P during summer when the availability of light is sufficient for extensive growth of this species. P resorption was 43% in D. trifarius, whereas no N resorption was recorded, and P proficiency ranged between 0.02 and 0.04% DW, which is comparable to the concentrations measured in other plant types under P limitation, suggesting that P resorption is an advantageous trait for the existence of moss in this oligotrophic environment. 5. Our results suggest that predicted prolongation of snow cover and consequent decrease in net radiation in lakes will decrease annual net primary production. Our results also suggest that species able to exploit elevated nutrient contents have an advantage in more nutrient-rich lakes in future and indicate that dominance patterns may change. We therefore foresee that climate change will have dramatic effects on the annual net production of mosses and thus whole lake primary production in Lake Sommerfugleso and similar lake types dominated by mosses in the high Arctic.