Reconstruction of annual growth in relation to summer temperatures and translocation of nutrients in the aquatic moss Drepanocladus trifarius from West Greenland

Mosses often dominate the submerged vegetation in Arctic lakes and ponds, making them essential contributors to the primary production in these habitats. However, little is known about the factors controlling annual growth of Arctic mosses and their sensitivity to climatic changes. It has been suggested that nutrient translocation occurs in mosses, and that annual growth of mosses therefore depends strongly on weather conditions and less on local environmental conditions. In this study, we examined annual growth of Drepanocladus trifarius ((F. Weber and D. Mohr) Broth.) from two ponds in West Greenland in relation to weather conditions. A reconstruction of annual growth increments from 2009 to 2014 was made in 200 individual mosses, and biomass and length were related to different weather parameters. In addition, we examined whether there would be an indication of nutrient translocation across annual growth segments. We found a positive relationship between mean summer temperature and growth segment length, which indicates the importance of temperature during seasons with sufficient light levels for growth of the D. trifarius. Weather parameters associated with light conditions had no significant effect on growth, which probably reflect that D. trifarius in two shallow ponds were not light limited. The nutrient stoichiometry showed that phosphorus (P) contents in the tissue were low (0.04-0.11% DW), and nutrient resorption efficiencies of P amounted to 11-29%. This suggests that D. trifarius was P limited during its growth season, but appears capable of nutrient translocation across annual segments, possibly to maintain growth in oligotrophic environments. Despite low nitrogen (N) contents (0.94-2.09%), no resorption of N was found, which indicates that D. trifarius was not N-limited in order to sustain growth. In conclusion, this study shows that growth of D. trifarius in small high Arctic ponds are mainly controlled by summer temperatures.