Macrophyte performance in a low arctic lake: effects of temperature, light and nutrients on growth and depth distribution

Submerged macrophytes are important contributors to primary production in clear water arctic and low arctic lakes. However, their production and potential coverage are hampered by the harsh climate conditions. In this study, we investigated the potential for increased macrophyte production and coverage in arctic lakes in a future warmer climate. In situ growth experiments with Callitriche hamulata were performed at 2, 4, 8 and 12 m depth in combination with nutrient assay experiments at 2 m depth. In addition, growth experiments were performed in the laboratory at four temperatures (5, 10, 15 and 20 degrees C) under saturated and light-limited conditions (150 and 25 mu mol m(-2) s(-1)). The results show that macrophyte growth in the low arctic lake, Badeso, is phosphorus limited, but they also indicate that nutrients are not the limiting factor for the macrophyte depth distribution. Rather the short growing season combined with low summer temperatures may limit the expansion of C. hamulata. Our study also shows that C. hamulata is a very temperature-sensitive plant, particularly around 10 degrees C. In a future warmer climate in the arctic, the thermocline in clear lakes is expected to expand deeper into the water column. Thus, at the present light conditions we can expect an expansion of both colonisation depth and coverage, which may affect the overall primary production in arctic lakes and the carbon flux and carbon cycling in the lake systems. Our findings strongly support recent predictions of increased growth and a more northerly distribution range of cold-temperature submerged macrophytes.