GROWTH RECONSTRUCTION AND PHOTOSYNTHESIS OF AQUATIC MOSSES - INFLUENCE OF LIGHT, TEMPERATURE AND CARBON-DIOXIDE AT DEPTH

1 The mosses Sphagnum subsecundum and Drepanocladus exannulatus domina te the vegetation in the oligotrophic, softwater Lake Grane Langso, De nmark, even at great depths where light and temperature are low. We us ed seasonal changes in morphology to reconstruct the annual growth and the longevity of the mosses and measurements of photosynthesis and re spiration to evaluate the importance of light, temperature and CO2 for the growth patterns at depth in the lake. 2 The reconstruction techni que revealed that the mosses had a relatively fast growth rate (90-250 mm shoot(-1) year(-1)) and were short lived (0.7-2.9 years). The shoo ts of both moss species grew faster in deep than in shallow water. Gro wth experiments in summer confirmed that Sphagnum grew more slowly and decayed more rapidly in shallow than in deep water. 3 Fast growth of mosses in deep waters can be accounted for by lower temperature, exten sive CO2 supersaturation and nutrient enrichment in the hypolimnion du ring summer stratification. Maximum rate of light-saturated photosynth esis in July was 3.3-fold higher and of dark respiration 1.3-fold lowe r in Sphagnum from 9.5m incubated at the ambient 8 degrees C than in S phagnum from 0.7m incubated at 20 degrees C. The net daily carbon fixa tion was greater in deep than in shallow water despite the much lower irradiance at depth. Extensive CO2 supersaturation stimulated photosyn thesis several-fold relative to the rates observed in air-saturated wa ter. Tissues of Sphagnum were richer in nitrogen in deep than in shall ow water during summer, but the importance of nutrient availability to annual moss growth remains unclear. 4 Reconstruction techniques are r ecommended for comparative studies on annual and interannual growth pa tterns of mosses within lakes and among lakes of different altitude, l atitude and water chemistry. This information can be based on just a s ingle collection and can therefore include remote sites with adverse c limate.