Chara hispida forms dense beds (0.78–0.95 kg DW m 2) in Colgada Lake. The ability of Chara meadows to act as a nitrogen source or sink was evaluated by the following methods: (1) investigating Chara growth, (2) nitrogen incorporation and decomposition laboratory experiments and (3) relating experimental results to field conditions. Sediment oospores were germinated in large aquaria and observed growth rates were 0.001 m day 1 (shoot length) and 0.0002 g day 1 (dry weight). Nitrogen uptake rates were determined by addition of K15NO3 during two different periods of Chara growth and the rates were 1.21 and 3.86 mM g DW 1 h 1 when charophytes were 166 days old (not sexually mature) and 323 days old (sexually mature), respectively. After the uptake experiments, the same charophytes were allowed to decompose within two types of litter bags (3 mm-pore litter bags and entire, non-porous plastic litter bags). Decomposition rates of Ch. hispida were 0.016 and 0.009 day 1 in perforated and non-perforated bags, respectively, and fit a negative exponential model. The nitrogen release rate, calculated as the disappearance of N content from Chara tissues, was 0.012 day 1 and there were no statistically significant differences between the values from the two different bag types. The dissolved organic nitrogen concentrations in aquarium and non-perforated litter bags waters increased linearly with time due to the leaching of soluble compounds from Chara. The rate of N loss from Chara tissues, total nitrogen and dissolved organic nitrogen release rates and the decrease in initial dry weight rate were all lower than the daily rate of Chara N uptake. By extrapolating laboratory data to field situations, we determined that approximately 38% of the N taken up by charophytes in Colgada Lake during the growth period is retained. Given the high charophyte biomass in the lake, its ability to incorporate nitrogen, its low decomposition rate and its ability to over-winter, we conclude that Chara beds could be acting as nitrogen sinks in this ecosystem.

The authors wish to thank the Spanish Ministry of Education and Science for funding the research projects (REN2002-00558 and CGL2006-2346).We are also very grateful to C. Caballero, L. Cabrera, S. Pin˜eiro, J. Larrosa, E. Lo´pez Delgado and J.C. Rodrı´guez-Murillo

8 pages, and tables stadistics, and figures.

Peer reviewed