Effects of hyposalinity in Thalassia testudinum (Hydrocharitaceae) from Parque Nacional Morrocoy, Venezuela

The year 1996 had a high pluviosity in Morrocoy National Park (western coas tal zone, Venezuela) and low salinity in December 1996 affected the seagras s beds, dominated by Thalassia testudinum. Patches without T. testudinum we re observed in localities of the park that used to have very dense populati ons of this plant. Sampling was done at Las Luisas to determine leaf produc tivity, turnover rate, short shoot density and relative biomass of plant se ctions, in order to compare with data obtained in September 1996, previous to the event. Green leaves, roots and rhizomes were the most affected parts . Mean green leaf biomass decreased in January and February 1997 to 5 % of the plant's total biomass; mean root biomass decreased in March to 40 % and mean rhizome biomass decreased in February to 30 %. The density of the act ive short shoots decreased to a minimum in February, but in April it reache d a value similar to that of September 1996 The density of the inactive sho rt shoots decreased to a minimum in March, and they dissappeared in April, matching the increasing density of the active short shoots between these tw o months. In February 1997, 56 % of the inactive short shoots showed eviden ce of leaf initiation. In January 1997 the leaf productivity and turnover r ate values (2.72 +/- 0.35 g/m(2)/d and 2.15 % leaf DW/d) were similar to th e annual mean previously determined from Las Luisas (2.35 +/- 0.72 g/m(2)/d and 1.96 % leaf DW/d). Nevertheless, the values of productivity and turnov er rate detected at Las Luisas in April 1997 (4.88 +/- 2.14 g/m(2)/d and 4. 66 % leaf DW/d) were higher than those values previously reported for this location. In response to the mortality episode, the leaf productivity and t urnover rate of T. testudinum increased and the leaf initiation was activat ed in the inactive short shoots.