Dynamics and stability of Chara sp. and Potamogeton pectinatus in a shallow lake changing in eutrophication level

The submerged vegetation in Veluwemeer (The Netherlands) has shown large changes over the past 30 years. Potamogeton pectinatus L. remained present in the lake even during the hypertrophie period with total phospohorus (P) levels of about 0.6 mg P 1−1. Furthermore, the data suggest that there may be one critical level of Secchi depth for the presence of charophytes (about 0.4 m), whilst there may be two critical P levels (0.3 mg P 1−1 for loss and 0.1 mg P 1−1 for their return). Logistic regression of detailed vegetation maps over the past 10 years (1988–1997) showed that occurrence of Chara spp. was positively related with spring averages of Secchi depth, while the occurrence of P. pectinatus was slightly negatively related with Secchi depth. Both species had a negative sigmoid response to water depth. Sites without vegetation can be easily predicted by water depth and Secchi depth (97% for Chara and 99% for P. pectinatus correctly predicted cases). The sites where the species will appear are difficult to predict (50% for Chara and 3% for P. pectinatus correctly predicted cases, respectively). If the vegetation of a year prior to the census was included as variable, the predictability of the occurrence of P. pectinatus and Chara can be improved to 71% and 53% correctly predicted cases, respectively. Grid cells which had been covered 1 year earlier with dense Chara vegetation showed a different relationship with Secchi depth than the grid cells without earlier cover. Previous dense Chara (>50% cover) stands returned always in the subsequent year independent on water depth or Secchi depth. Measurements in 1995 showed that dense charophyte beds decrease the turbidity to below 1 m−1 (estimated light attenuation, K d ) in high summer, while outside the vegetation the water remained turbid (estimated K d ~3 to 5 m−1). The positive effect on transparency, together with formation of dense propagule banks, may stabilize the Chara vegetation. Dominance of P. pectinatus may be restricted to turbid water, since Chara appears to be a stronger competitor than P. pectinatus in clear water.