A HYDROLOGICAL MODEL FOR PREDICTING THE EFFECTS OF DAMS ON THE SHORELINE VEGETATION OF LAKES AND RESERVOIRS

The species richness of shoreline vegetation of unregulated lakes in N ova Scotia, Canada, is known to increase as a function of catchment ar ea, a topographic variable governing water level fluctuations. Predict ions based on catchment area however, fail to account for richness pat terns at the margins of lakes enlarged by darns. Here, we compare the vegetation and hydrological regimes of regulated and unregulated syst ems. Hydrological regimes of regulated systems deviated from natural s ystems of similar catchment area by being either hypovariable or hyper variable for both within-year and among-year fluctuations in water lev el. Plant communities of dammed systems were less diverse, contained m ore exotic species, and were, with one exception, devoid of rare shore line herbs. Data from ''recovering,'' or previously dammed systems ind icated that shoreline communities can be restored upon return of the a ppropriate hydrological regime. Using observed within-year and among-y ear water level fluctuation data, we propose a general model for the m aintenance or restoration of diverse herbaceous wetlands on shorelines of temperate lakes or reservoirs. Managers can manipulate the within- year water level variation within prescribed limits (1-2 m), while ens uring that among-year variation (SD of summer levers) is less than 25% of within-year variation. This preliminary model is based on data fro m low-fertility temperate lakes in river systems. To calibrate the mod el, plant community data from other regions are needed, as are long-te rm water-level data for unregulated lakes, data which are essential bu t largely lacking in many areas.