Effect of wave exposure on vegetation abundance, richness and depth distribution of shallow water plants in a New Zealand lake

1. The effects of physical disturbance in terms of wave exposure, shore slope and substrate mobility on the presence, species richness, cover and depth limits of the low-growing, shallow water macrophyte community ( called the low mixed community) were examined at 41 shore sites in Lake Wanaka, South Island, New Zealand. 2. Wave exposure at a site was described by the previous year's maximum depth of sediment motion and maximum vertical extension of waves, determined from a computer wave model. Shore slope at each site was recorded from) 1 to 0 m depth, and sediment stability was assessed as the cover of small gravel. 3. The low mixed community was only present on sites where the previous year's maximum depth of sediment motion was < 8 m, maximum wave run-up was < 0.3 m, shore slope < 0.12 m m(-1), and small gravel cover < 78%. 4. Species richness, cover and depth limits of the low mixed community decreased with increasing disturbance on the sites. Sixty-two percentage of the variation in species richness could be explained by physical disturbance variables when all sites were included (N = 41). When only sites with a low mixed community were included (N = 22), only 18% of the variation was explained. Species richness within sites supporting a low mixed community is thus poorly explained by physical disturbance, whereas presence or absence is better explained. There was no evidence to support the intermediate disturbance hypothesis in this study. 5. Disturbance as a result of waves explained 86% of the variation in cover among sites with a low mixed community and accounted for 68 and 58% of the variation in upper and lower depth limit of the low mixed community, respectively. 6. The models obtained in this study can be used as predictive models for the low mixed community in New Zealand lakes in relation to natural physical disturbance on the shore. By integrating these results with previous studies on the effect of water level fluctuation, we describe a generalised optimum physical habitat for the low mixed community in New Zealand lakes.