Optimum water depth ranges of dominant submersed macrophytes in a natural freshwater lake.

Macrophytes show a zonal distribution along the lake littoral zone because of their specific preferred water depths while the optimum growth water depths of dominant submersed macrophytes in natural lakes are not well known. We studied the seasonal biomass and frequency patterns of dominant and companion submersed macrophytes along the water depth gradient in Lake Erhai in 2013. The results showed that the species richness and community biomass showed hump-back shaped patterns along the water depth gradient both in polydominant and monodominant communities. Biomass percentage of Potamogenton maackianus showed a hump-back pattern while biomass percentages of Ceratophyllum demersum and Vallisneria natans appeared U-shaped patterns across the water depth gradient in polydominant communities whereas biomass percentage of V. natans increased with the water depth in monodominant communities. Dominant species demonstrated a broader distribution range of water depth than companion species. Frequency and biomass of companion species declined drastically with the water depth whereas those of dominant species showed non-linear patterns across the water depth gradient. Namely, along the water depth gradient, biomass of P. maackianus and V. natans showed hump-back patterns and biomasses of C. demersum displayed a U-shaped pattern in the polydominant communities but biomass of V. natans demonstrated a hump-back pattern in the monodominant communities; frequency of P. maackianus showed a hump-back pattern and C. demersum and V. natans maintained high frequencies in the two types of communities. We can speculate that in Lake Erhai the optimum growth water depths of P. maackianus and C. demersum in the polydominant communities are 2.5-4.5 m and 1-2 m or 5-6 m, respectively and that of V. natans is 3-5 m in the polydominant communities and 2.5-5 m in the monodominant communities. This is the first report that the optimum water depth ranges in the horizontal direction of three dominant submersed macrophytes in a natural freshwater lake were determined.