Floating mats: their occurrence and influence on shoreline distribution of emergent vegetation

1. A study was conducted on the northern shore of Lake Victoria (Uganda) to determine the factors controlling the occurrence of floating root mats and the influence of the floating mats on the distribution of emergent vegetation. 2. Environmental conditions within 78 bays in the study area were characterised using bay size, wave exposure, water depth, littoral slope, sediment characteristics and water level fluctuations. Emergent plants that form floating root mats occur along the shores of these bays. The way in which commonly occurring shoreline vegetation was distributed across a wave-exposure gradient was compared with their distribution across a water level fluctuation gradient. 3. Results suggested that wind-wave action and water level fluctuations are important factors determining the occurrence of floating mats. Mat-forming plants occur in the most sheltered locations along the shore and in waterbodies with modest water level fluctuations. 4. The ability to form mats facilitated the lakeward expansion of emergent plants. Plants forming floating root mats had a larger depth range than non-mat forming plants. 5. The initiation mechanisms for the floating mats of emergent vegetation in Lake Victoria appear to be: (i) invasion of mats of free-floating plants by emergent vegetation; and (ii) detachment of emergent plants from the lake bed following flooding. 6. The formation of floating mats comes with a cost and benefit to emergent plants. The cost is increased vulnerability to damage by water level fluctuations or wind-wave action, leading to reduced horizontal distribution. The benefit is that deep flooding is avoided, thus increasing vertical distribution. The net effect may be to lead to dominance of mat-forming plants in low-energy environments and non-mat-forming plants in high-energy environments.