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 matforming plants in low-energy environments and non-mat-forming plants in high-energy environments.

1. The major optical components controlling the attenuation of photosynthetic available radiation in nearshore areas of Lake Victoria (Uganda and Kenya) were examined and their impact compared. It was found that chromophoric dissolved organic matter and tripton play a dominating role in many nearshore areas, indicating that the coastal areas of Lake Victoria cannot be considered as Case I waters.2. Concentrations of chromophoric dissolved organic matter declined with distance from the coast in an exponential manner indicating dilution and degradation of terrestrial sources of organic matter rather than in situ production. The importance of tripton was found to follow a similar pattern, while the relative importance of phytoplankton biomass in overall attenuation of photosynthetic available radiation was found to increase with distance from the coast. A specific attenuation coefficient for phytoplankton biomass was determined (0.019 m 2 mg Chl a )1 ). 3. Using a light limitation approach based on carrying capacity, it was possible to map areas that are closer to being light limited. Light limitation appears to occur throughout most bays and some coastal areas receiving catchment waters. This spatial information, geographically referenced to bathymetric and catchment conditions, was utilized to understand the importance of environmental conditions in limiting phytoplankton biomass.

Abstract:We investigated the presumption that wind-wave exposure is a major regulator of vegetation distribution within lakes. Along a 675-km stretch of shore in northern Lake Victoria (Uganda), the pattern of vegetation distribution in relation to shoreline features, and the variation of shoreline swamp area along a gradient of wave exposure were examined. The ability of wave exposure, when combined with bay morphometric characteristics, to predict the lakeward limit of vegetation distribution was assessed. Data were collected through a shoreline survey and from maps. Maximum effective fetch, computed from topographic maps, was used as a surrogate for wave exposure. Our results reinforce and amplify the notion that wave exposure is an important regulator of the within-lake distribution of vegetation. We found shoreline plants to either occupy stretches of shore shielded by coastal islands or hidden by convolutions of the lake margin. The area of shoreline swamps declined exponentially with increasing wave exposure. Of the coastal characteristics examined, bay area had the strongest influence on the lakeward expansion of vegetation. Wave exposure acting together with bay area, accounted for 64.4% of the variance in the limit of lakeward vegetation advancement.

[1] Temperature-conductivity diagrams are shown as a valid instrument to analyze the hydrographic structure of freshwater ecosystems, even along the surface waters. We put this method in practice in the Ugandan inshore waters of Lake Victoria. A complementary parameter (T-C anomaly) was used to differentiate between upland water intrusions. The relative value of the T-C anomaly provided information about the nature of the water intrusions and showed a considerable correlation with the biological characteristics of the water masses. The results indicated that the connections between catchment attributes, water characteristics, and biological community are quite direct in the inshore waters of Lake Victoria.

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