Abstract
One of the important functions of the wetland is the retention of catchment nutrients and improving lacustrine water quality. This study analyzed how much nutrients were retained in the Nyashishi wetland, southern part of Lake Victoria, and went further to analyze the processes which were responsible in the reduction of nutrients. Three major processes were analysed in this study, namely nutrients uptake by three macrophyte species (Eichhornia crassipes, Cyperus papyrus and Typha domingensis) dominating the Nyashishi wetland, nitrification and denitrification processes. The study demonstrated high nutrients retention especially phosphorus nutrients of which up to 98% were retained. In some occasions, particularly for nitrogen nutrients, there was 0% retention especially during wet season. In some other instances, the inflow exceeded the outflow meaning that, more nutrients were generated by the wetland itself. Among the three processes analyzed, nutrients uptaken by macrophytes were more efficient in reducing nutrients in wetland water. Biological nitrification and denitrification which are believed to be the major pathway for ammonia removal in both natural and constructed wetlands were less important in this study. Among the macrophyte species, Eichhornia crassipes demonstrated higher uptake rate than the other two species possibly due to its high turnover rate. This ability can be exploited in removing excess nutrients from runoff by frequent harvesting of the mature plants.
Highlights
Wetlands constitute an important part of the catchment area of Lake Victoria, and perform a number of beneficial roles that have direct consequences on the lake ecosystems [1]
Water samples for nutrients analysis were collected from the inlet (SSW) and outlet sites (OW), while the nitrification and denitrification experiments were conducted within the wetland (Swamp)
The electrical conductivity (EC) at station SSW ranged from 122.4 ± 4.1 μS/cm to 1107 μS/cm and at station OW the EC values ranged from 109.23 ± μS/cm to 364.3 ± 3.6 μS/cm The mean (±SE) percentage reduction of EC was 47.9% ± 11.1% in wet season and 74.5% ± 2.6% in dry season respectively
Summary
Wetlands constitute an important part of the catchment area of Lake Victoria, and perform a number of beneficial roles that have direct consequences on the lake ecosystems [1]. Through the uptake of nutrients by wetland sediments and plants, and by sedimentation of nutrient-rich particulate matter, wetlands can act as buffers against Lake Eutrophication [1] [3]. Another identified fate of nutrients once they enter the wetland is loss through degassing, and direct flow through the wetlands [2]. If wetland macrophytes are sustainably harvested, this could form a permanent removal mechanism for nutrients and other substances incorporated in the plant tissue. Several macrophytes, such as Water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes) and duckweed (Lemnaceae) [4] have been reported for the efficiency removal of nutrients
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