Macrophytes Community Structure, Relation to Sedimentation, Impact on Macro-Invertebrates and Level of Heavy Metal Contamination in Lake Koka, Ethiopia
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Date
2020-01-01
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Addis Ababa University
Abstract
Lake Koka is a multipurpose artificial lake in the Ethiopian Rift Valley and it is among the first to be invaded by Eichhornia crassipes (water hyacinth). The lake is under pressure from intensive anthropogenic activities and sedimentation problem. Like most Ethiopian water bodies, previous studies on the lake focused on plankton, fish and pollution neglecting the importance of macrophytes in the lake ecology. Thus, this study aimed at assessing the macrophyte abundance and diversity, efficiency of the dominant macrophytes in trapping sediment, impact of Eichhornia crassipes on water quality and macro-invertebrates and heavy metal contamination level in Lake Koka, Ethiopia. Data for macrophyte community was collected during the dry and wet seasons of 2017 using a line transect method and sediment trap experiment was conducted from July to September, 2017 to assess the efficiency of macrophytes in reducing sedimentation problem. The impact of E. crassipes on macro-invertebrates of the lake was investigated by collecting macro-invertebrates from E. crassipes mats and Echinochloa stagnina stand using a D-frame net (500μm). Heavy metal contamination of the lake was investigated by collecting water, sediment and E. crassipes organs (leaves and roots) samples from the lake and inflowing rivers and analyzing using ICP-OES. Data for physicochemical
parameters were collected by in situ measurement and laboratory analysis according to standard methods.
A total of 28 macrophyte species belonging to 15 families were identified. The most dominant families were Asteraceae and Cyperaceae each represented by 6 species. E. crassipes was the dominant macrophyte followed by Leptochloa caerulescens, E. stagnina, Cyperus dives and Typha angustifolia. The dry season had significantly higher species richness (28) and diversity (H=1.00) (p<0.05) than the wet season and most families were absent during the wet period. RDA analysis indicated that conductivity, pH, and nitrate were the main factors that governed the distribution of macrophytes in the lake. The result from the sedimentation study indicated that all macrophytes significantly facilitated sedimentation and reduced re-suspension than the non-vegetated area (P < 0.05) with the order E. stagnina = T. angustifolia > E. crassipes = L. caerulescens for sedimentation and E. crassipes = L. caerulescens > E. stagnina = T. angustifolia for re-suspension. So development and maintenance of wetlands from E. stagnina and T. angustifolia after conducting further studies on the effect of wind velocity and direction on their efficiency might be used as a biological management option for sedimentation problem. Except its significant reduction of macro-invertebrate evenness, E. crassipes did not affect water quality of the lake and macro-invertebrate diversity. But to have a full picture of its impact on the lake ecosystem, its effect on macrophytes, fishes and other aquatic lives should be investigated.
Heavy metal levels in water samples from Modjo downstream (MODD) site were significantly higher than the other sites while there was a uniform distribution of all the
metals in the lake sites. Levels of As, Pb, Cr, Mn and Ni at most sites were beyond the WHO guideline limit for drinking water while As, Cd, Cr, Cu, Pb and Zn at all sites were beyond the threshold values for the protection of aquatic life. Similarly the concentration of heavy metals in sediment samples from MODD had significantly higher concentration of Cr and Mn than most sites on the lake and there was uniform distribution of As, Co, Cr, Mn, Pb and Zn on the lake sites. Levels of As, Cd, Cr, Cu, Mn, Pb and Zn from sediments samples of Lake Koka and its inflowing rivers were beyond the sediment quality guideline suggesting the possibility to pose a significant effect to aquatic organisms. Hence profound measures against a further pollution and continuous monitoring of the heavy metal concentrations of the lake are recommended. Heavy metal analysis of leaf and root material proved E. crassipes to be a high accumulator of most measured metals with the concentrations in the roots being significantly higher than those in the leaves. Therefore, while considering its appropriate disposal, E. crassipes can be recommended for utilization as phytoremediation agent and bio-indicator of heavy metal pollution in the lake. Further investigation on the efficiency of the dominant macrophytes in the lake as a phytoremediation agents is necessary.
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Keywords
Eichhornia Crassipes, Heavy Metals, Macro-Invertebrates, Macrophytes, Phytoremediation, Sedimentation