Decomposition rates and nutrient Leaching efficacy of the Dominant Macrophytes in Lake Ziway, Ethiopia

Document Type: Primary Research paper

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Abstract

Decomposition of macrophytes is an important process that facilitates nutrient and carbon cycle in aquatic ecosystems. The role of macrophytes decomposition in nutrient dynamics of a lake was studied in slightly alkaline tropical lake, Lake Ziway, using laboratory and field incubation experiment (litter bag technique) between January to October 2013. The objective of this study was to assess the role of macrophytes to nutrient cycle and organic matter accumulation in Lake Ziway.  Potamogeton schweinfurthii decomposed faster (K= 0.0409 d-1) than other macrophytes while Arundo donax was the most resistant to decaying (K = 0.0073 d-1). Net increase in total nitrogen concentration was observed for the macrophytes’ litters during field decomposition experiments. Increase was highest (4.38 fold) for Cyperus papyrus. Phosphorous concentration decreased for all macrophyte litters, except Cyperus articulatus. The highest reduction was observed for P.schweinfurthii (82.6%). Laboratory incubation experiments were also done by incubating macrophyte litters with the lake water in bottles and an increment in soluble reactive phosphate concentration was observed at the end of the experiments except in two bottles, but the opposite trend was observed for nitrate in all bottles. The overall results of the experiments showed that most of the macrophytes decomposed faster and serve as net source of phosphorous, although the contribution of decomposition of these macrophytes to the change of water quality of the lake seems to be low. On the other hand, A. donax plays little role in the dynamics of major nutrients and may contribute to organic matter accumulation in the lake. Both laboratory and field experiments showed that all the macrophytes, except C. articulatus, served as net SRP to the lake system. In spite of higher decomposition rate of the macrophytes in the lake, the contribution of decomposition in changing the physico-chemical condition of the lake is still insignificant.

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