Will water hyacinth become established in Lake Ziway?

摘要

Several research projects by graduate students (mostly PhD) and staff are ongoing on water resources dynamics in Lake Ziway under Sub-Thematic Research 2 entitled “Enhancing peoples’ livelihoods in Lake Ziway through research and training in Aquaculture, Wetlands, Water quality and Adaptation and Mitigation of Climate Change”. The study on macrophytes (Work Package 5) focuses on the ecology and livelihood support of aquatic plants in Lake Ziway. Some studies on the biology of the major macrophytes   have   been   undertaken.   Another   focus   of   WP5   is development of monitoring tools for nutrient loadings in the lake, using diatoms,  and  lake  biotic  integrity monitoring  using  macro-invertebrate indices. Long-term observations in Lake Ziway have documented oligotrophication  trend  with  decreasing chl-a  levels, despite  increasing nutrient loadings. It was suggested that the excess nutrients in Lake Ziway could lead to the expansion of emergent macrophytes such as Arundo donax and possible establishment of floating ones (water hyacinth, Eichhornia crassipes). Similar condition   was noted in another shallow large lake   in 2008 and we predicted   the   potential establishment of water hyacinth in Lake Tana based on  three scenarios  (models) depicting

papyrus removals. Surprisingly most model predictions were proved correct as water hyacinth became established in the northern part of the lake in 2012 and expanded to cover more than 15% of the lake littoral. To assess whether a similar situation might happen in Lake Ziway, three nutrient loading models were considered, and while two models indicated that P-loading levels were below the growth requirement of water hyacinth in the lake yet (OECD and large watershed models) the third model (Vollenweider) projected that P-load was already twice more than growth requirement of the plant.  The paper cautions that already there are some signs of point source- nutrient pollution in Lake Ziway, such as near floriculture plants where water-lilies are flourishing and water hyacinth  is  already  established  in  the  nearby  Lake  Koka.  Hence  it concludes that regular monitoring for nutrients and macrophyte rehabilitation measures should be undertaken to avoid threat of possible establishment of water hyacinth in Lake Ziiway and prevent similar ecological crisis that was observed in Lake Tana.