Since 1989, the water hyacinth has been extremely destructive to both the species of Lake Victoria and the humans who depend upon the lake for food and water.  However, for roughly the last decade, the density of the water water hyacinth has decreased.  The reasoning behind this decrease has been hotly contested.  According to Wilson et al. (2007), the decline of the water hyacinth is the result of biocontrol.  On the contrary, Williams et al. (2007), attributes climate change caused by El Nino to be the main destroyer of the water hyacinth.

Wilson et al. (2007) believes that the introduction of the weevil in 1995 has played a pivotal role in the reduction in the number of water hyacinth.  Although the study does not deny that El Nino weather pattern in the first of 1998 help decrease the size of the water hyacinth infestation in Lake Victoria, the study also points to the fact that after El Nino, the water hyacinth population once again grew until 2000, at which point the weevil population grew large enough to be able to control the water hyacinth problem.  Williams et al. (2007) counters that yes, the weevil is helping fight the water hyacinth, but  that if it were not for El Nino in 1998, the same dramatic decrease in water hyacinth would not have occurred.  Additionally, Williams et al. (2007) cites that in 2006, the water hyacinth has made a return, as seen from satellite images of Lake Victoria (NASA Earth Observatory  2007).  And argues that if the weevil was the reason behind the decline of the water hyacinth, the 2006 resurgence would not have occurred.

I agree more strongly with the Wilson et al (2007) study.  This study concedes that the drop in the water hyacinth in 1998 was mostly due to El Nino, but then goes on to explain why it believes the weevil is now the dominant force driving down the water hyacinth population.  Williams et al (2007) fails to give reasoning for why, even after the 1998 El Nino, the water hyacinth population has not experienced the same expansion rate that it did prior to El Nino.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 2010.

Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Water hyacinth decline across Lake Victoria – Was it caused by climatic perturbation or biological control? A reply. Aquatic Botany 87:94-96.

Wilson, J. R. U., O. Ajuonu, T. D. Center, M. P. Hill, M. H. Julien, F. F. Katagira, P. Neuenschwander, S. W. Njoka, J. Ogwang, R. H. Reeder, and T. Van. 2007. The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp. Aquatic Botany 87:90-93.