In Lake Victoria, an invasive species is covering the water’s surface with green mats and blocking plants underneath it from direct sunlight, causing their death and many other related problems such as decrease in water quality and spread of waterborne disease. This species, the water hyacinth, has been a great puzzle for scientists. The original method of biological control, weevils (or Neochetina spp.), were introduced to reduce the water hyacinth population in 1995. A sharp decline of water hyacinth in 1998 and 1999 and again in 2000 is attributed to the weevils’ destruction by Wilson et al. (2007). However, according to a rebuttal article from Williams et al. (2007), the weather patterns brought by El Niño in 1997 and 1998  were mostly responsible for the water hyacinth’s decline.

According to Wilson et al. (2007), it is apparent that weevils caused the decline of the water hyacinth population because after their introduction, they normally take about three to four years to act, consistent with their introduction in 1995 and the decline in 1998 and 1999. They also claim that the resurgence shown on their graph of Lake Victoria hyacinth levels in 2000 occurred simultaneously with a low weevil population.

In contrast, Williams et al. (2007) argue against the claims made in Wilson et al. (2007). An interesting fact cited by Williams et al. states that El Niño caused a 1.70 meter rise in Lake Victoria’s water levels, which led to stable shoreline stands of water hyacinth washing into the lake. This compelling evidence, as well as individualized graphs for each region of the lake and explanations of how low light levels from El Niño’s stormy effects combined with other factors, including biological control using weevils, reduced water hyacinth populations, solidify Williams et al.’s conclusions.

Considering MODIS satellite images from 2005 and 2006, the re-invasion of water hyacinth adds another piece to the puzzle. The NASA Earth Observatory (2007) states that the reappearance of the invasive species coincided with extremely heavy rains. Reexamining Wilson and Williams’ claims, it is clear that Williams et al.’s (2007) worries about stormy El Niño and cloud cover are relevant in this situation as well. In my opinion, this correlation of information between two sources in two different time frames shines in favor of Williams’ argument. Also, Williams et al. (2007) use more area-specific graphs which seem to more accurately depict the water hyacinth problem. I agree with Wilson et al. (2007) that biological control is essential in controlling the water hyacinth invasion, but Williams et al. (2007) present more convincing data. Hopefully, we will be able to find the ideal balance and solution to maintain control in Lake Victoria.

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.