Michael Di Nunzio

09/13/2010

Water hyacinth has posed a problem for Lake Victoria since first being reported there in 1989. The plant forms dense mats of vegetation that inhibit the movements of fishermen, block sunlight to native plants, and obstruct irrigation systems.  The invasive weeds can also deplete the water’s oxygen levels, suffocating the indigenous flora and fauna of the lake and in turn disrupting the local ecosystem. To control the hyacinth populations, invasive weevils (Neochetina spp.) were introduced with the intention of suppressing the noxious weed.

Using satellite image samples, Wilson et al. (2007) estimated the proliferation of water hyacinth over Lake Victoria and fluctuations in the plant’s presence over time. However, Williams et al. (2007) warns that this method of gathering data is oversimplified for such a complex environment. Regardless, the results indicated that hyacinth levels dropped after the 1998 El Niño disturbed the lake. Following the initial drop in 1998 came a steady rise until 1999 when hyacinth levels again began to decline dramatically. Hyacinth levels stayed at close to zero from 2000 until 2002.

Wilson et al. (2007) reasoned that the drop in 1999 was a result of the control weevils introduced in 1995 becoming effective after four years. They also note that the weevils used indirectly lower plant buoyancy as a means of sinking and controlling the weed, and that wind and wave action from El Niño could have facilitated this process. Because El Niño would inevitably blow some hyacinth into new areas, Wilson et al. (2007) suspected that local reports of hyacinth resurgences might have actually been false. Valid reports of resurgence may have resulted if weevils died due to a lack of buoyant hyacinth leaving the plant temporarily uncontrolled. According to Wilson et al. (2007), there is no substantive evidence to link low light levels with any of the withdrawals of hyacinth.

Williams et al. (2007) places less emphasis on the importance of the weevils in regards to water hyacinth control. Rather than weevils controlling the plants, El Niño more likely pulled hyacinth from the shoreline along with native plants and destroyed them in the lake with wave action. This would account for the decline in 1998. Furthermore, the current nadir in hyacinth is thought to be a fleeting byproduct of the weevils efficacy after 1999 and “suboptimal light.” Williams et al. (2007) points to the River Kagera as a source for resurgence, as hyacinth from this region is untainted with weevils and can float freely into the lake. This means that there is a delay before the biomass control can take effect.

While Wilson et al. (2007) offers the more optimistic outlook on the data set, Williams et al. (2007) is unfortunately the most realistic. Williams et al. (2007) provides the most coherent argument, and reasonably parallels the situation in Lake Victoria with that of sub-tropical climates plagued by water hyacinth. They assume that the current lack of hyacinth is a part of a cyclic process involving a balance between weevils and weeds that will invariably lead to hyacinth resurgences. Wilson et al. (2007) seems to make convenient excuses for all reported instances of resurgence, rather than offering any real insight into their possible validity. The satellite images from MODIS vindicate the argument of Williams et. Al (2007), as resurgence obviously took place by 2006. Thus the relationship between adequate light, the presence of weevils, and the predominance of hyacinth must be a continued subject of study at Lake Victoria if definite conclusions about the hyacinth resurgence cycle are to be drawn.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nas/IOTD/view.php?id=7426. Viewed 12 Sep 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