Biotic and abiotic factors driving water hyacinth, Pontederia crassipes, phenology in Nyanza Gulf, Lake Victoria, Kenya

Abstract

While isolated patches of water hyacinth have persisted in most parts of Lake Victoria since the rapid decline in biomass beginning in 1999/2000, Nyanza Gulf has maintained extensive but cyclical coverage over the years. High water hyacinth productivities were found to coincide with the western region associated with upwelling and regenerated nutrients, and also with periods of complete lake turnover. River discharge sites, which are numerous in the eastern region, expressed relatively low water hyacinth productivities. High productivity meant better nutritional quality, a state preferred by the Neochetina weevils. Although the difference in productivities between the eastern and western regions were statistically insignificant, the feeding scars were notably different. This implies that any slight change in water hyacinth’s nutritional quality as a result of change in resource water quality, significantly affects the weevil’s productivities. During complete lake turnover, water hyacinth productivity is highly enhanced, likely due to upwelling of hypolimnetic nutrient enriched waters with subsequent spontaneous weevil invasion. During times of such high nutrient quality, the rates of leaf turnover outstrips the herbivory by the weevils. The nutrient dynamics from primary sources coupled with secondary regenerated loads through lake turnovers influences the water hyacinth’s varying nutritional quality and weevil population, which ultimately determines the water hyacinth phenology within the semi-enclosed Nyanza Gulf. The objectives of this study were to study the spatial influence of resource water quality to the consumer water hyacinth productivity and how temporal hydrologic dynamics drives its cyclicity.