Spatial and Temporal Dynamics of Water Hyacinth and Its Linkage with Lake-Level Fluctuation: Lake Tana, a Sub-Humid Region of the Ethiopian Highlands

Abstract

Water hyacinth originated from the Amazon Basin and has expanded to other parts of the world since the 1800s. In Ethiopia, the weed is affecting the socio-economic activities of the people whose livelihood is directly or indirectly dependent on Lake Tana. Still, the area covered by water hyacinth and the impact of water level fluctuation on the expansion of water hyacinth has not been known clearly. Therefore, the main objective of this study was to determine the spatiotemporal distribution of water hyacinth and relation with lake-level fluctuation. The area covered by water hyacinth was determined using monthly Sentinel-2 images, which were collected from November 2015 to December 2019. The impact of water level fluctuation on the expansion of water hyacinth was evaluated using hourly water level data converted to a monthly average to correlate with the area covered by the water hyacinth. In addition, MOD13Q1.006 data was used to evaluate the trend of the Normalized Difference Vegetation Index (NDVI) and its linkage with the weed. The maximum areas covered by water hyacinth were 278.3, 613.6, 1108.7, 2036.5, and 2504.5 ha in Feb 2015, October 2016, September 2017, December 2018, and in December 2019, respectively. Its areal coverage was declining from the northern corridors and increasing in eastern shores of the lake. The lake-level fluctuation was observed in the range of 1.5 to 3.98 m in this study. The annual mean maximum spatial values of the NDVI were in the range of 0.27 and 0.47. The area covered by water hyacinth was increasing significantly (P < 0.05) and positively correlated with the seasonal lake-level fluctuation. High water level enabled the expansion of the weed by extending its suitable habitat of shallow water to the flood plain. Based on the results of this study, lake-level fluctuations can have an adverse impact on the expansion of the weed.

Socio-ecological impacts of water hyacinth, Eichhornia crassipes (MART.) in Lake Tana, Gulf of Gorgora, Ethiopia

“…accidental (through flood, boat transport and fishing gears) and intentional (due to its ornamental and aesthetical value) introduction of the species are considered as the main routs of infestation into Ethiopian highland lakes (Dersseh et al, 2020). Once the species is established, it has a strong ecological and socio-economic impacts in the vicinity.…”

“…Once the species is established, it has a strong ecological and socio-economic impacts in the vicinity. Ecologically, its infestation in the form of floating dense mat leads to drastic deterioration of water quality such as decline in the amount of dissolved nutrients, temperature, pH and reduced dissolved oxygen in the water column leading to mass destruction of the fish fauna and decline in net primary production of the system (Chapungu et al, 2018;Dersseh et al, 2020;Enyew et al, 2020). Moreover, it affects fishing, high accumulation of organic matter and higher biological oxygen demand, less phytoplankton abundance and diversity, outcompete other riparian and aquatic vegetations.…”

“…The lower number of species recorded in the infested site (Table 2 and 3) could possibly be ascribed to the low light incident striking the water surface availability which is blocked by the canopy cover of the macrophyte. Thus, it hampers rate of photosynthesis resulting in limited growth and reproduction (Dersseh et al, 2020).…”

“…Vegetative structure of the hydrophyte consists of a rosette of six to ten leaves attached to a rhizome with a welldeveloped fibrous root system where reproduction can be both sexual and asexual (Jones et al, 2018). Since the late 1800's anthropogenic activities play a significant role in the spreading of the plant for ornamental purpose due to the inflorescences of flowers (Admas et al, 2017;Dersseh et al, 2019;Dersseh et al, 2020). Water hyacinth is an invasive species that has invaded freshwater systems in over 50 countries on five continents.…”

“…However, there is no clear record of how, why and when water hyacinth was introduced to water bodies outside of its native range. In Africa, the species was first introduced in Egypt between1879 and 1882 and then observed in White Nile in 1958; consequently, it was infested in the rest of Nile River and other East African regions aided through flooding and boat transport (Yifru et al, 2007;Dersseh et al, 2020). In Ethiopia, infestation of water hyacinth was first reported in 1965 in Lake Koka and its main tributary River Awash (Yifru et al, 2007;Admas et al, 2017;Dersseh et al, 2019).…”