Physical and chemical characteristics of the Tanzanian inshore and offshore waters of Lake Victoria in 2005–2008

Abstract

Assuming that the inshore and offshore waters of Lake Victoria are impacted differently by human activities in its catchment, this study investigated the water quality dynamics of the lake. A total of 29 stations were sampled in 2005-2008 for dissolved oxygen (DO), pH, Secchi transparencies, temperature, turbidity, chlorophyll a, NO 3 , SRSi, TN and TP. There was a decreasing trend of the measured parameters towards offshore sites, except for Secchi transparency and NO 3 , which increased towards the offshore waters. DO concentrations (mean ± SD) varied between 6.97 ± 0.57 mg O 2 l -1 and 5.80 ± 0.72 mg O 2 l -1 in the inshore and offshore waters, respectively. Turbidity values were comparatively higher in the inshore (3.73 ± 2.21 NTU) than the offshore waters (2.19 ± 1.81 NTU). Chlorophyll a concentrations varied between 17.36 ± 6.13 μg l -1 and 8.09 ± 4.38 μg l -1 in the inshore and offshore waters, respectively. Increases of unsustainable human activities in the lake and its catchments, plus increased degradation of wetlands, are among the causes of the observed water quality changes. In order to be fruitful and sustainable, the management of Lake Victoria and its catchment needs to take an ecosystem approach, and to involve all key stakeholders.

Spatio‐temporal trends of nutrients and physico‐chemical parameters on lake ecosystem and fisheries prior to onset of cage farming and re‐opening of the Mbita passage in the Nyanza Gulf of Lake Victoria

“…(2013). The SRSi and chlorophyll‐ a concentration are higher than those reported for the Tanzanian inshore and offshore waters (Ngupula, Ezekiel, Kimirei, Mboni, & Kashindye, 2012), with an SRSi range of 0.2 ± 0.1 mg/L to 1.7 ± 0.2 mg/L a chlorophyll‐ a range of 2.3 ± 0.1 µg/L to 32.0 ± 3.6 µg/L), with a comparatively lower range of nitrate concentration of 60.1 ± 4.6 µg/L to 201.5 ± 46.3 μg/L. Reported median values of the TN:TP ratios in the study area (Sitoki et al., 2012) range from 6.1 to 10.3 (gulf) and 14.2 (main lake), being considered lower than the 20:1 ratio at which N generally becomes limiting for phytoplankton growth (Guildford & Hecky, 2000).…”

Dynamics and limitations of phytoplankton biomass along a gradient inMwanzaGulf, southernLakeVictoria (Tanzania)

“…() in Mwanza Gulf and by Ngupula et al . () in Tanzanian waters, but were in general two‐ to threefold lower than in highly eutrophic gulfs in Uganda, such as Napoleon and Murchison Gulf (Lehman & Branstrator, ; Mugidde, ; Yasindi & Taylor, ; Silsbe et al ., ; Cózar et al ., ; North et al ., ; Okello et al ., ; Haande et al ., ). The decrease in chlorophyll concentration from shallow to deep water suggests that nutrients in shallow water were in excess, enabling a high chlorophyll concentration until light became limited.…”

“…Furthermore, phytoplankton dynamics have rarely been studied for southern Lake Victoria. The few studies conducted have focussed on large spatial scales between Gulfs or depth strata covering half the lake (Shayo, Lugomela & Machiwa, ; Ngupula et al ., ). Therefore, this study focussed on the dynamics of phytoplankton biomass and water quality on a small spatial scale in the Mwanza Gulf in southern Lake Victoria, covering a depth gradient from 3 m in inshore water in the south to 30 m in offshore water near the entrance of the Gulf.…”

How Does Increased Eutrophication and Pollution in the Lake Victoria Waters Impacts Zooplankton?

“…The quality of water in Lake Victoria (the largest in Africa and the second largest in the world) ecosystem has been deteriorating fast, due to mainly increased disturbances in its catchments (Sitoki et al, 2010;Ngupula et al, 2012). The impacts of the observed deterioration have been observed mostly on changed composition and abundance of plankton and benthic macroinvertebrates, leading into dominance of robust and less important species.…”