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Abstract
Lake Wamala (Uganda) is a small shallow lake (maximum mean depth = 4.5m) that has periodically undergonefluctuations in depth and area because of changes in rainfall, temperature and wind speed. No study, however,has been done to assess how these changes, in addition to catchment degradation, which are likely to intensify in future, have affected aquatic productivity processes. Data on Secchi depth (SD), turbidity, conductivity, dissolved oxygen (DO), total phosphorous (TP), soluble reactive phosphorus (SRP), soluble reactive silicon (SRSi), ammonia (NH4-N), nitrite nitrogen (NO2-N), nitrate nitrogen (NO3-N), Chlorophyl a, composition and abundance of algae, and invertebrates were compared between periods 1998-2000 and 2011-2013. Results showed a twofold increase in conductivity and TP, a threefold increase in SRSi, chlorophyll a > 25 ?g l-1, and persistence by low water transparency (SD < 0.7 m). In consequence, algal biomass increased by >70%, with emergence of new species, especially among the dinoflagellate and euglenophyte groups. Although rotifers, which are known to have capacity to withstand stressful habitat conditions, dominated the zooplankton, their density decreased by >80%. The phantom-midge, chaoborus, and the midge, chironomid, larvae dominated macro-invertebrate, but did not show clear trends between the two periods.
Highlights
Physical and chemical environment of water, including water transparency, conductivity, dissolved oxygen and nutrients, plankton productivity, and feeding interactions determine composition and yield of fishes [1, 2]
Turbidity was 20.3±2.13 NTU and 14.3±2.84 NTU during July 2012 and July 2013, respectively. These values five times higher those recorded for Napoleon gulf, Lake Victoria, during September 2011 (NaFIRRI unpublished data)
Between 1998 and 2013, Lake Wamala shifted in status from a seemingly mesotrophic state to hypertrophic state
Summary
Physical and chemical environment of water, including water transparency, conductivity, dissolved oxygen and nutrients, plankton productivity, and feeding interactions determine composition and yield of fishes [1, 2]. Seasonal and interannual variations in these conditions are known to reflect changes in the watershed as well as climate [3, 4], but limited work has been done with respect to small and shallow wetland-dominated tropical lakes For such lakes, studies that attempt to attribute changes in fish stocks to watershed disturbance and climate variability are subject to conjectures because of missing information on influence of the these factors on primary aquatic productivity. Cell size and the proportions of different elements that constitute the phytoplankton are some of the ecophysiological traits affected by variability in climate variables [10, 11] The effect on these traits can cascade to higher trophic levels and influence diversity, size, composition and distribution of zooplankton, macroinvertebrates and fish communities. These changes are exacerbated by land use and land cover change within the catchment area [3]
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