Seasonal variation in primary production of a large high altitude tropical lake (Lake Tana, Ethiopia): effects of nutrient availability and water transparency

Abstract

Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May-November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m -3 to 8.5 mg m -3 (mean: 4.5 mg m -3 ) in the offshore zone. Primary production was measured using the light-dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P max . Gross primary production in the openwater averaged 2.43 g O 2 m -2 d -1 and ranged between 0.03 g O 2 m -2 d -1 and 10.2 g O 2 m -2 d -1 ; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct-Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P max . Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z mix ‡ Z t ) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.

Planktonic production and respiration in a subtropical lake dominated by Cyanobacteria

“…The Peri Lake features, such as its position along shorelines, its exposure to local wind, and its relative shallowness, prevented water column stratification, as was observed by Hennemann and Petrucio (2011) in Peri Lake, as well as in other environments (Amarasinghe and Vijverberg, 2002;Wondie et al, 2007;Montero et al, 2011).…”

“…According to light extinction coefficient in water column, generally at the surface occurs photoinhibition, followed by subsurface of maximum phytoplankton density and primary production, which decreases towards the bottom, where the decomposition processes are more intense (Wondie et al, 2007;Tilahun and Ahlgren, 2010). However, some Cyanobacteria species are capable of regulating their position in the water column through gas vesicles in order to improve their photosynthetic efficiency (Moura et al, 2011).…”

Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)

“…Based on its chlorophyll content it may be classified as mesotrophic (Dejen et al, 2004), but because of its high turbidity, gross primary production rate (annual average 2.43 g O 2 m -2 day -1 ) is relatively low (Wondie et al, 2007). It is well-mixed because of wind exposure and its small depth.…”

“…; Nagelkerke, 1997). The piscivorous birds will be especially effective during the period of low water levels at the end of the dry season in May-June when water levels are 2.0-2.5 m lower than during the rainy season (Wondie et al, 2007). This could lead to increased mortality during the dry season.…”

Seasonal variation in the diversity and abundance of phytoplankton in a small African tropical reservoir

“…Analysis of the phytoplankton community in the small reservoir of Adzopé reveals some similarities with the situation observed in reservoirs in Central and North Côte d'Ivoire (Bouvy et al, 1998;Arfi et al, 2001), Lake Guiers in Senegal (Bouvy et al, 2006) and Lake Tana in Ethiopia (Wondie et al, 2007). These authors described the occurrence of many species of cyanobacteria.…”