Limnological and ecological characteristics of tropical highland reservoirs in Tigray, Northern Ethiopia

Abstract

The semi-arid highlands of Northern Ethiopia (Tigray) have numerous small reservoirs that have been created by microdams in an attempt to provide water supply for irrigation and livestock drinking. Although the reservoirs have substantial added value to residents, their use as water resource is jeopardized by eutrophication and a high occurrence of blooms of toxic cyanobacteria. So far, there is no systematic information available on the limnological and aquatic ecological characteristics of these dams. We carried out a standardized survey of 32 reservoirs and assessed a wide set of morphometric, abiotic and biotic variables. The sampling was performed during two seasons, September-October 2004 (end of the wet season/start of the dry season) and April-May 2005 (towards the end of the dry season). Using multivariate analysis, we revealed dominating patterns of variable associations and compared the variability in these patterns among seasons. According to standardized PCA and RDA analyses, the most important axis of variation was mainly represented by a gradient in nutrients and altitude that was also positively A. Pals associated with phytoplankton biomass, suspended matter and oxygen concentration, and negatively with water transparency. For most variables, correlations between the wet and dry season were weak, which suggests that individual reservoirs behaved rather differently in their response to seasonal changes. Nevertheless, a Mantel correlation (r = 0.32; P = 0.035) showed a weak but significant overall concordance in the variable association patterns among seasons. A number of reservoirs became very shallow or fell dry in the dry season, a process that was associated with an increase in suspended matter and conductivity and a decrease in transparency. These reservoirs contained lower amounts of fish and tended to be less eutrophic than the deeper, permanent systems, as they had lower levels of phosphorus and chlorophyll-a. With multiple regression analysis, we constructed most parsimonious models in an attempt to explain the variation in key biotic variables: phytoplankton and cyanobacteria biomass, cladoceran biomass, fish biomass and the abundance of submerged vegetation. Phytoplankton and fish biomass tended to be positively related with the concentration of total phosphorus, whereas cladoceran biomass was not associated with nutrient concentrations. The positive association of fish and phytoplankton with nutrient concentrations suggests a bottom-up control, whereas the absence of an association between zooplankton and nutrient concentrations may be indicative for top-down control. The biomass of cyanobacteria was negatively related to the biomass of cladocerans (Daphnia), which likely reflects a topdown effect. Most reservoirs were turbid. The occurrence and abundance of macrophytes tended to be positively related to water transparency and was negatively associated to TP and the amount of livestock frequenting the reservoirs. However, macrophytes were not limited to clear-water reservoirs.

Genetic diversity of Microcystis blooms (Cyanobacteria) in recently constructed reservoirs in Tigray (Northern Ethiopia) assessed by rDNA ITS

“…The annual rainfall ranges between 546 and 879 mm, with an average of approximately 700 mm. The period from October to May is classified as the dry season and the period from June to September as the wet season (Dejenie et al 2008). Most of the reservoirs have a surface area between 2 and 20 ha and were constructed after 1994 to provide local communities with water for irrigation and livestock watering.…”

“…Most of the reservoirs have a surface area between 2 and 20 ha and were constructed after 1994 to provide local communities with water for irrigation and livestock watering. A description of the limnological characteristics, year of construction and aquatic food web structure of these reservoirs is given in Dejenie et al (2008).…”

“…Microcystis, which produces the hepatotoxin microcystin, is one of the most common bloom forming cyanobacteria (Huisman et al 2005). Microcystis blooms are generally limited to summer in temperate regions but may occur year-round in (sub)tropical regions (Ballot et al 2003;Kardinaal et al 2007a;Dejenie et al 2008;Van Wichelen et al 2010). High densities of this cyanobacterium can intoxicate a variety of eukaryotes (Rohrlack et al 1999;Ferrão-Filho et al 2000;Pflugmacher 2004;Codd et al 2005).…”

“…Microcystis blooms containing high microcystin concentrations have been reported in Kenya (Ballot et al 2003), Morocco (Oudra et al 2001) and South Africa (Scott 1991), and a recent study has shown a link between a toxic Microcystis bloom and mortality of wild animals in South Africa (Oberholster et al 2009(Oberholster et al ). A survey in 2004(Oberholster et al -2006 showed that Microcystis blooms are very frequent in man-made reservoirs in the semiarid highlands of Tigray, Northern Ethiopia (Dejenie et al 2008). Microcystis blooms occur year-round in these meso-to hypereutrophic reservoirs, and biotic and abiotic conditions in the reservoirs differed between the wet and dry season of 2004 and 2005, respectively: water temperature, conductivity and Daphnia biomass were higher in the dry season than in the wet season.…”

Phytoplankton functional groups in a tropical reservoir in the Brazilian semiarid region

“…This factor is critical for sustaining the long-term dominance of certain phytoplankton species (Huszar, Silva, Marinho, Domingos, & Sant'Anna, 2000). Cyanobacterial species forming perennial blooms have been identified in eutrophic reservoirs in the semiarid region of Brazil (Chellappa, Chellappa, & Chellappa, 2008;Dantas, Moura, & Bittencourt-Oliveira, 2011) and in many other places worldwide (Naselli-Flores, Barone, Chorus, & Kurmayer, 2007;Dejenie et al, 2008;Douma et al, 2010). Factors that contribute to the distribution and stability patterns of phytoplankton in these reservoirs include trophic states (Barone & Naselli-Flores, 1994;Naselli-Flores, 2013), multidimensional environmental gradients (Fabbro & Duivenvoorden, 2000), and survival strategies (Reynolds, 1998).…”