Food web structure and trophic interactions of the tropical highland lake Hayq, Ethiopia (original) (raw)
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The Food and Feeding Ecology of Nile Tilapia, Oreochromis niloticus, in Lake Hayq, Ethiopia
International journal of ecology and environmental sciences, 2015
We investigated the food and feeding habits of Nile tilapia, Oreochromis niloticus, in Lake Hayq, Ethiopia from August 2008 to March 2009. A total 931 fish were collected by gillnets of various stretched mesh sizes, of which 326 individuals of Oreochromis niloticus stomachs contained food. The stomach contents were analyzed using frequency of occurrence, numerical methods and the Geometric Index of Importance (GII). The food items in the stomach covered a wide variety, ranging from various types of phytoplankton to zooplankton and to macrophytes. The major food items in terms of frequency of occurrence were Microcystis (87.7%), Cosmarium (65.13%), Navicula (64.2%) and Daphnia (71%) genera. Numerically, Cosmarium (38.5%) and Microsystis (31%) dominated the food of O. niloticus. However, Geometric Importance Index GII suggested that the most consumed group was Microcystis (83.93%). A monthly variation was also noted in the stomach contents of O. niloticus over the period of investigat...
Zooplankton community structure and ecology of the tropical-highland Lake Hayq, Ethiopia
Limnologica - Ecology and Management of Inland Waters, 2011
Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been done sporadically documenting a change in zooplankton composition including the disappearance of cladocerans, hypothesizing the stocked planktivorous fish could be a cause. Nevertheless, no detailed research was conducted to identify potential effects of fish stocking predominantly due to its remote location. The article presents data about zooplankton composition, abundance and biomass done between October 2007 and January 2009 on short-time intervals including the underlying limnological variables. The zooplankton community was depauperate comprising two copepods, three cladocerans, and six rotifers taxa, as typical for tropical lakes. Total mean standing biomass of all crustacean zooplankton was 237 mg dry mass m −3 , which gave Lake Hayq an intermediate position when compared with other tropical lakes. Of copepods, Thermocyclops ethiopiensis was almost an exclusive species, and its temporal variation was influenced by food supply and water temperature. We refute the hypothesis that Tilapia was the cause for the seasonal disappearance of cladocerans, and attribute it to the adverse effect of episodic mixing. Nevertheless, the planktivorous fish probably plays a key role in structuring the cladocerans in particular the large-sized Daphnia magna. In January 2008, we observed a massive planktivorous fish mortality that triggered high algal biomass, which was later grazed by large-sized D. magna demonstrating the trophic cascade hypothesis in a natural ecosystem.
Key drivers for phytoplankton composition and biomass in an Ethiopian highland lake
Limnologica - Ecology and Management of Inland Waters, 2014
We studied the temporal phytoplankton community pattern of the deep crater Lake Hayq in the highlands of Ethiopia from October 2007 to October 2008. Earlier sporadic surveys indicated that the phytoplankton community was predominantly characterized by heavy diatoms, which characteristically suffer from rapid sedimentation. The trophic status of Lake Hayq was reported to have changed from oligotrophic to eutrophic in 1992. The present study addresses the potential reasons for the diatom dominance as well as causes of the trophic change. Net and integrated water samples were used for determination of physico-chemical parameters and phytoplankton biovolumes. Our results revealed that diatoms and chlorophytes dominated during most of the study period in Lake Hayq and seem to be favored by the mixing regime of the water body, which can be described as partial atelomixis with daily mixing of the epilimnion maintaining the algae within the euphotic depth via regular re-suspension. However, the epilimnion may be decoupled from the hypolimnion by a seasonal chemocline. Nutrients were not limiting in the lake with an overall mean concentration of soluble reactive phosphorus of 22 g L −1 and total phosphorus of 58 g L −1 and of dissolved inorganic nitrogen of 305 g L −1 , with ammonium being the primary form. In the 1940s only diatoms were reported, but since the 1990s other phytoplankton groups and taxa have become relevant. Canonical correspondence analysis showed that chlorophytes were mainly associated with nutrients and rainfall, euglenophytes with elevated alkalinity and the diatoms with silica and zooplankton. Chlorophyll a as measure of total phytoplankton biomass was significantly influenced by seasonality and underwater light supply, reflecting the significant role of atelomixis in persistent occurrence of heavy taxa in the epilimnion. The lake is still categorized as a eutrophic system, demonstrating that the trophic change reported in 1992 was not short-lived. In addition to changes in the catchment the eutrophication process was probably primarily triggered by a previous introduction of Tilapia in the lake, causing a cascading effect in the food-web interactions. This implied that the phytoplankton composition and biomass of this tropical deep tropical lake can be controlled through biomanipulation, as has been demonstrated for temperate lakes.
Trophic analysis of Lake Awassa (Ethiopia) using mass-balance Ecopath model
Ecological Modelling, 2007
A user-friendly software model, Ecopath with Ecosim, version 5.0 Beta, was used to construct energy flow and mixed trophic impact (MTI) for the Lake Awassa ecosystem (Ethiopia). We used data from the literature and also several parameters were estimated from the present study done from November 2003-August 2004. Thirteen functional groups including two ontogeny ones were used in the present analysis, which assessed the trophic relationship, energy flow and interactions between them. The producers particularly phytoplankton and detritus are under exploited; hence energy transfer from lower trophic levels is low. On the contrary, all consumers have ecotrophic efficiency (EE) close to 1 indicating that consumers are heavily exploited in the system. Flow from detritus was as important as flow from phytoplankton. Flow from both herbivorous and carnivorous zooplankton to consumers was high. MTI analyses indicate that phytoplankton and detritus have positive impact on most other groups while zoobenthos has negative impact on some groups. Lake Awassa has low ecological efficiency with a value of 0.00144 for the gross efficiency of the fisheries. The system primary production/respiration (P/R) ratio of Lake Awassa is 5.834 showing that the lake is at developmental stage, with high autotrophy, and some attention should be given to human impacts. This trophic model analysis also enabled us to confirm/refute previous studies and pinpoint critical gaps in the present knowledge about Lake Awassa.
Food and Feeding Biology of Nile Tilapia (Oreochromis niloticus) in Lake Langeno, Ethiopia
Sustainability
This study aimed to investigate the natural feeding behavior of Nile tilapia in Lake Langeno, Ethiopia, with emphasis on potential spatial, size and seasonal effects on ingested food items. This study of the food and feeding biology of O. niloticus in Lake Langeno, Ethiopia, was conducted from March 2016to February 2017. Fish samples were collected monthly from six different sampling sites using different mesh sizes of gillnets. A total of 610 fish specimens with full stomachs were considered for the assessment of feeding biology. In total, seven food items, namely phytoplankton, zooplankton, insects, detritus, macrophytes, fish parts and nematodes, were identified from the fish stomach contents. Phytoplankton was the most commonly consumed food prey, followed by detritus, zooplankton and macrophytes. The other food items were occasionally and randomly consumed. Phytoplankton and detritus were the dominant food prey in the dry season, with zooplankton and macrophytes the main prey d...
Plankton- Based Assessment of the Trophic State of Three Tropical Lakes
Journal of Environmental Protection, 2011
In developing countries, lakes being important sources of water supply and fishing are vulnerable to anthropogenic impact, yet knowledge of their trophic state in relation to changes in species composition, and environmental variables, are limited. This study is aimed at assessing the trophic status of lakes by monthly sampling of three lakes located along the floodplain of Cross River, Nigeria between January 2008 and December 2009. Samples were analyzed for water quality parameters, zooplankton and phytoplankton composition and distribution. Results were subjected to community structure analysis using trophic state index, species richness and diversity indexes. Essential primary productivity nutrients, nitrates, sulphates and phosphates were highest in Ejagham Lake, and lowest in Ikot Okpora Lake. Dominant phytoplankton species Oscillatoria lacustria (Cyanophyceae), Cyclotella operculata (Bacilliarophyceae) and zooplankton Keratella tropica, Keratella quadrata, Filinia longiseta, Branchionus anguillaris and Trichocerca pusilla (rotifers) all typical of eutrophic communities were recorded in high densities in Ejagham Lake in both dry and wet seasons while Cladocerans, Bosmina longirostris and Moina micrura and copepods considered indicators of oligotrophy and mesotrophy were recorded in large numbers in Ikot Okpora and Obubra Lakes respectively. Higher values of species richness, Evenness and Shannon-Wiener diversity index for both phytoplankton and zooplankton, were recorded in Ejagham Lake during the dry season than wet. Also values of the Trophic state index were generally highest at the Ejagham Lake in the savanna region of the floodplain and lowest at Ikot Okpora in the forest region of the floodplain. Forest region is therefore a limiting factor in the productivity of lakes in the tropics.
2018
The study aims to describe the energy flow and trophic structure, to assess the ecological stage of maturity, the ecosystem impact of the introduced exotic carp species and the current fishery on the Lake Koka ecosystem using a mass-balance trophic model. The Ecopath approach and software were used to represent the lake ecosystem, and input parameters were obtained from in-situ surveys and from published lake specific sources. The model was balanced as per the recommended ecological and thermodynamic constraints. The low ecotrophic efficiency found for adult catfish, primary producers and detritus suggest that these resources are not fully utilized, while other groups have moderate to high values. The annual fishery catch represents only about one sixth of the total fish biomass, and the exploitation rates of target species are well below 0.5 suggesting under-exploitation. The dietary intake of fish-eating birds is 5.8 t km-2 year-1 almost twice the catch, showing the lake's important role for sustaining the bird populations and hence, caution needs to be taken before implementing any further fisheries development intervention. The mean trophic level of the catch (2.55) is intermediate when compared to other tropical lake systems and reflects the mixed fishery of the lake. Mixed Trophic Impact and Niche Overlap analysis reveals that, contrary to what may be expected, the impact of the introduced exotic carp species on the lake system is less significant than that of the native species. Besides its socioeconomic significance, the detritivorous carp seems to fill an unutilized niche and helps the system in recycling organic matter. The P/R value (3.2) and other system maturity indices indicate that Lake Koka is as yet an immature, developing system. The model can be used as a reference for exploring ecosystem-based fisheries management scenarios aimed at sustaining fish stocks, ecosystem services as well as ecological integrity.
Plankton-Based Assessment of the Trophic State of Three Tropical Lakes Open Access
Journal of …, 2011
In developing countries, lakes being important sources of water supply and fishing are vulnerable to anthropogenic impact, yet knowledge of their trophic state in relation to changes in species composition, and environmental variables, are limited. This study is aimed at assessing the trophic status of lakes by monthly sampling of three lakes located along the floodplain of Cross River, Nigeria between January 2008 and December 2009. Samples were analyzed for water quality parameters, zooplankton and phytoplankton composition and distribution. Results were subjected to community structure analysis using trophic state index, species richness and diversity indexes. Essential primary productivity nutrients, nitrates, sulphates and phosphates were highest in Ejagham Lake, and lowest in Ikot Okpora Lake. Dominant phytoplankton species Oscillatoria lacustria (Cyanophyceae), Cyclotella operculata (Bacilliarophyceae) and zooplankton Keratella tropica, Keratella quadrata, Filinia longiseta, Branchionus anguillaris and Trichocerca pusilla (rotifers) all typical of eutrophic communities were recorded in high densities in Ejagham Lake in both dry and wet seasons while Cladocerans, Bosmina longirostris and Moina micrura and copepods considered indicators of oligotrophy and mesotrophy were recorded in large numbers in Ikot Okpora and Obubra Lakes respectively. Higher values of species richness, Evenness and Shannon-Wiener diversity index for both phytoplankton and zooplankton, were recorded in Ejagham Lake during the dry season than wet. Also values of the Trophic state index were generally highest at the Ejagham Lake in the savanna region of the floodplain and lowest at Ikot Okpora in the forest region of the floodplain. Forest region is therefore a limiting factor in the productivity of lakes in the tropics.
Aquatic Living Resources, 2016
We examine phytoplankton community structure and how it is influenced by commercial tilapia (Oreochromis niloticus L.) cage farming in three tropical lakes in eastern Brazil. Sampling occurred during both the wet and dry seasons for two treatments-within tilapia cages and outside the cages in the lake. Total ammonia, total phosphorus, chlorophyll a, phycocyanin and phytoplankton were measured in all water samples. In the phytoplankton community, we estimated species richness and total abundance. Thirty-three genera of algae distributed in 8 classes were identified. The most abundant classes were Chlorophyceae and Cyanophyceae (the latter of which was the most species-rich class). In Palmas Lake, chlorophyll a was constant over time (summer and winter) and location (within and outside cage farms), which may reflect the fact that fish farming is more recent in this lake. The richness and abundance of species were similar among the lakes, but Palminhas Lake tended to have more extreme values, apparently associated with the longer time fish have been farmed there. Fish farming clearly influences water quality as measured by algae species richness and abundance but is also moderated by the unique conditions in each lake and the amount of time over which the fish have been farmed. Understanding and preventing the impacts and detrimental consequences of tilapia cage farming on both the farming yield and the water quality in tropical lakes require continual monitoring.