Key words: Flood, soils, vegetation, fish, dambo, wetlands, sand, hydrograph
Introduction
The focus area for this post was within the vicinity of Sioma, Western Zambia, some 140 km North from the border between Zambia and Namibia at Katimo Mulilo. The area is within the Southern Miombo Woodlands ecosystem type. Floodplains in the Upper Zambezi River are rainwater fed from the Angolan and Zambian highlands and drain an extensive wetland area until cascading over the Ngonye Falls, the second biggest waterfall on the Zambezi River.
These wetlands play a central role in the local communities, with the environment providing the essential resources to sustain villages within the wood-and-wetland mix landscape of the area.
Annual flooding, typically between March and June, provides suitable conditions for high yield recession agriculture and a rich fishery. This is demonstrated below at the Chavuma Falls Flow Station, located near the border of the Angolan/ Zambian floodplains
The soils along the Zambezi River, near Ngonye Falls, are typically well-drained, sandy and nutrient poor. Shallow soils are common particularly around weathered areas, such as the Zambezi's rocky riparian area, or rock outcroppings such as inselberg's (koppies). The geomorphological conditions at the Ngonye Falls are such that weathered sediment has deposited within the bays formed in the incised river channel. These white sandy beaches tucked into the rock-edged bays of the region produce a unique bark when walked on, due to their silican rich and near perfectly round composition.
At the Ngonye Falls the Zambezi River is channeled through multiple cascades, rapids and smaller channels. Fast clear flowing waters form the dominant velocity class, with sandy and bedrock boulder surfaces the most common substrate. The smaller channels are typically vegetated and protected by woody riparian cover, well adapted to flooding.
Further upstream of the Ngonye Falls, source tributaries known as Dambos within the Barotse Floodplains have characteristic soils with a high clay content, baking to a black-grey during low flow periods. The thick clay of the wetlands function as effective sponges that regulate water flow throughout the year. The resultant effect of the permanent inundation are dense mats of aquatic vegetation, these form unique habitat types.
This survey was undertaken during April 2019, a period of uncharacteristic low flow in the Upper Zambezi, with heavy flooding usually expected. Two sites were visited during the April 2019 period, one at the Ngonye Falls, the other in a Dambo wetland close to Mongu.
Fish of the Ngonye Falls
The fish community of this area is typical of the Zambezian freshwater ecoregion, characterized by the historical lake conditions, where lentic systems were dominant. River capture from the lower Zambezi provided the change into the lotic systems that exist today. The resultant fish community consists of both rheophilic taxa as well as lotic adapted species. As such, diverse Cyprinidae and Cichlidae fauna dominate the fish fauna.
Enteromius are one of the more diverse genera of fishes in the Upper Zambezi River. To date, myself and Simone have been able to observe at-least 16 species of Enteromius in the Upper Zambezi River. The Enteromius genera have a wide range of habitat preferences ranging from the flowing open waters, to densely vegetated backwaters and floodplains. At the Ngonye Falls, through cast nets and underwater videos we observed the dominant Enteromius species present was E. poechii - the Dashtail Barb (pictured below).
The rapid habitats between boulders as well as shallow fast flowing reaches on the margins of the main channel was habitat to another common species in the river, Opsaridium zambezense - Barred Minnow (pictured below). The species were observed both in cast nets and underwater videos. Nymphing (angling) with a GHRE fly also captured numerous of these fierce toothless species, sometimes capturing the same individual on multiple occasions.
The rocky substrates of the side channels in the Ngonye Falls, provide great cover for two species of Labeo that are common throughout the Zambezi River system. These include Labeo cylindricus and Labeo lunatus (both pictured below). These species have adapted and have specialized lips which aid them to feed on algae and aufwuchs from the rocky substrates. I have also seen them graze off of underwater vegetation and sticks.
Four species of Alestidae are known to occupy the Ngonye Falls, these include Brycinus lateralis (Striped-Robber), Micralestes acutidens (Silver Robber), Rhabdalestes maunesnsis (Slender Robber) and the tertiary predator Hydrocynus vittatus (Tigerfish). The B. lateralis and M. acutidens were found to be common at the Ngonye Falls side channels, whilst H. vittatus were caught in the backwaters of the main Zambezi River channel (pictured below).
We are sure there are more fish in the area and we would have undoubtedly caught more should we have applied different techniques. We hopefully cover this gap through the other reports we provide such as the report of the Kwando River and Chobe River rapids.
Flora of the Ngonye Falls
The Barotse Floodplains area is a declared Ramsar site and make-up the largest wetland area in Zambia. It is appropriately named the Zambezian Flooded Grasslands Ecoregion, Unique Dambo wetland systems provide a distinct contrast to the fast-flowing, whirlpool dominated Ngonye Falls river reach. Wetland and species adapted to poor nutrient conditions are common.
Shallow Rapids Shaded Runs Aquatic Macrophytes
The Sioma area is dominated by Miombo/Kalahari Woodland, this is the focus area of the study, with a concentration on adjacent riparian areas. The soil is largely sandy, with white beaches along the torrent of the Zambezi and diverse aquatic macrophytes in the rapids below the Ngonye Falls. Riparian and wetland plants abound in the bubbling rapids, sheltering in the creeping roots of the Waterberry (Syzygium sp) trees anchored in the hard Sandstone ledges.
The woodlands consist of a diverse mix of woody species, typically Miombo is characterised by the species of: Brachystegia sp (Common Name: Miombo) and Fabaceae (Legume Family).
Trees
FABACEAE:
1-2. Bauhinia petersiana cf.
3-5. Baikiaea plurijuga (Common Name: African Teak)
6-7. Peltophorum africanum (African Black Wattle)
8. Vachellia erioloba (Camel Thorn)
1. BIGNONIACEAE Markhamia obtusifolia (Golden Bean Tree): The Golden Bean Tree bears elongate seed pods densely covered in golden hairs
2. APOCYNACEAE Diplorhyncus condylocarpon (Horn-pod Tree): Two drooping green & red/brown splayed fruit. Wavy leaves darker above than underneath
3-4. CHRYSOBALANACEAE Parinaria curatellifolia (Mabola Plum): fruit is eaten and hangs from drooping branches, brown-mottled and rough. Leaves alternate.
5-6. Phyllanthaceae (Palmately compound leaves): Hymenocardie acida (Heart Tree): The heart-shaped fruit, turning dark red once mature.
EUPHORBIACEAE: Schynziophyton rautenii (Manketti Tree): Fruit & seed edible, singular and large green fruit.
Riparian Flowering Plants
1. AIZOACEAE Gisekia sp.(Gisekia)
2. FABACEAE Indigofera newbrowniana cf.; 3.Mimosa pigra (Puff-ball Mimosa) 4.Tophrosia purpurea (Ash vetch); 5. Aeschynomene indica (Indian Sensitive Plant)
6. ASTERACEAE Flaveria trinervia
7. BULBINELLA Bulbinella sp. (Afrikaans: Katstert - Cat-tail)
8. CUCURBITUCEAE Cucumis zeyheri (Spiny African Cucumber)
9-10. CRASSSULARIACEAE Kalacnhoe lanceolata (Sneeze Plant)
11. LAMIACEAE Leonotis nepetifolia (Wild Dagga); 11. Acrotome inflata (Small Wild Dagga)
13-14. OLEACEAE Jasminum fluminense (Wild Jasmine)
15. RUBIACEAE Spermacoce sinensis (Coffee family - Spermacoce)
16. Ceratotheca sp.
17-18. OLACEAE Jasminum fluminense (Wild/River Jasmine)
Aquatic Plants
Dense mats and clumps of aquatic vegetation thrive in the side channels directly downstream of the Sioma falls.
1-2. Ludwigia repens
3. Najas Horida
4. Periscaria sp
Fish of the Wetland Dambos
The hydrology and conditions of Dambos is provided in the detailed review by von der Heyden (2004). In April 2019, with permission from the local government and community, we were able to conduct fish sampling in a dambo near the town of Mongu, Zambia. The goal of sampling the Dambo's was to investigate the presence of a fish species only known from its type locality, the species known as Neolebias lozii - Banded Neolebias. N. lozii is currently classified as Critically Endangered by the International Union for the Conservation of Nature (IUCN), where its range is restricted to the wetland systems south of Mongu (IUCN, 2019). The watercourse this fish species occupies is currently being canalised in order to facilitate effective drainage for agriculture, a known threat to the fish species. It is noted that N. lozii will undergo a revision to be re-categorized into the Dundocharax genus in the near future.
Using a small seine and dip net we sampled the fish assemblage present in the dense vegetation of the watercourse. Fish observed in the seine net pushed into the edges of the dense vegetation included Nannocharax multifasciatus - Multibar-Citharine and Enteromius barnardi - Blackback-Barb, common fish species across the Zambezi system in the well vegetated areas (pictured below). We were also able to observe Pseudocrenilabrus philander Tilapia sparrmanii and a juvenile Serranochromis species in the open water and against the vegetation edges, common fish across the region Africa. Several top-minnows such as Micropanchax katangae - Striped-Topminnow were also sampled in the dense vegetation and open water. Due to excitement and focused attempts to find Dundocharax and he lack of experience in the area to us at the time, we likely observed the Pigmy Topminnow, a recently described Micropanchax which we confused for the more common Micropanchax huteria - Meshscaled Topminnow.
None of the seine net sampling was able to capture any N. lozii. Simone used the dip net in the dense-shallow vegetation to capture the N. lozii away from the open water as pictured below. Although some parasites were found on some specimens, the population appeared healthy.
Flora of the Wetland Dambos
Dambos make-up the headwaters of the Zambezi, covering more than 10% of the surface area of Zambia. The waterlogged grasslands provide the basis for dense wetland vegetation growth often forming mats of floating reeds interspersed with smaller species. The site visited was densely vegetated with floating mats of Bladderwort (Utricularia sp) as well as floating beds of mixed Phragmites sp (Below: P. mariatiana), sedges and Ludwigia sp.
From top right to bottom left: 1-2. Ludwigia stolonifera, 3-4. Utricularia gibba (Bladderwort) distinct aquatic plant with bladder-like protrusions from the stolons trapping zooplankton for added nutrients, 5-6. Nymphaea nouchali (Blue Lotus), purple/blue to white flowers with a heart of yellow.
Sansevieria hyacinthoides (Mother-in-Laws Tongue); Red rimmed and mottled dark-light green tiger-like stripes. In wet and dry areas.
Floscopa glomerata, occurring in both the meandering Dambos and rushing waters of the Ngonye Falls, it can tolerate a variety of flow-depth classes. Flowers bright purple with yellow anthers, rounded clumps when not open and hairy leaves sheathes at the base.
REFERENCES
Manning J. 2009. Field Guide to Wild Flowers of South Africa.
Roodt V. 2011. Wild Flowers, Water Plants and Grasses from the Okavango Delta and Kalahari. The 'Did you Know' Nature Series.
Vetter S. WWF. 2020. Western Zambezian grasslands. www.worldwildlife.org/ecoregions/at0724. Accessed 16 December 2020.
von der Heyden J. 2004. The hydrology and hydrogeology of dambos: a review. Progress in Pyhsical Geography: Earth and Environment. 28: 544-564. https://doi.org/10.1191/0309133304pp424oa