The macroinvertebrate fauna of water hyacinth fringes in the Sudd swamps (River Nile,southern Sudan)

The water hyacinth now forms an almost ubiquitous fringe to open-water habitats in the permanent wetland of the Sudd. In a typical river-lake the fringe varied between 9 and 16 m in breadth with highest plant biomasses in the centre. Largest quantities of macroinvertebrates in hyacinth root-mats were obtained in drop-trap samples within 6 m of the open water. They were dominated by coleopterans, odonatans, gastropods andCaridina nilotica. Outer edges of the fringe offer easy access, good dissolved oxygen concentrations, and a variety of potential food resources for aquatic invertebrates. By contrast the less hospitable landward zone contained a reduced fauna in which coleopterans, hydracarines and gastropods were prominent. A marked reduction in the numbers of invertebrates in lift-net samples of detached hyacinth rafts was attributed to browsing by fish. Hyacinth root-mats appear to have replaced the niches formerly provided by those of the Nile cabbage for aquatic invertebrates in the Sudd.     

Fish communities in the eastern,seasonal-floodplain of the Sudd,Southern Sudan

Annual flooding of the permanent Sudd swamps innundates large areas of Oryza longistaminata grasslands. Fish migration into a southeast section of the seasonal floodplain was led by Clarias gariepinus, Polypterus senegalus and several small species which were intent on spawning. Channa obscura and juveniles of Oreochromis niloticus were prominent among later entrants. From a total of 23 species caught in seasonal habitats, 7 contributed over 97% of total estimated numerical and biomass densities. The latter ranged from 435–580 kg ha^–1. In the 1982–83 flood-season entry to the floodplain was essentially restricted to species tolerant of oxygen-deficient waters and thereby able to penetrate the papyrus and Typha swamps of the permanent wetland. Migrants became stranded in temporary pools and subject to heavy cropping by birds.     

Uses,abuses and management of lakes and rivers

Freshwater basins are vulnerable to human-induced change for a number of reasons. They lie at the bottoms of catchments and therefore are subject to the run-off of a large variety of dissolved and particulate substances; shallow basins provide fertile agricultural soils if drained; and public perception, particularly of wetland basins, is of inhospitable places, so that lobbies for drainage are more likely to be effective than those seeking to preserve the habitat. On the other hand, water is so important to human life that there is also strong concern about the fate of freshwater habitats. Freshwater systems are also often open systems capable of faster recovery from disturbance than their terrestrial counterparts.Three lake, river and wetland ecosystems, the Sudd in the Sudan, the Everglades and adjacent swamplands of southern Florida, USA, and the Norfolk Broadland in the UK are examined from the viewpoints of how they are or were used by their indigenous human populations, and the problems they may face or already face. This sites are used to suggest a general model of human impact and to establish approaches to the wise future use and conservation of these and other similar systems.     

Permanent swamp vegetation of the Upper Nile

This paper describes vegetation in the nothern Sudd area of the Sudan. A visit by boat was made in April 1978 along the White Nile from Malakal to Lake No; to the inflow regions of the Bahr el Jebel, the Bahr el Zeraf and to the Bahr el Ghazal. Only the vegetation along the fringe of the rivers was observed as plants were tall and dense and it was not possible to penetrate into the inner swamps.The outer edge of the vegetation was fringed by extensive mats of free-floating Eichhornia crassipes which was easily dislodged and formed floating islands to be carried downstream. The main rivers, excluding the Bahr el Ghazal, were too deep and turbid to support beds of submerged species but plants such as Ceratophyllum demersum could be found amongst floating plants. Vossia cuspidata usually produced a broad zone to the landward of Eichhornia. It dominated the shallow water with a floating raft of stems growing out from the shore over the water surface. Massive stands of Cyperus papyrus and/or Phragmites karka, over 5 metres tall, dominated the vegetation behind and sometimes excluded the Vossia zone. The papyrus could form dense, floating rafts of rhizomes at the water's edge, but Phragmites always remained firmly rooted to the bottom. Floating papyrus and Vossia would become detached from the main beds and drifted downstream with Eichhornia.Creepers were common amongst papyrus but less common amongst Phragmites where the vegetation was generally more dense. Large stands of Typa domingensis were observed behind the fringe vegetation.The vegetation of the Bahr el Ghazal, in marked contrast to the rest, was stunted, and reasons for this are suggested. Species diversity was greater and the less turbid waters allowed considerable development of floating-leaved and submerged species.Estimates of fresh weights and heights of dominant vegetation types were attained but methods were very laborious and alternative ways have been suggested. The fresh weight of Eichhornia from a one metre square quadrat in the White Nile was around 70 kg. The shoots of Phragmites weighed 18.5 kg and Cyperus papyrus, 11.0 kg m^–2.