CI's AquaRAP team undertook a multi-disciplinary survey of 37 georeference sites in four main localities of the Okavango Delta between June 5 and 22, 2000. The survey attempted to provide an integrated baseline analysis of biodiversity and conservation status of this unique mosaic of aquatic ecosystems.
Areas of analysis included water quality, vegetation, aquatic invertebrates (both benthic organisms that live in or on the bottom sediments, and planktonic organisms that live in the water), fish diversity, fisheries and aquatic birds. The survey also conducted some assessments of the perceptions and concerns of residents regarding perceived environmental problems and threats that face the Delta.
The four focal areas were the Upper Panhandle (Drotsky's Cabins), and the northern, southern, and western regions of the Delta (respectively Guma Lagoon, Xakanaxa, and Oddballs). The survey broadly coincided with arrival of the annual floods from the upper catchment in Angola, superimposed on unusually heavy local rains in the Delta. Although there are interpretative constraints associated with the unusual hydrology, as well as the limited sampling coverage in space and time, the following preliminary findings can be offered.
Several water quality variables were measured on site: temperature, electrical conductivity (EC, a measure of the salinity or total dissolved salts), pH (an indicator of the water's acidity or alkalinity), dissolved oxygen and water clarity. All of the measures apart from dissolved oxygen reflected benign and healthy conditions throughout the Delta. Dissolved oxygen concentrations were often low, especially in deeper lagoon waters of the upper Delta. This can be attributed to natural flood-induced flushing of accumulated organic matter from beneath papyrus mats into these lagoons. This is undoubtedly the cause of the natural annual fish kills experienced at Guma Lagoon.
The team found, with localized exceptions, a general 'downstream' increase in dissolved oxygen levels. This trend is probably caused by the increased numbers of submerged and emergent aquatic plants, as well as algae, that produce oxygen during photosynthesis. Water temperatures reflected a corresponding trend, with surface water values rising from approximately 17°C in the faster-flowing upper region, to values around 19°C in the shallower and quieter lower reaches of the Delta.
Low salinity values are characteristic of the Okavango system. However, EC increased almost three-fold along the hydraulic axis of the system, from values around 30 µS/cm in the upper reaches to slightly over 80 µS/cm at Oddballs. This increase is attributed to the evaporative concentration of salts along the system's hydraulic axis, associated with evapotranspiration of both aquatic plants (reeds, grasses and sedges) and the riparian vegetation complex found on the multitude of islands. Greatly elevated salinity values were recorded at certain confined locations, particularly in isolated rain-fed pools.
The pH of water throughout the Delta was at or near neutral, apart from localized areas that exhibited slight acidity (a natural consequence of the dissolution of natural humic compounds) and fewer areas exhibiting slight alkalinity. A range of additional water quality determinations will require laboratory analysis, including major anions and cations (sodium, potassium, magnesium, chloride, sulphate, carbonate, etc), plant nutrients, and concentration of total and organic suspended solids.
Semi-quantitative vegetation surveys were undertaken at 122 sites in the focal areas. These revealed the expected increase in plant species richness down the delta. In addition, the primarily aquatic assemblage in the upper reaches, dominated by grasses (Vossia cuspidata and Echinochloa pyramidalis), sedges (Cyperus papyrus), rushes (Typha capensis) and reeds (Phragmites mauritianus) changed to a much more patchy mosaic of aquatic, semi-aquatic, and terrestrial habitats and species in the lower reaches. A remarkably high proportion (about one-third) of the approximately 800 plant species known from the Delta were encountered during this brief cool-season survey.
Macro-invertebrate surveys focused on a selected subset of representative organisms in the aim of ensuring a rapid evaluation. Groups targeted were the aquatic molluscs (snails and mussels), adult dragonflies, mayfly nymphs, bugs and beetles, and leeches, along with crabs and shrimps. Both diversity and abundance of the macro-invertebrates were surprisingly low throughout the open system.
Isolated and ephemeral pools showed somewhat greater diversity of aquatic bugs, sometimes matched by apparently greater abundance. Freshwater shrimps (Caridina) declined strikingly down the system. Analysis of the basis for this decline may be ecologically very informative, in terms of energy sources, and predation pressure. It is relevant to note that Biomphalaria pfeifferi and Lymnaea natalensis, the snail intermediate hosts of intestinal bilharzia and liver fluke disease respectively, were the most common snails in all focal areas. This situation may have important public health implications.
In part, the sparsity of macro-invertebrates may be linked to inhospitable habitat (depressed oxygen levels, and/or shade-depressed primary productivity in much of the papyrus and other swamps) and predation pressure by a multitude of opportunistically predatory fishes.
While laboratory microscopic analysis is required, subjective visual assessments reflect considerable variation in diversity and abundance of planktonic and benthic micro-invertebrates. Diversity was minimal in flowing main channels, slightly higher in vegetated side channels, and increased in quiet vegetated backwaters and lagoons. The highest levels were found in some of the isolated and ephemeral pools. The underlying general trend of increases further downstream implies an inverse pattern to that shown by the freshwater shrimp Caridina.
At least 65 of the 71 fish species previously recorded from the Botswanan part of the Okavango system were collected using gill-, D- and seine-netting, and limited electro-fishing. Establishment of such diversity in such a brief survey indicates that sampling methods and selected sites effectively covered the diversity of Delta habitats. There were differences between the four sites and this may indicate real differences in community diversity; however, the sites were at different phases in the flood cycle, which affected our collection efficiency. Extensions of known ranges for certain species within the system were recorded, and one undescribed species of topminnow (Aplocheilichthys sp.) was collected at Xakanaxa. No exotic species were recorded.
Gill net catches were analyzed for over 3,000 individual fishes, documenting catch per unit effort by species and by different mesh size panels in the experimental gill nets. Gill net catches comprised a comparable assemblage at many sites. Obligate or opportunistically predatory species – principally catfish (Clarias), butter-barbel (Schilbe), and pike (Hepsetus) – formed the major part of the catch, together with the small omnivorous species Brycinus lateralis. The cichlids caught were also largely predaceous taxa; few herbivorous or detritivorous cichlid species were gill-netted.
The information gathered on the fish stocks has implications for the management of fisheries, including exploitation for food and for tourist angling in the Delta region.
Independent aquatic bird surveys were conducted at the four focal areas. The aquatic avifauna appeared much less diverse than expected, though some species (e.g. Fish eagle, Halietus vocifer) were more abundant in the upper than in the lower Delta region, while the reverse was true for other species such as Reed cormorant (Phalacocorax africanus). Additional studies should evaluate the possible reasons for these trends.
Sixty-one aquatic bird species were recorded during the survey. One site in particular – the breeding colony at Gadikwe Lagoon at Xakanaxa-was an important discovery. This heronry was occupied by 17 species, including African spoonbill, Sacred ibis, several species of egrets (Great white, Yellow billed, Black) and herons (Grey, Rufous-bellied, Green-backed), and Reed cormorants. In addition, two species of rare and endangered aquatic birds were recorded breeding here: the Yellow-billed stork and the Slaty egret. Based on the team's observations, Gadikwe Lagoon represents the most important breeding site for these species in southern Africa. It is notable that the breeding activity at Gadikwe Lagoon is earlier than usual, perhaps because of the unusually heavy rains experienced throughout the region.
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