Impressions
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| Invertebrate Team Brings in its 'Catch'. |
Carting scores of samples, damp life vests and luggage, the AquaRAP team arrived back in Maun today. We left the Delta with mixed emotions, since the thrill of collecting samples in the spectacular setting of the Delta will soon be replaced by the tedious labor of data entry in a hotel conference room.
Even so, this is an exciting time. With input from each of the teams, we're beginning to compare our findings at each of the sites and arrive at a picture of how the Delta's aquatic life changes as we move from north to south.
Some of the preliminary findings:
The Invertebrates team reports that certain snail species common at Drotsky's, the first sampling area, had dropped out of the system by the time we reached the final site. Similarly, freshwater shrimp common in the north are uncommon further south. To find out what's driving these changes, the team will look at data collected by other teams, such as Water Quality, to see if pH changes might be responsible.
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| Aerial View of the Delta on our way back to Maun. |
The Fish team collected extraordinary quantities at the final sampling area – in the words of team member Denis Tweddle, "Our catch yesterday was the biggest I've ever seen for the length of net we were using."
However, many of the fish were small specimens; Ben van der Waal noted in particular the absence of Largemouth Bream of a large size. Is this because the larger specimens are migrating out of the area? Further research will be required to find out.
The Plants team noted many more species as the expedition progressed; where papyrus was the dominant plant in the north, there was no single dominant species in the south. Fred Ellery commented that in the southern sampling sites, the vegetation is strongly "zoned" depending on water fluctuations, meaning that changing water levels create a variety of habitats for different species.
The Aquatic Birds team also reported more species diversity as they moved south, noting in particular the abundance at the final sampling site. More than 35 aquatic bird species were recorded in short transects there, including several pairs of fish
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| Fish Team Untangles Fish Gill Net . |
eagles. However, Seteng Motalaote found more fish eagles in the north and south than he did in the Xakanaxa, the area in between. More study will be needed to find out what's behind these reproductive fluctuations.
The Water Quality team observed that water flows declined as we moved south, becoming higher in salt content. Read more about salt concentrations and other water quality changes below, in AquaRAP in Action.
Although the expedition is drawing to a close, we'll post additional observations in the next few days. The team is writing up a summary statement on each of the sites, which we'll post along with some final thoughts on our adventure in the Okavango Delta.
– Reported by Clare Nielsen
PEOPLE: Budzanani Tacheba, Botswana's Budding Ecologist.
TOOLS: Preparing a Specimen.
ISSUES: Tourism from a Provider's Perspective.
SPECIES: AquaRAP Team Favorites.
AquaRAP in Action: Water Quality
"We tend to focus on the obvious – the things we can see and touch. We collect specimens of plants, fish and invertebrates, admiring their individual color patterns and textures," said Pete Ashton, water quality team leader. "Yet, the things we can't see are just as significant to the ecosystem, if not more so."
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AquaRAP's Water Quality
Team at Work. |
I sat down with Pete Ashton, to find out more about the abstract side of AquaRAP. Throughout the expedition, his team has been taking samples of the water for later chemical analysis, as well as field measurements of water temperature, dissolved oxygen, pH, and electrical conductivity. All of these tests are important in helping us to determine the "health" of the Okavango Delta. For instance, an aquatic ecosystem is considered to be "healthy" when high levels of dissolved oxygen are present.
Water temperature helps us to determine how much oxygen can be dissolved into the water: As temperature increases, less oxygen can be dissolved. Also, pH values that range from slightly acidic to neutral are ideal for wetland plants and animals to thrive. The pH values that were measured throughout the Delta are all within this range, a good sign for the living organisms found here.
Pete explained that Guma Lagoon (the second sampling site) is an example of an ecosystem that can be considered "unhealthy." Guma Lagoon was part of an old oxbow lake that was cut off from the main Okavango River channel and later enclosed by fringing swamps. Because the lagoon is cut off from the main channel, the flow of oxygenated water is blocked. When low oxygen content is combined with a large number of oxygen-consuming bacteria, low levels of dissolved oxygen are available for fish and other forms of aquatic life. On the other hand, Xakanaxa Lagoon and the main river channels in the Upper Panhandle always have high levels of dissolved oxygen and are healthy habitats for fish.
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"I wondered how a river system with so much salt flowing through it, especially at the current site, could still be considered fresh water." |
Electric conductivity is another factor that influences how the Delta looks and functions. These measurements give us a good idea of the concentration of dissolved salts that are present in the water. The northern regions of the Okavango Delta, especially the main river channel, normally have low conductivity values (in other words, lower concentrations of dissolved salts). This contrasts with the conductivity values found in the lower reaches of the Delta; these were almost double the values found at the upper Panhandle.
I wondered how a river system with so much salt flowing through it, especially at the current site, could still be considered fresh water. In a normal situation, when water is lost by evaporation, the salts in the water would remain behind, becoming gradually more concentrated in the system. However, Fred Ellery explained that
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| Testing Water Quality. |
most of the water leaves the system by evapotranspiration, or through the leaves of plants. Trees on the fringe of islands suck in water from the surrounding lagoons through their roots, filtering out the salts that then accumulate in the soil. In this way, the trees are behaving like kidneys, removing waste and toxins that would otherwise poison the aquatic ecosystem.
Pete elaborated by stating, "the trees concentrated on islands are creating their own funeral by taking up all of the salt into the soil." Rainwater washes these salts into slightly lower depression areas, usually in the middle of the island. This is why the middle regions of most islands tend to be void of aquatic life forms, leaving large pans of bleached, crusty salt. Islands are therefore one of the most important habitats in terms of ecological functioning of the Delta, as they are the only place where these massive trees can grow.
I started the day, tasting the fresh water and wandering around the island – barren of trees 100 meters from the fringes – not realizing the significance of these activities. Not until I sat down with the scientists, studying these things I never thought to even think about, did I start to really understand the true nature of the Delta.
– Reported by Sharon Safran
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