<p>The 20.000 km<sup>2</sup> swamp of the Delta is organized into islands, flood plains and permanent and seasonal channels. Most of these islands display a surprising vegetation distribution composed of tree rings surrounding the islands and limiting an inner domain with scarce vegetation. Whereas the hydrology of the Okanvango wetlands is governed by a series of drivers such as, sedimentation, climate, tectonic and biological processes, the potential of the use of multi-chemical tracing has not been so far much investigated. The conducted study as part of a multidisciplinary project dedicated to the understanding of the functioning of the Delta, involved water samples collected both upstream and downsream the river, close to one of these islands and also recovered within the island as well. The main objective of this geochemical investigation was to better constrain the interactions prevailing in between these islands and the water chemical record. pH, conductivity (C), dissolved organic and inorganic carbon (DOC & DIC) concentrations were measured as well as those of major anion and cation and trace cation concentrations as well. Whichever the tracers are considered, two contrasted groups of samples were evidenced depending on their sampling positioning regarding the island. The samples recovered only within the island displayed pH around and over 9 and higher conductivities, whereas the other showed lower circumneutral pH values and conductivities as well. The high conductivities of the water samples fom the island also correspond to the highest DOC and DIC concentrations. The strong relationship linking the high DIC values and the high pH in the island samples records probably alkaline CO<sub>3</sub><sup>2-</sup> et HCO<sub>3</sub><sup>- </sup>-rich waters resulting from water-rock interactions with carbonates. The marked DOC enrichment has mostly to be related to microbial or photo-degradation of plant-derived organic matter and/or hydrological condition variations promoting DOC release. Significant, Cl<sup>-</sup>, SO<sub>4</sub><sup>2-</sup>, NO<sub>2</sub><sup>-</sup>enrichments as well as major cation ones were also evidenced in the same group of samples within the island. However, the most surprising results are sourced in the trace element fingerprinting. This latter includes huge enrichment in heavy, critical metals and metalloids as well (e.g. Cr, Pb, V, REE, U, Th or As). Beyond the only marked REE-spike, Upper Continental Crust-normalized REE patterns displayed markedly contrasted shapes exhibiting two types of waters with circumneutral pH ones with MREE-enrichment, whereas the alkaline waters evidenced a classical continuous enrichment throughout the whole series from LREE to HREE and a positive Ce anomaly. The use of such multi-tracing allowed an efficient fingerprinting of two distint types of waters to get clues to further constrain both the dynamics of such islands and the functioning of the water system. Still in progress, the study will be completed by (i) the stable isotope analysis, (ii) the modeling of the minerals possibly at equilibrium with the waters and of the organic matter-trace element interactions, (iii) the speciation analysis of some enriched elements, (iv) the comparison between water and solid samples analyses and (v) the understanding of the relations in between the concentrations and locations in the hydrological system.</p>
We investigated the influence of hydroperiod variation on soil nutrient content in the Okavango Delta seasonal
floodplains. Soil samples were collected from eight zones of homogenous vegetation cover after low and high
floods and analysed for pH, Na, Mg, Ca, K and P content. A Student's
t
-test was used to test for differences
in pH, Na, Mg, Ca, K and P between soils after low and high floods. The Kruskal-Wallis test was used to
compare means of flooding duration and depth between low and high floods. Na, K, Mg, P and pH levels in
soils were significantly different (
p
<0.05) after low floods compared with after high floods. Na content was
lower (
p
<0.05) in Zones 2 and 8 and higher (
p
<0.05) in Zones 4, 5 and 6 during high flood than during
low flood. Ca content was lower (
p
<0.05) in Zones 1, 2, 3, 4 and 5 and higher in Zone 7 (
p
<0.05) under
high flood than during low flood. Mg content was lower (
p
<0.05) in Zones 1 and 5 and higher (
p
<0.05) in
Zones 6, 7 and 8 during high flood than during low flood. K content was lower in Zones 1 and 3 and lower
in Zones 6, 7 and 8 (
p
<0.05) during high flood than during low flood. pH was significantly lower and higher
after a high flood in Zones 1 and 6, respectively, than during low flood. P content was significantly (
p
<0.05)
higher in all zones after high flood than after low flood. Flooding depth and duration increased (
p
<0.05)
in all vegetation zones during high flood. Our results have direct implications for
molapo
(flood recession)
farming. We recommend that farmers plough immediately after the onset of flood recession when the soil is
still moist and rich in nutrients.
Data-poor tropical wetlands constitute an important source of atmospheric CH4 in the world. We studied CH4 fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH4 fluxes and controlling environmental factors in the delta's seasonal floodplains. Net CH4 emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg a−1. Microbial CH4 oxidation of approximately 2.817 × 10−3 ± 0.307 × 10−3 Tg a−1 in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH4 emissions from seasonal floodplains. The observed microbial CH4 sink in the delta's dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH4 fluxes in both the seasonal and occasional floodplains. The optimum SWC for soil CH4 emissions and oxidation in the delta were estimated at 50% and 15%, respectively. Electrical conductivity and pH were poorly correlated (r2 ≤ 0.11, p 4 fluxes in the seasonal floodplain at Nxaraga.This article is part of the discussion meeting issue 'Rising methane: is warming feeding warming? (Part 1)'.
The Okavango Delta in northern Botswana is a mosaic of water paths, floodplains and arid islands. The delta sits in the Okavango river basin, which spans three African countries: Angola, Namibia and Botswana.
