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Hot-spots in biological transformation of silica

Final Report Summary - HOBITS (Hot-spots in biological transformation of silica)

Goal of the project

The overall objective of the proposed research was to increase our understanding of the biological silicon (Si) processing in tropical river systems. We investigated Si cycling in large tropical wetlands and focussed on hot-spots in biological Si cycling. The objectives were met both through well-planned sampling expeditions and analysis of existing samples that have been collected previously as part of other projects. While the original objective included a campaign in the Okavango delta (Botswana) and the Fly River delta in Papua New Guinea (NG), in the end we decided to focus on the Okavango delta and perform two campaigns here. Cooperation was established with University of Botswana and Lund University and campaigns were co-financed by National Geographic, as actual costs doubled funds provided by HOBITS. We also analysed historically collected samples in the Fly River basin.

Campaigns

In September 2011 and 2012, we performed two 21 day sampling campaigns in the Okavango delta, in collaboration with University of Botswana (ORI) and Lund University (Sweden). The campaigns were co-financed by a Marie-Curie reintegration grant (HOBITS) and National Geographic. We sampled three transects along typical island-floodplain gradients in the Okavango delta at the end of the seasonal flooding in September 2011. We sampled soil water, vegetation, soil and sediments and surface water along these transects. Dissolved Si, N and P fractions were measured in all water samples, as well as biogenic Si in surface water. In vegetation, total nitrogen (N), phosphorus (P) and Si contents were analysed. In soils, biogenic Si content, total nitrogen and total P were analysed.

Using HOBITS money, we funded a two year Master of Philosophy student at ORI, Mosimane Keotshephile. He visited University of Antwerp from half March to half May 2012 to perform analyses. Our sampling campaign in September 2011 was highly successful: through cooperation with Lund University we also sampled for analysis of germanium (Ge) to Si ratios and silicon isotopes, useful tools to trace biological pathways of Si. Current results indicate that the delta is a huge bio-filter for Si, with vegetation and hydrology playing a crucial role in the storage and processing. The study will push forward understanding on Si cycling in tropical wetlands.

In September 2012, we performed a full trans-Okavango transect, sampling water at 30 stations to study in detail biogeochemical processing through the delta. Jonas Schoelynck joined the campaign to study the impact of aquatic macrophytes on Si and organic matter processing in the delta. This Okavango delta transect is unique: no research team has before studied Okavango delta water quality in this detail.

We also performed additional ex situ experiments for the functioning of aquatic macrophytes in wetland nutrient cycles, focussing specifically on Si. In September 2010, I got the chance to join a campaign on the Berg River in South-Africa, to study the riparian ecosystems and nutrient storage.

Results

Both Okavango delta campaigns produced a large set of samples. For both campaigns, not all samples have already been analysed. Especially concerning isotope measurements and Ge/Si ratios, the results are not yet available for campaign one. These results are analysed by Lund University. For campaign two, the analysis phase is even more in progress. We additionally started cooperation with University of Dresden (Joerg Schaller) to analyse the fate of trace metals in the delta.

In the first campaign, we investigated the typical island-floodplain gradient in the delta. We specifically targeted the distribution of Si, but also N and P in the surface 30 cm. While the precipitation of minerals in deeper layers in the islands has been documented before, the distribution of elements in the biologically influenced top layers was poorly known at best. Regarding Si, our results showed that large amounts of biogenic Si are stored in the top 30 cm of sediments and soil, especially in grass dominated plots. High concentrations up to 5 % of dry weight were observed. Our extrapolation shows that the Okavango delta stores about 4 kg of biogenic Si per m2 in the biologically active top layers. This shows the gigantic capacity of tropical wetlands to retain biogenic Si. In total, storage equals about 600 years of Si into the delta. This clearly indicates the large importance of internal recycling of nutrients in the delta to maintain the huge biodiversity in the Delta; the most abundant primary producers in the delta, diatoms and grasses, need Si for their production. Input is by far insufficient to sustain this food web base.

In the second campaign, we travelled the full length of the Okavango Delta, starting in the Panhandle near Namibia, finishing at Maun seven days later. No scientific campaign has ever collected a dataset of soil, sediments and water through the delta in the detail we did: in total we sampled at 30 spots along this transect, for dissolved nutrients, Si and a whole range of other elements, including magnesium (Mg), potassium (K), sodium (Na) and calcium (Ca) that are important for island formation processes. Although analyses are ongoing, it is already clear that large variation in concentrations exist through the delta that cannot only be explained by evaporation. To track the fate of Si and other elements, we sampled deep soil cores (up to 6 m) along an island-floodplain gradient. Our results show that the importance of surface biological processing of elements has been underestimated for the formation of the unique island-floodplain structure.

Conclusions

The Okavango delta, the largest inland wetland in Africa, hosts a unique biodiversity. Its productivity is largely due to a unique island-floodplain pattern. The storage and cycling of nutrients and salts across this gradient prevents salinification of the surface water and ensures efficient recycling of essential nutrients. While Si plays an important role in the island formation processes, its biogeochemistry in tropical wetlands remained unstudied before this project. We show that vegetation plays a previously unaccounted role in Si retention in the delta. The retention of Si by vegetation is essential to support the basic food web. The biogeochemical island-floodplain patterns we observed shed new light on the potential effects of hydrological and nutrient management in upstream regions. Increasing need for water as well as increasing nutrient input due to agriculture expansion might occur in the future. We show that nutrients will likely mostly be retained in the upper delta, in the large Papyrus floodplains. A reduction in water delivery would have crucial effects on the delta, as it would potentially reduce the surface area of high production wetlands, which could interfere with the food web in the delta.

Our research and our experiments have opened up new cooperation and research horizons for research in tropical wetlands and have allowed me to reintegrate efficiently in my old research group, by providing immediate chances to start a new research direction in the field of Si biogeochemistry. It also provided the opportunity to perform new research on interactions between Si cycling and other biogeochemical cycles, including the Si, N and P cycle.