Interpreting the effects of sand mining in Lwera wetland using landscape metrics and testing a rehabilitation approach

Abstract

Wetlands are one of the most valuable ecosystems but are fast getting degraded. Increasing threats of environmental degradation to wetlands has driven adoption of ecological rehabilitation as one of the tools for conservation. Rehabilitation has further been emphasized to compensate for the biodiversity values lost in carrying out development projects in fragile ecosystems. For a mined wetland at Lwera, an attempt was made to: (i) study the spatial patterns of wetland fragmentation using landscape metrics computed from satellite-based land cover classification, and (ii) test differences in establishment of the dominant wetland vegetation in the area (Cyperus Typha angustifolia and Phragmiles). To assess the mining footprint, the active mining wetland area was mapped and constrained around some area for landscape analysis using sensor data. To test a rehabilitation strategy, a northerly transect was selected with random intervals at pits where experimental pits were set. In each of the pits, six replicate plots were set up, that is, for each species two arrays of four platforms, one arranged parallel and the other conterminous at the edges. The platforms were each lined with perforated sisal bags anchored on supports at the platform corners. Rhizomes of the plants were then planted on the platforms where wetland soil material had been placed. The results of landscape analysis showed fragmentation of the wetland, mostly by mining activities. For instance, using a representative metric, namely vegetation patch number, it was shown that the number of patches under vegetation increasesd from 400 in 2016 to 620 in 2017 and then to 7 10 in 2018, depicting a discontinuous and patchy landscape and with this was a change in landscape structure. Further, the results on testing establishment of the native wetland species showed that the species rapidly established, but Papyrus was particularly more successful growing up to a shoot length of 94cm as compared to Typha (80. I 4cm) and Phragmites (64cm). It was also shown that the distance from the lake had a significant effect on establishment of the three species tested (P S 0.05). The results were beneficial in reporting the effects of mining on the wetland and demonstrated the possibility of using remote sensing to quantify spatial changes in the wetland. It was however clear that expansion dynamics of the planted species cannot be studied in a short period of time which calls for a long term study.