Wetland mapping at 10 m resolution reveals fragmentation in southern Nigeria

Wetland ecosystems play key roles in global biogeochemical cycling, but their spatial extent and connectivity is often not well known. Here, we detect the spatial coverage and type of wetlands at 10 m resolution across southern Nigeria (total area: 147,094 km(2)), thought to be one of the most wetland-rich areas of Africa. We use Sentinel-1 and Sentinel-2 imagery supported by 1500 control points for algorithm training and validation. We estimate that the swamps, marshes, mangroves, and shallow water wetlands of southern Nigeria cover 29,924 km(2) with 2% uncertainty of 460 km(2). We found larger mangrove and smaller marsh extent than suggested by earlier, coarser spatial resolution studies. Average continuous wetland patch areas were 120, 11, 55 and 13 km(2) for mangrove, marsh, swamp, and shallow water respectively. Our final map with 10 m pixels captures small patches of wetland which may not have been observed in earlier mapping exercises, with 20% of wetland patches being < 1 km(;)(2) these were clustered around urban centres, suggesting anthropogenic wetland fragmentation. Our approach fills a knowledge gap between very local (< 400 km(2)) studies reliant on field studies and aerial photos, and low resolution (> 250 m pixel dimensions) global wetland datasets and provides data critical for both improving land-surface climate models and for wetland conservation.

Automated summary

This study used Sentinel-1 and Sentinel-2 imagery, supported by 1500 control points, to detect the spatial coverage and type of wetlands in southern Nigeria. The wetland area was estimated to be 29,924 km(2), with smaller patches of wetland observed compared to previous studies. 20% of the wetland patches were found to be clustered around urban areas, indicating anthropogenic wetland fragmentation. This research fills a data gap for land-surface climate models and wetland conservation.