Urbanisation gradients, riparian-loss and contaminant effects on macroinvertebrate distribution within a tropical river (Nigeria)

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Abstract
Riparian vegetation and built-up density along the urbanised section of the Calabar river were estimated using NDVI (Normalised Differential Vegetation Index) and NDBI (normalised differential built-up index), respectively. Four sampling stations representing riparian density and built-up gradients i.e. Station 1 (sparse riparian + high-density built-up), 2 (sparse riparian + high to intermediate-density built-up), 3 (moderate riparian + intermediate to low-density built-up) and station 4 (moderate riparian + low-density built-up) were selected. Station-specific variations of metal (Pb, Cd, Cr, Fe and Cu) in surface water and benthic macro-invertebrate communities were examined between January and December 2013 and correlated using redundancy analysis (RDA). Buffer analysis revealed that stations having low NDBI with over 120 m dense riparian areas showed a marked abundance of muddy/coarse substrate while sparse riparian areas showed siltier and sandier substrate. Redundancy analysis revealed a marked delineation of macroinvertebrate occurrence with the riparian density of stations. High diversity macroinvertebrate assemblages showed a strong positive association with alkaline pH, the density of riparian vegetation and substrate type. This first report highlights the critical relationship between riparian density, water quality and benthic macroinvertebrates in a tropical river.

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