Trends in macrobenthal biotypes of Imo River in a Nigerian Delta region

Dynamics in macrobenthos abundance and diversity in sand-mined Imo River in Etche, South-eastern Nigeria has been investigated. While 14 representative species, comprising 3 classes (insect, oligochaeta, and gastropoda) were earlier identified in 2003, 9 species of these classes and 5 species comprising 2 classes only (insecta and oligochaeta) were identified in 2009 and 2010, respectively. Though Margalef's diversity had increased from 1.67 in 2003 to 2.30 in 2009, it sharply decreased to 0.69 in 2010. Over these periods, entomofauna (class insecta) predominated the faunal assemblages with 8 species from 6 families in 2003, 7 species from 5 families in 2009, and 3 species from 2 families in 2010. Significant spatial variance inequality in macrobenthos abundance was observed at P<0.05 [F(240.54)>Fcrit(3.89)]. Further, there was variance inequality in means of macrobenthos abundance between 2003, 2009, and 2010 [F(6.54)>Fcrit(4.03)] at P<0.05. A further structure detection of mean difference revealed that inequality was most accounted for by the larva of the insect- Sialis luteria that was conspicuously absent in the 2009 and 2010 identifications. There was generally high interspatial community dissimilarity, except an average similarity of 50% (0.50) recorded between locations 2 and 6, and 3 and 7 in 2009, as well as 50% between locations 1 and 3 and 2 and 6, and 67% between locations 2 and 7, and 3 and 7 in 2010. Benthic sediments were slightly acidic (6.95±0.01), with low nitrate (0.48±0.03), phosphate (0.72±0.04), and sulphate (44.35±1.57) concentrations. We suspect declining macroinvertebrate diversity and abundance in the river, and that ongoing anthropogenic perturbation (sand mining) contributes immensely to this paucity through the excavation and removal of benthal nutrients necessary for a thriving and diverse sediment community.