Investigating the influence of habitat structure and hydraulics on tropical macroinvertebrate communities

Abstract

The influences of habitat structure and hydraulics on tropical macroinvertebrate communities were investigated in two foothill rivers of the Udzungwa Mountains (United Republic of Tanzania) to assist future Environmental Flow Assessments (EFAs). Macroinvertebrate samples, hydraulic variables and habitat structure were collected at the microhabitat scale (n=90). Macroinvertebrate communities were first delineated (i.e. clustered) through Poisson and negative binomial mixture models for count data in a semi-supervised mode by taking into account the sampled river. Then, genetically optimised Multi-Layer Perceptrons (MLPs) were used to identify the relationship of the most relevant variables with the delineated communities. Between the three delineated communities exclusively one community was shared between both rivers. The first and third communities presented similar values of richness (i.e. number of families) and diversity but the first was characterised by high abundance and was dominated by Baetidae (43.2%) while Hydropsychidae (36.3%) dominated the third community. The second community was dominated by Baetidae (33.4%), but it involved low abundance, richness and diversity samples and encompassed the microhabitats where no-macroinvertebrates were found. The performance of the MLP acknowledged the quality of the delineation and it indicated that the first community shows a clear affinity for microhabitats with aquatic vegetation and woody debris and the third for unshaded, fast flowing and shallow microhabitats on intermediate-sized substrate. Conversely, the second community occurred in deep and shaded microhabitats with low flow velocity and coarse substrate. We demonstrated that habitat structure and hydraulics are able to properly discriminate the macroinvertebrate communities, which, in turn, underlines their importance as drivers of community composition and abundance. Aquatic vegetation, woody debris, velocity and substrate index, followed by depth and shade, emerged as the most discriminant variables to understand macroinvertebrate communities in these tropical running waters. These results should enhance the implementation of ongoing and future EFA studies.