Abundance- and biomass-based metrics of functional composition of macroinvertebrates as surrogates of ecosystem attributes in Afrotropical streams

The composition of macroinvertebrate functional feeding groups (FFGs) has been used as surrogates of ecosystem attributes in aquatic ecosystems, but studies that utilize such knowledge are still limited in the tropics. This study investigated the suitability of abundance- vs. biomass-based metrics of macroinvertebrate FFGs as surrogates of ecosystems attributes of the Sosiani–Kipkaren River in western Kenya. Macroinvertebrates were sampled in wet and dry seasons, classified into five FFGs and used to derive five metrics that are surrogates of ecosystem attributes; (1) a balance between autotrophy and heterotrophy, (2) linkage between riparian inputs of coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM), (3) top-down predator control, (4) geomorphic channel stability, and (5) relative dominance of fine particulate organic matter (FPOM) in transport compared to FPOM deposited in the sediments. Taxon richness, abundance and biomass of shredders were higher in forested sites, and scrapers were numerically dominant in mid-order streams, whereas collectors dominated agricultural and urban sites. Abundance-based metrics were better predictors of ecosystem attributes and displayed greater response to changes in stream size than biomass-based metrics. Moreover, there was incongruence between abundance- and biomass-based indicators for P/R and CPOM/FPOM. Catchment land use did not influence metric performance, suggesting that reach scale influences played a predominant role in structuring communities and determining ecosystem functioning. Although the use of FFGs as indicators of ecosystem integrity and functioning in this river show promise, lack of agreement between abundance- and biomass-based measures suggests that more studies are needed to refine the metrics used.