The Zambian macrophyte trophic ranking scheme, ZMTR:a new biomonitoring protocol to assess the trophic status of tropical Southern African rivers

Abstract

tThe Zambian macrophyte trophic ranking system (ZMTR) is a new bioassessment scheme to indicate thetrophic status of tropical southern African river systems. It was developed using a dataset of 218 samplesof macrophytes and water chemistry, collected during 2009–2012, from river sites located in five worldfreshwater ecoregions primarily represented in Zambia. A typology based on these ecoregions, and three stream order categories, was used to determine soluble reactive phosphate (SRP) reference conditions. Zambian Trophic Ranking Scores (ZTRSsp) were calculated for 156 species, using direct allocation fromSRP data for 80 species, in samples for which sufficient available SRP data existed. An indirect quantitative procedure, based upon occurrence of species in six sample-groups, of differing mean SRP status, produced by TWINSPAN classification, allocated provisional ZTRSsp values for the remaining 76 species. Additional data for nitrate, pH, alkalinity and conductivity were used to help assess the trophic preferences of macrophyte species showing differing ZTRSsp values. ZMTR samplevalues were calculated as the meanZTR Sspscore of species present per sample. ZMTR indicated trophic status reasonably accurately for83.1% of Zambian samples, and for all samples within a test dataset from Botswanan rivers. Examples of application of the methodology, and its potential for hindcasting river trophic status are provided. The scheme currently underestimates highly-enriched conditions, and, to a lesser extent, overestimates the trophic status of some very low-nutrient rivers, but at this pilot stage of development it generally predicts the trophic status of tropical southern African river systems quite well.

Keywords:Biomonitoring,Aquatic macrophytes,Eutrophication,Tropical Africa Southern African river assessment scheme