The Zambian Macrophyte Trophic Ranking scheme, ZMTR: A new biomonitoring protocol to assess the trophic status of tropical southern African rivers

The Zambian Macrophyte Trophic Ranking system (ZMTR) is a new bioassessment scheme to indicate the trophic status of tropical southern African river systems. It was developed using a dataset of 218 samples of macrophytes and water chemistry, collected during 2009–2012, from river sites located in five world freshwater 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 from SRP 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. ZMTRsample values were calculated as the mean ZTRSsp score of species present per sample. ZMTR indicated trophic status reasonably accurately for 83.1% 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.