Environmental drivers of freshwater macrophyte diversity and community composition in calcareous warm‐water rivers of America and Africa
Authors: Tapia Grimaldo Julissa , O'Hare Matthew T. , Kennedy Michael P. , Davidson Thomas A. , Bonilla‐Barbosa Jaime , Santamaría‐Araúz Betzy , Gettys Lyn , Varandas Martins Sara , Thomaz Sidinei M. , Murphy Kevin J. 
Published: June 2017

This study assessed the hypothesis that spatial and environmental drivers of river macrophyte diversity and community composition differ in relative importance in calcareous river systems located in warm regions of America versus Africa.

We collected aquatic vegetation and spatio‐environmental data, during 2006–11, from >200 hardwater rivers, and associated floodplain waterbodies, located up to 30° north or south of the Equator, in México, Trinidad, Brazil, Argentina, U.S.A. (Florida), South Africa, Botswana and Zambia.

Species rarefaction procedures were used to assess the impacts of differing sampling effort in the two continents upon estimation of γ‐diversity (“species pool”). We then used a cluster analysis approach (two‐way indicator species analysis:TWINSPAN) to classify samples into groups based upon species composition. Variation in species richness, community composition and six spatial and environmental variables, among samples making up these groups, were compared usingANOVAand Kruskal–Wallis procedures. Regression trees and redundancy analysis were used to infer the relative importance of spatial and environmental drivers in explaining variation in local species richness and species community composition between the two continents. Sorensen's index (Cs) was calculated to estimate species turnover (β‐diversity) between African and American samples.

In total, 378 macrophyte taxa were recorded, with no significant difference in mean macrophyte α‐diversity between African and American sites, but with evidence for high species turnover between the two continents (Cs = 0.17). Rarefaction analysis confirmed the existence of a larger macrophyte species pool in the hardwater rivers sampled in Africa compared to America.TWINSPANclassification identified seven sample end‐groups, only one of which contained a mix of sites from both continents.PERMANOVAand non‐metric multidimensional scaling ordination analysis confirmed significant differences in community composition present in these sample‐groups. There were substantial differences between the sample‐groups for α‐diversity, and for spatial and environmental variables.

The high species turnover between Africa and America may be accounted for by geographical segregation, along with differences in aquatic habitat characteristics, and varying long‐distance dispersal capacities of individual species. The relative importance of spatial and physicochemical drivers (latitude,pH, altitude, alkalinity and electrical conductivity but not flow) differed between the continents in influencing variation in both macrophyte diversity and community composition. Latitude was a significant, although nonlinear and rather complex, spatial driver of macrophyte α‐diversity in both American and African hardwater rivers. Water chemistry variables varied in relative importance as drivers of macrophyte α‐diversity for African and American sites individually, and for all sites combined, butpHand/or electrical conductivity were more important than alkalinity in each case. In all three cases, altitude was consistently the third most important driver of α‐diversity. Spatial and environmental variables played important roles in structuring macrophyte community composition in warm‐water calcareous rivers in both America and Africa, with latitude being the strongest individual driver. Thus, this spatial variable, which is a surrogate for numerous enviro‐climatic variables, appears to be of importance in determining macrophyte distributions at large spatial scales, for the ecosystem type examined here.