The biodiversity of aquatic ecosystems is under threat and there is an urgent need to quantify the various facets of biodiversity to assess the conservation value of freshwater ecosystems. The effects of taxonomic relatedness have so far not been taken into account in biodiversity assessments of lake macrophytes. We therefore tested the response of species richness and average taxonomic distinctness (AvTD) of aquatic macrophytes along environmental gradients using linear regression models and Bayesian Information Criterion variable selection method. We selected data from four regions, each with 50–60 lakes, situated in northern Europe (Finland and Sweden) and northern America (Minnesota and Wisconsin). We separately studied all macrophyte species, hydrophytes and helophytes. Species richness and AvTD of aquatic macrophytes were generally negatively related in all regions, although it was not statistically significant. Both biodiversity measures responded to environmental gradients to various degrees among the studied macrophyte groups and regions. Species richness was best explained by alkalinity and lake area in Finland, by elevation, annual mean temperature and total phosphorus in Minnesota, and by alkalinity in Wisconsin. AvTD was best explained by alkalinity, annual mean temperature and total phosphorus in Finland and by alkalinity in Wisconsin. Very weak relationships were found in Sweden. Our findings strongly suggest that complementary indices are needed to indicate more comprehensively the effects of environmental conditions on freshwater biodiversity. Species richness was found to be a better measure than AvTD to account for conservation value in freshwaters. However, further research is required to evaluate the usefulness of AvTD to indicate conservation value (e.g. randomisation tests), because alternative measures are clearly needed for those freshwater taxa lacking complete information on true phylogenetic diversity.
Bibliographical noteFunding Information:
Comments from Benoit Demars and two anonymous reviewers helped us to considerably improve this work. Sampling of Finnish macrophyte data was a joint contribution of Biological Monitoring of Finnish Freshwaters under diffuse loading project (XPR3304) financed by Ministry of Agriculture and Forestry and national surveillance monitoring programmes of lakes. Swedish macrophyte data were surveyed within the Swedish Monitoring Program of macrophytes in lakes funded by the Swedish Agency for Marine and Water Management. We are grateful for Minnesota and Wisconsin Departments of Natural Resources for collecting the macrophyte data. We especially thank Carol Reschke from the University of Minnesota Duluth for her work in combining and performing quality control for the Minnesota macrophyte data used in the analysis, and the Minnesota DNR staff for collecting the macrophyte data. This study was supported by grants from the Academy of Finland (267995 and 285040). This is contribution no. 607 of the Natural Resources Research Institute of the University of Minnesota Duluth.
- aquatic biodiversity
- aquatic plants
- freshwater macrophytes
- taxonomic diversity