Influence of historical landscape on aquatic plant diversity

Questions The historical composition of landscapes is recognized as an important factor for explaining plant diversity, because species assemblages are not only patterned by current ecological conditions, but also represent legacies of the past. Contrary to terrestrial ecosystems, the influence of history has rarely been considered for aquatic communities. Here, we examined the effect of past land use on aquatic plant communities of freshwater shallow lakes. We first aimed to identify the relevant scale for studying landscape effects on macrophyte communities and then to test the relative influence of environmental variables and past and recent land use on aquatic plant assemblages. Location Atlantic coastline, southwestern France. Methods We surveyed shoreline macrophyte communities of 17 lakes and used species accumulation curves to estimate taxonomic richness. We used water chemistry and lakes' morphological features as environmental variables and reconstructed land-use occupancies of lakes' watershed with aerial pictures of 1945, 1965, 1985 (past) and 2002 (recent). We examined landscape change with principal component analysis and taxonomic composition with non-metrical multidimensional scaling. We then evaluated the relative influence of morphological, chemical, past and recent land-use factors on taxonomic richness and composition with variation partitioning methods. Results We found that the studied landscapes exhibit a trend toward an increase of urban and arable fields areas at the expense of semi-natural ones. Water chemical composition and past land use of the whole watershed (particularly urban and semi-urban areas) were strongly inter-correlated, and explained the major part of the variation of taxonomic richness and composition. Conclusions These results underline the importance of historical factors in explaining aquatic plant diversity and suggest delayed responses of communities to anthropogenic pressures. Integrating historical factors in future analyses of aquatic ecosystems would thus greatly contribute to understanding ecological processes, and is crucial for the conservation and management of macrophyte communities.