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 landuse 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 semiurban 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.
Freshwater ecosystems are among the systems most threatened and impacted by anthropogenic activities, but there is still a lack of knowledge on how this multi-pressure environment impacts aquatic communities in situ. In Europe, nutrient enrichment and temperature increase due to global change were identified as the two main pressures on lakes. Therefore, we investigated how the interaction of these two pressures impacts the community structure of the two extreme components of lake food webs: phytoplankton and fish. We modelled the relationship between community components (abundance, composition, size) and environmental conditions, including these two pressures. Different patterns of response were highlighted. Four metrics responded to only one pressure and one metric to the additive effect of the two pressures. Two fish metrics (average body-size and biomass ratio between perch and roach) were impacted by the interaction of temperature and eutrophication, revealing that the effect of one pressure was dependent on the magnitude of the second pressure. From a management point of view, it appears necessary to consider the type and strength of the interactions between pressures when assessing the sensitivity of communities, otherwise their vulnerability (especially to global change) could be poorly estimated.
Les auteurs tiennent particulièrement à remercier l'agence de l'eau Adour-Garonne pour son soutien financier, Kevin Petit et Daniel Uny pour leur soutien avec le système d'information géographique, Sébastien Boutry pour son aide sur la réalisation des graphiques, Hilaire Drouineau pour ses discussions statistiques et Jeanne Dachary-Bernard pour son aide dans la recherche de littérature socio-économique.Rôle de l'histoire du paysage sur la diversité des macrophytes dans les lacs ...
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