Geographic patterns of lake macrophyte communities and species richness at regional scale

QuestionsAre community composition and species richness of aquatic macrophytes determined primarily by local (habitat heterogeneity and water quality) or regional (climate) patterns at regional scale? Do two macrophyte functional groups (i.e. emergent and submerged macrophytes) respond similarly to local and regional patterns? Are lake macrophytes and explanatory variables geographically structured? LocationThe US state of Minnesota. MethodsThe community composition and species richness of aquatic flora was studied using presence-absence data in 454 lakes, covering the entire US state of Minnesota. In addition, community composition and species richness of emergent and submerged macrophytes was investigated separately. Variation partitioning based on partial redundancy analysis and partial linear regression was used to study the relative roles of water quality, habitat heterogeneity, climate and sampling effort in explaining community composition and species richness of lake macrophytes, respectively. ResultsMacrophyte community composition and species richness (all taxa and two functional groups) were explained by water quality and climate. Alkalinity and total phosphorus were water quality variables that most affected community composition of aquatic flora, and macrophyte species richness decreased with increasing concentrations of these two variables. Maximum temperature of the warmest month and mean annual temperature most affected plant community composition, whereas species richness had a negative relationship with minimum temperature of the coldest month. Most significant explanatory variables (e.g. alkalinity, total phosphorus and temperature) were geographically structured, showing a latitudinal change. ConclusionsCommunity composition and species richness of macrophytes were congruently influenced by regional (climate) and local patterns (water quality) at regional scale. Community composition and species richness of helophytes and submerged macrophytes were equally explained by environmental gradients. The latitudinal change in these most significant environmental variables was related to calcareous soils and intensive agriculture, which were situated in the southern part of the state. Macrophyte species richness showed a reverse latitudinal gradient, which was likely due to high nutrient concentrations in southern latitude lakes. Water quality primarily filters species from the regional species pool, allowing only species tolerating high nutrient concentrations, e.g. invasive plants, to survive at southern latitudes.