Do interactions between eutrophication and CO2 enrichment increase the potential of elodeid invasion in tropical lakes?
Understanding the roles of eutrophication and CO2 enrichment in the invasive success of aquatic plants is an ecological challenge with relevance to climate change. We tested the hypotheses that (1) eutrophication of freshwaters increases the invasive success of the submersed aquatic plant Hydrilla verticillata; (2) CO2-enrichment makes freshwater systems more prone to H. verticillata invasion; and (3) interactions between eutrophication and CO2 enrichment increase the potential of H. verticillata invasion. We carried out a factorial experiment crossing eutrophication (nutrient and phytoplankton addition) and CO2 enrichment to compare the growth attributes of the invasive H. verticillata with those of the native Egeria najas (both Hydrocharitaceae). We found that eutrophication and CO2 enrichment had different effects on growth attributes. Eutrophication decreased the growth rate and lateral branch formation of both species, but the invasive species continued producing more branches than the native one. These results suggest that the invasive species has an increased advantage for spreading under eutrophic conditions. In contrast, CO2 enrichment increased the growth rate and lateral branch formation of both species, but only under eutrophic conditions. Consequently, CO2 enrichment largely counteracted the negative effects of eutrophication on these two macrophytes. Our results indicate that the interaction between freshwater CO2 enrichment and eutrophication may enhance the invasion success of H. verticillata, and potentially other Hydrocharitaceae, due to an increase plant growth and in propagule pressure.