Aquatic vascular plants (macrophytes) play an important role in nutrient cycling in aquatic systems. Macrophytes incorporate nitrogen (N) and phosphorus (P) from the substrate into their above-ground tissues and then leach these nutrients into the water column upon senescence, thereby acting as biological nutrient pumps. The flux of nutrients is influenced by both species identity and life form (e.g., submerged vs. emergent). Life form, however, is often linked to species confounding our ability to examine the relative importance of these two factors. Here, we examine three aquatic plant species capable of expressing both submerged and emergent life forms to determine the relative influence of form versus species on N and P cycling in aquatic systems. We used an outdoor microcosm array with two factors: life form (submerged vs. emergent) and species (Hippuris vulgaris, Megalodonta beckii, or Myriophyllum heterophyllum). Plants were treated with 10 μM abscisic acid (ABA) to induce the emergent form. Each species-life form treatment combination was replicated 25 times, using randomly distributed 15 L tanks each containing one individual plant (150 independent tanks and individuals in total). Water depth, light (PAR), temperature, and sediment and water nutrient concentrations did not differ significantly among experimental tanks.
Results/Conclusions
In total, 149 of 150 plants survived the 12 week experiment and average biomass increased by 141%. ABA hormone application resulted in 60 of 75 (80%) of the treated plants expressing emergent life forms; all but 3 of the 74 (96%) non-treated plants remained in submerged form. Overall biomass differed significantly by species (F: 23.1, p-value: < 0.0001; ANOVA) with Hippuris being larger than Megalodonta and Myriophyllum. Emergent biomass alone varied significantly with hormone treatment and species (F: 9.7, 24.7; p-values: 0.002, <0.0001 ; two-way ANOVA), and showed only marginal interaction (p-value: 0.054). Preliminary chemical analysis of submerged Myriophyllum revealed average tissue nutrient concentrations of 23.92 (+/- 0.13) mg N/g dry mass (DM) and 3.18 (+/- 0.04) mg P/g DM. Emergent Hippuris showed tissue nutrient concentrations of a similar magnitude to Myriophyllum: 20.22 (+/- 0.44) mg N/g DM and 5.73 (+/- 0.14) mg P/g DM. These species-form combinations significantly differ in nutrient concentration by species (F: 22.4; p-value: < 0.0005; ANOVA) and differed in their N:P ratios. Analysis of remaining samples will allow us to determine if species or life form is a more important determinant of nutrient cycling potential in aquatic plants.