Growth response to nitrate enrichment helps facilitate success of an alien Potamogeton in New Zealand streams

Motivated by stream ecosystem degradation by eutrophication, we mimicked slow flowing lowland stream conditions with a novel experimental setup to further our understanding of aquatic plant responses to increases in nitrate and light. We conducted a mesocosm growth experiment of two species from the genus Potamogeton: P. crispus (alien) and P. ochreatus (native), grown at four nitrate and four light levels. We hypothesised that (i) internal nutrient status of the plants would scale with water column nutrient concentration, and that (ii) plant performance would reflect the nutrient status of the plant. Furthermore, we hypothesised that (iii) a low irradiance level would negate the effects of an increased nitrate level. In relation to (ii) we hypothesised that (iv) the traits of the alien species would enable it to outperform the native species where both the availability of light and nutrient resources was high. Internal tissue N content was broadly similar in the two higher (>250 μg NO(3)(−) L(−1)) and the two lower nutrient treatments (<20 μg NO(3)(−) L(−1)) in both species and plants were therefore collapsed into high and low N-groups. High-N individuals had higher growth rates than low-N ones regardless of species or light treatment and plants had reduced growth rates at the lowest light treatment, however this response was less evident for P. crispus. The highest growth rate was found at the high-N individuals of P. crispus at the highest light treatment, and correspondingly, in this treatment this species exhibited an increase in branching degree and lateral spread from the low-N plants. As P. crispus spreads by fragmentation, our results show it to be a highly effective competitor in anthropogenically impacted areas compared to its native counterpart. Our study exemplifies how light can influence eutrophication responses of plants and how both need to be accounted for in management decisions.