Experimental evidence that water-exchange unevenness affects individual characteristics of two wetland macrophytes Phalaris arundinacea and Polygonum hydropiper

Abstract

Water-exchange unevenness (WEU) is the temporal variation in water exchange rate that can be characterized by the coefficient of variation of water exchange rates across time. Although WEU is recognized as an important hydrological feature, little is known about its effects on aquatic ecosystems. Herein, two laboratory experiments were conducted to test the effects of WEU on 1) environmental conditions in the absence of plants, and 2) both environmental conditions and plant growth in the presence of two wetland macrophytes Phalaris arundinacea and Polygonum hydropiper. A two-factor factorial design comprising WEU (low-, intermediate-, and high-level of unevenness) and burial depth (0 and 5 cm) was used for both experiments. Compared to turbidity, pH, and dissolved carbon dioxide concentration of the waterbody, dissolved oxygen concentration showed the highest sensitivity to WEU, which decreased significantly with increasing WEU regardless of the absence or presence of plants. For the sediment, oxidation-reduction potential (ORP), NO3-N, total N, available P, and total P decreased consistently with increasing WEU, regardless of burial depth and the absence or presence of plants. Furthermore, deeper burial led to further decreases in soil ORP for all three WEU levels. Biomass accumulation decreased with increasing WEU and burial depth in both plant species. Additionally, plants developed shorter and thicker roots, and the root alcohol dehydrogenase activity increased, but there was a decrease in the root mass ratio, likely due to the plants adapting to anaerobic stress under higher WEU levels and/or deeper burial conditions. However, under deeper burial conditions, increased WEU generally did not lead to further root adjustments due to the limited nutrient absorption capacity. Consequently, plant biomass decreased by 49.82% for P. arundinacea and 76.94% for P. hydropiper from low to high WEU levels at 5 cm burial depth compared to values of 39.68% and 45.71% at 0 cm burial depth, respectively. Overall, our results demonstrated that WEU strongly influences the performances of wetland macrophytes through modulation of oxygen and nutrient availability. Considering that anthropogenic disturbance and climate change are markedly altering the WEU of lake ecosystems, we call for more research into this hydrological element to assess its potential ecological impacts.