Response Of Ceratophyllum Demersum To Cabomba Caroliniana Invasion Under Phosphorus Stress

The invasion of Ceratophyllum demersum by Cabomba caroliniana seriously destabilizes the ecosystem of our water bodies,and the eutrophication of water bodies will intensify the reproduction and expansion of alien aquatic plants,and phosphorus is a more important factor leading to the eutrophication of water bodies.In this study,using Ceratophyllum demersum as the main research object,we investigated the response of Ceratophyllum demersum to Cabomba caroliniana invasion under phosphorus stress by simulating different concentrations of phosphorus and different invasion levels of Cabomba caroliniana,and measured the physiological,growth indicators of Ceratophyllum demersum,Cabomba caroliniana and water quality indicators.The main conclusions are as follows:1.Under the single stress of phosphorus,the growth of Ceratophyllum demersum followed the principle of"low promotion and high inhibition".The fresh weight of the control group(0 mg·L^-1)increased with time,with an increase of 7.17%at the 50 d compared to the 0 d.The low phosphorus(0.2 mg·L^-1)promoted the growth of Ceratophyllum demersum,with an increase of 9.64%compared to the control group（the 50 d）;The highest biomass growth was observed in the medium phosphorus group(1.0 mg·L^-1),with an increase of 21.31%compared to the control group（the 50d）;the high phosphorus group(5.0 mg·L^-1)showed a negative biomass growth of6.03%compared to the control group（the 20 d）,with a decrease in chlorophyll and soluble protein（SP）,and a significant increase in peroxidase（POD）activity and malondialdehyde（MDA）.2.Under the single stress of Cabomba caroliniana invasion,the low（25%）,medium（50%）and high（75%）invasion intensities all had a certain inhibitory effect on the growth of Ceratophyllum demersum in the first 40 d,which were reduced by1.88%,1.80%and 1.80%,respectively,compared with the control group（the 40 d）;at the 50 d,the medium and high invasion intensities of Cabomba caroliniana had a certain promoting effect,but the difference was not significant compared with the control group,while the survival rate of Cabomba caroliniana was higher under high invasion intensity,so Cabomba caroliniana with high invasion intensity was more likely to invade Ceratophyllum demersum.3.Under the combined stress of phosphorus and Cabomba caroliniana invasion,the different invasion levels of Cabomba caroliniana had a significant inhibitory effect on the fresh weight of Ceratophyllum demersum until the 20 d of low phosphorus,which were 2.87%,3.24%and 3.46%lower than the control group,respectively.In the 50 d of medium phosphorus,the fresh weight of Ceratophyllum demersum was significantly promoted when the invasion level of Cabomba caroliniana was 50%,which increased by 3.53%compared with the control group,and the fresh weight of Ceratophyllum demersum was significantly inhibited when the invasion level of Cabomba caroliniana was 25%and 75%,which decreased by 1.41%and 1.47%compared with the control group,respectively.The fresh weight of Ceratophyllum demersum under the same treatment time of high phosphorus increased significantly with the increase of Cabomba caroliniana invasion degree,and increased by 8.84%,11.04%and 16.86%respectively compared with the control group at the 50 d.Low invasion level of Cabomba caroliniana did not change the highest peak value of phosphorus-stressed Ceratophyllum demersum,but the peak value of Ceratophyllum demersum fresh weight gradually shifted back under medium and high invasion intensity,which improved the tolerance of Ceratophyllum demersum to phosphorus.4.The phosphorus adaptation of invasive Ceratophyllum demersum also follows the rule of"low promotion and high inhibition".It is more suitable for low phosphorus,no phosphorus,medium phosphorus,high phosphorus when the stress is severe,rapid decomposition,rapid reduction in biomass,reduced chlorophyll content,POD activity,MDA content continues to increase;high phosphorus invasion degree of75%of the Cabomba caroliniana decomposition faster,compared with the without phosphorus group of fresh weight decreased by 20%（the 30 d）,the competition effect on Ceratophyllum demersum weaker.5.Changes in water phosphorus after 50d of Cabomba caroliniana invade Ceratophyllum demersum under phosphorus stress showed that total phosphorus removal and particulate phosphorus（PP）increased with increasing phosphorus concentration under phosphorus mono-stress;there was no significant difference in water column phosphorus between groups under Cabomba caroliniana invasion mono-stress.Under the combined stress of phosphorus and Cabomba caroliniana invasion,the total phosphorus removal increased and PP decreased with the increase of Cabomba caroliniana invasion at high phosphorus level,and the high invasion level increased by 340.00%and decreased by 58.24%,respectively,compared with no Cabomba caroliniana invasion.It indicates that the increase of phosphorus will promote plant decomposition,and the invasion of Cabomba caroliniana under high phosphorus increases the total phosphorus removal and alleviates the phenomenon of plant decomposition.6.Correlation analysis showed that the growth and physiological response of Ceratophyllum demersum were significantly correlated with the degree of Cabomba caroliniana invasion and phosphorus concentration.Unlike the significant change of chlorophyll a of Chrysophyceae,the change of chlorophyll-b of Cabomba caroliniana was highly significantly correlated with phosphorus in the water column,so that Cabomba caroliniana utilizes weak light more strongly.In addition,the increase of invasion degree of Cabomba caroliniana promotes the synthesis of chlorophyll of Ceratophyllum demersum,which increases its stem length thus increasing the fresh weight,which may be one of the important reasons for the symbiosis of Cabomba caroliniana and Ceratophyllum demersum.In conclusion,the invasion of Cabomba caroliniana inhibited the growth of Ceratophyllum demersum to some extent at low and medium phosphorus,while the phosphorus stress on Ceratophyllum demersum was alleviated at high phosphorus,probably as a result of the combined effect of lower biomass of Cabomba caroliniana,higher total phosphorus removal,and stronger weak light utilization ability of Cabomba caroliniana than Ceratophyllum demersum.This study provides directions to protect the habitat of Ceratophyllum demersum and to more rationally utilize and protect our aquatic vegetation resources and manage water ecosystems.