In lake floating treatment wetlands could provide algae control through unsuspected mechanisms.

摘要

Cultural eutrophication has been negatively impacting both artificial and natural water bodies by triggering (at times harmful) algal blooms. These blooms are typically controlled by algaecides, which can collaterally affect aquatic organisms thus further damaging the hydrosystem. Preventing point and non-point nutrient sources from entering hydrosystems may not be sufficient to prevent cultural eutrophication since internal nutrient loading is generally high. Internal loading can however be reduced, often at great expense, through i) demucking, ii) chemical treatment, and iii) hypolimnetic aeration. Floating Treatment Wetlands (FTWs) are a new type of phytoremediation being used to reduce the impact of eutrophication. FTWs consist of plants grown hydroponically on a floating mat which uptake water nutrients from their roots. The nutrients become tied up in the tissues of the young growing plants, which tissues are then harvested before fully mature to remove the excess nutrients from the system to reduce external and internal loading. Nutrient removal performed by these FTWs is limited unless they cover 5-10% of the surface of the water body. However, most hydrosystems use a lower, e.g. 3% cover rate with often great algal control. Thus, poorly understood or identified mechanisms must be at play. A 2-year investigation was therefore led in three subtropical manmade urban ponds (Pond A, Livingston Pond and Collier Pond) of about an acre and located within the City of Naples, FL to examine whether the dense root network from FTWs' plants, Juncus effusus and Canna flaccida, would i) have allelochemicals able to control algae, ii) offer diurnal zooplankton protection against predation so that nocturnal grazing would be intensified and iii) harbor beneficial bacteria able to remove water nutrients. For the allelopathy experiments, two methods were used: a liquid culture assay as well as an agar diffusion assay, both in which wells containing algae were inoculated with methanolic extracts from the roots potentially containing allelochemicals. Zooplankton presence/absence studies were completed by performing vertical tows underneath the FTW and in the open water column both during the day and night to explore their nycthemeral horizontal migrations. J. effusus and C. flaccida root microbial communities were characterized by extracting DNA from the biofilm living on root samples using the phenol-chloroform extraction method. The purified DNA was used for 16S rRNA gene high-throughput sequencing to compare community structure. Results showed that chemicals present in the two plants examined could either control algae (especially Cyanophyceae) but in some cases also enhance algae growth (especially Chlorophyceae). When inhibition was found, the agar diffusion assay displayed stronger inhibition than the liquid culture assay in which allelopathic compounds were diluted in liquid medium. In Livingston Pond and Collier Pond, it was determined that zooplankton did not utilize the root systems of FTWs, which is typical of subtropical systems. The bacterial community of Pond A FTWs was typical of oxic as well as anoxic and even anaerobic environments despite the presence of dissolved oxygen in the water underneath the FTWs. Based on the microbial community composition, it is hypothesized that the microbial biofilm growing on the root changed from oxic, to anoxic and anaerobic from its surface to its basal layer. Thus, FTW biofilms provide an environment in which major biologically mediated reactions could potentially occur (e.g. nitrification, denitrification, and sulfate reduction). Based on the results of this study it appears that FTWs have potential mechanisms to control algae growth aside from nutrient uptake. Allelopathy and a microenvironment prone to degradation were such mechanisms but more investigation should be conducted to fully understand the net algal control and impact on nutrient cycling. Future research should include the investigation of these parameters on FTWs in different hydrosystems to determine if the findings of this study are common for all FTWs, or if each FTW provides its own unique environment.