Assessing floating treatment wetland effects on water quality at two scales and their potential to restore meadow marsh habitats

Abstract

Freshwater wetlands are important ecological habitats that are often degraded by water quality issues from runoff events. Floating treatment wetlands (FTWs) can be an effective method to reduce and retain excess phosphorus in watersheds through plant and microbial uptake. This study aims to support the literature on FTWs and fill gaps in knowledge by testing specific plant and substrate treatments on phosphorus reduction and investigating the sustainability of FTWs. I examined changes in phosphorus concentrations in mesocosms and retention ponds using wetland plant species native to northeastern North America: Carex stricta (tussock sedge), Iris versicolor (northern blue flag), Juncus effusus (common rush), and Eleocharis palustris (common spikerush), as well as with coconut coir substrate and no substrate. Plant combinations of the FTWs included tussock species (tussock sedge/northern blue flag), reed species (common rush/common spikerush), and a mixture of reed and tussock species. I measured the percent change in concentrations for total phosphorus (TP), orthophosphate, chlorophyll-a, dissolved oxygen, specific conductivity, and pH. I also looked at extending FTW lifecycles by investigating overwintering and establishment success of FTW plants for reuse in future projects. The mesocosm study was conducted as two separate experiments during the 2020 and 2021 summer field seasons. The 2020 mesocosm study investigated changes in phosphorus and water quality metrics between soaked coconut coir substrate and no-substrate FTWs, as well as between no-substrate planted FTWs and unplanted controls. I found that FTWs with coconut coir substrate had significantly greater orthophosphate reduction than FTWs with no substrate, but no significant differences in phosphorus reduction between no-substrate plant treatments and controls. The 2021 mesocosm study investigated changes in phosphorus and water quality metrics between FTW functional groups with unsoaked coconut coir substrate. I saw an increase in TP and orthophosphate concentrations, and no significant differences between the plant treatments due to phosphorus leaching from the unsoaked coconut coir substrate. For the retention pond study, I did not find any significant differences in changes in phosphorus concentrations between FTW plant treatment ponds and FTW control ponds due to the low FTW coverage ratio and sample size. During the establishment study, I found that tussock species were most effective in overwintering and establishment, while reed species had the greatest number of flowering individuals when planted in soil following the end of the experiment. The findings suggest that FTW studies and applications should use coconut coir substrate since it significantly reduced orthophosphate in mesocosms and improved overall growth of plants, though researchers should be sure to flush out any phosphorus that may leach from the coir. Floating treatment wetland applications can also use any native wetland plant species since no real differences were found in TP or orthophosphate concentrations between reed species, tussock species, or mixtures. Any FTW study conducted on larger scales should be sure to have FTW coverage ratios of at least 5% or greater to see any contribution of FTW plants to changes in phosphorus concentrations or impact on water quality metrics. Lastly, it is encouraged that any FTW study or application take into consideration the sustainability of the FTWs, and reuse plants for future applications or for wetland restoration efforts.