Phosphorus Removal Through Plant Assimilation in Floating Treatment Wetlands

By Mia Oliveira, Sara Sampieri Horvet, Micheline Labrie 

Affiliation: Coastal Systems Program, Department of Estuarine and Ocean Science, School for Marine Science and Technology 

Abstract 

Excess phosphorus (P) in freshwater systems contributes to the degradation of aquatic habitats through algal blooms and low dissolved oxygen levels. This study investigates the deployment and effectiveness of floating treatment wetlands (FTWs) as a method for phosphorus removal in Long Pond, located in Barnstable, MA. Preliminary results demonstrate species-specific differences in P assimilation and overall plant biomass increase during the first year of growth. 

Introduction 

Freshwater ponds and lakes in Massachusetts are degraded by excess inputs of phosphorus (P). Bioavailable dissolved forms of P, such as ortho-phosphate (PO43-), can stimulate phytoplankton blooms, potentially containing toxins. These blooms lead to low water column dissolved oxygen, resulting in poor habitat health. 

Long Pond (Barnstable, MA) is currently impaired by excessive P loading, primarily from wastewater sources (Eichner et al. 2022). Although sewering is expected to significantly reduce P inputs, it will not be implemented for ~25 years. The Town of Barnstable is exploring low-cost, non-infrastructural alternatives that can be implemented within 5 years. One such option is the deployment of floating treatment wetlands (FWTs). FTWs transform bioavailable P into plant biomass, which can be permanently removed upon harvest, effectively reducing P levels in the water (Lane et al. 2016). 

Using guidance from the Cape Cod Commission (Eichner et al. 2003), the target total P in the water column is 7.4 kg, consistent with levels measured in 2013 (7.8 kg) and 2011 (6.4 kg). This target contrasts sharply with the 2021 measurement of 16.2 kg, reflecting the need for additional water quality data collection and P reduction. 

Objective: Quantify in-pond P removal by FTW plant biomass via assimilation by seven aquatic plant species. Ultimately, success will be evaluated based on total P removal (kg/year), in situ P reduction from water quality measurements, and cost per kg P removal for full-scale implementation. 

Figure 1.

(Left) Aerial image of Long Pond, a relatively shallow, ~50-acre Great Pond. It is a public resource and subject to MA and federal regulations. The gray square represents the FTWs deployment location. Blue stars represent 2024 water quality sampling sites.

(Right) September 2024 Photo of the FTW rafts. 

Materials and Methods

Study Site

Ten FTW rafts were installed in the northern section of Long Pond in April 2024. Each raft measured 2.0 m × 2.1 m and consisted of a buoyant, high-density polyethylene matrix with designated planting holes for 36 (n=6) or 72 (n=4) plants per raft (Figure 2). The selection of plant species and planting configurations was directed by the Town of Barnstable, with 5 cm plugs sourced from New England Wetland Plants, Inc. 

Figure 2. Aquatic plant species and planting configuration determined by the town. The town obtained 5 cm plugs from New England Wetland Plants, Inc. 

Plant biomass measurements and tissue samples were collected in May (pre-planting) and September 2024 (end of Year 1 growing season). Sub-samples included whole plant wet weight, and wet and dry weights of above and below-water biomass. At the Year 1 season’s end, the ten rafts were surveyed to determine plant survival. 

Biomass was dried to a constant weight of 65°C and ground using a ball mill. Total P was measured via persulfate digestion and analyzed by the ascorbic acid-molybdenum blue method (Method 4500-PE) using a spectrophotometer set at 882 nm at the Coastal Systems Program Analytical Facility. 

Figure 3. Mia Oliveira conducting persulfate digestions at the Coastal Systems Program, SMAST-West. 

Results 

Plants deployed in rafts with 36 planting holes showed slightly higher survivability compared to rafts with 72 planting holes (Table 1). Preliminary data from the town indicated that most plant mortality occurred during July. Higher plant mortality coincided with greater incidence of weed growth. Species with higher survival rates contributed proportionally more to total P removal. 

Table 1. Summary of plant species remaining at the end of Year 1 for each raft. On average, rafts had 67% and 55% survival with 36 and 72 planting holes, respectively. 

Species	Raft #
	36 planting holes						72 planting holes
	1	2	3	4	5	6	7	8	9	10
C. lurida	9	5	9	2	8	17	9	0	7	14
J. effusus	6	8	8	10	8	3	9	21	13	11
P. cordata	0	0	0	0	0	0	0	0	0	0
A. americanus	3	2	2	4	13	3	5	10	11	10
A. incarnata	4	5	5	2	0	0	12	8	1	8
S. atrovirens	3	0	0	0	0	0	1	0	0	0
V. noveboracensis	0	0	1	2	1	1	2	0	3	4
Percent Survival	69%	56%	69%	56%	83%	67%	53%	54%	49%	65%

Initial P content varied across plant species (Figure 4). Acorus americanus displayed the highest P concentration in shoot biomass and significant root P content. Viburnum noveboracensis exhibited higher root P levels compared to shoots, though data was limited (n=1). Comparing initial and final phosphorus contents, phosphorus (umol/g dry plant biomass) decreased over the growing season. On average, root and shoot phosphorus content decreased by 58% and 52%, respectively. 

Whole plant biomass increased across all species from deployment to the end of Year 1, except for P. cordata, which showed no growth. C. lurida had the highest biomass increase, reaching 500 g wet weight. Sub-sampling in September 2024 identified a total of 80 C. lurida individuals, accounting for a 37 kg biomass increase. Similarly, J. effusus contributed 30 kg of biomass over the growing season. Overall, given the wet biomass increase (Figure 5) and the total number of plants remaining at the end of the growing seasons, the total wet biomass increase was approximately 100 kg. 

Figure 4. Total phosphorus content (umol/g; dry wt. basis) of plant roots and shoots subsampled at the time of deployment (a. Initial) or at the end of the Year 1 growing season (b. Final). 

Figure 5. Whole plant biomass (g wet wt.) measured at the time of deployment (May 2024) and at the end of the first year growing season (September 2024). 

Discussion 

The results highlight the different phosphorus uptake capacities among studied aquatic plant species. The growth and P assimilation data demonstrate the potential effectiveness of FTWs as a P mitigation strategy. However, the feasibility of FTWs will likely depend on the biomass increases observed during the second year growing season.

C. lurida and J. effusus showed promise for selection for future FTW implementations, given their biomass increase. It is likely that biomass increase, rather than phosphorus content, will control total P assimilation. The lower performance of P. cordata suggests that careful species selection is important for optimizing FTW efficiency. Factors such as planting density, environmental stressors (competition, predation), and species-specific nutrient uptake mechanisms likely influenced survival and growth outcomes.

Additionally, the cost-effectiveness of FTWs should be assessed relative to other mitigation strategies. Integrating FTWs with other methods, such as aluminum sulfate treatment, sediment dredging, or aeration systems, could provide a more comprehensive and reliable approach to P management in Long Pond. Future research should focus on long-term monitoring of FTWs, including multi-year survival rates, biomass increase potential, and the ecological impacts of large-scale deployments. Additional work is also needed to quantify in situ water column P reductions and evaluate potential secondary benefits, such as enhanced habitat for aquatic organisms and sedimentation. 

Acknowledgements 

We thank the Office of Undergraduate Research and the Town of Barnstable Department of Public Works for their invaluable support. Special thanks to the individuals and organizations dedicated to the protection and restoration of Barnstable’s aquatic ecosystems.  

References

Eichner, E.M., T.C. Cambareri, G. Belfit, D. McCaffery, S. Michaud, and B. Smith. (2003).

Cape Cod Pond and Lake Atlas. Cape Cod Commission. Barnstable, MA.

Eichner, E., Howes, B., & Schlezinger, D. (2022). Long Pond management plan and diagnostic assessment. Town of Barnstable, Massachusetts. Coastal Systems Program, School for Marine Science and Technology, University of Massachusetts Dartmouth. New Bedford, MA. 110 pp.

Lane, S., Sample, D., Lazur, A., Winston, R., Streb, C., Ferrier, D., Linker, L., & Brittingham, K. (2016). Recommendations of the expert panel to define removal rates for floating treatment wetlands in existing wet ponds. Final Report to the Urban Stormwater Work Group.

Standard Methods for the Examination of Water and Wastewater, 19th edition. Method 4500- PE.