Introduction
Semi-intensive common carp (Cyprinus carpio) pond aquaculture in Central and Eastern Europe provides valuable ecosystem services, including water retention and biodiversity support, yet also contributes to nutrient pollution and the risk of eutrophication. In 2022, European (EU27) carp production reached 63,865 t with a market value of nearly ���190 million (FAO, 2024). Despite regulatory limits of 1,500 kg biomass ha-1 (in Poland), effluent loading from semi-intensive systems can exceed the thresholds of the EU Water Framework Directive (EU-WFD, 2000) for total nitrogen (TN) and total phosphorus (TP). Phytoremediation — the use of aquatic macrophytes to remove nutrients from water — offers a promising, low-cost nature-based solution (NbS) that can simultaneously improve discharge quality and generate marketable biomass. Watercress (Nasturtium officinale) is a nutrient-dense, semi-aquatic perennial valued for its vitamins A, C, and K, as well as antioxidant and cancer-protective properties (Gill et al., 2007). Duckweed (Lemna minor) is a fast-growing floating macrophyte that contains up to 45% protein and is widely recognised for its efficient removal of nitrogen and phosphorus (Xu et al., 2023). The integration of both species into carp farm infrastructure has the potential to diversify farm income while mitigating eutrophication at scale (Eljasik et al., 2026).
Materials and Methods
The study was conducted at a commercial 120 ha (80 ha water surface) carp farm (Maliniec, northwest Poland, 53°42���N 15°21���E) producing approximately 28 t of carp annually in a traditional 3-year semi-intensive cycle. Four production systems were evaluated: watercress cultivated in (i) an unused earthen breeding pond (w-pond, 102.95 m2), (ii) a concrete discharge channel (w-channel, 169.93 m2), and (iii) a raft aquaponics system (w-aquaponics, 25.44 m2); and (iv) naturally established duckweed in the farm discharge channel (d-channel, 145.5 m2). Water quality parameters (pH, dissolved oxygen (DO), oxidoreduction potential (ORP), conductivity, total dissolved solids (TDS), and temperature) were measured in situ. Water samples were analysed for TN, nitrogen forms (NH4+-N, NO3--N, NO2--N), TP, total reactive phosphorus (TRP), total carbon (TC), inorganic carbon (IC), and potassium (K). Harvested plant biomass was analysed for dry matter, ash, N, P, K, macro- and microelements, and heavy metals. Nutrient retention efficiency (%) was calculated as: (outflow conc. ��� inflow conc.) / inflow conc. × 100, following Kajgrov�� et al. (2024), and compared against EU-WFD thresholds for safe riverine discharge (≤3.5 mg TN L-1; ≤106 µg TP L-1).
Results and Discussion
Watercress achieved production yields of 11.92 ± 6.71 kg m-2 (w-pond), 7.99 ± 0.99 kg m-2 (w-channel), and 11.54 ± 3.73 kg m-2 (w-aquaponics). Duckweed yielded 2.54 ± 0.77 kg m-2 in the d-channel. TN was reduced by 5.93% (w-pond) and 8.63% (w-channel), while TP removal reached 23.43% via watercress (w-pond). Duckweed outperformed watercress in nutrient sequestration, removing 19.15% of TN and 57.89% of TP, along with significant reductions in NH4+ (���44%), NO3- (���3.8%), NO2- (���13.8%), TRP (���68%), and organic phosphorus (���39.8%). Combined across all systems, the average retention relative to inflow was ���9.9% for N and ���25.6% for P, with negligible effect on carbon. All outflow waters met EU-WFD nitrogen limits; duckweed-treated water also approached compliance with phosphorus discharge thresholds. Both macrophytes remained within EU food-grade limits for heavy metals (Pb <0.30 mg kg-1 w.w.; Cd <0.10 mg kg-1 w.w.; As <20 µg kg-1 w.w.), confirming their safety for human consumption (EC, 2023). Watercress nutrient uptake patterns varied across systems, reflecting the stoichiometric N:P ratio of the inflow water. TRP removal was observed in the w-pond (N:P ratio 40:1) but not in the w-channel (N:P ratio 11:1), consistent with the importance of nutrient stoichiometry for macrophyte phosphorus uptake. An unexpected increase in K concentration (+170.8%) in duckweed channel outflow was attributed to cation exchange, stress-induced K+ efflux, and leaching from decomposing biomass. The duckweed C:N:P ratio of 185:9:1 — shifted from the aquatic plant optimum of 119:17:1 (Hillebrand & Sommer, 1999) — indicates considerable remaining capacity for N and P removal under optimised harvesting regimes.
Conclusions
Both watercress and duckweed demonstrated significant phytoremediation potential in effluents from semi-intensive carp aquaculture, with duckweed showing superior nutrient removal efficiency. Integration of these macrophytes into existing carp farm infrastructure (unused ponds, inter-pond channels, discharge channels) requires minimal investment while producing food- and feed-grade biomass that can diversify farm income. The multitrophic design further enhances the ecosystem services value of European fishponds, already estimated at >1,257 ��� ha-1 (Fr��lichov�� et al., 2014), and supports the water quality goals of the EU Water Framework Directive. Scaling these NbS across the Central and Eastern European aquaculture landscape could substantially reduce nutrient loads to inland waterways and help prevent eutrophication.
Acknowledgments
This research is part of the EU Horizon Europe project SAFE, funded under grant agreement no. 101084549. Additional support was provided under the 'Regional Excellence Initiative' Programme (RID/SP/0045/2024/01) and the NAZV project QL25020009 (K. Roy).
Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.
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