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Frontiers in Aquaculture ·

Volume 5 - 2026 | https://doi.org/10.3389/faquc.2026.1813061

Comparative analysis of nutrient dynamics, growth performance, and economic returns of lettuce (Lactuca sativa) in coupled aquaponic, decoupled aquaponic, and conventional hydroponic systems

  • Mary Clarisse M. Surmion1,2
  • Frenz Charish B. Hechanova1,2
  • Fredson H. Huervana1,3*corresponding author
  • Rizza Mae T. Guyapale1,4
  • Vyenge Erre D. Gayosa1
  1. 1Institute of Aquaculture, College of Fisheries and Ocean Sciences, University of the Philippines Visayas, Miagao, Iloilo, Philippines
  2. 2Department of Agriculture, National Fisheries Research and Development Institute, Fisheries Biotechnology Center, Science City of Muñoz, Philippines
  3. 3Brackishwater Aquaculture Center, Institute of Aquaculture, College of Fisheries and Ocean Sciences, University of the Philippines Visayas, Leganes, Iloilo, Philippines
  4. 4Department of Agriculture, Bureau of Fisheries and Aquatic Resources Region 6, Iloilo, Philippines
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Abstract

Quantitative information on the fate of macronutrients derived from fish effluents, their concentrations in culture water, and their assimilation by plants across hydroponic and aquaponic systems remains limited. To bridge this gap, this study examined the growth response of lettuce (Lactuca sativa) and the dynamics of key nutrients, nitrogen (N), phosphorus (P 2 O 5), potassium (K 2 O), calcium (Ca), and magnesium (Mg), in water and plant tissues under three production strategies: coupled aquaponics (T1), conventional hydroponics (T2), and nutrient supplemented decoupled aquaponics (T3) integrated with African catfish (Clarias gariepinus). Over a forty-day cultivation period, lettuce grown in conventional hydroponics achieved higher biomass values than plants in either of the aquaponic systems. T1 produced intermediate growth, while T3 showed the lowest biomass. Morphological observations revealed greater shoot development in T2, moderate shoot formation in T1, and comparatively higher root biomass in T3, indicating differences in nutrient uptake and allocation among treatments. Although measured macronutrient concentrations suggested sufficient availability, plant productivity was not directly proportional to total nutrient levels, highlighting the role of nutrient assimilation efficiency. Economic evaluation indicated that although T2 achieved the highest vegetable yield, T1 generated the highest gross income due to the combined fish and crop sales, while T3 achieved the highest return on investment and the shortest payback period. Overall, this study provides quantitative insight into nutrient dynamics, plant uptake, and economic performance across hydroponic and aquaponic systems. These findings contribute to a better understanding of resource use efficiency and support the optimization of nutrient management and system design for sustainable integrated aquaculture production.

Keywords

1 Introduction

Aquaculture currently supplies more than 50% of the world’s aquatic animal food, surpassing capture fisheries (FAO, 2024). This rapid expansion has been driven primarily by the intensification of production systems and increased feed inputs to maximize output. However, intensified aquaculture generates substantial nutrient loading within culture units, particularly in the form of nitrogenous and phosphorus compounds derived from fish metabolism and uneaten feed (Nederlof et al., 2022; Dalbem Barbosa et al., 2024). The accumulation of these compounds necessitates frequent water exchange or advanced filtration to maintain optimal water quality. Such practices increase operational costs and contribute to environmental nutrient discharge, raising concerns regarding sustainability and resource-use efficiency in modern aquaculture systems (Taufik et al., 2024; Symeonidou and Mente, 2024).

Improving nutrient retention and recovery within aquaculture has therefore become a central research priority. Recirculating aquaculture systems (RAS) were developed to reduce water consumption and manage wastewater through biological filtration and controlled production environments (Engle et al., 2020; Brown et al., 2025) Building upon this concept, aquaponics integrates RAS with hydroponic plant production to create a closed-loop, multi-trophic system in which nutrient-rich aquaculture effluent is utilized for crop cultivation (Espinosa Moya et al., 2016). In these systems, dissolved nutrients originating from fish feed and fecal matter are biologically transformed and subsequently absorbed by plants (Goddek, 2019), thereby reducing nutrient accumulation in culture water while generating an additional agricultural output. This integration enhances water-use efficiency and represents a potential strategy for effluent valorization in intensive aquaculture.

Despite its promise, nutrient management remains one of the principal constraints in aquaponic production (Reyes Lastiri et al., 2018; Yang and Kim, 2019). Commercial fish feed rations are formulated and designed to optimize fish growth rather than to satisfy plant nutrient requirements. As a result, aquaculture effluents may present nutrient imbalances or deficiencies when directly used as hydroponic nutrient solutions (Madady et al., 2025). Several studies have reported that plant yields in aquaponics are often lower than those obtained under conventional hydroponic fertilization unless mineral supplementation is applied (Lobanov et al., 2021; Nozzi et al., 2018). However, supplementation strategies vary widely and may alter system chemistry, nutrient availability, and overall efficiency. Further, nutrient dynamics differ between coupled and decoupled aquaponic configurations, yet comparative characterization of nutrient profiles across these systems remains limited.

Critically, quantitative information on the distribution of key macronutrients supplied through fish-derived effluents, their resulting concentrations in culture water, and their uptake by plants across different aquaponic configurations remains limited. Most studies focus on system performance or yield outcomes, while comparative assessments of water and plant nutrient profiles in coupled and decoupled aquaponic systems relative to conventional hydroponics are scarce. A clearer characterization of macronutrient availability and plant nutrient accumulation is necessary to improve nutrient use efficiency and guide the design of aquaculture-integrated production systems. Thus, the present study evaluated and compared the nutritional characteristics of plant and water in conventional hydroponics, coupled aquaponics, and nutrient-complemented decoupled aquaponics systems. By elucidating nutrient dynamics across these production strategies, this study seeks to contribute to the optimization of nutrient recovery and resource efficiency in integrated aquaculture systems.

2 Materials and methods

2.1 Plant and fish materials

Lettuce (Lactuca sativa) was used as the plant component of the experiment. Seeds (Olmetie, Rijk Zwaan Batavia, India) from the same batch were sown in seedling trays and grown under uniform conditions for 11 days prior to transplantation. Initial plant biomass was standardized to ensure comparable starting conditions across all treatments. For the fish component, a total of 509 apparently healthy African catfish (Clarias gariepinus) were stocked in a circular tank under continuous aeration. The fish system primarily served as the nutrient source for the aquaponic treatments. The fish had an average body weight of 120 g and were maintained at a stocking density of 10 kg m-3 in a 6 m-3 circular tank (Goddek, 2019), resulting in a total biomass of 61.08 kg. They were fed a commercial diet (Catfish Starter Feeds, 1.8–2.0 mm pellets; TATEH Aquafeeds, SANTEH Feeds Corporation, Bulacan, Philippines) three times daily at 3% of their body weight, corresponding to a daily feed ration of 1.83 kg. The proximate composition of the feed is presented in Table 1. Water sourced from the tilapia ponds was used to fill the catfish culture tank. Additional water from the same source was added as needed to maintain a water depth of 0.8 m.

Table 1. Proximate composition of commercial feed.

Crude protein(% min.)Crude fat(% min.)Crude fiber(% max.)Crude ash(% max.)Moisture(% max)
31661212

Manufactured by: TATEH Aquafeeds, SANTEH Feeds Corporation, Bulacan, Philippines.

2.2 Experimental setup and production systems

The experiment was carried out in the Freshwater Aquaculture Station (FAS) of the Institute of Aquaculture, College of Fisheries and Ocean Sciences, University of the Philippines Visayas, in Miagao, Iloilo, Philippines. Using a system-level approach (Goddek and Vermeulen, 2018; Rodgers et al., 2022; Harabiš et al., 2026; Subah et al., 2025; Heo et al., 2024), a single integrated experimental system was established consisting of three treatments: T1 – coupled aquaponics, T2 – conventional hydroponics, and T3 – nutrient-complemented decoupled aquaponics. These treatment components represented distinct production strategies within the same experimental setup and were not replicated as independent systems. Further, since there is no system level replication, individual lettuce from each treatment were considered as independent experimental units. For T1 and T3, the aquaponics systems consisted of a catfish recirculating aquaculture system (RAS) unit and a lettuce hydroponic unit. The RAS component included a circular fish tank with a diameter of 3 m, a water depth of 0.8 m, and a total working volume of 5.7 m³. The fish tank water is continuously recirculated through a settling tank (1 × 1 m), a biofilter (1 × 1 m), and a sump (1 × 1 m), resulting in a 400% daily water exchange volume. The plant production units were housed in a separate greenhouse and consisted of bamboo plant boxes fitted with a sump tank (Figure 1). Each plant box measured 1 m in width and 3 m in length, was constructed from bamboo, and lined with black canvas to ensure water retention. A total of 145 lettuce plants were grown per treatment, with individual plants placed in perforated polystyrene foam cups (7.62 cm in diameter) to allow for root contact with the nutrient solution. Plants were arranged in a floating raft made of polystyrene foam, with a spacing of 15.24 cm × 7.62 cm within each box. The experiment ran for 40 days until the lettuce reached harvestable size.

Design of the plant production unit of coupled aquaponics (T1), conventional hydroponics (T2), and nutrient-complemented decoupled aquaponics (T3).
Figure 1. Design of the plant production unit of coupled aquaponics (T1), conventional hydroponics (T2), and nutrient-complemented decoupled aquaponics (T3).

For T1—coupled aquaponics, the filtered water from the RAS was directed to a sump tank and then pumped into both the fish tank and the plant-growing units. The water from the plant box flows into the sump and is then pumped back into the fish tank.

For T2—conventional hydroponics, the system consisted of a plant growing box connected to a sump reservoir. Treated tilapia pond water obtained from the FAS was used as the culture water and recirculated continuously between the plant box and the sump throughout the production cycle. At the start of the experiment (Day 0), 1,500 mL each of SNAP A and SNAP B hydroponics nutrient solutions (water-soluble fertilizer, University of the Philippines Los Baños, Laguna, Philippines) and 540 mL of pH-adjusting solution (NutriHydro pH Down, Quezon City, Philippines) were added directly to the system. The pH of the nutrient solution was maintained between 5.5 and 6.5 (Meselmani, 2022). On Day 10, approximately 200 L of treated tilapia pond water were added to compensate for water loss, together with 500 mL each of SNAP A and SNAP B nutrient solutions and 260 mL of pH Down solution. On Day 30, an additional 100 L of treated tilapia pond water and 250 mL each of SNAP A and SNAP B nutrient solutions were applied. The SNAP hydroponics solutions served as the primary nutrient source for plant growth in this treatment. The nutrient composition of SNAP solutions A and B is presented in Table 2.

Table 2. Nutrients present in SNAP dissolved in 85% distilled water.

SNAP A Nutrients% wt/vol
Nitrogen (N)6.10
Calcium (Ca)4.25
Potassium (K)3.09
SNAP B Nutrients% wt/vol
Phosphorus (P)0.38
Magnesium (Mg)0.50
Iron (Fe)0.15

Microelements: Boron (B), Manganese (Mn), and Molybdenum (Mo) in trace amounts.

For T3—nutrient-complemented decoupled aquaponics, the system consisted of a plant growing box connected to a sump reservoir. Treated tilapia pond water sourced from the FAS ponds served as the base water and was circulated between the sump and plant box throughout the culture period. An essential component for the decoupling of the plant and fish components is the mineralization unit where catfish sludge obtained from the settling tank of the RAS was aerobically mineralized for 15 days, adapting the method from Guyapale et al. (2024). Molasses was supplemented in the procedure as a carbon source to hasten the mineralization process. An organic nutrient solution was prepared by combining mineralized catfish sludge with targeted chemical fertilizers and a pH-adjusting solution. Approximately 1.7 L of mineralized catfish sludge was added daily directly to the sump to supply nutrients derived from aquaculture waste. In addition, chemical supplementation was applied at approximately 50% of the rate used in the conventional hydroponics treatment (T2). At the start of the experiment (Day 0), 500 mL each of SNAP A and SNAP B nutrient solutions were added to the system, along with 240 mL of pH Down solution. On Day 30, an additional 500 mL each of SNAP A and SNAP B were applied together with 1,500 mL of pH Down solution to maintain nutrient availability and pH stability.

2.3 Water parameters

Optimal water parameters for C. gariepinus culture were maintained in the rearing tank, with dissolved oxygen (DO) > 5 ppm, temperature 27–28 °C, and pH 7.9–8.1. The pH of the sump water supplying the plant-growing units was monitored twice daily using a calibrated digital pH meter to maintain suitable conditions for plant growth.

2.4 Sample collection and analyses

Water and plant samples were subjected to nutrient analysis. A 500 mL water sample was collected from each system on Days 0, 15, and 40. For plant nutrient analysis, pooled lettuce samples equivalent to 15 g fresh weight were collected at Day 0 for the initial nutrient concentration. At Day 40, three individual lettuce plants were collected from each treatment for analysis. Although multiple plants were measured, they originated from a single system per treatment and therefore do not constitute independent experimental replicates. Samples were sent to the Laboratory Facilities of the Sugar Regulatory Administration (SRA), Agro-based Laboratory, Bacolod City, Negros Occidental, for nutrient analysis. The concentrations of nitrogen (N), phosphorus (P2O5), potassium (K2O), calcium (Ca), and magnesium (Mg) were determined following standard analytical procedures. Total nitrogen was analyzed using the Kjeldahl digestion method, a standard procedure for quantifying organic nitrogen, utilizing the Foss Tecator Digestion System and the Foss Kjeltec 8200 Auto Distillation Unit (AOAC, 1990), while phosphorus was measured via a standard volumetric method. Potassium concentration was determined using the sodium tetraphenylborate (STPB) volumetric method, and calcium and magnesium concentrations were quantified by ethylenediaminetetraacetic acid (EDTA) titrimetric analysis. All titrations were performed using calibrated glassware and analytical-grade reagents. Sample digestion and analyses were conducted under controlled laboratory conditions, and each parameter was analyzed in triplicate for each treatment to ensure accuracy, reproducibility, and data reliability. At the end of the trial, 20 randomly selected plant samples were evaluated for leaf length, root length, total plant length, number of leaves, leaf weight, root weight, total weight, and leaf color.

2.5 Data analysis

All data were analyzed using the SPSS 26.0 package (IBM, Chicago, IL, USA) software and expressed as mean ± standard error of the mean (SEM). Crop growth values were log-transformed to meet the assumptions of parametric testing (normality and homogeneity). One-way analysis of variance (ANOVA) was used to compare treatment means, with statistical significance set at p < 0.05. Post hoc multiple comparison tests (Tukey’s HSD) were used to determine the significant differences among treatment groups when ANOVA indicated significance. Statistical tests were applied to subsample data; however, because of pseudo-replication, p-values may overestimate treatment effects.

3 Results

3.1 Crop growth performance

Crop growth rate, total fresh weight, and root-to-leaf weight ratio of Lactuca sativa after 40 days were affected by the production system employed (p < 0.05) (Figure 2). Plants grown under T2 exhibited the highest crop growth rate and total fresh weight, followed by T1, whereas T3 produced the lowest values. Conversely, the root-to-leaf weight ratio showed an opposite trend, with T2 recording the lowest ratio and T3 the highest, indicating differences in biomass allocation among treatments.

Crop growth rate performance of *Lactuca sativa* under different treatments, T1 - Coupled aquaponics, T2 - Hydroponics, and T3 - Nutrient-complemented decoupled aquaponics. **(A)** Crop growth rate (CGR); **(B)** Total weight of crop (TWC); **(C)** Root-to-leaf weight ratio (R:L). Values are shown as fresh weights (mean ± SEM). Superscripts indicate observed differences (One-way ANOVA with Tukey’s HSD, CGR = *p* < 0.001; TWC = *p <* 0.001; R:L = *p* < 0.001).
Figure 2. Crop growth rate performance of Lactuca sativa under different treatments, T1 - Coupled aquaponics, T2 - Hydroponics, and T3 - Nutrient-complemented decoupled aquaponics. (A) Crop growth rate (CGR); (B) Total weight of crop (TWC); (C) Root-to-leaf weight ratio (R:L). Values are shown as fresh weights (mean ± SEM). Superscripts indicate observed differences (One-way ANOVA with Tukey’s HSD, CGR = p < 0.001; TWC = p < 0.001; R:L = p < 0.001).

Similarly, root and leaf growth parameters of L. sativa were influenced by the different production systems (Table 3). T1 produced the greatest number of leaves compared to T2 and T3. However, root biomass showed observed differences in T3 than in T1 and T2. In contrast, leaf fresh weight was highest in T2, followed by T1, and lowest in T3. Root length was greatest in T2, intermediate in T3, and shortest in T1. Leaf length was lowest in T1, while T2 and T3 exhibited comparable and higher values. Similarly, total plant length was highest in T2, followed by T3, and lowest in T1. The root-to-leaf length ratio showed a pattern consistent with total plant length, with T2 exhibiting the highest ratio.

Table 3. Root and leaf parameter of L. sativa after 40 days of culture under different systems.

IndexT1T2T3p - value
No. of leaves18.85 ± 0.58a15.15 ± 0.47b13.00 ± 0.27c< 0.001
Root weight (g)4.18 ± 0.35b2.43 ± 0.19c6.31 ± 0.41a< 0.001
Leaf weight (g)21.78 ± 0.96b32.26 ± 2.23a12.17 ± 0.45c< 0.001
Root length (cm)7.73 ± 0.51b32.38 ± 1.23a10.15 ± 0.35b< 0.001
Leaf length (cm)20.18 ± 0.37b25.27 ± 0.54a25.54 ± 0.62a< 0.001
Total length (cm)27.91 ± 0.64c57.65 ± 1.39a35.69 ± 0.85b< 0.001
Root-to-leaf length ratio0.38 ± 0.03b1.29 ± 0.05c0.40 ± 0.01a< 0.001

Weight values are all fresh weight of lettuce. Data are shown as mean ± SEM. T1, coupled aquaponics; T2, conventional hydroponics; T3, nutrient-complement decoupled aquaponics. Superscripts indicate significance (One-way ANOVA with Tukey’s HSD, p < 0.05).

3.2 pH levels in the plant-growing unit sumps

Sump pH varied among the different production systems throughout the 40-day culture period (Figure 3). Notably, an overall declining trend in pH was observed across all treatments. The conventional hydroponic system (T2) exhibited the most pronounced acidification, with pH decreasing from 6.7 at the start of the experiment to a minimum value of 5.2 during the culture period. In contrast, the coupled aquaponic system (T1) and the nutrient-complemented decoupled aquaponic system (T3) maintained comparatively higher pH values than T2 throughout the experiment. The pH in both T1 and T3 ranged from 6.45 to 7.75 throughout the experimental period.

pH levels measured in the sump of coupled aquaponics (T1), conventional hydroponics (T2), and nutrient-complemented decoupled aquaponics (T3) over a 40-day experimental period.
Figure 3. pH levels measured in the sump of coupled aquaponics (T1), conventional hydroponics (T2), and nutrient-complemented decoupled aquaponics (T3) over a 40-day experimental period.

3.3 Essential nutrient composition

3.3.1 Lettuce

Nutrient composition of plants also varied across different systems, with marked changes from the initial concentrations (Table 4). The total nitrogen (N) content of the samples increased substantially from the initial 1,800 mg kg-1. At the end of the cultivation period, T3 attained the highest N concentration (3,015 mg kg-1), which was statistically comparable to T1 (2,980 mg kg-1), while the lowest value was observed in T2 (2,350 mg kg-1). The phosphorus (P) content, expressed as phosphate pentoxide (P2O5) due to its stability and well-defined stoichiometry, followed a similar trend. T3 recorded the highest P concentration (22,013 mg kg-1), statistically comparable to T1 (21,645 mg kg-1), whereas T2 had the lowest concentration (19,403 mg kg-1). Potassium (K) content also differed among treatments, although values were relatively close. T2 exhibited the highest K concentration (13,296 mg kg-1), followed by T3 (12,638 mg kg-1) and T1 (11,966 mg kg-1). The higher K uptake in T2 corresponded with the availability of K from the SNAP solution, indicating efficient absorption by the plants. Calcium (Ca) content of the plant samples was generally lower than other macronutrients but differed among treatments. T2 had the highest Ca concentration (9,700 mg kg-1), followed by T3 (8,736 mg kg-1), while T1 recorded the lowest value (4,800 mg kg-1). The observed Ca levels were proportional to the Ca concentrations in the respective water systems. Magnesium (Mg) content showed a distinct pattern. T3 obtained the highest Mg concentration (403 mg kg-1), representing a marked increase from the initial level (<170 mg kg-1). In contrast, Mg levels in T1 and T2 remained below 170 mg kg-1 and were not different from each other. The elevated Mg concentration in T3 may be attributed to the contribution of mineralized fish sludge, which likely increased Mg availability in the culture water and contributed to the higher Mg concentration observed in lettuce tissues.

Table 4. Nutrient composition of L. sativa after 40 days of culture under different systems.

Nutrient (mg kg-1)InitialT1T2T3p - value
Nitrogen (N)1,800.00 ± 7.21c2,980.00 ± 59.91a2,350.00 ± 26.35b3,015.00 ± 14.84a< 0.001
Phosphorus (P2O5)14,171.00 ± 80.33d21,645.00 ± 45.00b19,403.00 ± 67.16c22,013.00 ± 13.58a< 0.001
Potassium (K2O)9,307.00 ± 9.00d11,966.00 ± 120.83c13,296.00 ± 105.95a12,638.00 ± 24.13b< 0.001
Calcium (Ca)2,400.00 ± 18.33d4,800.00 ± 29.67c9,700.00 ± 44.30a8,736.00 ± 29.61b< 0.001
Magnesium (Mg)170.00 ± 0.00b170.00 ± 0.00b170.00 ± 0.00b403.00 ± 3.51a< 0.001

Values are shown as mean ± SEM. Different superscripts indicate significance (One-way ANOVA with Tukey’s HSD, p < 0.05).

3.3.2 Water

The essential macronutrient concentrations in the culture water exhibited distinct temporal trends among the aquaponic systems throughout the experimental period. Total nitrogen consistently remained highest in T3, followed by T1, and lowest in T2 across the 40-day trial (Figure 4A). This pattern was maintained from the beginning until the end of the experiment, indicating a stable difference among treatments. Phosphorus concentrations followed a similar and consistent trend (Figure 4B). T3 recorded the highest phosphorus levels throughout the culture period, T1 showed intermediate values, and T2 consistently had the lowest concentrations. In contrast, potassium concentrations showed a changing trend over time (Figure 4C). During the initial phase from day 0 to day 15, T3 and T2 had higher potassium concentrations compared with T1. However, by day 40, potassium levels in T1 increased and became comparable to those in T3, while T2 exhibited relatively lower concentrations than the other treatments. Calcium concentrations demonstrated a gradual shift during the culture period (Figure 4D). T3 initially showed relatively higher values, whereas T1 remained stable throughout the experiment. T2 increased from day 15 to day 40, eventually reaching concentrations similar to T3 by the end of the trial. Magnesium concentrations increased in all treatments over time (Figure 4E). T1 showed an increase from day 0 to day 15, followed by a plateau until day 40. In contrast, both T2 and T3 exhibited a steady and continuous rise in magnesium concentration throughout the entire experimental duration.

**(A)** Nitrogen concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point(one-way ANOVA: Day 0 = *p* < 0.001; Day 15 = *p* < 0.001; Day 40 = *p* < 0.001). **(B)** Phosphorus concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = *p* < 0.001; Day 15 = *p* < 0.001; Day 40 = *p* < 0.001). **(C)** Potassium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = *p* < 0.001; Day 15 = *p* < 0.001; Day 40 = *p* < 0.001). **(D)** Calcium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = *p* < 0.001; Day 15 = *p* < 0.001; Day 40 = *p* < 0.001). **(E)** Magnesium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = *p* < 0.001; Day 15 = *p* < 0.001; Day 40 = *p* < 0.001).
Figure 4. (A) Nitrogen concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point(one-way ANOVA: Day 0 = p < 0.001; Day 15 = p < 0.001; Day 40 = p < 0.001). (B) Phosphorus concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = p < 0.001; Day 15 = p < 0.001; Day 40 = p < 0.001). (C) Potassium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = p < 0.001; Day 15 = p < 0.001; Day 40 = p < 0.001). (D) Calcium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = p < 0.001; Day 15 = p < 0.001; Day 40 = p < 0.001). (E) Magnesium concentration (mg/L) in water at days 0, 15, and 40 in three experimental systems. Data are presented as mean ± SEM (n = 3 per treatment per time point). Different superscript letters indicate apparent changes among treatments within each time point (one-way ANOVA: Day 0 = p < 0.001; Day 15 = p < 0.001; Day 40 = p < 0.001).

3.4 Economic analysis

Economic estimates involving fish production are based on assumed or standard values and were not supported by measured growth performance data. Price assumptions used in this study were based on prevailing market prices as of June 2023 (Philippine Statistics Authority, 2023, https://psa.gov.ph/price-indices) and the investment items and depreciation costs of the three systems, as well as the operation costs were based on actual procurement costs during the study period. The economic evaluation of the three systems revealed distinct differences in investment (Table 5), operational costs (Table 6), and profitability (Table 7). Total capital investment was highest in T1 due to the cost incurred by the fish tank. T2 and T3 have comparable start-up costs. Operational costs mirrored this trend, with T1 incurring the highest recurring expenses due to catfish fingerlings and feed inputs, followed by T2 because of its greater nutrient solution requirements, while T3 had the lowest operational costs. Annual operation costs were estimated based on four production cycles per year for T1 and eight production cycles per year for T2 and T3. Revenue from lettuce production was similar among treatments on an annual basis. However, T1 generated additional revenue from catfish sales, resulting in highest gross income and net income among the three production systems. T3 recorded the highest annual return on investment (ROI) and the shortest payback period of investment. In contrast, T1 exhibited the shortest payback period of operating costs, while T2 showed the lowest economic performance in terms of net income, annual ROI, and payback periods (Table 8). Sensitivity analysis using a ±10% variation was conducted to evaluate the effects of fish pricing (Table 9), feed cost (Table 10), and lettuce market price (Table 11) on total production cost and income.

Table 5. Investment items and depreciation costs of the three systems.

Items/Economic life (yrs)Value (Php)Depreciation per cropping (Php)
T1T2T3T1T2T3
Fish Tank25,000.00––2,083.33––
Plant box and Greenhouse8,350.008,350.008,350.00695.83347.92347.92
Pump (70 Watts)1,000.00––83.33––
Pump (22 Watts)500.00500.00500.0041.6720.8320.83
Blower5,000.00––416.67––
Airhose2,500.00––208.33––
Water hose600.00300.00300.0050.0012.5012.50
Grow lights900.00900.00900.0075.0037.5037.50
Total43,850.0010,050.0010,050.003,654.17418.75418.75
Annual Depreciation14,616.673,350.003,350.00

Table 6. Operation costs of the three systems.

CostsValue (Php)
T1T2T3
Variable Costs
Lettuce Seeds580.00290.00290.00
Catfish fingerlings2,500.00––
Catfish Starter Feeds4,104.00––
pH down–375.00637.50
SNAP solution–1,620.00720.00
Activated Sludge (w/molasses)––250.00
Electricity6,272.64253.44253.44
Depreciation3,654.17418.75418.75
Fixed Cost
Salary (1 person)6,000.002,000.002,000.00
Total23,110.814,957.194,569.69
Annual Operation Costs92,443.2439,657.5036,557.50

Annual operation costs were calculated based on 4 production cycles per year for the coupled aquaponic system (T1) and 8 production cycles per year for the conventional hydroponic (T2) and nutrient-complemented decoupled aquaponic (T3) systems.

Table 7. Product price, revenue, and profit of the three systems per crop and annually.

IncomeT1T2T3
Per cropPer yearPer cropPer yearPer cropPer year
Product Volume
Lettuce290 cups1,160 cups145 cups1,160 cups145 cups1,160 cups
Catfish100 kg400 kg––––
Product Price (Php)
Lettuce40/pc40/pc40/pc40/pc40/pc40/pc
Catfish200/kg200/kg––––
Revenue (Php)
Lettuce11,600.0046,400.005,800.0046,400.005,800.0046,400.00
Catfish20,000.0080,000.00––––
Profit (Php)8,489.1933,956.76842.816,742.481,230.319,842.48

Table 8. Income, returns, and payback period of the three systems.

Economic efficiencyT1T2T3
Per cropPer yearPer cropPer yearPer cropPer year
Gross Incomea (Php)31,600.00126,400.005,800.0046,400.005,800.0046,400.00
Net Incomeb (Php)8,489.1933,956.76842.816,742.481,230.319,842.48
Return of Investmentc (%)19.3677.448.3967.0912.2497.94
Payback period of Investmentd (yr)–0.90–1–0.76
Payback period of operating costse (yr)-1.90–3.93-2.77

Price of the product x Volume of product sold.

Gross income – total production cost.

Net income/investment cost x 100.

Investment cost/(annual net income + depreciation).

Total production cost/(annual net income + depreciation).

Table 9. Sensitivity analysis on the effect of fish selling price.

Scenario 1.A: Fish selling price +10%
Economic efficiencyT1T2T3
Per cropPer yearPer cropPer yearPer cropPer year
Gross Incomea (Php)33,600.00134,400.005,800.0046,400.005,800.0046,400.00
Total Cost (Php)23,110.8192,443.244,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)10,489.1941,956.76842.816,742.481,230.319,842.48
Scenario 1.B: Fish selling price -10%
Gross Incomea (Php)29,600.00118,400.005,800.0046,400.005,800.0046,400.00
Total Cost (Php)23,110.8192,443.244,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)6,489.1925,956.76842.816,742.481,230.319,842.48

aGross income – total production cost. bNet income/investment cost x 100.

Table 10. Sensitivity analysis on the effect of fish feed cost.

Scenario 2.A: Fish feed cost +10%
Economic efficiencyT1T2T3
Per cropPer yearPer cropPer yearPer cropPer year
Gross Incomea (Php)31,600.00126,400.005,800.0046,400.005,800.0046,400.00
Total Cost (Php)23,521.2194,084.844,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)8,078.7932,315.16842.816,742.481,230.319,842.48
Scenario 2.B: Fish feed cost -10%
Gross Incomea (Php)31,600.00126,400.005,800.0046,400.005,800.0046,400.00
Total Cost (Php)22,700.4190,801.644,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)8,899.5935,598.36842.816,742.481,230.319,842.48

aGross income – total production cost. bNet income/investment cost x 100.

Table 11. Sensitivity analysis on the effect of lettuce market price.

Scenario 3.A: Lettuce pricing +10%
Economic efficiencyT1T2T3
Per cropPer yearPer cropPer yearPer cropPer year
Gross Incomea (Php)32,760.00131,040.006,380.0051,040.006,380.0051,040.00
Total Cost (Php)23,110.8192,443.244,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)9,649.1938,596.761,422.8111,382.481,810.3114,482.48
Scenario 3.B: Lettuce pricing -10%
Gross Incomea (Php)30,440.00121,760.005,220.0041,760.005,220.0041,760.00
Total Cost (Php)23,110.8192,443.244,957.1939,657.524,569.6936,557.52
Net Incomeb (Php)7,329.1929,316.76262.812,102.48650.315,202.48

aGross income – total production cost. bNet income/investment cost x 100.

4 Discussion

The variation in lettuce (L. sativa) growth performance across the different treatments, coupled aquaponics (T1), conventional hydroponics (T2), and nutrient-complemented decoupled aquaponics (T3), can be fundamentally attributed to the complex interplay of nutrient solubility, pH-mediated availability, and subsequent morphological adaptations. Differences in lettuce growth were observed among treatments after 40 days of culture, with T2 producing the highest total fresh weight and crop growth rate, followed by T1. Conversely, T3 exhibited the lowest biomass accumulation.

Leaf development of lettuce mirrored the overall growth pattern. T2 produced the longest and largest leaves, as reflected in the leaf length and weight data, while T3 exhibited reduced shoot biomass despite relatively high water nutrient concentrations. These differences are linked to root morphology across treatments. T2 developed longer roots indicative of an efficient absorptive system (Li et al., 2024; Guia et al., 2023), whereas T1 showed intermediate root and foliage characteristics, consistent with its moderate growth. In contrast, T3 produced shorter roots with greater biomass, which may indicate osmotic stress and ionic antagonism (Yuan et al., 2021), which likely created a physiological “bottleneck” limiting shoot growth. Overall, these results demonstrate that plant biomass was not directly proportional to total nutrient concentration in the culture water, highlighting that nutrient availability and assimilation efficiency, rather than abundance alone, influence crop productivity.

While recent studies suggest that aquaponic systems produce greater crop yields compared to hydroponic systems (Subah et al., 2025; Heo et al., 2024), the results of the present work suggest otherwise. The present study coincides with the meta-analysis of Ayipio et al. (2019), suggesting that aquaponics yields inferior crops than hydroponics. This discrepancy underscores the influence of nutrient inputs, system configuration, and aquaculture species. Supporting this variability, Rodgers et al. (2022) demonstrated that nutrient supplementation in decoupled systems enhanced the growth of basil (Ocimum basilicum) to levels comparable with conventional hydroponics, suggesting that plant species, nutrient formulation, and system management are critical in shaping outcomes. In a similar manner, Goddek and Vermeulen (2018) reported improved lettuce performance in RAS-based hydroponics, attributing this to more stable nutrient availability, further emphasizing the importance of system design, crop type, and nutrient management strategies in determining productivity.

The superior performance of T2 in the present study is consistent with the premise that optimized nutrient solutions provide more immediate bioavailability compared to aquaculture effluents, which are inherently variable and influenced by aquaculture management practices (Yang and Kim, 2019; Dhal et al., 2022). Meanwhile, the intermediate growth observed in T1 reflects the typical “compromise” environment of integrated systems, where the physiological requirements of fish, plants, and nitrifying bacteria often prevent the maintenance of plant-optimal pH conditions, a phenomenon widely documented in aquaponic literature (Wang et al., 2023; Sinurat et al., 2016).

The mechanisms driving these growth differences may potentially be rooted in the interplay between water pH and nutrient solubility. Water pH serves as a critical regulator of nutrient speciation and microbial nitrification (De Rijck and Schrevens, 1998, 1999; Huang and Fisher, 2025; Trientini et al., 2023). In this study, the pH observed in T2 remained within or close to the recommended range for lettuce production (5.5-6.5) for much of the culture period, whereas T1 and T3 generally exhibited higher pH values. T2 exhibited the lowest sump pH due to the acidic nature and high buffering capacity of the SNAP nutrient solution. This slightly acidic environment may have prevented the precipitation of essential elements such as iron (Fe), manganese (Mn), calcium (Ca), and magnesium (Mg), which typically become bio-unavailable at pH values above 7.0 (Colombo et al., 2014; Resh, 2022; Huang et al., 2025). This enhanced solubility may have potentially facilitated superior root-to-shoot translocation, supporting the higher biomass and larger leaf expansion observed in T2 (Anderson et al., 2017; Al Tawaha et al., 2025). In contrast, the higher pH levels in T3 and T1 may have potentially restricted the mobility of essential nutrients, limiting their transport from the roots to the shoots even when root zone concentrations were sufficient (Anderson et al., 2017; Huang et al., 2025; Huang and Fisher, 2025; Bugbee, 2004). While these mechanistic links are plausible, the present study did not comprehensively analyze these factors. Therefore, a detailed micronutrient profiling is recommended in future work to enable more accurate interpretation and to validate the proposed hypothesis.

Beyond physicochemical factors, feed input plays a central role in shaping nutrient dynamics within aquaponic systems. As fish biomass increases, feeding rates are adjusted accordingly, resulting in fluctuations in nutrient loading. Subramanian et al. (2025) demonstrated that increased feed inputs enhance the availability of key nutrients, including Ca, Mg, K, Fe, Mn, and Zn, thereby improving plant nutrient uptake. In addition, nutrient transformation efficiency is influenced by both system design and crop type, with improved nitrogen utilization observed when microbial activity is effectively regulated (Alkhoori et al., 2026). Thus, optimizing feeding strategies and nutrient inputs can enhance nitrogen conversion efficiency while minimizing losses.

In aquaponic systems, nutrient availability is largely dependent on the transformation of fish-derived wastes. Approximately 95% of feed is consumed, with 60–70% of ingested nutrients subsequently excreted as waste products, including ammonia (FAO, 2014; Khakyzadeh et al., 2015). In the present study, a daily feed input of 1.83 kg corresponds to approximately 90.77 g of nitrogen, a substantial proportion of which may subsequently be excreted and becomes available for plant uptake. Through microbial nitrification, ammonia is converted into nitrate (NO3-) and ammonium (NH4+), the primary nitrogen forms assimilated by plants (Khiari et al., 2020; Edgar et al., 2018). However, nitrogen losses may also occur via denitrification, accounting for up to 78.5% loss as N2O emissions, as previously reported (Zou et al., 2016). This nutrient-loading mechanism may partially explain the observed variation in nitrogen levels in both water and plant tissue across different treatments.

Phosphorus and potassium exhibit distinct dynamics within aquaponic systems. Although fish feed, particularly fishmeal, is the primary phosphorus source, a substantial portion (30–65%) is lost as solid excreta (Yep and Zheng, 2019; Schmautz et al., 2016; Prabhu et al., 2007). Its availability as ionic orthophosphate depends on microbial mineralization, particularly by phytase-producing bacteria such as γ-proteobacteria commonly found in fish sludge (Prastowo et al., 2024; Jorquera et al., 2008; Kasozi et al., 2021). System design further influences this process, as improved oxygenation enhances phosphorus mineralization and plant uptake efficiency (Nishanth et al., 2025), which may explain the higher phosphorus availability observed in T3.

In contrast, potassium is not a primary requirement for fish health, and fish-derived effluents are typically deficient in K (Suhl et al., 2016). To address this limitation, potassium hydroxide (KOH) buffers are commonly used to stabilize pH while simultaneously supplementing potassium levels (Duarte and Cerozi, 2024; Tsoumalakou et al., 2022). Aquaponic systems often require such supplementation to match hydroponic nutrient availability, particularly in nutrient film technique systems where controlled nutrient delivery enhances plant growth (Al Jenaid et al., 2026). This explains the elevated potassium levels observed in T3, which received supplemental nutrients through SNAP solution in addition to nutrients released during sludge mineralizataion (Guyapale et al., 2024; Otani et al., 2023). Interestingly, the increase in potassium in T1 without external supplementation may potentially be attributed to enhanced mineralization of organic matter, releasing intracellular potassium into the system (Delaide et al., 2017, 2019). Unlike nitrogen, which can be lost through denitrification, potassium remains highly mobile and tends to accumulate in closed-loop systems (Tang et al., 2026).

Despite the presence of sufficient nutrients, plant uptake may still be constrained by ionic interactions. The uptake of Ca, Mg, and K is inherently competitive, such that an excess of one element can inhibit the absorption of others (Ali Al Meselmani, 2023). While Ca contributes to cell wall stability and Mg plays a central role in chlorophyll formation (White and Broadley, 2003; Ishfaq et al., 2022), changes in pH during aerobic mineralization can accelerate their release into the system (Delaide et al., 2017). In T3, the addition of SNAP nutrients may have altered solution chemistry and nutrient interactions (Ishfaq et al., 2022). Furthermore, elevated calcium concentrations may have induced nutrient lockout, limiting potassium and magnesium absorption (Ozaki et al., 2005; Hawkesford and Barraclough, 2011). Consequently, even though these nutrients were present in the water, plants were physiologically unable to absorb them, leading to the reduced biomass observed in T3 and, to some extent, in T1. While the proposed underlying mechanism is plausible and supported by previous studies, it requires further validation through targeted analyses.

Furthermore, the economic evaluation highlights that profitability is not solely determined by plant biomass. While T2 achieved the highest plant yield under the present work’s conditions, its profitability was constrained by reliance on a single product. In contrast, the coupled aquaponic system (T1), despite its moderate vegetable yield, generated the highest gross income and net income through the combined sale of fish and lettuce. It should be noted that the economic analysis incorporated estimated catfish production rather than direct measurements of fish growth and harvest yield (Djokosetiyanto et al., 2007; Permatasari et al., 2023). This dual-revenue stream generated higher gross income and net income. Although T3 produced the lowest lettuce biomass among the evaluated systems, it achieved the highest annual ROI and the shortest investment payback period. These results suggest that the nutrient-complement decoupled aquaponics system utilizes capital more efficiently than the other production systems. The relatively low investment and operational costs associated with T3 compensated for its lower crop yield, indicating that further refinement of nutrient management and pH regulation may substantially improve its commercial viability. Thus, while T1 generated the highest overall income from integrated fish and vegetable production, T3 demonstrated considerable potential as a cost-efficient production strategy with favorable economic returns and rapid capital recovery.

Overall, lettuce productivity in these systems is governed by the interaction of nutrient availability, pH-mediated solubility, and root-to-shoot translocation. Conventional hydroponics maximizes plant growth through optimized nutrient availability, whereas coupled aquaponics offers a more integrated and economically resilient production strategy. However, economic outcomes remain sensitive to fluctuations in fish prices, feed costs, and the market value of lettuce. The sensitivity analyses (Tables 9-11) indicate that the profitability in T1 is strongly influenced by fish market price, whereas T2 and T3 are more dependent on lettuce market value due to reliance in a single crop. Nevertheless, the diversified income stream in T1 provides greater resilience against market fluctuations compared with the plant-only systems.

From a sustainability perspective, each system presents distinct trade-offs. Coupled systems enhance nutrient recycling and reduce reliance on external inputs, but they require tighter environmental control due to the competing needs of fish, plants, and microbial communities. In contrast, decoupled systems offer greater operational flexibility by allowing independent optimization of fish and plant production, although this may increase dependence on external inputs. These differences influence resource-use efficiency, system stability, maintenance requirements, and scalability, all of which affect farmer adoption. To enhance farmer adoption, future aquaponics systems should be designed with greater simplicity, operational flexibility, and ease of management while maintaining productivity and resource-use efficiency.

In summary, by linking water nutrient concentrations to plant tissue accumulation across hydroponic, coupled, and decoupled aquaponic systems, this study provides a clearer understanding of nutrient recovery and resource-use efficiency in aquaculture-integrated production. The findings highlight the importance of optimizing system design and nutrient management strategies to balance plant productivity with sustainable effluent utilization. However, these results are specific to the lettuce–African catfish system, and responses may differ across other crop–fish combinations. Future research should include micronutrient profiling, comprehensive nutrient characterization of fish effluents, as well as providing focus on the effects of feed input variability, additional sampling points to capture temporal nutrient dynamics, and evaluation of alternative crop–fish pairings. Long-term studies with multiple production cycles and system replicates are also recommended to validate economic performance under practical farming conditions. Furthermore, as the present study focused primarily on plant growth and nutrient dynamics, future work should incorporate detailed assessments of fish growth performance.

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Footnotes

Edited by: Aya S. Hussain, Purdue University, United States

Reviewed by: Radhakrishnan Subramanian, United Arab Emirates University, United Arab Emirates

Rajib Das, Krishi Vigyan Kendra Khowai, India

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research study was funded by the Commission on Higher Education (CHED) through the International Affairs Service under the Study in the Philippines Education Tourism Project. This study is an output of the project UPV Aquaponics Agritourism: Technology Demonstration Farm and Experiment Station for Integrated Farming (Agriculture, Freshwater & Marine) under the Tourism Studies on Island-Based Opportunities for Growth (TSIBOG) in Western Visayas program.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethics approval

Ethical approval was not required for the study involving animals in accordance with the local legislation and institutional requirements.

Author contributions

MS: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. FH: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. RG: Data curation, Writing – original draft, Writing – review & editing. FH: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. VG: Data curation, Writing – original draft, Writing – review & editing.

Acknowledgements

The authors gratefully acknowledge the essential technical assistance, provision of facilities, and overall support provided by the Institute of Aquaculture, College of Fisheries and Ocean Sciences, University of the Philippines Visayas, and the support of the Office of the Vice Chancellor for Research and Extension (OVCRE). The authors sincerely thank Dr. Rex Ferdinand M. Traifalgar, Dr. Ricardo P. Babaran, and Dr. Ram P. Bautista II for their guidance, encouragement, and valuable assistance throughout the implementation of the project. The authors also thank the project staff—Mr. Carl Luigi M. Cervantes, Ms. Christine N. Fabrigas, Mr. Reynaldo T. Nuñeza, and Mr. Philip C. Nim—for their dedicated technical assistance and invaluable contributions to this research. The authors extend their sincere appreciation to all individuals who contributed, directly or indirectly, to the successful completion of this study. The authors also note the use of AI-assisted tools (Grammarly and QuillBot) to improve the clarity, grammar, and readability of the manuscript. The authors reviewed and edited all outputs and accept full responsibility for the content.

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Citation

Surmion, M. C. M., et al. (2026). Comparative analysis of nutrient dynamics, growth performance, and economic returns of lettuce (Lactuca sativa) in coupled aquaponic, decoupled aquaponic, and conventional hydroponic systems. Frontiers in Aquaculture, Volume 5. https://doi.org/
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5 - 2026

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Copyright © 2026 Surmion, Hechanova, Huervana, Guyapale and Gayosa.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Fredson H. Huervana

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