The study was conducted in the coastal zone of Tien Hai district, Thai Binh province, within the RRD in Vietnam (Fig. 1). The RRD is located in the tropical monsoon area; the dry season coincides with the winter season, and the rainy season coincides with the summer season. The rainy season occurs from May to October, with a total rainfall of 1600-1800mm. The dry season from November to April with total rainfall was approximately 180-200mm. The sampling area belongs to the Red River Delta Biosphere Reserve and the Tien Hai Nature Reserve, which play essential roles in biodiversity conservation and supporting local communities (Viet Dung et al. 2021).

Figure 1.

Sampling area in the Red River Delta.

Sediment cores were collected from mangrove forests at low tide using a Russian Peat Corer during the rainy season in September 2020. A total of six sediment cores were obtained from different mangrove stands along two transects extending from aquaculture ponds to the river estuary (Table 1 and Fig. 1). At each sampling location, we recorded the composition of mangrove species and counted the number of dead trees within a 5-meter diameter circular plot, with measurements conducted in triplicate alongside the core sampling. Sediment cores were taken with a length of 100cm and sliced into 5 cm and 10 cm intervals for 0-50cm and 50-100cm in depth of each sediment core, respectively. All sediment samples were kept in polyethylene (PE) bags and immediately stored in cool boxes during field sampling. Sediment samples were frozen within 24h after collection in the field and transported to the laboratory for further analysis.

In the laboratory, the sediment samples were dried at 60^oC until constant weight by an electric oven Nuve KD-400. Approximately 2 grams of each sediment sample was ground to fine powder with an agate mortar and pestle to determine organic matter content. The loss on ignition method was applied for organic matter (OM) analysis, with the burning temperature kept at 550 ^oC for at least 3 hours by an electric furnace (Daihan Lab) (Viet Dung et al. 2021). The sediment grain size was determined using the laser diffraction method with the Horiba LA-950V2 system. The organic matter content and sediment grain size from core TH-C1, TH-C2 and TH-C3 were gathered from the previous study in the RRD (Viet Dung et al. 2021). Ammonium, nitrate, and phosphate concentrations were analyzed using a CFA Skalar SAN++ continuous flow automated analysis system (BV, Skalar Analytical 2019) at the University of Science, Vietnam National University, Hanoi.

The available phosphate in samples was extracted following the Bray II method, with the extractants being a mixture of NH₄F 0.03M and HCl 0.1M solutions (Irving and McLaughlin 1990). The sample: extraction ratio was 1:10, and the extraction time was 5 minutes. The ammonium and nitrate were extracted by KCl 2N solution in 1 hour with the same extraction ratio as phosphate analysis (Binkley and Vitousek 1989, Griffin et al. 1995). Before conducting CFA analysis, the extracted solution was filtered through a quantitative filter paper to remove suspended matter larger than 10 µm. The filtered samples were kept in an ice box and analyzed on the same day of sample filtration.

The analysis of ammonium (NH₄–N) is based on the Berthelot reaction, in which ammonium is chlorinated to monochloramine and reacted with phenol. Sodium nitroprusside and Sodium hypochlorite are used as catalysts for this reaction. The reaction product formed a green complex and was measured with an optical probe with a wavelength of 630 nm. Samples with expected ammonium concentrations higher than 500 µg/L should be diluted at least twice before analysis. The cadmium reduction method for nitrate analysis (NO3–N) involves automatically mixing samples with a pH 8.2 buffer, then passing them through a copper-coated cadmium U-column to convert nitrate to nitrite. The post-reaction nitrite was determined by diazotization with sulfanilamide and combined with N–(1–naphthyl) ethylene diamine dihydrochloride to form a pink color complex which was measured at 540 nm. Phosphate (PO₄–P) was determined by the reaction between ammonium heptamolybdate and potassium antimony (III) oxide tartrate with the phosphate-containing solution to form the Ammonium-Phospho-Molybdate complex. This complex in the reduction process has a dark blue color when reacted with L(+)–Ascorbic acid and is measured at 880 nm. All analysis process was operated automatically through the CFA SAN++ system and Skalar's FlowAccess V3 software (BV, Skalar Analytical 2019). The analytical detection limit for nitrate and ammonium is 10 µg/l, and phosphate is 2 µg/l. After ten samples in a batch were analyzed, a standard sample was repeated to correct the sensor's signal variation (if any) during analysis. This process is performed automatically in the FlowAccesss V3 software (BV, Skalar Analytical 2019). The duplicate samples of the standard sample were evaluated to ensure that the concentration difference was not more than 5% and the R-value of the analytical standard curve was in the range of 0.99 to 0.999.

The two-factor ANOVA was applied to test the differences in sampling location and depth profiles of nutrient parameters in the mangrove sediment cores. The location factor was based on the distance from the aquaculture pond (Near the aquaculture ponds, transition, and near estuary). The depth profile factor was 0-20cm, 20-50cm, and 50-100cm depth group of sediment cores. The interaction between location and depth profile factors was also tested in the present research. The IBM SPSS 20.0 software was used to run the ANOVA test and the Person correlation coefficient matrix. The statistical analysis is significant if the p-value < 0.05.

Sediment composition is mainly silt, accounting for 81.66 - 97.47%, with an average value of 88.93%. The clay content ranged from 2.31 to 15.27%, with an average value of 9.55%, and the rest is the sand particle, ranging from 0 - 12.30%. The median sediment grain size (Md) tends to increase from top to bottom of all sediment cores, except core TH-C1 and TH-C2 (Fig. 2). The sediment samples ranged from 6.6 to 15.4µm, with an average value of 9.8±1.8µm. The Md values and grainsize distribution of sediment samples indicated the muddy environment in the mangrove forest from RRD. Results from ANOVA analysis showed that the location factor only affected clay content, whereas no statistical differences were observed for other factors (ANOVA, p>0.05, Table 2). The sand and clay contents also significantly relate to the Md value of sediment samples (Pearson, p<0.05, Table 3). The organic matter content of mangrove sediments ranged from 4.88% to 9.79%, with an average value of 7.10 ± 0.94%. Organic matter content significantly decreased from the top to bottom of sediment cores (ANOVA, p<0.05, Table 2). The OM content in the transect from TH-B1 to TH-B3 is slightly higher than those of the transect from TH-C1 to TH-C3 (Fig. 3). The OM content has also significantly correlated with Md, Sand, and Silt content at the medium level (Pearson, p<0.05).

Figure 2.

The depth profiles of Md in sediment cores in RRD.

Figure 3.

Variation of organic matter content in sediment cores in RRD.

The ammonium concentration in sediment ranged from 0.925 to 32.278 mg/kg with an average value of 9.879±4.485 mg/kg. The average values of ammonium concentration were 11.281±6.362 mg/kg, 9.206±2.784 mg/kg, and 9.148±3.231 mg/kg for near aquaculture pond, transition, and near main estuary zone, respectively. The ammonium concentration decreased from top to bottom of the sediment core, with average values of 12.539±7.236 mg/kg, 9.229±2.416 mg/kg, and 8.530±2.213 mg/kg for 0-20cm, 20-50cm, and 50-100cm in depth, respectively (Fig. 4). The ammonium concentration varied significantly by location, depth, and location x depth interaction (ANOVA, p<0.05).

Figure 4.

Ammonium concentration in sediment cores in RRD.

The nitrate concentration in sediment ranged from 0.236 to 7.240 mg/kg with an average value of 0.823±0.745 mg/kg. The mean nitrate concentration values were 0.714±0.184 mg/kg, 0.973±0.1.203 mg/kg, and 0.641±0.119 mg/kg for near aquaculture pond, transition, and near main estuary zone, respectively. Nitrate concentration decreased from top to bottom of sediment cores, with average values of 1.001±1.338 mg/kg, 0.826±0.393 mg/kg, and 0.677±0.218 mg/kg for 0-20cm, 20-50cm, and 50-100cm in depth, respectively. In the RRD, the nitrate concentration showed a large variation in the surface layer of sediment cores, with the highest nitrate concentration observed in the core TH-C2 from 0 to 5cm in depth (Fig. 5). However, the present study did not observe the statistical difference in nitrate concentration between location and depth. The phosphate concentration in sediment spatialy varied in the large range from 0.047 to 9.124 mg/kg with an average value of 1.134±1.294 mg/kg. The phosphate concentration was 1.066±1.773 mg/kg, 0.754±0.634 mmol/kg, and 1.585±1.107 mg/kg for near aquaculture ponds, transition, and near main estuary zone, respectively. The phosphate concentration decreased from top to bottom of the sediment core, with average values of 1.908±2.001 mg/kg, 1.076±0.856 mg/kg and 0.584±0.450 mg/kg for 0-20cm, 20-50cm, and 50-100cm in depth, respectively (Fig. 6). The phosphate concentration varied significantly by location and depth interaction (ANOVA, p<0.05).

Figure 5.

Nitrate concentration in sediment cores in RRD.

Figure 6.

Phosphate concentration in sediment cores in RRD.

In the present study, the ammonium and phosphate concentrations showed medium-level correlation with organic matter content (Table 3, Pearson, p<0.05). However, this pattern was not observed between the nitrate concentration and organic matter contents. The sediment characteristic was not statistically related to the sediment grain size and Md values. The ammonium also showed a weak significant relationship with phosphate concentration in sediment samples (Table 3, Pearson, p<0.05). Overall, the sediment core depth profiles showed two main trends, including nutrient levels decreasing from the aquaculture area to the estuary, and nutrient parameters declining consistently from the top to the bottom of the sediment cores. These results support our hypothesis that aquaculture activities influence nutrient levels in the mangrove sediment cores of nearby forests.

Ammonium and phosphate concentrations in mangrove sediments decreased from sampling plots near aquaculture ponds to those closer to the estuary. The spatial variation patterns indicated that nutrient dynamics in these sediments are affected by aquaculture sewage discharge. The high availability of nutrients in sediment cores collected from areas near high-density aquaculture activities further supports this assumption. Additionally, high level nutrient concentrations were found at the top of all sediment cores, suggesting direct contributions from external sources, including aquaculture, domestic sewage, and rice cultivation. Compared to mangrove forests in regions like the Indo-Pacific and South America, the concentrations of NH₄-N, NO₃-N, and PO₄-P observed in this study are lower than other regions with high density of anthropogenic activities. The average NH₄-N concentration is 9.879±4.485 mg/kg, which is lower than the recent reported values in similar research (Table 4). This pattern is consistent with previous research on seawater quality in the Red River estuary, which showed that seawater ammonium, nitrate, and phosphate sources are strongly linked to aquaculture activities (Le et al. 2022,Dung et al. 2025). Most aquaculture ponds in this area use extensive shrimp farming methods and lack equipment for water monitoring. Consequently, it is challenging for local communities to assess water quality before discharging sewage into tidal creek systems. The sewage from these ponds, which is rich in organic matter, can deposit nutrients into mangrove sediments through tidal dynamics (Dung et al. 2025). Nitrogen from the deposited organic matter was converted to ammonium through the ammonification process in mangrove ecosystems (Reis et al. 2016). Mangrove stands were also considered as a factor influencing coastal nutrient dynamics. Pore-water sediments in the low intertidal zone of mangrove forests typically contain lower nutrient levels compared to those in the high intertidal zone (Middelburg et al. 1996). However, the differences in nutrient concentration between near aquaculture ponds and transition areas were not observed in the present study, indicating that the aquaculture sewage may strongly influence the nutrient parameters in tidal creeks and adjacent mangrove forests (Le et al. 2022,Dung et al. 2025). Sampling sites near aquaculture ponds likely receive substantial deposits of nutrient-rich materials from sewage, leading to high concentrations of ammonium and nitrate in the top layer of sediment stratum. However, this pattern was not observed in phosphate concentration, with the highest value observed in transition areas of transect C and near aquaculture ponds in transect B. Sampling sites in the transition area and near the coastal estuary are faced with tidal flushing, which may transport nutrients from mangrove forests to coastal waters (Wadnerkar et al. 2019). The slight change of Md values from top to bottom of cores TH-C2 and TH-C3 indicated the more stable deposition environment in transect C compared to transect B, potentially facilitating the accumulation of nutrients from adjacent aquaculture activities. Additionally, the changes in Md may reflect the historical evolution of the mangrove forest in the coastal estuary, which plays a crucial role in trapping sediments and filtering pollutants.

In all sampling sites, the nutrient concentration decreased from the top to the bottom of sediment cores. Nitrification, ammonification, and denitrification processes controlled the depth variation of nitrogen in mangrove ecosystems (Reef et al. 2010). The anaerobic environment and high organic matter from litterfall and roots in mangrove forests could support ammonification process in the sediment stratum, causing the high concentration of ammonium in mangrove sediment (Nedwell et al. 1994, Ray et al. 2014). The top layers of sediment cores may be also affected by aquaculture sewage, causing the increasing trend of ammonium and nitrate (Prasad and Ramanathan 2008, Wu et al. 2014). This pattern was also consistent with the phosphate concentration in mangrove sediment cores, with a high phosphate concentration observed in the top sediment layers. The other factors influencing nutrient contents are the biological activities of mangrove crabs and direct groundwater flows in mangrove areas (Susilo et al. 2005, Laverock et al. 2014, Ray et al. 2014). The growth of mangrove trees may also affect the nutrient concentration in the sediment stratum, with the nutrient absorption of mangrove roots in the 0-50cm depth layer (Reef et al. 2010, Ray et al. 2014, Taillardat et al. 2019). The roots and fine roots of dominant mangrove species K. obovata and S. caseolaris were highly developed in the 0-50 cm in the depth of sediment stratum (Alongi et al. 2005, Poungparn et al. 2015) and may lead to increasing the nutrients' exchange in mangrove forests (Ray et al. 2014). Organic matter content was another factor related to nutrient parameters in mangrove sediment. In the present study, the organic matter has a medium positive correlation with ammonium and phosphate in mangrove sediments (Table 3). This pattern can be explained by the organic mineralization process and microbial activities in mangrove sediments, which leads to an increase of nutrient parameters such as nitrogen and phosphorus in sediment (Holmer and Olsen 2002, Joseph et al. 2010). The other factor influencing phosphate concentration in sediment is anaerobic condition in mangrove sediments, causing the high mineralization rates of organic phosphorus (Bridgham et al. 1998, Joseph et al. 2010).

Mangrove forests are essential in the nutrient exchange between estuarine and coastal waters. Mangrove forests in RRD were also considered as net sinks of nutrients, which used nutrients for biomass growth, and the residual nutrients can be preserved in the sediment layer (Wösten et al. 2003). Unfortunately, high loading of nutrient discharge causes various negative impacts, such as increasing trees' mortality rate and coastal ecosystem degradation (Vaiphasa et al. 2007, Lovelock et al. 2007). The effluent and wastewater from aquaculture ponds may accumulate in RRD's mangrove sediments, estuaries, and coastal waters. High nutrient discharge may lead to eutrophic risks in adjacent coastal areas, strongly influencing nutrient dynamics in coastal zones (Robertson and Phillips 1995, Paez-Osuna 2001, Duong et al. 2019, Queiroz et al. 2019). The high concentration of nutrients in the top layers of a sediment core from this sampling plot also indicates the influence of anthropogenic activities (aquaculture) on nutrient dynamics in the mangrove forest. The high concentration of ammonium in sediment in the RRD was consistent with a recent study in mangrove forests from China, which suggested that ammonium is the largest inorganic nitrogen trapped in mangrove forests (Wu et al. 2014).

The high loading of nutrients from aquaculture effluent can alter detritus-based food webs in coastal areas and may change ecosystem structure (Vaiphasa et al. 2007, Serrano-Grijalva et al. 2011). This pattern is also consistent with previous research on Thailand mangroves, which indicated that mariculture areas might negatively impact surrounding mangrove forests (Vaiphasa et al. 2007). In the sampling plot near the aquaculture pond, the number of dead trees was higher than the others, suggesting that the mangrove ecosystems may have unstable conditions (Vaiphasa et al. 2007). The changes in the biogeochemical characteristics of nitrogen and phosphorus can also alter tree growth and forest structure. The enrichment of nutrient resulted in lower C:N ratio in mangrove sediment, which significantly influence the decomposition processes of organic matter and coupled with the anaerobic conditions of mangrove sediments, may create an environment that favors the proliferation of methanogenic bacteria (Rani et al. 2021). The high availability of organic nitrogen and phosphorus from aquaculture activities will increase mineralization rates of nutrients in sediment cores, which provide significant sources of inorganic nitrogen and phosphorus for the coastal area (Li et al. 2007, Ray et al. 2014). The fertilized experiment of nitrogen and phosphorus in the mangrove forests of Twin Cay suggested that the small amount of nutrients altered the tree growth, decomposition rates, and nutrient burial in the sediment stratum (Feller et al. 2003). Changes in nutrient dynamics and the structure of mangrove forests will enhance carbon oxidation and mineralization processes, quickly releasing GHGs into the atmosphere, such as carbon dioxide, methane, and nitrous oxide (Bhomia et al. 2016, Dung et al. 2016, Rani et al. 2023). In a large estuary like the RRD, the high sedimentation rates and nutrient accumulation will significantly enhance tree growth and mangrove forest expansion (Lovelock et al. 2007). Even mangroves are net sinks of nutrients in the estuaries, more significant inputs of nutrients than uptake capacity cause several ecosystem disturbances, such as enhancing algal growth and reducing the development of mangrove seedlings (Sarker et al. 2021). The aquaculture's impacts on mangrove forests were comprehensive and essential for long-term monitoring and assessment. Understanding these interactions is important for proposing effective conservation strategies in the context of environmental and climate changes. Future research should focus on quantifying methane emissions from mangroves under different nutrient conditions, linking nutrients dynamics with mangrove biomass and carbon storage (Wu et al. 2014).