Sedimentary Nutrient Dynamics in Homestead Fishpond Systems from a Subtropical Coastal Area

Abstract

Homestead ponds are small seasonal ponds that are rarely utilized for fish farming. Culturing fish in these small ponds can boost household fish consumption and cash inflow. The availability of nutrients in the water and sediment, however, plays a significant role in the pond’s natural productivity. This study was conducted to assess nutrient contents and some key physical parameters in the bottom sediments of 30 homestead ponds from the central coast along the Northern Bay of Bengal. Analyses of results showed the average values (±SD) of temperature, pH, electrical conductivity, organic matter (OM), organic carbon (°C), nitrogen (N), phosphorus (P) and sulphur (S) were 17.8 ± 1.12 °C, 7.29 ± 0.11, 0.41 ± 0.09 mS/cm, 29,615.48 ± 10,528.9 mg kg⁻¹, 11,958 ± 6107 mg kg⁻¹, 1030.6 ± 252 mg kg⁻¹, 17.1 ± 13.5 mg kg⁻¹ and 32.6 ± 19.7 mg kg⁻¹ during winter and 27.2 ± 1.81 °C, 6.73 ± 0.12, 0.38 ± 0.17 mS/cm, 19,100.01 ± 13,739.07 mg kg⁻¹, 11,079 ± 7969 mg kg⁻¹, 955 ± 229.4 mg kg⁻¹, 26.6 ± 20.2 mg kg⁻¹ and 23.4 ± 15.9 mg kg⁻¹ during pre-monsoon, respectively. One-way ANOVA revealed no significant differences in the mean value of sediment quality parameters among the selected ponds (p > 0.05) except for phosphorus in the winter season (p < 0.05). The sediment quality variables were found to be in the suitable range for fish culture. Pearson’s correlation coefficient values (r) showed that only organic carbon and nitrogen had a strong significant correlation with organic matter (p < 0.05). Based on Cluster Analysis (CA), two major associations among the nutrients were attained at a 15% similarity level: C, OM, and N in one cluster and P and S in another. The findings showed that the parameters were within the preferable range for aquaculture, and the homestead ponds are average productive ponds.

1. Introduction

Nutrients remain in the bottom sediment of an aquatic habitat [1], and their release and mixing into water influences the biological cycle of an ecosystem [2,3] including inevitable nutrients such as carbon (C), nitrogen (N), and phosphorus (P), which underlie diverse biogeochemical processes, control water quality [4] and ecosystem production [5]. In addition, most detrital-based systems may view nitrogen as the most crucial nutritional regulator [6]. These nutrients can exist in various forms, such as organic, inorganic, dissolved, or particulate, in water and sediment [7]. Sediments are considered active and dynamic characteristics may also affect water quality [8], resulting from a wide range of biogeochemical interactions and transformations. The sedimentary column may serve as the reservoir to accumulate nutrients [9]. To support significant algal growth, sediment discharge could preserve nutrient concentrations in surface waters [10]. According to Ramachandra et al. [11], the physicochemical characteristics of sediments significantly influence the type of food, feeding, and other life activities of benthic forms. Understanding the sediment–water interaction requires knowledge of the sedimentary dynamics of the water body, which affects the productivity of the surface water [12,13,14].

The coastal aquaculture sector in Bangladesh is the fastest growing industry; like other tropical or subtropical countries, it is privileged with suitable sites, cultivable species, and favorable climatic conditions and is essential for supplying nutrition, creating jobs, reducing poverty, and earning foreign currencies [15], or, to put it another way, for socio-economic development [16]. Almost every house in the coastal areas of Bangladesh has a pond in its courtyard. These homestead ponds are commonly used for extensive aquaculture practices, creating income-generating opportunities, mitigating malnutrition [17], and, most importantly, offering women an excellent chance to participate in the fish culture system [18]. The homestead fish production system and associated aquatic communities’ productivity and diversity primarily depend on natural food sources [19], which is also contingent on the trophic status [20] and quality [21] of water and sediment. In addition, the adaptability, growth, and reproduction of aquatic species are also subject to the quality of water and sediment [22] of the concerned habitat. Considering the importance of sediment nutrients for aquatic ecosystems, several studies on different water bodies such as beels [23,24], aquaculture ponds [25], lakes [26], micro-basin [4], rivers [27,28,29], channel [30], and estuary [31] were conducted. Despite their relevance in the coastal region of Bangladesh, extensive research on the dynamics of nutrients and the physical features of sediments from homestead ponds is lacking. Therefore, the primary aim of this study was to evaluate the potential of the homesteads ponds as aquaculture ponds by measuring the sediment nutrients, e.g., organic matter (OM), organic carbon (OC), nitrogen (N), phosphorus (P), sulphur (S) in different seasons, and correlating the values with some key environmental variables, e.g., temperature (T), pH and electrical conductivity (EC).

2. Methodology

2.1. Study Site and Sampling Protocol

Noakhali is a district in the coastal belt of Southern Bangladesh along the northern Bay of Bengal. The annual temperature varies from 15 °C to 34 °C, and the annual average rainfall was 2200 mm in 2021 [32]. Thirty homestead ponds in the five sub-districts (Sadar, Hatiya, Subarnachar, Kabirhat and Companigonj) of Noakhali between 22°07′ and 23°08′ N and 90°53′ and 91°27′ E were chosen at random for this investigation where the distance between two ponds was kept at least 1.5–3 km (Figure 1). A total of 180 sediment samples including 3 replicates per stations were collected during two seasons: the winter (December 2020) and the pre-monsoon (April 2021), with 90 samples per season. The samples were collected from the bottom surface of the homestead ponds by a mud corer.

2.2. Sediment Analysis

In the laboratory of the Soil Resource Development Institute in Noakhali, Bangladesh, the soil nutrient content was measured. After collecting the sediment samples in plastic bags, the samples were transported into the lab. The samples were dried before examination in indirect light to maintain the microstructural quality of the soil. It was pulverised after drying, and its nutrient concentration was assessed. A soil EC Tester (HANNA instrument: HI98331) was used during sampling to determine the temperature and EC of the pond sediment. According to Jackson [33], soil pH was measured using a glass electrode pH meter (Adwa pH/mV Meter: AD 131). The electrode was adjusted in the clamp of the electrode holder. pH was determined when the electrode was submerged in a partially settled soil suspension with a soil to water ratio of 1:2.5. The measurement of soil pH in water was given as the outcome. Titrimetry was used to measure organic carbon using Walkley and Black’s method as modified by Ghosh et al. [34]. The organic matter was calculated by multiplying the percentage of organic carbon with the standard Van-Factor Bemmelen’s of 1.724 [35]. The formula is as follows:

(OM) content = % (OC) × 1.724

(1)

The total N content of the sediments samples was determined by the Micro-Kjeldahl method as suggested by Jackson (1962) [36]. P-PO₄ of soil was determined by two methods on the basis of pH: Olsen’s method was followed for the ponds’ sediment whose pH is greater than six i.e., pH > 6 [37], and Bray and Kurtz’s method was followed for the ponds’ sediment whose pH is less than 7 i.e., pH < 7 [38]. In addition, the Bray and Kurtz method was also followed for the sediment samples, whose pH fell between 6 and 7. According to both methods, the absorbance was measured on a spectrophotometer at 890 nm. The determination process for SO₄—S was followed as described by Fox et al. [39], where the absorbance was measured at 535 nm.

2.3. Statistical Analysis

The statistical analysis was accomplished by PAST (Paleontological Statistics; version 4.03, Oslo, Norway) and Microsoft Office Excel 2016, and the map was plotted by using ArcGIS (version 10.3, Brisbane, Australia). Here, one-way ANOVA was performed to determine the significant difference among the ponds [40], and Pearson’s correlation coefficient was accomplished to determine the relationship among all nutrients [41] and physio-chemical parameters of samples. Cluster analysis was conducted in this study to identify the association among nutrients [42].

3. Results and Discussion

3.1. Physical Parameters and Nutrient Concentrations

3.1.1. Temperature (°C)

The temperature varied from 16.2 ± 0.4 °C to 20.3 ± 1.3 °C during winter (mean = 17.8 ± 1.12 °C) and 24.2 ± 0.4 °C to 31.2 ± 0.8 °C during pre-monsoon (mean = 27.2 ± 1.81 °C) where the highest temperature was recorded at station 5 (Sadar) in winter and station 6 (Sadar) in the pre-monsoon (Figure 2). The lowest level was shown in stations 13 and 16 (both in Subarnachar), respectively. There was no significant variation in temperature across the sites during the winter (H = 0.0054, p = 1) and pre-monsoon (H = 0.0049, p = 1) seasons (

Table 1. Result of one-way ANOVA of the physio-chemical parameters and nutrients.

Variables	Source of Variation	Winter			Pre-Monsoon
		df	x2	p	df	x2	p
Temperature (°C)	Stations	29	0.0054	1	29	0.0049	1
pH	Stations	29	0.0021	1	29	0.01	1
EC (mS/cm)	Stations	29	0.0709	1	29	0.2737	0.9996
OM (mg/kg)	Stations	29	0.4859	0.9724	29	0.9612	0.5416
OC (mg/kg)	Stations	29	0.4853	0.9726	29	0.9609	0.542
N (mg/kg)	Stations	29	0.4855	0.9725	29	0.9609	0.542
P (mg/kg)	Stations	29	4.728	2.96 × 10−5	29	0.6309	0.8908
S (mg/kg)	Stations	29	0.4312	0.987	29	0.5817	0.9259

). Even while pond temperatures often do not fluctuate as much as air temperatures, the amount of sunlight and air temperature have a major impact on the temperature of ponds. Some environmental factors can also change the temperature of the water. These components include turbidity, stream confluence, solar radiation, heat transfer from the atmosphere, and sunlight. These elements have a greater impact on shallow and surface waters than deep water. In tropical coastal areas, the temperature usually widely varies from 10 °C to 40 °C, which was also reflected in this study. This finding was completely consistent with the previous study [9].

3.1.2. pH

The value of pH varied from 6.75 ± 0.23 to 7.98 ± 0.17 during winter (mean = 7.29 ± 0.11) and 5.45 ± 0.07 to 8.43 ± 0.45 during pre-monsoon (mean = 6.73 ± 0.12) where the highest pH value was shown at station 13 (Subanachar) during winter and at station 25 (Companiganj) during pre-monsoon (Figure 3). The lowest value was found at Kabirhat (Stations 24 and 22, respectively) in both seasons. There were no significant differences in the mean value of pH among stations during the winter (H = 0.0021, p = 1) and pre-monsoon (H = 0.01, p = 1) (Table 1). For any aquatic ecosystem, pH is the most crucial element because it maintains productivity by regulating the majority of chemical processes [23]. Pond water pH changes throughout the day and season due to carbon dioxide being taken from the water by plants and animals during photosynthesis and respiration. At dusk and dawn, pH often reaches its highest and lowest values, due to increased carbon dioxide concentrations brought on by nocturnal respiration, which react with water to produce carbonic acid and reduce pH. The use of carbon dioxide and other inorganic compounds by algae for photosynthesis is a common source of high pH levels during the day. In most situations, a soil pH of around 7 is ideal for appropriate nutrient availability, according to BARC [43]. The pH of the environment has an impact on other factors both directly and indirectly [25]. According to Trussell [44], a high pH sharply reduces the ammonia ionization constant, increasing the percentages of the hazardous unionized part. Rapid photosynthesis can occasionally result in a pH that is dangerously high in the water with little alkalinity [25]. With a range of 6.75 ± 0.23 to 7.98 ± 0.17 and 5.45 ± 0.07 to 8.43 ± 0.45, respectively, the examined area had slightly basic pH during the winter (mean = 7.29 ± 0.11) and slightly acidic pH during the pre-monsoon. The pH ranged from 5.7 to 7.1 in Hilna beel and from 5.3 to 8.2 in Kumari beel, with an average of 6.33 ± 0.431 and 6.78 ± 0.684, respectively. This was closer to the findings of Alam et al. [23]. The ideal pH range for pond sediments is 6.5 to 7.5, while the permissible pH range is 5.5 to 8.5, according to Banerjea [45]. Jhingran [46] further recommended that pH in the range of 6.5–7.5 suggests average to high production, whereas pH below 6.5 indicates poor production. For freshwater species to be healthy and to grow quickly, a pH between 6.5 and 9.0 is ideal [47]. According to Banerjea [45] and Jhingran [46], the examined region displayed good pH values, indicating that the ponds are averagely productive. Additionally, during the winter, pH and sulphur showed a substantial inverse link; however, during the pre-monsoon, pH and other indicators showed no significant correlation.

Since soil bacteria are dormant during the winter (due to the low temperature), sulphur cannot be converted to sulphuric acid, which lowers pH.

3.1.3. Electrical Conductivity

The EC value varied from 0.18 ± 0.02 mS/cm to 0.63 ± 0.06 mS/cm during winter (mean = 0.41 ± 0.09 mS/cm) and 0.18 ± 0.04 to 1.13 ± 0.12 during pre-monsoon (mean = 0.38 ± 0.17 mS/cm) where the highest EC value was shown at station 8 (Hatiya) during winter and station 2 (Sadar) during pre-monsoon (Figure 4). The lowest value was found at Station 5 (Sadar) during winter and Station 20 (kabirhat) during the pre-monsoon season. The EC rises as the amount of ions increases. This is because conductivity rises when more ionic substances are dissolved by water because ions carry an electrical current across solutions. With rising temperatures, they become more conducting. Inorganic dissolved particles, such as chloride, nitrate, sulphate, and phosphate anions or sodium, magnesium, calcium, iron, and aluminium cations, have an impact on conductivity in water. There was no significant difference in the mean value of EC among stations during winter (H = 0.0709, p = 1) and pre-monsoon (H = 0.2737, p = 0.9996) (Table 1). Although many species have developed mechanisms for regulating EC, there are numerous ways that EC can have a direct physiological influence on aquatic creatures [25]. The optimal value, according to Gul et al. [48], is an EC of 2.90 dS/m. According to Gul et al. [48], the EC values of pond sediments ranged from 0.18 to 0.63 mS/cm in the winter and 0.18 to 1.13 mS/cm in the monsoon, which is not desirable. In contrast, EC showed a substantial positive association with P and S during the pre-monsoon but not with temperature, pH, OM, OC, N, P, or S during the winter. According to Das, soluble ions in the soil water system were either released or depleted when Electrical Conductivity (EC) changed.

3.1.4. Organic Matter and Organic Carbon

Concentrations of OM varied among the stations/ponds from 8066.6 ± 400 mg kg⁻¹ to 52,533.7 ± 4500 mg kg⁻¹ during the winter season (mean = 29,615.48 ± 10,528.9 mg kg⁻¹), where the highest concentrations were recorded at station 22 (Kabirhat) and the lowest concentrations were from station 5 (Sadar) (Figure 5). Again, the concentrations of OM varied from 7108.05 ± 397 mg kg⁻¹ to 62,367.4 ± 3529 mg kg⁻¹ during pre-monsoon (mean = 19,100.01 ± 13,739.07 mg kg⁻¹), where the highest concentrations were recorded at station 4 (Sadar) and the lowest at station 20 (Kabirhat) (Figure 5). As precipitation increases, soil organic matter levels often rise, and it also carries organic matters from land. Increased biomass production occurs when soil moisture levels are high, which produces more organic detritus. In general, soils with a higher natural clay content hold onto organic matter longer than sandy soils, which holds onto more organic carbon. In a pond, too many nutrients and organic growth will result in too much organic matter, which needs to be digested. This can cause anaerobic decomposition, which causes bad pond odour and reduced oxygen levels during the decomposition process. Additional presence may result in the generation of harmful gases such CO2, H2S, and NH3. In this case, ANOVA showed no statistically significant differences in organic matter among stations during winter (H = 0.4859, p = 0.9724) and pre-monsoon (H = 0.9612, p = 0.5416) (Table 1). Soil organic matter influences the physical, chemical, and biological properties of soils and improves the physical conditions of soils such as soil structure, water holding capacity, aeration, etc. [23]. Given that it serves as a source of metabolic energy that powers soil biological activities involved in nutrient availability, it is crucial in preserving long-term soil fertility [25]. According to Alam et al. [23], it provides food and energy for helpful organisms such as N₂ fixing bacteria. The OM contents in this study ranged from 0.81 ± 0.04% to 5.25 ± 0.45% during the winter and from 0.71 ± 0.04% to 6.24 ± 0.35% during the pre-monsoon, with an average of 2.06 ± 1.05% and 1.91 ± 1.37%, respectively. This result was quite comparable to that of Haque et al. [49], who found organic matter to be 1.67% in rice fields in Mymensingh that had been flooded for paddy cum fish farming. Low OM concentrations of 0.72 and 2.62% in Hilna beel and 0.69 to 2.22% in Kumari beel, with an average of 1.426 ± 0.642% and 1.338 ± 0.504%, respectively, were noted by Alam et al. Less than 0.86% of the organic matter in soil is regarded as too low, 0.86 to 2.58% as average, and 2.58 to 4.31% as extremely productive [46]. As a result, the ponds used in this study are regarded as an average level of productive. Additionally, during both seasons, organic matter and organic carbon and nitrogen revealed strong connections.

Concentrations of OC varied among the stations from 4679 ± 234 mg kg⁻¹ to 30,472 ± 6110 mg kg⁻¹ during the winter season (mean = 11,958 ± 6107 mg kg⁻¹), where the highest concentrations were recorded at station 22 (Kabirhat) and the lowest concentrations were from station 5 (Noakhali Sadar) (Figure 6). Again, the concentrations of C varied from 4123 ± 230 mg kg⁻¹ to 36,176 ± 2048 mg kg⁻¹ during the pre-monsoon season (mean = 11,079 ± 7969 mg kg⁻¹), where the highest concentrations were obtained at station 4 (Noakhali Sadar) and the lowest at station 20 (Kabirhat) (Figure 6). ANOVA detected no significant variation in the mean value of organic carbon among stations during winter (H = 0.4853, p = 0.97) and pre-monsoon (H = 0.9609, p = 0.54) (Table 1). Less than 0.5% organic carbon in pond sediments is regarded as insufficient or unproductive, 0.5–1.5% as typical, and 1.5–2.5% as extremely productive [46]. According to Banerjea [45], aquatic ecosystems with higher organic carbon content are more productive, and the appropriate range for aquaculture ponds is between 0.5 and 2.5%, with 1.5 to 2.5% being the ideal range. Based on percentage composition, the average OC level of pond sediments was 1.20 ± 0.61% and 1.12 ± 0.80% during winter and pre-monsoon, respectively, ranging from 0.47 ± 0.02% to 3.05 ± 0.26% and 0.41 ± 0.02% to 3.62 ± 0.20% indicating a typical pond for aquaculture. At commercial aquaculture ponds in Noakhali, Tapader et al. [25] found that the average OC content of sediments was 1.56 0.60% in new ponds and 1.39 0.47% in old ponds. Alam et al. [23] found lower OC content in two Rajshahi beels, and Kumar et al. [50] and Hoque et al. [26] found higher OC content in Kaptai Lake. However, during the winter and the pre-monsoon, organic carbon displayed highly substantial connections with organic matter and nitrogen.

3.1.5. Nitrogen

Concentrations of nitrogen varied among the stations from 403 ± 20 mg kg⁻¹ to 2627 ± 223 mg kg⁻¹ during the winter season (mean = 1030.6 ± 252 mg kg⁻¹), where the highest concentrations were recorded at station 22 (Kabirhat), and the lowest concentrations were from station 11 (Hatiya) (Figure 7). The concentrations of nitrogen varied from 355 ± 20 mg kg⁻¹ to 3118 ± 176 mg kg⁻¹ during pre-monsoon season (mean = 955 ± 229.4 mg kg⁻¹), where the highest concentrations were recorded at station 4 (Sadar) and the lowest from station 20 (Kabirhat) (Figure 7). Although nitrogen is present in large amounts in the environment naturally, it is also added through fertilisers and sewage. Around the world, crop fields are treated with fertilisers and other chemicals. Excess chemicals may enter water bodies through runoff and impair the water’s quality. The breakdown of large quantities of animal manures and slurry is another factor. ANOVA showed no significant variation in the mean value of nitrogen among stations during the winter (H = 0.4855, p = 0.9725) and pre-monsoon (H = 0.9609, p = 0.54) (Table 1). Less than 75 ppm (or mg/kg) of phosphorus in the soil is considered low, 76 to 150 mg/kg is medium, and 151 to 300 mg/kg is high [29]. Averaging 1030.6 ± 252 mg kg⁻¹ and 955 ± 229.4 mg kg⁻¹ during winter and pre-monsoon, respectively, the N level of pond sediments during the research period was greater than that reported by Alam et al. [23] and Sugunan et al. [51] and lower than that reported by Bragadeeswaran et al. [31]. Additionally, during both seasons, nitrogen demonstrated a strong association with organic matter and organic carbon.

Low organic carbon can result in low nitrogen in soil, and vice versa, establishing a strong positive relationship. The majority of the nitrogen in soil is present in organic form, which is the cause of this [52].

3.1.6. Phosphorus

Phosphorus levels varied among the stations from 5.0 ± 1.6 mg kg⁻¹ to 62.6 ± 80.9 mg kg⁻¹ during the winter season (mean = 17.1 ± 13.5 mg kg⁻¹), where the highest levels were obtained at station 1 (Sadar) and the lowest from station 5 (Noakhali Sadar) (Figure 8). The phosphorus levels in some ponds of Sadar were high, since the ponds in Sadar were densely forested and obtain runoff and debris from nearby municipalities that might contain phosphate. The levels of this nutrient can differ according on the surrounding environmental conditions, including rainfall, fertiliser runoff, and more. The amount of P can also rise when there is leftover fish food, debris, leaves, and even tap water. Less phosphates are typically released by higher-quality fish meals. Aquatic weeds or pond scum are encouraged to flourish by this elevated P level. The water, fish, and aquatic species may develop illnesses as a result of this buildup. Based on the seasons, the concentrations of phosphorus varied from 2.3 ± 0.3 mg kg⁻¹ to 91.2 ± 5.8 mg kg⁻¹ during the pre-monsoon season (mean = 26.6 ± 20.2 mg kg⁻¹), where the highest concentrations were recorded at station 2 (Noakhali Sadar) and the lowest at station 14 (Subarnachar) (Figure 8). ANOVA showed significant differences in the mean phosphorus levels among stations during winter (H = 4.7280, p = 2.96 × 10⁻⁵) (Table 1). However, during pre-monsoon, there was no significant difference in phosphorus among stations (H = 0.6309, p = 0.8908). To thrive, aquatic plants and algae must have access to phosphorus, and the interaction of sediments with the water column in natural environments affects the phosphorus cycle [53]. According to Jhingran [46], sediments with phosphorus levels of less than 30 ppm (or mg/kg) are regarded as poor, 30–60 ppm (or mg/kg) are average, and over 60 ppm (or mg/kg) are optimal. Phosphorus concentrations during the research period ranged from 5.0 ± 1.6 mg kg⁻¹ to 62.6 ± 18.9 mg kg⁻¹ and 2.3 ± 0.3 mg kg⁻¹ to 91.2 ± 5.8 mg kg⁻¹, with an average of 17.1 ± 13.5 mg kg⁻¹ and 26.6 ± 20.2 mg kg⁻¹, respectively, in winter and pre-monsoon. This indicates that the phosphorus level of the pond sediments was low. Both Alam et al. [23] and Bragadeeswaran et al. [31] observed lower concentrations of phosphorus in the Arasalar estuary, India, which is closer to the current study. While phosphorus did not significantly correlate with any other metrics during the winter, it did significantly correlate with EC and Sulphur during the pre-monsoon.

The variance could also result from processes such as the adsorption and desorption of phosphates and the sediment’s ability to operate as a buffer in a variety of environmental circumstances [54].

3.1.7. Sulphur

Levels of sulphur varied among the stations from 4.3 ± 1.4 mg kg⁻¹ to 82.7 ± 3.3 mg kg⁻¹ during the winter season (mean = 32.6 ± 19.7 mg kg⁻¹), where the highest concentrations were recorded from station 2 (Sadar) and the lowest concentrations from station 13 (Subarnachar) (Figure 9). Seasonally, the concentrations of sulphur varied from 3.0 ± 0.6 mg kg⁻¹ to 59.3 ± 2.6 mg kg⁻¹ during pre-monsoon (mean = 23.4 ± 15.9 mg kg⁻¹), where the highest concentrations were recorded at station 2 (Noakhali Sadar) and the lowest from station 27 (Companigonj) (Figure 9). The main reason for the elevated amount of sulfur is land erosion. Rainfall erodes rocks and soil components with a similar makeup over time. Rainfall continues to fall, causing sulfur runoff into nearby rivers as well as the discharge of sediments through erosion. Poor agricultural practises, runoff from cities and lawns, leaking septic systems, and sewage treatment plant discharges can all contribute to high sulfur concentrations. ANOVA results showed no significant difference in phosphorus among stations during winter (H = 0.4312, p = 0.987) and during pre-monsoon (H = 0.5817, p = 0.9259) (Table 1). Less than 12 ppm (or mg/kg) of soil is regarded as low, between 13 and 25 ppm (mg/kg) as medium, and between 26 and 75 ppm (mg/kg) as high [29]. Additionally, acid-sulfate soils are indicated by sulphur concentrations exceeding 0.75% (7500 ppm or mg kg⁻¹), and these soils typically have considerable acidity [55,56]. Sulfur concentrations in the sediments of the study ponds ranged from 4.3 1.4 mg kg⁻¹ to 82.7 3.3 mg kg⁻¹ and 3.0 0.6 mg kg⁻¹ to 59.3 2.6 mg kg⁻¹, respectively, with an average of 32.6 19.7 mg kg⁻¹ and 23.4 15.9 mg kg⁻¹. This typical concentration may be found in Rajshahi’s Hilna beel (24.05 ± 7.846 ppm) and Kumari beel (22.167 ± 7.241 ppm) [23]. In the aquaculture ponds of Noakhali, Tapader et al. [25] recorded extremely high sulphur concentrations. In this study, sulphur revealed an inverse significant link with pH during the winter but a positive significant correlation with the contents of EC, OM, OC, and P during the pre-monsoon. This result is supported by the study of Singh [57].

3.2. Relationship between Nutrients and Physical Parameters

A Pearson’s correlation coefficient test from all sediment types showed positive and highly significant correlations among organic matter, organic carbon, and nitrogen during both seasons where the correlation was significant at the 0.05 level, and highly significant at the 0.01 level (

Table 2. Result of Pearson coefficient correlation (r-value) test among all nutrients content and physicochemical parameters.

		T	pH	EC	OM	OC	N	P	S
Winter	T
	pH	0.03
	EC	−0.22	−0.09
	OM	−0.31	−0.42	0.12
	OC	−0.31	−0.42	0.12	1.00
	N	0.11	−0.33	0.02	0.59	0.59
	P	−0.19	0.06	0.03	0.14	0.14	−0.20
	S	−0.03	−0.51	0.09	0.42	0.42	0.06	0.37
Pre-monsoon	T
	pH	0.06
	EC	−0.21	0.09
	OM	−0.36	−0.34	0.19
	OC	−0.36	−0.34	0.19	1.00
	N	−0.19	−0.15	0.29	0.71	0.72
	P	0.15	0.35	0.65	0.13	0.13	0.25
	S	−0.27	0.04	0.56	0.55	0.55	0.38	0.56

T: temperature; EC: electrical conductivity; OM: organic matter; OC: organic carbon; N: nitrogen; P: phosphorus and S: sulphur. Significant values are shown in bold characters.

). However, sulphur showed an inverse significant correlation with pH during winter and a positive significant correlation with electrical conductivity, organic matter, organic carbon and phosphorus during pre-monsoon. Phosphorus also showed a significant correlation with electrical conductivity.

Cluster analysis showed almost the same similarities among the nutrients during the winter and pre-monsoon seasons (Figure 10). At 15% similarity, two major clusters were attained from five nutrients in both seasons. From these clusters, one cluster contained organic matter, organic carbon and nitrogen, and the other cluster contained phosphorus and sulphur, which means organic matter and organic carbon have a strong relationship and phosphorus and sulphur might have come from similar sources. The most probable sources of these nutrients may be different household effluents.

4. Conclusions

This preliminary study summarizes key sediment quality parameters (T, pH, EC, OC, OM, N, P and S) of small seasonal homestead ponds from the Noakhali Coastal area. Mean values (±SD) of the key parameters such as organic matter (OM), nitrogen (N), phosphorus (P) and sulphur (S) were 19,100.01 ± 13,739.07 mg kg⁻¹, 955 ± 229.4 mg kg⁻¹, 26.6 ± 20.2 mg kg⁻¹ and 23.4 ± 15.9 mg kg⁻¹, respectively. The findings of OC, OM, N, P and S indicated that most of the studied homestead ponds are medium to highly productive in nature. Statistical analyses showed no significant variation in the mean value of sediment quality parameters except for phosphorus. Among the variables, only organic carbon and nitrogen had strong significant correlation with organic matter, and two major groups were clustered at a 15% similarity level as revealed by cluster analyses. Considering all the studied variables, pH, OM, OC and S contents were at an acceptable level for fish culture. It may be suggested that the homestead ponds from the area are deemed to be suitable for the development of homestead aquaculture, which can increase the household consumption and revenue.