Physicochemical characteristics of shrimp ponds on mangrove ecosystems in Kannur District of Kerala, India

Mangrove forests are one of the most productive ecosystems in the world and are known for their ecological, economic, and social importance. Large-scale shrimp farming severely threatens mangrove ecosystems since shrimp productivity is correlated with mangrove ecosystems. The present study was carried out to determine the physicochemical characteristics of soil and water of traditional and non-traditional shrimp ponds near mangroves for understating the variation from the natural mangrove ecosystems of the Kannur district. Different parameters such as pH, electrical conductivity, BOD, DO, alkalinity, acidity, fluoride, iron, sulphate, nitrate, calcium, magnesium, and total hardness of collected water sample and pH, electrical conductivity, nitrogen, phosphorus, potassium, organic carbon and heavy metal contents of collected soil samples were determined by respective instruments and methods. From the analysis, it was found that all the parameters of traditional, as well as non-traditional shrimp ponds showed significant variation from the natural mangrove ecosystems. The result from the statistical analysis such as multivariate analysis (partial eta squared) concluded that the parameters of water such as pH, electrical conductivity, temperature, turbidity, alkalinity, nitrate, sulphate and iron contents of traditional shrimp ponds, and electrical conductivity, temperature, nitrate, sulphate and total hardness of non-traditional shrimp ponds significantly varied from its corresponding natural mangrove ecosystems. In addition, the potassium content in soil samples from traditional shrimp ponds and the pH, electrical conductivity, phosphorus, lead, and chromium in soil from non-traditional shrimp ponds significantly varied from corresponding natural mangroves. Changes in those physicochemical parameters of soil and water will affect the associated organisms and restrict further restoration of mangroves in the long term.


Introduction
Mangrove is the most productive and biodiversityrich ecosystem (Tomlinson 1986) rather than a halophyte growing in tropical or subtropical coastline areas (Giri et al. 2011;Hema and Devi Hema and Devi 2014). Since they are located in the transition zone between two different ecosystems, mangroves have a higher genetic diversity than other vegetation types (Jithendra et al. 2014). Hence All the plant, animal, and microbial communities in this ecosystem are associated with these intertidal zones and can adapt to their changing environment (Edwin 2002).
According to the global Forest Resource Assessment (2020), mangroves represent a total of 0.15 billion km 2 across 123 countries worldwide. India covers 4992 km 2 of mangrove forest, accounting for approximately 5% of the world's land area vegetation (CRZ 2019;FSI 2021). Kerala constitutes 17.82 km 2 of mangrove forests, scattered as small patches along the coastal and intertidal region (Sreelekshmi et al. 2021).
However, mangroves are rapidly disappearing on a global scale due to anthropogenic interventions such as aquaculture, urbanization, tourism, and agriculture (Peng et al. 2009;Hema and Devi 2015;Sreelekshmi et al. 2021). The digging and leveling of the mangrove habitats for shrimp farms' preparation have significantly impacted mangrove status worldwide (Giri et al. 2008;Jayanthi 2018). Shrimps positively correlate with mangrove ecosystems, and the habitats are ideal for farming (Barbier and Cox 2004). Traditional and non-traditional/scientific shrimp farms are two types of farming methods that have been well managed in Kerala. Traditional shrimp farms depend on diurnal tidal inundation to supply the larval shrimp and their food nutrients into the ponds. In contrast, non-traditional shrimp farming focuses on modern techniques such as intensification of culture operation by innovative changes in pond size, increasing stocking rate, employment of aeration, application of feed, etc. (Bhattacharya 2010).
The highest extent of mangrove forests in Kerala is distributed along the coastline of the Kannur district, known to be the capital of the mangroves in Kerala (Vidyasagaran 2014). More than 60% of these forests in the Kannur district are privately owned and, therefore, highly threatened (Preethy 2019). The primary threat to the mangroves of Kannur is the continuous destruction due to the construction of shrimp ponds/ aquaculture ponds (Bijith et al. 2022) and recorded that the mangroves in the vast intertidal tracts of Kannur district have been reclaimed for the construction of such farms. They reported 140 active shrimp farms with a total extent of 524.4 km 2 , including traditional and non-traditional shrimp farms.
The physical and chemical properties of the water and soil determine the welfare of organisms in the mangrove ecosystem. The construction of such shrimp ponds may alter the physicochemical characteristics of the soil and water of mangrove ecosystems and the dependent communities (Mishra et al. 2008). Mishra et al. (2008) studied water quality assessment of aquaculture ponds in Orissa's Bhitarkanika mangrove ecosystem. The significant findings were that the pH of the aquaculture ponds varied from 5.63 to 8.5 due to the application of chemical additives and that the DO contents of water samples were high compared to the standard (4 mg/L) because of mechanical aerators. They have investigated high levels of chlorine in water due to the inflow of waste, and those aquaculture ponds showed more range in calcium and magnesium due to the addition of lime and other pesticides. The observed nitrate concentration in aquaculture ponds is much below BIS's prescribed standard of 10 mg/L (Mishra et al. 2008). The quality of soil and water, the growth of floral-faunal components, and the entire structure and function of mangrove ecosystems are determinedly affected by shrimp farms (Primavera 1997;Primavera 2006;Ashton 2008;Biao et al. 2009;Hamilton 2011;Preethy 2019).
This study analyses the physicochemical characteristics of the soil and water of traditional and non-traditional shrimp ponds near the mangrove vegetation of the Kannur district to understand the critical variation from the natural mangrove ecosystems. Farmers have to be aware of the coming consequence of such extensive shrimp farming, and they should be acquainted with sustainable management strategies for mangrove-friendly shrimp farming in the future.

Study area
This study was conducted in Kannur (Cannanore) district, which lies between the latitudes 11֯ 40 and 12֯ 08 N and the longitudes 75֯ 11 and 76֯ 08 E with an area of 2966 km 2 . Shrimp pond locations in Kannur were selected by referring to the study by Bijith et al. (2022). Soil and water samples were collected from the places Cherukunnu, Kunhimangalam, and Thalassery in March, and April 2022, the location map was given in Fig. 1.

Methodology
Soil and water samples were collected from the natural mangrove ecosystems, traditional shrimp ponds, and non-traditional shrimp ponds (scientific shrimp ponds) of Kannur district. A total of 24 water and 6 soil samples were collected from the sample sites. 6 water samples were taken in 1L sample bottle from both type shrimp pond which adjacent to mangroves and from a natural mangrove ecosystem. DO and BOD samples were collected in separate sample bottles from the same area. DO samples were fixed itself from the sampling sites and BOD samples were wrapped by black paper to avoid the further reaction with sunlight. Fixed DO samples were titrated immediately from the laboratory to get the initial DO value, whereas BOD samples were analysed after 3 days of incubation at 27 °C. Turbidity, pH, electrical conductivity, temperature of each water samples was determined by using a multi-parameter. Alkalinity and acidity determined by using titration method. Fluoride, iron sulphate and nitrate were measured by using the instrument spectrophotometer. Fluoride in water samples was measured by SPANDS colorimetric method and read the absorbance at 570 nm. Iron content in water samples measured by the phenanthroline method and read the absorbance at 510 nm, Sulphate in water samples was measured by the turbid metric method and read the absorbance at 410 nm, Nitrate in water samples was measured by the ultraviolet spectrophotometric screening method of absorbance at 220 nm. Chloride is measured by the argentometric method. Total hardness is measured by the titration method. Calcium and magnesium can be determined directly by using atomic absorption spectrophotometer.
Soil samples were collected up to 15 cm depth below the surface from the same sites in polythene covers. Those soil samples were dried, powdered, and sieved through a 2 mm sieve. The different parameters such as pH, electrical conductivity, nitrogen, phosphorus, potassium, and organic carbon and heavy metal content of collected soil samples are determined by using different materials and methods. pH is measured directly in a calibrated pH meter. Electrical conductivity is determined by using an electrical conductivity meter. The nitrogen content of the soil is determined by the Kjheldal method. Potassium was measured in the flame photometer and phosphorus in the spectrophotometer. Organic carbon is measured by Walkley-black wet digestion method. Heavy metals were measured The physicochemical analysis of soil and water were done with the concerned instruments/methods given in Tables 1 and 2. Multivariant analysis of variance (MANOVA) was done to determine the significance of physicochemical parameters (Anderson 2001) by using statistical software SPSS version 20.0. The magnitude of the effect of each variable of the natural mangrove ecosystem on traditional and non-traditional shrimp ponds was checked by partial eta squared values ranging from 0 to 1 where 0-0.2 is a minor effect, 0.2-0.5 is medium and over 0.5 is a larger effect on soil and water (Cohen 1988).

Result
Water quality analysis of mangroves ecosystems, traditional shrimp ponds, and non-traditional shrimp ponds Mean value of the physicochemical parameters of water samples such as pH, electrical conductivity, BOD, DO, alkalinity, acidity, fluoride, iron, sulphate, nitrate, calcium, magnesium, and total hardness of collected water samples from traditional shrimp pond, non-traditional shrimp pond, and natural mangrove ecosystem are given in Tables 3 and  4    Atomic absorption spectrometer (AAS) Heavy metals (Cr, As, Hg) Inductively coupled plasma atomic emission spectrometer (ICP-AES) The physicochemical analysis of soil was carried out by appropriate instruments/methods is given in Table 2. In the results, the mean value of pH, electrical conductivity, nitrogen, phosphorus, potassium, organic carbon, and heavy metal content of traditional shrimp pond, non-traditional shrimp pond, and natural mangrove ecosystem is given in   a higher effects on natural mangrove ecosystem (Fig. 2).

Discussion
Physicochemical characteristics of water samples of shrimp ponds and natural mangrove ecosystems in Kannur district The direct discharge of the effluent to the adjacent mangrove ecosystems from the shrimp ponds shown in Fig. 2. The pH value of the water sample collected from the traditional shrimp pond is more alkaline than that of the mangrove due to the application of lime to the shrimp pond with an objective of better production and phytoplankton bloom activity (Venkateswarlu et al. 2019). The nitrate concentration was higher in mangroves than in traditional shrimp ponds since the mangroves are an excellent source of nitrate (Mishra et al. 2008). Iron content was found to be higher in the traditional shrimp than in the mangrove ecosystems. Electrical conductivity, and temperature of traditional shrimp ponds were high whereas sulphate, alkalinity, and turbidity high at natural mangrove ecosystems. The excessive use of lime and dolomites, probiotics, etc., during the pre-construction stage of nontraditional ponds, might be the reason for the higher pH in non-traditional ponds concerning mangroves. Total hardness is mainly due to the presence of ions like calcium (Ca 2+ ) and magnesium (Mg 2+ ) (Mishra et al. 2008). In the present study, non-traditional shrimp pond water was found to have less calcium and magnesium content; thus, it indicated a relatively lower value of total hardness compared to mangroves. The nitrate concentration was lower in non-traditional shrimp ponds than in mangrove ecosystems since the mangroves and other aquatic plants contribute nitrate to the ecosystem. Electrical conductivity was high in mangrove ecosystems, whereas temperature and sulphate were high in non-traditional shrimp ponds.

Physicochemical characteristics of Soil samples of shrimp ponds and natural mangrove ecosystems in Kannur district
Soil is considered the base of the mangrove ecosystem because it determines the status of the ecosystem in terms of dependent flora and fauna (McKee 1993). Non-traditional shrimp farming has been the major cause of the change in the physicochemical characteristics of the mangrove soil than traditional shrimp farms. Several ponds with an approximate depth of 1.5 m using modern techniques and mechanized tools were constructed in mangrove land (Bijith et al. 2022), making mangrove restoration more challenging. The pH value of the soil sample collected from non-traditional ponds was more alkaline than that of mangroves which can be attributed to the addition of lime in the water. The electrical conductivity of soil near non-traditional shrimp ponds was found to be lower compared to mangroves. It may affect aquatic animals' physiological effects . Potassium in the soil taken from the non-traditional shrimp ponds was low compared to mangroves. The accumulation of heavy metals in the sediments has been reported to be closely related to the frequency and duration of the tidal flood (Sarangi 2002), rather than the effect of the aquaculture practices. The heavy metals like lead and chromium in the soil showed relatively lesser values compared to mangroves. The non-traditional shrimp ponds had high phosphorus content because a significant portion of the input phosphorus is released into the surroundings rather than being transformed into shrimp biomass (Briggs and Funge-Smith 1994;Funge-Smith and Briggs 1998;Paez-Osuna et al. 1997;Teichert-Coddington et al. 2000;Xia et al. 2004;Paez-Osuna 2005). A significant phenomenon known as eutrophication, or the exposure of coastal waters to excessive nutrients, has been linked to the release of aquaculture effluents in numerous locations around the world (Herbeck 2013;Nóbrega et al. 2013).

Conclusion
The study in the Kannur district of Kerala showed a significant change in the physicochemical parameters of the traditional and non-traditional shrimp ponds compared to that of natural mangrove ecosystems. The physicochemical parameters of water collected from the traditional shrimp ponds, such as pH, electrical conductivity, temperature, turbidity, alkalinity, nitrate, sulphate, and iron, varied from natural mangroves, mainly due to the addition of organic feed, fertilizers, and other chemicals. It indicates that the mangrove species adjacent to the traditional shrimp farm have wider tolerances to varying levels of these parameters. Soil from the non-traditional shrimp ponds has the most significant changes compared to traditional shrimp ponds. The parameters like phosphorus, pH, electric conductivity, lead, and chromium of soil samples collected from the non-traditional shrimp ponds were also significantly changed concerning natural mangroves. Therefore, it reduces the prospects for further mangrove restoration in abandoned shrimp ponds.
The abiotic condition of the soil and water of the mangrove habitats were found to have considerably changed by both traditional and non-traditional shrimp farms in the Kannur district. Applying several chemicals, fertilizers, and feeds in shrimp farms will affect the mangrove habitats' flora, fauna, and microorganisms. Hence it is crucial to pay close attention to sustainable management strategies for mangrove-friendly shrimp farming by devising sustainable strategies for shrimp farming and implementing them with people's participation.