Nutrient Level in the Lower Gangetic Estuaries

Abstract

Very little research work is available on the comparative account of natural and man-made (anthropogenic) sources of nutrients in the aquatic ecosystem. The estimate made by Ferguson is presented in Table 4.1.

Annexure 4A.1: Decadal Variation of Nutrient Level in Two Major Estuaries in the Indian Sundarbans

Abstract

 The impact of nutrient level on water quality in the Hooghly and Matla estuarine complex in Indian Sundarbans was assessed for three decades (1984–2014). Nitrate, phosphate and silicate were used as indicators of nutrient-related water quality in the estuarine water. The present research reflects significant spatio-temporal variations of selected nutrients with relatively higher values in the Hooghly estuary (in the western Indian Sundarbans) compared to the Matla estuary (in the central Indian Sundarbans). Significant variations were observed in dissolved nitrate, phosphate and silicate concentrations between stations and years (p < 0.01). Such pronounced variations may be attributed to the location of highly industrialized and urbanized city of Kolkata, Howrah and Haldia port-cum-industrial complex adjacent to the Hooghly estuary. The sudden rise of selected nutrients during premonsoon, 2009 (irrespective of sampling stations), is directly related to Aila, a supercyclone that occurred in the lower Gangetic delta during 22–25 May 2009.

Keywords 

Indian Sundarbans · Nutrients · Aila · Spatio-temporal variation

4.1.1 4A.1.1 Introduction

Coastal waters and estuaries are facing a variety of adverse impact affecting both the ecosystem and human health through sewage wastewater discharge and disposal practices that often lead to introduction of high nutrient loads, hazardous chemicals and pathogens causing diseases. In countries like India, shrimp culture practices and aquaculture waste have magnified the adverse situation (Mitra 1998; Mitra and Zaman 2015). In regional level like Sundarbans deltaic ecosystem (located at the tail end of the mighty River Ganga), a phenomenon like erosion and subsequent washing of the topsoil from the intertidal mudflats of mangroves also contribute considerable amount of nutrients in the adjacent aquatic ecosystem. The adverse public health, environmental, socioeconomic, food quality, security and aesthetic impact from sewage contamination in coastal areas are well documented (Luger and Brown 1999; Tyrrel 1999; Danulat et al. 2002; WHO 2003). Apart from these causes, erosion of riverbanks due to tidal surges also conveys nitrate, phosphate and silicate in the estuarine water (Mitra et al. 2009), which may have a far-reaching effect on the environment. However, very few researches have focused on the trend of nutrient load in the estuarine waters based on past long-term data bank. The present paper is a road map towards this direction in the frame work of Indian Sundarbans.

4.1.2 4A.1.2 Materials and Methods

4.1.2.1 4A.1.2.1 Study Area

The mangrove-dominated Indian Sundarbans in the lower Gangetic delta region at the apex Bay of Bengal is inhabited by some 4.2 million populations. Two major estuaries in this delta complex are Hooghly (in the western sector) and Matla (in the central sector). There is a multitude of industries located on the western bank of the Hooghly estuary. Apart from this, large numbers of tourism units and shrimp culture farms are also located adjacent to the Hooghly estuary . A number of literatures are available on the salinity profile, surface water temperature and pH of the area (Mitra 1998, 2000; Mitra and Bhattacharyya 1999; Majumder et al. 2002; Panja et al. 2003; Banerjee et al. 2005; Mitra and Banerjee 2005; Mitra et al. 2011; Zaman and Mitra 2014; Mitra and Zaman 2015; Ray Choudhuri et al. 2015; Trivedi et al. 2015, 2016; Chaudhuri et al. 1994). However, very few researches have addressed the long-term variation of nutrient level in the estuarine waters, although the area is presently experiencing population growth, industrial activities, mushrooming of shrimp farm, growth of tourism units and establishment of fish landing stations (Mitra 2013). Apart from these primary sources of nutrients, churning action of bed substratum due to wave action and currents conveys silica to the overlying aquatic phase.

4.1.2.2 4A.1.2.2 Sample Collection

Sampling of surface water was done during high tide from three stations, namely, Diamond Harbour , Namkhana (in the Hooghly estuary) and Ajmalmari (in the Matla estuary ) (Table 4A.1.1). Sample collection was carried out during May (premonsoon), September (monsoon) and December (postmonsoon) for a period of 31 years (1984–2014).

Table 4A.1.1 Location of sampling stations

4.1.2.3 4A.1.2.3 Nutrient Analysis

Surface water samples collected for nutrient analysis were filtered through a 0.45 μm polycarbonate filters (Millipore 47 mm diameter) and then deep frozen for further analysis in the laboratory. The standard spectrophotometric method of Strickland and Parsons (1972) was adopted to determine the nutrient concentrations in surface water. Nitrate was analysed by reducing it to nitrite by means of passing the sample with ammonium chloride buffer through a glass column packed with amalgamated cadmium filings and finally treating the solution with sulphanilamide. The resultant diazonium ion was coupled with N-(1-naphthyl)ethylenediamine to give an intensely pink azo dye. Estimation of the phosphate was carried out by treatment of an aliquot of the sample with an acidic molybdate reagent containing ascorbic acid and a small proportion of potassium antimony tartrate. Dissolved silicate was analysed by treating the sample with acidic molybdate reagent. The resultant silicomolybdic acid was reduced to molybdenum blue complex by ascorbic acid, and incorporating oxalic acid prevented the formation of similar blue complex by phosphate.

4.1.2.4 4A.1.2.4 Statistical Analysis

ANOVA was performed in order to analyse whether the selected nutrients vary significantly between years and stations (p < 0.01).

4.1.3 4A.1.3 Results

Significant spatio-temporal variations of nitrate, phosphate and silicate in the study region were noted. Also sudden rise of the nutrient level during premonsoon, 2009, is attributed to supercyclone Aila that contributes nutrients through massive erosion of riverbanks, washing of topsoil of intertidal mudflats along the estuaries and churning of the river bed (vide Figs. 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, and 4.10 in this chapter).

4.1.3.1 4A.1.3.1 Dissolved Nitrate

At Diamond Harbour, dissolved nitrate concentration ranged from 20.84 ppm (during premonsoon in 1984) to 48.15 ppm (during premonsoon in 2009). At Namkhana, dissolved nitrate concentration ranged from 18.90 ppm (during premonsoon in 1984) to 43.44 ppm (during premonsoon in 2009). In case of Ajmalmari, the values ranged between 9.44 ppm (during premonsoon in 1984) and 24.89 ppm (during premonsoon in 2009).

In both Diamond Harbour and Namkhana of the Hooghly estuarine system, nitrate has increased 9.49 μg at l⁻¹/decade and 10.57 μg at l⁻¹/decade, respectively. In Ajmalmari, located in the Matla estuarine complex, the increase is relatively low (5.98 μg at l⁻¹/decade).

4.1.3.2 4A.1.3.2 Dissolved Phosphate

At Diamond Harbour, dissolved phosphate concentration ranged from 1.54 ppm (during premonsoon in 1984) to 6.99 ppm (during premonsoon in 2009). At Namkhana, dissolved phosphate concentration ranged from 0.98 ppm (during premonsoon in 1984) to 6.05 ppm (during premonsoon in 2009). Ajmalmari, the station in the Matla estuary exhibited a phosphate value from 0.56 ppm (during premonsoon in 1984) to 2.96 ppm (during premonsoon in 2009). In both Diamond Harbour and Namkhana of the Hooghly estuarine system, phosphate has increased 1.96 μg at l⁻¹/decade and 2.11 μg at l⁻¹/decade, respectively. In Ajmalmari, located in the Matla estuarine complex, the rate of increase is 0.92 μg at l⁻¹/decade.

4.1.3.3 4A.1.3.3 Dissolved Silicate

At Diamond Harbour, dissolved silicate concentration ranged from 101.83 ppm (during premonsoon in 1984) to 242.78 ppm (during premonsoon in 2009). At Namkhana, it ranged from 44.68 ppm (during premonsoon in 1984) to 211.49 ppm (during premonsoon in 2009). In case of Ajmalmari, the silicate ranged from 31.43 ppm (during premonsoon in 1984) to 111.99 ppm (during premonsoon in 2009). In both Diamond Harbour and Namkhana of the Hooghly estuarine system, silicate has increased 64.57 μg at l⁻¹/decade and 54.56 μg at l⁻¹/decade, respectively. The rate of increase is 31.18 μg at l⁻¹/decade in Ajmalmari, which is relatively low compared to stations selected in the Hooghly estuary.

4.1.4 4A.1.4 Discussion

The enrichment of the aquatic system by nutrients has both natural and anthropogenic origin . The main sources of nutrient input in the present study area are runoff from the adjacent landmasses (Mitra 2013), erosion and leaching (Mitra et al. 2009), sewage from cities and industrial wastewater (Mitra and Choudhury 1993; Mitra 1998; Bhattacharyya et al. 2013) and untreated sewage disposal from shrimp farms and tourism units (Mitra 2013; Zaman and Mitra 2014; Mitra and Zaman 2015). Atmospheric deposition of nitrogen (from combustion gases) can also be important for Hooghly estuarine system and its surrounding area as multifarious industries are concentrated in this estuarine region. The effects of nitrogen on marine and estuarine systems, the pathways for nitrogen transport between land and aquatic habitats and the positive correlation between nitrogen and primary production and often secondary production (i.e. fishery yields) have been widely reviewed (Hecky and Kilham 1988; Howarth 1988; Rabalais 2002).

Most of the phosphorus pollution in the present study area comes from the households and industries including phosphorus-based detergents which are widely used in thickly populated cities of Kolkata, Howrah and Haldia industrial complex adjacent to the Hooghly estuary.

Silicate being an important ingredient of bed material and soil substratum originates due to erosion, churning action of the water that amplifies during tropical cyclone, which is common in the present geographical locale.

The ratio of nitrogen and phosphorus plays a regulatory role in the phytoplankton diversity spectrum of brackish water. It determines which of the two elements will be the limiting factor and consequently which one has to be controlled in order to arrest the bloom condition (Table 4A.1.2).

Table 4A.1.2 Nitrogen/phosphorus ratios (expressed in weight) for various limiting conditions in freshwater and estuarine/coastal water

Large marine areas frequently have nitrogen as the limiting nutrient, especially in summer. Intermediate areas such as river plumes are often phosphorus-limited during spring but may turn to silica or nitrogen limitation in summer. When phosphorus is the limiting factor, a phosphate concentration of 0.01 mg l⁻¹ is enough to support plankton, and concentrations from 0.03 to 0.1 mg l⁻¹ or higher will be likely to promote blooms.

In coastal areas , the growth and proliferation of diatoms are promoted by the presence of silica. When the silica concentration is low, diatoms cannot develop. Then other opportunistic toxic algal species, which are no longer submitted to competition, can grow and form blooms. Species from the genus Phaeocystis and several dinoflagellates (Prorocentrum, Dinophysis, Gymnodinium) are known to proliferate under such conditions.

In this study, the N:P ratios in all the selected stations and seasons are greater than 10 (except monsoon season in Namkhana) (Table 4A.1.3). This implies that the aquatic phase of the present study area is P-limiting (WHO 2003). A high N:P ratio normally increases the standing stock of dinoflagellates and diatoms.

Table 4A.1.3 Average nitrogen/phosphorus ratios in different seasons in the three sampling stations

Significant variations in the level of dissolved nitrate, phosphate and silicate between years and between stations were observed (p < 0.01) which reveal the impact of season and anthropogenic pressure in the present study area (Table 4A.1.4).

Table 4A.1.4 ANOVA result showing temporal and spatial variations of dissolved nutrients (nitrate, phosphate and silicate)

4.1.5 4A.1.5 Conclusion

The core findings of the present study are listed here:

• 1. The main sources of nutrients in the present study area are primarily anthropogenic in nature, although natural disaster (like supercyclone Aila ) resulted in sudden rise in nutrient level during premonsoon, 2009.

• 2. The concentrations of nutrients exhibit a gradual increasing trend which is an evidence of rising anthropogenic pressure in and around the mangrove-dominated Indian Sundarbans.

• 3. The situation seems to be alarming in terms of nutrient enrichment if proper management/control measure is not adopted. The policymakers must foster appropriate action with true partnership with private sectors. The regulation should also be put in place to enforce the polluters (from urban areas, industries, shrimp farms and tourism units) to pay the principal cost and also foster a willingness to pay among polluters through provision of better and efficient services. This should ensure operational sustainability of the services.