Using stable isotopes and chemical markers to understand the history of Negombo Lagoon, Sri Lanka

Highlights

• The biomarkers and stable isotopes reveal the changes at Negombo lagoon.

• Switch from terrestrial to a marine signal in the late 19th century.

• PAH and oil hopanes pollution reached a peak concentration at the 1980s.

• Marine and terrestrial biomarkers reached the peak maxima at the 1980s.

• Reduction in terrestrial biomarkers in the last decade due to mangrove exploitation.

Abstract

Negombo Lagoon is a highly valued estuary in Sri Lanka due to its high socioeconomic value, but this estuary has undergone many changes since the early 1800s due to human impact. Records of carbon and nitrogen stable isotopes, and alkane, alcohol, polycyclic aromatic hydrocarbon (PAH) and hopane biomarkers from the mid-19th century to 2012 in a sediment core collected from Negombo Lagoon indicate that the lagoon went through two major changes during this 160-year time frame. The first change occurred from 1850 to 1900, where a switch from predominately terrestrial sources of organic material to marine sources is observed, after the Hamilton Canal was built in 1804 to connect the open ocean to the lagoon. The second change occurred post 1940s during Sri Lanka’s industrialization period where high anthropogenic inputs are seen in the estuary, specifically as increases in PAHs and oil-related, diagenetic hopanes. Overall, the Negombo Lagoon appears to be moderately polluted by PAHs with the highest concentrations of parent PAHs measured in the 1980s and likely derived from pyrolytic sources of solid fuel biomass.

Introduction

Estuaries around the world have faced severe human impact and, in particular, developing nations are more sensitive to these impacts due to the lack of adequate infrastructures. Sri Lanka, a small island nation located south of India, has experienced anthropogenic effects since the industrial revolution at the beginning of the 18th century. Among the estuaries in Sri Lanka, Negombo Lagoon is a shallow brackish lagoon located on the western coast (Fig. 1). It is particularly important due to its high socioeconomic value that provides a living for 3000 households from the fisheries of this lagoon (Silva, 1996, Samarakoon and Samarawickrama, 2012). The narrow inlet situated in the northern part of the shallow lagoon allows it to experience two tidal cycles a day. In addition, since the 1960s, population and urbanization has increased in the Gampaha District (Silva, 1996, Chandrasekara et al., 2014), making Negombo Lagoon very susceptible to eutrophication. These anthropogenic effects are mainly due to overfishing, wastewater discharge from the sewage treatment plant of Katunayaka Export Processing Zone, and urban and agriculture runoff (Asanthi et al., 2007, de Croos and Pálsson, 2013). Katunayake Export Processing zone, located in Negombo Lagoon’s watershed, has 86 industries including leather tanning, metal plating, manufacturing of metal and wastewater treatment plants, all of which contribute to the anthropogenic impact on the lagoon (Dasanayaka 2015). The watershed is mostly made up of urban areas, home gardens, and coconut and paddy cultivation. In 2001, 41% of the land cover was made up of urban areas and home gardens (Chandrasekara et al., 2014). Agriculture is one of the major land-based activities in Negombo Lagoon’s watershed, and shrimp farming and other aquacultures are other land-based activities that have negatively impacted the lagoon due to the amount of waste produced (Silva et al., 2013).

Due to lack of monitoring and surveys of the lagoon, past environmental conditions (i.e., type of pollutants and the quantity that have entered the waterway through time) are unknown. Measuring baseline and past conditions of a system is important for understanding the degree in which human activities have affected the lagoon. Only a few studies on heavy metal contamination in water (Chandrasekara et al., 2014) and sediment core (Dasanayaka, 2015) samples from Negombo Lagoon have been carried out. These studies concluded that metals were not from an anthropogenic origin until the end of the 19th century. Measuring multiple proxies in sediment cores are relevant tools to obtain information about past environmental conditions.

For studying past environmental conditions, organic matter buried in sediments contains a unique fingerprint of organic matter supplied from overlying waters through time. Natural and artificial radionuclide tracers such as ²¹⁰Pb and ¹³⁷Cs are common sediment dating tools for the estimation of sediment chronology (Kirchner, 2011, Mabit et al., 2014). Using dated sediment cores, the study of past environmental conditions and the role of eutrophication can be achieved by using different multiproxy and chemical parameters (Dalton et al., 2005, Smittenberg et al., 2005, Lu and Meyers, 2009, Carreira et al., 2011). One unique proxy is stable carbon and nitrogen isotopes, which are used to study the origin of organic matter (Graham et al., 2001, Zhang et al., 2007a, Zhang et al., 2007b, Alonso-Hernández et al., 2017, Alonso-Hernández et al., 2020) and enhance our understanding on the biogeochemical processes involved in the transfer of nutrients through food webs (Fry, 1988, Fry and Sherr, 1989, Peterson, 1999, Zanden and Rasmussen, 2001). Organic carbon originating from different end members (autochthonous vs. allochthonous) has distinctively different isotopic values (Thornton and McManus, 1994, Grey et al., 2001). The reason for this is due to the different fractionation effects that can occur in nature, for example marine phytoplankton has more enriched δ¹³C values (−19 to −23‰) compared to terrestrial plants (C₃ plants) that have more depleted values (−25 to −31‰; O'Leary, 1981, Hayes, 1993, Fry and Sherr, 1989, Bouillon et al., 2011). Another example is the enriched values of δ¹⁵N in treated sewage, which leads to the loss of the light nitrogen isotope due to ammonia volatilization and denitrification (Kendall et al., 2007, Risk et al., 2009, Rumolo et al., 2011, Archana et al., 2016).

The elementary C_org/N ratios are also complementary tracers used to differentiate between marine and terrestrial organic matter in sedimentary records (Graham et al., 2001). In general algae have C/N ratios that range from 4 to 10 and terrestrial plants contain less nitrogen than marine plants and their ratios are greater than 12 (Prahl et al., 1980, Meyers, 1994, Lamb et al., 2006). Sediment records need to be analyzed with caution since different sources of organic matter can be preserved differently and therefore giving biased results. For example, autochthonous marine organic matter is degraded much quicker than allochthonous terrestrial organic matter, with a potential bias towards a larger terrestrial organic signal (Meyers, 1994, Lamb et al., 2006). However, δ¹³C and C/N ratios have been shown to retain source signatures of the water column, with minor changes during sinking and possible digenesis at the sediment surface (Meyers, 1994, Meyers, 1997, Lamb et al., 2006).

Qualifying and quantifying lipid biomarker fractions and polycyclic aromatic hydrocarbons (PAHs) in sediments are another complimentary tool that can be used to determine past organic matter sources (Page et al., 1999, Kumar et al., 2018, Kumar et al., 2020). Lipids are ubiquitous and chemically stable which make them good biomarkers. Certain lipid biomarkers or a combination of biomarkers can be unique to certain types of plants, organisms or human sources. Biomarkers are made up of a variety of different chemical compounds, but this study focuses on aliphatic hydrocarbons, PAHs and alcohols. The number of carbon units in aliphatic compounds can reveal information about the terrestrial or aquatic sources. For example, long chain n-alkanes with odd number of carbon units (C₂₃-C₃₃) originate from terrestrial plants (Eglinton and Hamilton, 1967, Prahl et al., 1980), whereas algae are dominated by short, odd number of carbon units (C₁₅-C₁₇) (Youngblood et al., 1971, Volkman et al., 1992). Hopanes, which are pentacyclic triterpanes originating from precursors in bacterial membranes, are present in geologically old organic matter such as oil and heavier refined products but absent in lighter refined oil products such as diesel and gasoline (Wang et al., 1999). Bacterial and diagenetic αβ hopanes (i.e., oil hopanes) can be distinguished by the hopane stereoisomers. Bacterial hopanes and ancient shales tend to have a ββ stereochemistry whereas petroleum has an αβ configuration (Dastillung and Albrecht, 1976, Peters and Moldowan, 1993, Bost et al., 2001). Series of branched alkanes with odd carbon number preferences, such as the 5,5 diethylalkanes are biomarkers generally associated to benthic microbial mats with specific paleoenvironmental conditions (Greenwood et al., 2004, Zhang et al., 2007a, Zhang et al., 2007b, Wang et al., 2007).

PAHs are persistent organic pollutants derived from petrogenic, pyrogenic and diagenetic sources (Lake et al., 1979, Yunker et al., 2002, Tolosa et al., 2004, Yunker et al., 2015). Petrogenic PAHs, which derive from crude and refined petroleum, are mostly made up of alkylated PAHs. Pyrogenic PAHs are formed during incomplete combustion of organic matter such as burning of wood, fossil fuels, and industrial waste, and have a higher contribution of the parent PAHs (Yunker et al., 2002, Liu et al., 2009).

The alcohol fraction is comprised of source-specific and recalcitrant compounds such as steroid alcohols (sterols), triterpene alcohols and acyclic fatty alcohols. Sterols can give information about terrestrial plants, phytoplankton, bacteria, benthic algae and human activities (Volkman, 1986, Volkman et al., 1998, Grimalt et al., 1990, Vane et al., 2010). Many triterpene alcohols, such as α- and β-amyrin, β-amyrone, germanicol and taraxerol are derived from higher plants (Jaffé et al., 2006, Kumar et al., 2019). Taraxerol is also considered a specific marker in coastal mangrove vegetation (Killops and Frewin 1994). The C₂₇ steroid cholesterol has been found to be dominant in zooplankton and phytoplankton autochthonous organic matter (Rieley et al., 1991). There are also sterols such as coprostanol and epicoprostanol that are fecal derived and provide an indication of domestic sewage (Vane et al., 2010). Similar to n-alkanes, the number of carbon units in a saturate n-alkanol compound can also indicate origins. For example, long chain n-alcohols, derived from terrestrial plants, range from C₂₂ to C₃₂ with even predominance (Sever and Parker 1969) and microbial n-alcohols have short chains (≤C₂₀; Robinson et al., 1984).

The purpose of this study is to perform a comprehensive examination of Negombo Lagoon using stable isotopes, elemental composition of carbon and nitrogen, chemical biomarkers and radiometric dating to determine the past history from the mid-19th century to present day conditions. These measurements were made on a sediment core collected in Negombo Lagoon with the aim to identify the long-term changes on the origins and main sources of organic matter deposed in the lagoon. In addition, this study presents the first comprehensive study on stable isotopes and lipid biomarkers in the area and the PAH contents can be used as baseline data in future assessments.