Effects of insecticides on aquatic ecosystems in Bangladesh

In Bangladesh, the intensification of agriculture is indispensable due to its ever increasing population, the food security needs, and land scarcity. Severe agro-climatic events (e.g. flash floods, seasonal water scarcity and salinity intrusion in coastal land) pose further difficulties to crop production. To meet the growing demand of food under these harsh conditions, farmers are using a variety of pesticides indiscriminately; a sharp increase of their use was observed during the last decades. Residues of pesticide applied on agricultural land may enter into the aquatic environment through drain, runoff and spray drift, thereby contaminating this environment. Hence, this PhD thesis aimed to investigate the human health issues and ecological risks on aquatic ecosystems posed by the large scale use of pesticides in Bangladesh.

In Chapter 1 the current status of pesticide use in intensive agriculture in Bangladesh is described together with their associated potential risks on the aquatic environments posed by pesticides. The available studies on assessing the fate and effects of pesticides for the (sub-) tropical aquatic ecosystems are reported. Chapter 1 describes the knowledge gap regarding the environmental risks of pesticides in the context of Bangladesh and discusses the tired-based approach to take into account for the risk assessment in Bangladesh.

Chapter 2 outlines the information on the current status of pesticide use in rice-prawn concurrent systems of south-west Bangladesh and human health issues posed by the application of pesticides. The ecological risks of 10 pesticides for the aquatic ecosystems that support the culture of freshwater prawns (Macrobrachium rosenbergii) were assessed using exposure and effect models. The TOXSWA model calculated pesticide exposure (peak and time-weighted average concentrations) in surface waters of rice-prawn systems for different spray drift scenarios. The simple first-tier risk assessment for these 10 pesticides were performed using a risk quotient (RQ) method. The results of RQ method indicated that chlorpyrifos, cypermethrin, alpha-cypermethrin and malathion may pose a high to moderate acute and chronic risks for invertebrates and fish for all spray drift scenarios. The higher-tier PERPEST effect model confirmed the high risks of cypermethrin, alpha-cypermethrin and chlorpyrifos for insects and macro- and micro-crustaceans, which were previously derived by the RQ-based risk assessment approach.

Chapter 3 presents the results of a chemical monitoring in surface water and sediment samples of north-west Bangladesh. The residues of the 10 most commonly used organophosphate insecticides in surface water and sediment samples were measured in that region. Like Chapter 2 of this thesis, the risk assessment of the concentrations of these 10 insecticides for fish, Daphnia and algae was started with a deterministic RQ method based on measured environmental concentrations (MECs) and the threshold concentrations derived from single species toxicity tests. The results showed high acute and/or chronic RQs (RQ > 1) in surface water and sediment for chlorpyrifos, diazinon, quinalphos, malathion and fenitrothion. The higher-tier PERPEST effect model also confirmed the risks of chlorpyrifos, diazinon, quinalphos and fenitrothion for aquatic insects, micro- and macro-crustaceans.

Chapter 4 describes the fate and effects of imidacloprid on several structural and functional endpoints of freshwater ecosystems in Bangladesh as evaluated in freshwater outdoor microcosms. The safe threshold values (i.e. NOECs) of imidacloprid for the individual taxa, community and water quality variables were derived for (sub-)tropical Bangladesh. Single species toxicity tests were also performed using the two most responding species (e.g. Cloeon sp. and Diaptomus sp.) of the microcosm study. The sensitivity of several arthropod species to imidacloprid was much higher in sub-tropical country Bangladesh compared to their temperate counterparts.

Chapter 5 elucidates the acute toxicity of chlorpyrifos on the developmental stages of Banded Gourami (Trichogaster fasciata), which is a local freshwater fish species in Bangladesh. In this chapter, the effects of chlorpyrifos on the incubation period of embryo, hatching success, mortality of embryos and two-day old larvae of Banded Gourami are discussed. Several malformations of larvae including irregular head and eye shape, lordosis, body arcuation, notochordal abnormality and caudal fin damage when exposed to 10 and 100 μg/L chlorpyrifos were also demonstrated.

Chapter 6 investigates the toxicity of chlorpyrifos on the mortality and the reproductive tissues of male and female Banded Gourami (Trichogaster fasciata) over a period of 75 days. The threshold values (NOECs) for male and female mortality, GSI, histopathological alterations of ovary and testis for different time interval were derived in this chapter. The results show that the long-term exposure to chlorpyrifos affect the reproductive tissues of Banded Gourami at exposure concentrations that cause mortality also.

In chapter 7 the major findings of different studies are discussed and after an overview of the conclusions, this thesis recommends: (1) to promote the suitable averting behaviour by farmers during pesticide application, (2) to conduct future experimental, monitoring and model validation studies nationwide, in order to better characterize the risks posed by pesticides for Bangladeshi aquatic ecosystems, (3) to improve the technical facilities (i.e. analytical verification) for future laboratory studies, (4) to reduce the pesticide use based on the recommended dosage by agricultural extension officers, and (5) to seek alternatives of pesticide use through the adoption of integrated pest management (IPM) practices to avoid the risks posed by pesticides.