© by PSP Volume 29 – No. 09/2020 pages 7342-7348 Fresenius Environmental Bulletin 7342 ACUTE AND SOME CHRONIC EFFECTS OF NICKEL IN GAMBUSIA HOLBROOKI Ahmet Burak Dumlu, Utku Guner * Trakya University Science Faculty Department of Biology, Balkan Campus, 22030 Edirne, Turkey ABSTRACT Nickel, which has a wide usage area throughout history, spreads throughout the ecosystem. Although Ni is low in the ecosystem, its usage continues to increase due to human use. Nickel is used in different industrial areas and released into nature from the atmosphere, from underground and above groundwater resources to aquatic ecosystems through erosion. Acute and some chronic toxic effects of Ni on Gambusia holbrooki, mosquito biological control fish, were investigated. Acute toxicity of Ni was determined by the probit analysis method and 96- hour lethal dose test result was 6.811 mg/l for G. holbrooki .During acute and chronic toxicity experiments, some behavioral changes in concentration, water column distribution, clustering and mobility and some physical changes such as blood supply to the gills and blackening of the outer epithelial tissue were observed in test animals. KEYWORDS: Nickel, mosquitofish, Gambusia holbrooki, acute toxicity, LC50, lethal dose, lethal concentration INTRODUCTION The current number of human-made chemicals added to the aquatic ecosystems in the world is esti- mated to be about nine millions. Toxic effects of heavy metals such as copper (Cu), cadmium (Cd), silver (Ag) and zinc (Zn) on fish have received con- siderable research attention [1, 2]. However, nickel (Ni) has received much less attention although it has long been known as an important chronic and acute toxicity agent. Nickel enters the environment from natural and anthropogenic sources and is distributed throughout all compartments using chemical and physical processes and biological transport by aquatic animals [3]. The continuous increase in the use and quantity of heavy metals is one of the biggest environmental problems. Different definitions have been proposed for heavy metals based on density, atomic number or atomic weight, and chemical prop- erties or toxicity [4, 5]. Heavy metals can enter aquatic systems from different natural and anthropo- genic sources, including industrial, agricultural or/and domestic wastewater, stormwater flow, land- fill leaks, and transport [6, 7]. Heavy metals in such environments have been a threat to all aquatic organ- isms even at low levels [8]. The most important property of heavy metals is that they can accumulate in the exposed organism and the accumulation in aquatic organisms is 1.000-10.000 times more than water concentrations. Heavy metals not only have acute but also chronic effects on the entire life cycle of aquatic or- ganisms, especially as loss of growth and damage to reproductive functions. The type, age and sex of aquatic organisms affect aquatic toxicity [9, 10]. In aquatic ecosystems, fish are usually located at the top of the food chain and transfer some heavy metals from water to other food networks. Therefore, fish are the most decisive factor for estimating the risk potential of heavy metal pollution and human consumption in freshwater systems [11]. Some met- als such as Cu and Zn which are important for nor- mal metabolism of fish should be obtained from food or sediments. Non-essential metals are also known to be taken up by fish where they accumulate in tissues [12, 13]. Combating pests with biological control meth- ods is also a problem. Gambusia holbrooki (Girard, 1859) is one of the most widely used organism used in biological control of mosquitoes. At this point, it is very important to know how living organisms used in biological control are affected by pollution. Gam- busia holbrooki is commonly used in Thrace region in Turkey and is exposed to heavy metal pollution of agricultural and industrial origin. In this study, we determined the 96-hour acute toxicity and show some chronic toxicity of Ni on G. holbrooki. MATERIALS AND METHODS Gambusia holbrooki specimens were collected from Güllapoğlu Pond located in Balkan Campus of Trakya University (Edirne, Turkey) and brought to the laboratory. The collection was carried out fol- lowing the research permission obtained from the General Directorate of Nature Conservation and Na- tional Parks of the Ministry of Agriculture and For-