# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 5 Ecotoxicological Effects on Microplastics  
Ecotoxicological studies on MPs can be distinguished according to: (i) test organisms, (ii) type, shape, and size of the polymer, and (iii) the site of analysis (in situ or in laboratory). There are numerous organisms that can be used in MPs ecotoxicological assessment; however, \(>\)75% of studies were performed on marine (micro)organisms. The most common testing organisms applied are: fish, mollusks, small and large crustaceans, annelids, mammals and echinoderms, birds and cnidarians, sponges, reptiles, and rotifers. Fish is mostly used in in situ studies, while small crustaceans prevail among organisms tested in a laboratory [4; 19; 22; 23; 29; 104; 105; 106; 107; 108; 109; 110]. The most studied MPs shapes include spherical particles, fibers, and fragments. Although PE and PS are the most studied MPs types (dueto their ubiquitous presence in aquatic ecosystems), investigations also comprehended ecotoxicological effects of other MPs types such as PP, PES/PET, PVC, polyamide, acrylic, polyether, cellophane, and polyurethane [22]. Regardless of MP types, both direct and indirect toxic effects on aquatic organisms have been investigated. Results of ecotoxicological studies are commonly expressed as a concentration of tested compounds causing certain effects (in percentage) on measured population. The most common effect considered within these studies was the percentage of population mortality. Hence, in lethal toxicity testing, LD\({}_{50}\) represents the median lethal dose, while LC\({}_{50}\) represents the median lethal concentration. If a test end point is an adverse response other than death, an effective concentration (EC) or effective dose (ED) is used toxicity parameter. Concentration causing 50% of adverse effect in the tested population is commonly used (EC\({}_{50}\)), although other levels (i.e., EC\({}_{10}\)[111] or EC\({}_{20}\)[112]) can be applied if necessary. Effective and lethal values can be expressed per mass of tested organism; ED and LD are used for higher organisms. Besides, it is also possible to determine the No Observed Adverse Effect Level (NOAEL) and Lowest Observed Adverse Effect Level (LOAEL). NOAEL is the highest exposure level at which there are no biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered as adverse. LOAEL is the lowest exposure level at which there are biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. Growth inhibition test is used for the ecotoxicological purpose as well [113]. Table 3 summarizes ecotoxicological studies performed including (micro)organisms and MPs types used. MPs toxicological effects on freshwater aquatic (micro)organisms are still scarce since most studies applied marine (micro)organisms. Therefore, future studies should be focused on freshwater microorganisms such as algae and bacteria or other surface water and sediment organisms. Current data summarized in Table 3 reveal that researches are performed using a wide range of concentrations and types of MPs to study adverse effects to the range of organisms (from bacteria to fish). Accordingly, the graphical presentation in Figure 3 shows the range of ecotoxicological concentrations established for different organism levels studied, thus providing an insight into the sensitivity of tested organisms to changes in MPs concentrations affecting different trophic levels. Figure 3: The range of ecotoxicological concentrations for different organism levels.