## 1 Introduction
Apart from the international and European legislation for the discharge in the natural receivers, in Romania, the applicable national legislation is the regulations regarding the establishment of pollutant loading limits of industrial and urban wastewater when discharged into natural receivers [24]. This document lists chemical substances with maximum allowable concentrations for discharge and also stipulates that, for unlisted substances, dedicated studies should be performed to establish the discharge limits. Thus, the need for ecotoxicity studies derives from the lack of maximum admissible limits for discharge into the Black Sea of the concentrations of chemical substances used by operators from the applicable legislation in force, as well as the requirement for the user to present them when issuing the water authorization. Otherwise, the potential risks associated with unregulated chemicals may go unnoticed, posing significant threats to human health and the environment. Copepods are essential trophic links between primary producers and consumers in aquatic systems. By grazing on phytoplankton and serving as prey for fish, they play a key role in the bioaccumulation and biomagnification of toxic pollutants within aquatic food webs [25; 26; 27]. The calanoid copep, _Acartia_ (_Acanthacartia_) _tonsa_ [PERSON], 1849, is a widespread species found in temperate regions, typically abundant in coastal and estuarine areas [28] of the Indian, Atlantic, and Pacific Oceans, as well as in the Malay Archipelago, the Baltic Sea, and the Sea of Azov [29]. In the Black Sea, it has been recorded since the early 1970s [7; 30; 31; 32], and in the Mediterranean since 1986 [32]. Despite its widespread presence, _A. tonsa_ is considered a non-indigenous species in the Mediterranean Sea, having been introduced in the 1980s via ballast water [9]. The International Organization for Standardization (ISO) recommends using _A. tonsa_ for assessing the lethality and toxicity of various contaminants because it is sensitive to different types of contaminants and toxins [33; 34] and is considered a valuable model organism for studying the effects of pollutants on marine zooplankton communities [35]. Since the 1970s, research has increasingly focused on the physiology, reproduction, and feasibility of multi-generation rearing of _A. tonsa_[8; 36; 37]. Additionally, _A. tonsa_ has been widely used as a model organism in ecotoxicology studies to evaluate the quality of marine environments and detect hazardous compounds [38; 39; 40]. Currently, _A. tonsa_ is included in international standardized protocols for acute, semi-chronic, and chronic bioassays for environmental risk assessment in marine coastal areas [1; 33; 41; 34]. Recently, it has also been incorporated by the Italian Ministry of the Environment (DM, 173/2016) [42] and the Government of Canada [43] in tests for evaluating sediment toxicity. These bioassays involve either direct exposure of eggs to sediment or incubation in sediment-derived aqueous solutions (e.g., elutriate or pore water) and assess specific endpoints such as egg hatching success and larval viability. Furthermore, a chronic endpoint, the reduction of _A. tonsa_ fecundity, has been proposed for assessing the impact of emerging contaminants like nanomaterials on adult copepods [6]. Copepods of the genus _Acartia_ inhabit numerous coastal and offshore environments, where they are often among the most abundant zooplankton species. Multiple species can coexist simultaneously, but typically, winter species alternate with summer ones. Local populations of Acartifidae can form due to the highly variable conditions of temperature, salinity, and food availability [32]. In the Black Sea, two predominant species are present most of the year: the permanent eurothermic _Acartia clausi_ and the more recent warm-water species _A. tonsa_, which has replaced the stenothermic _Acartia latisteosa_ since the 1970s [7]. _A. tonsa_ is a thermophilic species that produces resting eggs. Consequently, the _A.