Critical body residues, Michaelis-Menten analysis of bioaccumulation, lethality and behaviour as endpoints of waterborne Ni toxicity in two teleosts

Traditionally, water quality guidelines/criteria are based on lethality tests where results are expressed as a function of waterborne concentrations (e.g. LC50). However, there is growing interest in the use of uptake and binding relationships, such as  Biotic Ligand Models (BLM), and in bioaccumulation parameters, such as Critical Body Residue Values (e.g. CBR50), to predict metal toxicity in aquatic organisms.  Nevertheless, all these approaches only protect species against physiological death (e.g. mortality, failed recruitment), and do not consider ecological death which can occur at much lower concentrations when the animal cannot perform normal behaviours essential for survival. Therefore, we investigated acute (96-h) Ni toxicity in two freshwater fish species, the round goby (Neogobius melanostomus) and rainbow trout (Oncorhynchus mykiss) and compared BLM, LC, and CBR parameters for various organs, as well as behavioural responses (spontaneous activity).  In general, gobies were more sensitive. Ni bioaccumulation displayed Michaelis-Menten kinetics in most tissues, and goby gills had lower K_d but similar B_max values relative to trout gills. Gobies also accumulated more Ni than did trout in most tissues at a given exposure concentration. Organ-specific CBR values tended to be higher in goby but CBR50 and CBR10 values in the gills were very similar in the two species. In contrast, LC50 and LC10 values were significantly higher in trout. With respect to BLM parameters, gill log K_NiBL values for bioaccumulation were higher by 0.4-0.8 log units than the log K_NiBL values for toxicity in both species, and both values were higher in goby (more sensitive). Gobies were also more sensitive with respect to the behavioural response, exhibiting a significant decline of 63-75% in movements per minute at Ni concentrations at and above only 8% of the LC50 value; trout exhibited no clear behavioural response. Across species, diverse behavioral responses may be more closely related to tissue Ni burdens than to waterborne Ni concentrations.