The aim of this study was to assess the toxicity of dispersants (OSD/SEACARE and OSD/LT) with different concentrations on the growth of Aspergillus and Rhizopus species in fresh water (Ocdee River) in Kegbara-DereGokana Local Government Area, Rivers State. The Fresh water sample was collected by submerging 10liters of jerry Can (sterile) into a depth of 30cm in K-Dere River in Gokana Local Government Area, Rivers State. The samples were transported to the laboratory in sterile containers packed in coolers in an ice pack for analyses within six hours of collection. Oil spill dispersants; OSD/SEACARE and OSD/LT were obtained from Barker and Hughes Nig. Ltd (formally Mil Park Nigeria Limited) in Port Harcourt Rivers State. The toxicity testing was done for the period of days at room temperature. Standard microbiological techniques were used; toxicity testing was prepared by setting up conical flask aseptically covered with cotton wool. For each dispersant (toxicant), the test was carried out in five (5) separate conical flasks containing appropriately sterile habitat water (freshwater). In each of the conical flask, four toxicant concentrations (%); 10, 25, 50, and 75 were added separately. These concentrations were obtained by aseptically transferring 10, 25, 50 and 75 ml of the dispersant into 90, 75, 50, and 25 ml of sterile freshwater. The control contained the water sample (habitat water) and the organism without toxicant. One millilitre (1ml) of the test organism was added to each toxicant concentration in test tubes containing (10, 25, 50, 75% and control respectively). Aliquot (0.1ml) from each concentration was then plated out using spread plate technique on Sabouraud Dextrose Agar immediately after zero (0) hour inoculation was done, inoculation and spreading continued after 4, 8, 12 and 24 hours for short time toxicity (Acute) while for long time (Chronic) inoculation was from 168 (7days), 336 (14days), 504 (21days) and 672 (28days) hours respectively and the plates were incubated for 72 hours at 28± 2ºC. After incubation, the total viable count on each plate was taken and converted to log₁₀.This process was repeated for the two test organisms and two dispersants separately giving a total number of twenty (20) experimental set-ups. The result obtained showed that the two dispersants employed in this study pose lethal threats to the biological population of the aquatic environment and tend to seriously alter the overall stability of the river ecosystem [²] as observed with the response of Aspergillus and Rhizopus species to the toxicity of dispersant with respect to other forms of life. However, the result of the percentage (%) log survival count showed that Aspergillus and Rhizopus species are quite tolerant to the toxicants at some concentrations and exposure times as seen in table 4.4 and 4.5 which showed that the organisms had reasonable growth at 10% and 25% concentrations of the toxicants at 0, 4, 8 hours, and to some extent, 12 and 24hours. The sensitivity of the fungi; Aspergillus and Rhizopus species to the toxicity of different toxicants (OSD/SEACARE AND OSD/LT) with the fresh water sample decreased in the following order (noting the lower lethal concentration (Lc50), the more toxic the toxicant); Aspergillus in OSD/SEACARE (56.15%), Aspergillus in OSD/LT (58.81%) and Rhizopusin OSD/SEACARE (58.05%), Rhizopusin OSD/LT (58.40%). The study revealed that the fungal species; Aspergillus and Rhizopus species responded differently to different concentrations of the two toxicants from time to time and the response of the organisms due to the presence of the toxicant may alter or affect the process of decomposition, mineralization and nutrient generation which will in turn disturb the nature of the ecosystem and biogeochemical cycle. Furthermore, the study also revealed that dispersants contain some chemical components that can be very hazardous to life, reducing productivity and promoting death of not just the aquatic forms of life (both the micro and macro flora) but also higher forms of life such as humans and other mammals that make use of them as source of food and other means of survival.

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