Chlorine is extensively used as a powerful oxidizing agent in the countries surrounding the Arabian Gulf for water treatments and biofouling control. The usage has been increasing significantly as demand for water grows considerably both in industry and domestic use. This is due to the fact that it is a well-tested technology, has had a history of long-term worldwide industrial use and is of acceptable cost. Sodium hypochlorite (NaOCl, commonly termed chlorine) is a common form of chlorine that is used and in all cases it is produced on site by means of an electro-chlorination plant (ECP). In seawater, chlorine produces a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl − ). These rapidly react with the bromide ion to form a mixture of hypobromous acid (HOBr) and hypobromite ion (OBr − ). The acute oxidants formed by chlorination are therefore short lived and are not persistent in seawater, but can be quite toxic. Further complicating the environmental concern of chlorination is the production of numerous, and more persistent, compounds formed by complex reactions between chlorine/bromine and the organic constituents of seawater, collectively described as chlorination by-products (CBPs). Many CBPs are persistent and may be toxic to marine organisms subjected to long-term exposures. The Gulf waters support a range of coastal and marine habitats including mangrove swamps, seagrass beds and coral reefs. However, marine organisms in the Arabian Gulf are living close to their tolerance limits due to the extreme environmental stressors of temperature and salinity. Anthropogenic stressors such as chlorine may further exacerbate these natural stressors.


In order to evaluate the risks of chlorine exposure to Arabian Gulf marine organisms, the aims of this study were to 1) develop protocols for acute and chronic toxicity tests involving native species at different trophic levels, 2) correlate sensitivity to other marine organisms used as indicator species in risk assessments, 3) draw conclusion from the results and explore ways that this could be used for informing environmental management activities.


Materials and methods

Stock solution of calcium hypochlorite was prepared in filtered artificial seawater (0.45 μm filter paper) daily in dark bottles. Concentration of chlorine in the experimental chambers were verified daily by the N,N-diethyl-p-phenlenediamine (DPD) colorimetric method. Different concentrations of chlorine were either administered as a continuous flow through or via daily renewal.

Exposure to test species

Species were collected from different location around the coastal areas of Qatar. They were then cultured in the laboratory using conditions of the Arabian Gulf. Species used in the tests include phytoplankton (Synechococcus sp.), zooplankton (Uterpina acutifrons), pearl oysters (Pinctada radiata), sea urchins (Diadema setosum) and killifish embryos (Aphanius dispar).


Sensitivity varied across the trophic levels for each species and was found to be in line with other test organisms that are used in established toxicity procedures. All five species used were found to be useful for certain types of toxicity testing. For example, Pinctada radiata and Diadema setosum were excellent in testing for successful fertilization and early development stages, while local Aphanius dispar embryos were useful for following sub-lethal effects such as malformation and hatchability. Embryos of oysters and urchins were found to be the most sensitive to chlorine (Fig. 1.). Effect of chlorine was only observed near or after hatching has occurred. Continuous exposure showed more effective than the semi-renewal method.

Discussion and conclusion

While chlorine continues to be used heavily globally, available toxicology data are either very limited or out dated especially those based on the Arabian Gulf. The purpose of this study was to increase the knowledge of chlorine chemistry and toxicity especially the sensitivity towards the Arabian Gulf species. Results obtained reveal that the sensitivities of all five species used in this study are in line with species used internationally in similar tests. LC50s were determined for all species and are currently being used in modelling tools to determine the fate and environmental risks of chlorine. Chlorine being a powerful oxidizing agent asserted its effect immediately after contact. It loses its potency within the first hour in seawater and this may contribute to the low effect shown in this study compared those in fresh water. Given the short-lived nature of chlorine as an oxidant in seawater, our attention has also been focused on the sub-lethal effects that may be attributed from the by-products. Work is ongoing in this area. Expanding tests with species used in this study to evaluate their sensitivity towards chlorine by-products will further increase our understanding of their chemistry and environmental risk in the Arabian Gulf and will provide a science-based tools for making management decisions.


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