The link between shrimp farm runoff and blooms of toxic Heterosigma akashiwo in Red Sea coastal waters

In May 2010 a copious bloom of the raphidophyte Heterosigma akashiwo was observed for the first time in Red Sea waters off the coasts of Saudi Arabia. This bloom was confined to an area where water and phytoplankton flow freely between the sea and a shrimp farm. The phytoplankton density and physico-chemical characteristics of the sea water were therefore investigated weekly at bloom and non-bloom sites in order to gain insight into the environmental factors prevailing at the bloom site and their link with the shrimp farm runoff. The bloom site showed higher nutrient concentrations than the non-bloom site, indicating the possible role of the shrimp farm in flushing nutrients into this site. The bloom appeared on 27 May, coinciding with a decrease in salinity (< 30‰) and an increase in temperature (> 19 °C). The results of toxicological assays showed that both bloom samples and batch cultures of H. akashiwo were toxic to Artemia salina and exhibited haemolytic activity with respect to rabbit erythrocytes. Bloom samples showed a higher toxicity (LC50 = 8.9 × 104 cells ml− 1) and haemolytic activity (EC50 = 3.64 × 104 cells ml− 1) than the batch cultures (LC50 = 11.6 × 104 cells ml− 1, EC50 = 5.1 × 104 cells ml− 1). In the light of the results of this study, the link between H. akashiwo blooms and shrimp farm runoff should be considered during the monitoring of Red Sea coastal waters for the presence of harmful algal blooms.