The occurrence and toxicity of a red tide caused by Noctiluca scintillans (Macartney) Ehrenb., in the coastal waters of Hong Kong

Abstract

A red tide caused by Noctiluca scintillans (Macartney) Ehrenb., affected numerous beaches in Hong Kong in June 1971. The red tide followed closely in the wake of a typhoon. It is suggested that the typhoon created optimum conditions suitable for the abnormal blooming of an already established population of Noctiluca. In this respect it has been suggested that the light southerly winds following the typhoon were responsible for concentrating the red tide on the beaches. Bright sunshine occurring before and after the typhoon increased the air and sea temperatures and perhaps provided optimum conditions for holophytic phytoplankton to bloom when the strong winds during the typhoon stirred up nutrient rich bottom deposits in the coastal waters. Noctiluca, a holozoic member of the plankton, was able to quickly flourish. The possibility also exists that Noctiluca was feeding directly on bacteria present in the polluted waters of Hong Kong. Heavy rainfall associated with the typhoon, in all probability further reduced the already lowered salinity of the sea water in these waters caused by the increased effect of the Pearl River. Noctiluca, contrary to earlier reports was found to possess a minimal toxicity and did not cause a significant fish mortality. The digestive diverticula of Atrina vexillum collected from the infected area was also found to possess a low toxicity. Sea water from which the Noctiluca had been filtered caused a significant change in the white cell picture of the blood of mice. This effect has hitherto been ascribed solely to bacterial endotoxins. It is suggested that this may also be a property possessed by an exotoxin-like substance produced by Noctiluca scintillans.