NaOCl produced by electrolysis of natural seawater as a potential method to control marine red tide dinoflagellates

Hae Jin Jeong^1*, Heung Rak Kim², Kwang Il Kim², Kang Young Kim³, Kwan Ha Park¹, Seong Taek Kim⁴, Yeong Du Yoo⁴, Jae Yoon Song⁴, Jae Seong Kim⁴, Seong Kyeong A⁴, Won Ho Yih¹, Se Jin Pae⁵, Chang Hoon Lee⁴, Min Do Huh⁶, Sang Ho Lee¹

KEY WORDS: bloom, copepod, diatom, dinoflagellate, fish, HAB, macroalgae, mitigation, protist, shellfish, sodium hypochlorite (NaOCl)

ABSTRACT

As part of the development of a method to control the outbreak and persistence of red tides using sodium hypochlorite (NaOCl), we investigated the effect of NaOCl on survival of red-tide dinoflagellates, diatoms, heterotrophic protists, planktonic crustaceans, fin-fish, shellfish, and macroalgae. Because NaOCl introduced into natural waters would be subject to dilution, as well as breakdown in sunlight to NaCl, the survival of organisms was determined after 10 min (10M) and 1 h NaOCl exposure (1H), and again after transfer to fresh seawater for 6 or 24 h. The lethal total residual chlorine (TRC) concentration that killed 50% of the test organisms (LC₅₀) for the red tide dinoflagellates Gymnodinium catenatum, Cochlodinium polykrikoides, Akashiwo sanguinea, Lingulodinium polyedrum, Prorocentrum micans, Alexandrium affine, and Gymnodinium impudicum ranged from 57-157 ppb for 10M and 30-106 ppb for 1H. Complete mortality of all red-tide species occurred at TRC concentrations ≥ 500 ppb. The LC₅₀s of the diatoms Skeletonema costatum and Thalassiosira rotula, 3,083-3,383 ppb for 10M and 3,128-3,433 ppb for 1H, were much higher than the red tide dinoflagellates. However, the LC₅₀s of the heterotrophic dinoflagellates Polykrikos kofoidii and Oxyrrhis marina were similar to the red tide dinoflagellates. The ciliate Strombidinopsis sp. had LC₅₀ of 306 ppb for 10M and 119 ppb for 1H, higher than that of the dinoflagellates. The LC₅₀s of the calanoid copepods Acartia spp. and Pseudodiaptomus sp. were 1,397-1,493 ppb for 10M and 744-987 ppb for 1H, and that for larvae of the brine shrimp Artemia franciscana was 4,905 ppb for 10M and 2,814 ppb for 1H. The LC₅₀s of juvenile gray mullet Mugil cephalus and juvenile black rockfish Sebastes schlegeli were 1,234-1,883 ppb for 10M and 1,234-1,440 ppb for 1H, while those of adult manila clam Ruditapes philippinarum and spat of the abalone Nordotis discus were > 20,000 ppb. The LC₅₀s of the macroalgae Griffithsia japonica (Rhodophyta) and Ulva pertusa (Chlorophyta) were 1,519-12,365 ppb for 10M and 1,085-12,558 ppb for 1H. The present study suggests that if NaOCl is introduced into waters containing red tide organisms at TRC concentrations of 300-500 ppb for 10M and 200-400 ppb for 1H, red tides can be effectively controlled without serious harmful effects on other marine organisms, except heterotrophic dinoflagellates.