Chlorine reactions with seawater constituents and the inhibition of photosynthesis of natural marine phytoplankton

Abstract

The time course of ‘chlorine’ disappearance was followed in filtered (I), non-filtered (II) and u.v.-oxidized (III) seawater. In I and II there was an initial rapid decline in both free and residual ‘chlorine’ followed by a slower decline of these species. No decline was observed in III, suggesting that the removal of organic matter and/or the oxidation of metal ions by u.v. oxidation, removes those molecular species that react with ‘chlorine’. Reaction rate was more rapid in II than in I, suggesting that particulate organic matter may also react with ‘chlorine’. Reaction kinetics of ‘chlorine’ were complex but approximated second order in the ‘slow’ phase. The inhibition of phytoplankton photosynthesis by ‘chlorine’ was studied by adding hypochlorite to seawater samples in the laboratory and in field samples collected adjacent to an open coast marine outfall of an electric generating station. ‘Chlorine’ concentrations required for 50% inhibition varied with exposure time. For 24-hour incubations such inhibition took place with residual ‘chlorine’ concentrations about 10 parts/10 9. However, there was no recovery of photosynthetic activity when residual chlorine had fallen to undetectable levels. The inhibition appears to be irreversible. As noted earlier by others the principal deleterious impact on marine phytoplankton of the use of seawater for cooling power plants lies in the use of ‘chlorine’ to prevent slime build-up in the condenser systems. Hypochlorite reacts rapidly with the bromide ion in seawater to from hypobromous acid and hypobromite. Perhaps these and ‘residual bromine’ are the physiologically active substances formed on chlorinating seawater (Dove, 1970). If so, then biocidal concentrations of ‘chlorine’ for marine organisms would not necessarily be the same as those reported for freshwater organisms.