Chlorine toxicity to Navicula pelliculosa and Achnanthes spp. in a flow-through system: The use of immobilised microalgae and variable chlorophyll fluorescence

Chlorination is a widely used antifouling method for freshwater and marine applications. Chlorine added to seawater reacts to form oxidants that are toxic to biofouling organisms. Further, the oxidants that result are short-lived, but may nevertheless affect non-target species in waterbodies receiving the antifouling effluent. This study evaluated the toxicity of chlorinated seawater (e.g. following sodium hypochlorite addition) on two different species of marine benthic diatoms (Achnanthes spp., and Navicula pelliculosa), which are representative of microphytobenthos communities - an important component in coastal habitats that may be exposed to chlorinated seawater. To evaluate the growth inhibition over a 72 h period, algae were immobilised in alginate beads and exposed to different levels of chlorination in a flow through system. Growth rates and physiological condition of the microalgae were evaluated using a Fast Repetition Rate fluorometer (FRRf). To determine whether alginate influenced the sensitivity of algal response, studies were also conducted in a static test system (without renewal of test solutions) using both free cells and immobilised cells with initial chlorine added to achieve a similar range of concentrations as those used in the flow-through study. Within the first hour of the exposure period there was an indication that, for both species, the free algal cells in the static system were more sensitive to exposure to chlorinated seawater than were alginate-immobilised cells in the flow through system. Immobilised cells in a static system with a single addition of chlorine were also less sensitive to chlorination than free algal cells. However, for periods of 24 h or more due to decay of TRO in the static system the exposure of immobilised algae in the flow through system had a greater impact and hence lower effect concentrations. For the flow-through studies Achnanthes spp. was the most sensitive after 72 h exposure with a potential no effect concentration EC10 value of 0.02 mg l−1 as Cl2 equivalents expressed as total residual oxidants (TRO) compared 0.04 mg l−1 TRO for N. pelliculosa. Immobilisation of algal cells in alginate was found to be an effective means of determining the impact of chlorination and is likely to be effective for other non-persistent substances. Based on the data produced, the extent and significance of ecological effects of chlorination upon algal species typical of microphytobenthos are likely to be limited providing discharges comply with a maximum allowable concentration of 0.01 mg l−1 TRO at the edge of an agreed mixing zone.