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Impacts of bleach on bryozoans: A framework to distinguish direct and indirect effects using chemical and physical manipulations

Mayer-Pinto, Mariana
The Science of the total environment 2017 v.599-600 pp. 58-67
Bryozoa, biocides, biofilm, field experimentation, organisms, photosynthesis
Chemical disturbances, caused by contamination, are a global issue and can cause changes in the abundance of populations of one or more species via direct and/or indirect effects. This, in turn, can have profound consequences on assemblages and/or systems. Understanding how contaminants affect functional groups or taxa, i.e. which ecological or biological processes are being affected, is necessary to better predict their consequences. To distinguish between direct and indirect effects of contaminants, however, specific experiments, physically manipulating the changing variable (in this case, biofilm) as well as the contaminant, are necessary. Here, I tested which processes were affected by bleach, a biocide commonly found in urban run-offs that caused a decrease in covers of bryozoans. Effects of bleach on recruitment and growth of bryozoans were variable, suggesting that impacts are complex. Nevertheless, results indicate that bleach reduced recruitment of bryozoans. Therefore, manipulative field experiments were done to test whether these effects were direct or indirect, through changes in the abundance of photosynthetic biofilms. Responses of biofilms varied with the duration and exposure to bleach, as well as the timing of experiments. Abundance of biofilms did not seem, however, to affect the number of recruits of bryozoans, which could suggest a direct effect of bleach on bryozoan recruitment. Given that bryozoans are a major component of subtidal benthic assemblages and an important food source for >300 species, decreases in their abundance, as those observed here, might have important knock-on effects on marine systems due to trophic cascades.Capsule.In situ manipulative experiments showed that bleach decreased covers of bryozoans, which is possibly due to a combination of effects on their growth and recruitment. Manual removals and chemically induced removals are necessary to disentangle direct and indirect effects of contaminants.