Phosphorus availability amplifies negative effects of silver nanoparticles on lake periphyton

Background/Question/Methods

Discharge of silver nanoparticles (AgNP) to freshwater ecosystems is an emerging environmental concern. AgNP are antimicrobial and have been shown to inhibit the growth of pelagic algae and bacteria. Less is known about its effects on other components of freshwater ecosystems, including the benthos. We investigated the effect of AgNP on littoral periphyton in three lakes at the Experimental Lakes Area in northern Ontario. Periphyton in ELA lakes has been shown to be phosphorus limited, so we tested the effects of AgNP under P limiting (ambient) and P-enriched conditions. We used nutrient diffusing substrates (NDS) filled with agar treated with PVP-capped AgNP (50 nm diameter) and phosphorus (P). NDS were constructed using a full factorial design with three levels of AgNP concentrations in the media: control (0 mg AgNP/L), low (3 mg AgNP/L), and high (12 mg AgNP/L), and three levels of P concentration in the media: control (0 M), low (0.0005 M), and high (0.005 M).  Three replicate NDS of each treatment combination were placed in each lake for ~28 days. Upon removal from the lakes, periphyton was gently scrubbed from NDS and sampled for carbon (C), nitrogen (N), P, chlorophyll a, and silver content. 

Results/Conclusions

Our results show that the effects of AgNP on the autotrophic component of periphyton varied with phosphorus availability. AgNP exposure decreased chlorophyll a standing stocks by 40% in the low P treatments and by almost 90% in the high P treatments. Periphyton C was not strongly affected by AgNP, such that exposure to AgNP and elevated P supply increased the C: chlorophyll a ratios in the periphyton. Periphyton C:N, C:P, and N:P were not influenced by AgNP exposure, but varied among lakes and with P supply. Our results demonstrate that the effects of AgNP discharge on benthic communities may be amplified in eutrophic lakes and rivers, such as areas below waste water treatment plants where AgNP is likely to be introduced.