Effects of fish and nutrient addition on emergent insect-mediated methyl mercury flux out of experimental ponds

I examined the top down effects of fish predation and the bottom up effect of nutrient addition on emergent insect-mediated methyl mercury (MeHg) flux from experimental ponds. To determine the effect fish, emergent insects were collected from ponds with (n=5) and without fish ( n=5) over a five to six-week period in 2011–2012. To determine the effect of nutrients, I fertilized six of the ponds in 2012 (three ponds with and without fish) and used four ponds as unfertilized controls (two ponds with and without fish). Fish reduced MeHg flux by suppressing emergence of large insect taxa (dragonflies and damselflies) but not small insect taxa (chironomids). Nutrient addition significantly reduced MeHg flux in herbivorous and predatory chironomids. Methyl mercury flux in emergent insects was controlled by both top down fish effects and bottom up nutrient effects.