Most small farm ponds are initially constructed to hold water for long periods of time (years). As ponds age, sediment infilling causes them to become more shallow, and through time they change from permanent with fish to semi-permanent without fish and eventually to temporary without fish as shown in the figure below. This filling with sediment often takes decades. About 47% of small ponds (less than 0.25 surface acres) in the southern Great Plains dry periodically and are semi-permanent or temporary.
Mercury transport by insect emergence is controlled in part by pond permanence. Permanent ponds do not dry, allowing them to sustain fish populations. Compared with fishless ponds, ponds with fish have a lower biomass of aquatic insects.
Semi-permanent ponds that contain water over long periods do not contain fish because of periodic drying. In semi-permanent ponds without fish, large-bodied insect predators like dragonflies replace fish as the top predators in the food chain. Semi-permanent fishless habitats that contain water throughout most years tend to support the highest overall abundance of large predatory insects like dragonflies, resulting in the highest transport of mercury by insects from semi-permanent ponds.
Temporary ponds contain water for up to a few weeks, but this is not long enough for dragonflies to complete their life cycle. Mosquitoes can complete their life cycle in temporary ponds and temporary ponds can produce high numbers of adult mosquitoes. Although temporary ponds can be responsible for high amounts of mosquito production, temporary ponds only have relatively low levels of mercury transport by insects.
Damselflies, dragonflies, and nestling red-winged blackbirds
We assessed the effects of month and pond permanence on the transport of mercury out of ponds by damselflies and dragonflies and its potential risk to nestling red-winged blackbirds. As shown in the figure below, transport of mercury by damselflies, aeshnid dragonflies, and libellulid dragonflies began in March and peaked in April, May, and June, respectively, and then declined throughout the rest of the summer. Mercury transport from semi-permanent ponds without fish was greater than mercury transport from permanent ponds with fish.
Nesting of red-winged blackbirds overlapped with peak mercury transport by damselflies and dragonflies (see figure above). The diet of nestling red-winged blackbirds can be dominated by damselflies and dragonflies. We found that mercury concentrations in dragonflies were high enough to pose a health risk to nestling red-winged blackbirds.
Future research should address whether concentrations of mercury in nestling red-winged blackbirds are high enough to affect their health and if mercury contamination may be one of the factors contributing to the population decline of red-winged blackbirds in the U.S.
Sources of information
- Blackwell, B. and R. W. Drenner. 2009. Mercury contamination of macroinvertebrates in fishless grassland ponds. The Southwestern Naturalist 54:468
- Henderson, B.L., M.M. Chumchal, R.W. Drenner, Y. Deng, P. Diaz, W.H. Nowlin. 2012. Effects of fish on mercury contamination of macroinvertebrate communities of grassland ponds. Environmental Toxicology and Chemistry 31: 870- 876.
- Chumchal, M.M. and R.W. Drenner. 2015. An environmental problem hidden in plain sight: small man-made ponds, emergent insects and mercury contamination of biota in the Great Plains. Environmental Toxicology and Chemistry, 34: 1197-1205.
- Chumchal, M.M., R.W. Drenner, K.J. Adams. 2016. Abundance and size distribution of permanent and temporary farm ponds in the southeastern Great Plains. Inland Waters, 6: 258-264.
- Williams, E.B., M.M. Chumchal, R.W. Drenner, J.H. Kennedy. 2017. Seasonality of odonate-mediated methyl mercury flux from permanent and semi-permanent ponds and potential risk to red-winged blackbirds (Agelaius phoeniceus), In press at Environmental Toxicology and Chemistry.
For more information about our mercury research, go to our Aquatic Ecology Lab website.
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