Assessing the toxicity of aquatic sediments using Japanese medaka (Oryzias latipes) embryolarval bioassays

Soil erosion from agricultural fields is a major environmental concern on Prince Edward Island (PEI). The Wilmot River watershed is one of the most intensively farmed regions of PEI with approximately 77% of its land designated to agriculture. The main objectives of this study were to test techniques that would provide more effective and reliable analysis of aquatic sediments. These techniques were used in two initial experiments; the first to assess different sediment storage methods and the second to assess the toxicity of three different carrier solvents.

The first experiment assessed different storage methods of field collected sediment. Japanese medaka (Oryzias latipes) embryos were exposed to Wilmot River sediments that had been refrigerated, frozen, and freeze-dried. Hatching success, time to hatch, length at hatch, and developmental abnormalities were used as endpoints. Medaka exposed to freeze-dried sediments had a drastic reduction in hatching with only 2 of 72 individuals surviving to hatch. There were no significant differences found between medaka exposed to refrigerated or frozen stored sediments with 75% and 69.4% of embryos hatching, respectively. These results indicate that short term freezing does not increase the toxicity of sediments and that freeze-drying is not an appropriate method of storage for sediments to be used in bioassays.

In the second experiment, Japanese medaka embryos were used to assess the toxicity of three different carrier solvents, ethanol, methanol and acetone. Heart rate, hatching success, time to hatch, length at hatch, and developmental abnormalities were used as endpoints. This was conducted in order to select the most appropriate carrier solvent for sediment extracts to be used when comparing the toxicity of Wilmot River sediments from different sites or time points. Methanol proved to be the least toxic to medaka. With increasing concentrations, individuals exposed to ethanol and acetone exhibited delayed heart rate, reduced hatching success, and delayed time to hatch.

Finally the sediment toxicity of an upstream site (8 I) and a downstream site (82) in a river heavily impacted by agriculture (the Wilmot River) was compared. The rate of developmental abnormalities from the downstream site was also compared with results from a previous study done on at the same site on the Wilmot River in 2005. Medaka exposed to downstream sediments showed decreased hatching success, increased time to hatch, and decreased length at hatching when compared to upstream spring sediments. When compared to results from a similar study done in 2005 there were fewer developmental abnormalities. When exposed to sediment extracts from the 2005 study as well as the present study, medaka exhibited no signs of exposure to toxicants. Pesticide analysis showed trace amounts of several pesticides (e.g. linuron and imidacloprid), with concentrations varying between the years. Analysis of metals (copper, cadmium, and zinc) showed that levels of all three remained constant between the two time points. The techniques assessed here represent a substantial contribution to our knowledge of aquatic sediment analysis.