Toxicity testing evaluates the effects of sediment samples on laboratory test organisms. Amphipods (Hyalella azteca) and midges (Chironomus dilutus) are exposed to sediment samples following EPA protocols, and their responses (mortality and growth) are recorded at the end of the exposure. Endpoints are measured as percent survival and growth in milligrams. Higher survival and growth indicate sediments are less contaminated and have lower potential to affect resident organisms.
Hyalella azteca is an epibenthic amphipod that is native to California streams. Wild populations have been known to become resistant to anthropogenic chemicals such as pesticides (Weston et al. 2013), but laboratory cultured populations are sensitive indicators of contaminants. In 2015, SPoT added the midge Chironomus dilutus toxicity test at sites in watersheds with greater urban land use and related contaminants. This midge is also native to California streams and builds tubes from sediment particles. Use of both organisms in tandem provides additional information on sample toxicity.
Amphipod toxicity at SPoT sites can be ranked to determine which sites are potentially impacted by contaminants. The percent survival table below takes the form of a heat map, with sites that have the most highly toxic samples listed toward the bottom (green indicates non-toxic, yellow indicates toxic, and red indicates highly toxic (survival < 38.6%)). Some sites are highly toxic year after year (403STCBQT), whereas others have intermittent samples that are non-toxic (312SMA). There are some sites with improving trends, including 801SDCxxx, which was historically toxic, but significantly increasing survival and a number of non-toxic samples in recent years.
Weston DP, Poynton HC, Wellborn GA, Lydy MJ, Blalock BJ, Sepulveda MS, Colbourne JK. 2013. Multiple origins of pyrethroid insecticide resistance across the species complex of a nontarget aquatic crustacean, Hyalella azteca. Proceedings of the National Academy of Sciences of the United States of America 110(41):16532-16537.
While toxicity has been observed at 20% of samples statewide, there were no significant trends for amphipod survival and growth, or for midge survival. The Hyalella survival and growth responses, as well as the Chironomus growth response, had significant negative correlations with urban land use, indicating increasing toxicity with increasing percent of urban land use. Midge growth has been significantly increasing since 2015. This trend indicates lower overall toxicity to the Chironomus growth endpoint.
In order to avoid subtle upward and downward trends, "No Trend" was recorded for sites that have not had any toxic samples. Sites with a history of significant toxicity tend to remain toxic, but overall few sites had significant trends for increasing or decreasing toxicity.