Toxicity of Ammonia and Nitrite to Aquatic Macroinvertebrates

The acute toxicity of ammonia was studied for six aquatic macroinvertebrate species (mayfly, stonefly, and caddisfly families). Two partial-chronic (24- and 30-day) tests were conducted on Pteronarcella badia. The acute toxicity of nitrite was studied for seven species, including one Diptera species; the mitigating effect of chloride ion on nitrite toxicity to two species was also investigated. For 6 tests on ammonia the median lethal concentration (96-hour LC50) values ranged from 1.8 to 5.0 mg/L un-ionized ammonia (NH3); in 19 tests less than 50% of the larvae died at the highest test concentration, so an LC50 could not be calculated. In the partial-chronic tests on P. badia, food consumption was not affected at concentrations up to 6.9 mg/L NH3, but concentrations in excess of 3.4 mg/L NH3 adversely affected nymphal survival rates and emergence of adults. For nitrite toxicity, test results showed a wide range of tolerance. The 96-hour LC50 for the single species of Diptera exceeded 123 mg/liter NO2-N; the 96-hour LC50 range for the other tests was between 0.25 and 2.4 mg/liter NO2-N. The addition of 10 mg/liter chloride ion in nitrite tests on P. badia and Ephemerella grandis resulted in a 3- to 10-fold decrease in 96-hour LC50 values. The tolerance to ammonia of the most sensitive of the insect species tested was greater than that reported in the literature for most species of fishes. Except for A. variegata, the range of acute toxicity of nitrite to the insects tested was similar to that reported for fishes

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements